GET THE APP

From Bench to Barn: Plant Model Research and its Applications in
Advancements in Genetic Engineering

Advancements in Genetic Engineering
Open Access

ISSN: 2169-0111

+44 1478 350008

Research Article - (2013) Volume 2, Issue 2

From Bench to Barn: Plant Model Research and its Applications in Agriculture

Andrea Pitzschke*
Department of Applied Genetics and Cell Biology, University of Natural Resources and Applied Life Sciences, Austria
*Corresponding Author: Andrea Pitzschke, Department of Applied Genetics and Cell Biology, University of Natural Resources and Applied Life Sciences, Vienna, Austria, Tel: +43 1 47654-6358, -6359, Fax: +43 1 47654-6392 Email:

Abstract

In view of the increasing world population and climate change, solutions are needed for improving agricultural productivity and energy supply. Biologists recognized the merits of employing model organisms as representatives of their species/subspecies to investigate molecular principles and pathways in great depth. For most model organisms, full genome sequence information and numerous bioinformatic resources are available. Protocols for transgenesis of model plants and various crops have been established. It is this jewelry of data and tools that helps to improve plant properties in a highly targeted manner.

Combining transgenic approaches with traditional plant breeding is a promising strategy to enhance agricultural productivity and the nutritional value of crops, to clean up contaminated environments and to improve efficiency in biofuel production. This review article provides an update on plant model organisms, including monocots and dicots; legumes and trees. It summarises current bioinformatic resources and transgenesis protocols. Recent achievements in genetic engineering are highlighted and theoretic scenarios for further optimization of plants are discussed.

<

Keywords: Plant model organisms; Symbiosis; Sustainable agriculture; Transgenesis; Bioinformatic resources

Plant Models and their Merits

The diversity of living species on our planet is immense. It is impossible to understand each of them in detail. Phylogenetically related species display strong similarities in their genetic set-up, physiology and behaviour. Moreover, at the molecular level, principle cellular signaling pathways are highly conserved. Biologists recognised the merits of employing model organisms as representatives of their species/subspecies to investigate the phenomena and mechanisms of development in great depth. The focused and extensive study in these experimental models has aided progress in microbiology, animal and plant science enormously [1].

 

As a result of these concerted efforts, for most model organisms full genome sequencing is accomplished and various omics data are available. The strategic use of bioinformatic resources makes the value of these data to disentangle biological complexity, to unravel networks of molecular interactions and finally to predict mechanisms in distantly related species indefinite [2].

Arabidopsis thaliana

Arabidopsis thaliana has emerged as the model organism of choice in plant biology [3]. Almost 30 years later, this modest weed is the by far best-studied plant. It was the first plant to be fully sequenced (Arabidopsis Genome Initiative, 2000), followed by rice [4], poplar [5] and others (Table 1). As a member of the mustard (Brassicacea) family, Arabidopsis is closely related to cultivated species, including oilseed rape and cabbages. A. thaliana offers important advantages for basic research in genetics and molecular biology: It is small in size and can be easily cultivated in restricted space.

>
Organism Importance, research area (haploid) genome
size
Reference
Arabidopsis thaliana general model plant 5 chromosomes,
diploid, 119 Mb
(Kaul et al., 2000)
Arabidopsis lyrata close relative of A. thaliana 5 chromosomes,
diploid, 207 Mb
(Hu et al., 2011)
comparison A.
thaliana vs. A. lyrata
functional, evolutionary and ecological studies    
Arabidopsis halleri metal-binding model organism 8 chromosomes,
diploid, estimated
size 255 Mb
(Johnston et al., 2005)
comparison A. lyrata
vs. A. halleri
identification of crucial factors in metal binding/
metal accumulation. biotechnological relevance:
phytoremediation, bio-fortification
   
Lotus japonicus model legumes
nitrogen fixation,
6 chromosomes,
diploid, 470 Mb
 
Medicago truncatula root nodule symbiosis and arbuscular mycorrhzia,
incl. overlap (common symbiosis pathway)
8 chromosomes,
diploid, 240 Mb
(Young et al., 2011)
Populus trichocarpa
(poplar, black cottonwood)
model tree, carbon sequestration
forest tree growth and development
phytoremediation
difficulties:
no self-pollination
long generation interval (4-8 years)
19 chromosomes,
diploid, 550 Mb
(Tuskan et al., 2006)
Brachypodium distachyon model monocot 5 chromosomes,
diploid, 227 Mb
(InternationalBrachy
podiumInitiative,
2010)
Oryza sativa ssp.
japonica
model monocot; food crop 12 chromosomes,
diploid, 420 Mb
(Goff et al., 2002)
Oryza sativa ssp.
indica
model monocot; food crop 12 chromosomes,
diploid, 466 Mb
(Yu et al., 2002)
Comparison O.s.
japonica vs. O.s.
indica
Identification of single nucleotide polymorphisms,
transcripts and proteins related to altered biotic and
abiotic stress tolerance levels in rice
  e.g. (Lee et al., 2003;
Ventelon-Debout et al., 2004; Liu et al., 2007)
 

Table 1: Model plants and their importance for main aspects in plant research.

The life cycle is short (6-8 weeks). A. thaliana has a small genome, comprising 119 Mb DNA and 26,000 genes. Extensive genetic and physical maps of all 5 chromosomes as well as a large number of mutant lines and genomic resources are available (Table 2). The plant is easily accessible to genetic manipulation (see below). The vast information arising from efforts of an international research community is collected and made accessible by The Arabidopsis Information Resource (TAIR).

Organism Bioinformatics data/ resources Link
Arabidopsis thaliana Central access point for Arabidopsis data; large  set  of  visualization  and  analysis TAIR
  Tools http://www.arabidopsis.org/index.jsp
(Lamesch et al., 2012)       
  gene mapping tool http://signal.salk.edu/cgi-bin/tdnaexpress
  AtTFDB, comprehensive database for
transcription factors
http://arabidopsis.med.ohio-
state.edu/AtTFDB/
Lotus japonicus various      links/tools      for      functional genomics http://www.kazusa.or.jp/lotus/
Medicago truncatula various links/tools for functional
genomics
http://www.medicago.org/
  Gene Expression Atlas: expression profile of all major organs and under many treatments http://mtgea.noble.org/v2/
MtGEA V2
  Medicago hapmap, sequence information of 330 inbred M.  truncatula accessions. For discovering single nucleotide polymorphisms insertions/  deletions  and copy number variants. http://www.medicagohapmap.org/home/view
legumes LegumeTFDB, comprehensive database for transcription factors http://legumetfdb.psc.riken.jp/
Populus    trichocarpa
(black cotton wood)
Overview,    various    links,    functional genomics tools http://www.ornl.gov/sci/ipgc/
http://popgenie.org/
Brachypodium
distachyon
EST sequence database, links to homology-based annotations www.brachypodium.org
Oryza sativa (rice) functional   genomics   database,   identify mutants in gene of interest / mapping tool http://signal.salk.edu/cgi-bin/RiceGE
  Obtain full-length cDNA clones (http://cdna01.dna.affrc.go.jp/cDNA/)
Monocots GRAMENE
for comparative genome analysis to study cross-species homology
http://www.gramene.org/
  GRASSIUS, collection of   databases, computational and experimental resources that   relate   to   the   control   of   gene expression in the grasses, and their relationship with agronomic traits http://grassius.org/
various  species,  incl.
most model organisms
extensive set of genomic, gene expression, protein and mapping tools BAR  –  Bio-Analytic  Resource  for  Plant
Biology http://bar.utoronto.ca/welcome.htm
  transcription factor database http://plntfdb.bio.uni-potsdam.de/v3.0/
  PLACE, database of cis-acting regulatory DNA elements  
  tools and resources for plant genomics http://plantgdb.org/

Table 2: Bioinformatic resources and tools for research in model plants and selected crops.

Arabidopsis halleri and Arabidopsis lyrata

Two species in the Arabidopsis genus have attracted the attention of plant scientists, Arabidopsis lyrata and Arabidopsis halleri. The A. lyrata genome is approximately 65% larger genome than that of A. thaliana [6]. Full genome sequencing was accomplished in 2011 [6]. A. lyrata and A. thaliana, which are phenotypically very similar, can now be compared systematically at the genome level. Such comparison will advance our understanding of mutation and selection in plants. Surprisingly, despite its much smaller size, the A. thaliana genome only contains 17% fewer genes than A. lyrata [6].

Since the divergence of these two species, approximately 10 million years ago, numerous small deletions, mostly in non-coding DNA and transposes have occurred in A. thaliana [6]. A. lyrata is very closely related to Arabidopsis halleri. The metal-accumulating ability of A. halleri is of interest both for nutritional (biofortification) and ecological (phytoremediation) reasons. A recent study has documented the usability of A. halleri for phytoextraction of cadmium-polluted soil [7].

Genome sequencing of A. halleri is in progress (the Joint Genome Initiative). Comprehensive analyses of the genetic set-up and transcription profiles of A. lyrata vs. A. halleri will facilitate the identification of factors and pathways involved in metal binding. Individual factors or even entire pathways that mediate metal binding in A. halleri can be revealed and functionally characterized in a more systematic manner.

Upon further optimization (e.g. replacing particular amino acid residues to enhance enzyme activities), such factors or pathways may be engineered to confer metal-binding properties on transgenic target plants. Potentially, this strategy presents a means of large-scale phytoremediation. Which target species to be used is determined by the specific characteristics of the affected area. Obviously, such target species must be accessible to genetic manipulation. Two recent studies in tomato and tobacco have shown the principle applicability of this approach [8,9].

Symbiosis research and model legumes

Not all key questions in plant science can be addressed in Arabidopsis. Most land plants form symbiotic associations with arbuscular mycorrhizal (AM) fungi. Arabidopsis is one exemption to this rule. Many agricultural crops are mycorrhizal, and despite marked differences in the organization of monocot and dicot root systems, the morphology of fungal colonization is similar. Endosymbioses (i.e. microbe lives within plant cells) are of imminent agricultural importance, because the host plant’s benefits from these associations directly correlate with harvest yield parameters. Benefits associated with AM symbiosis formation include improved phosphate nutrition, enhanced resistance to soil-borne pests and disease, improved resistance to drought, tolerance to heavy metals and better soil structure [10,11].

The second major type of endosymbiosis is the association of leguminous plants with nitrogen-fixing bacteria. This so-called root nodule (RN) symbiosis is much less abundant than AM and is restricted to some species belonging to the rosid I clade [12]. All legumes are found in one branch of this clade. RN symbiosis has a high economic and ecologic value and presents a solution towards sustainable agriculture. Biologically fixed nitrogen offers a number of benefits over the use of nitrogen fertilizer as the primary source of nitrogen input into crop production systems [13,14]. These benefits include: improved soil physical conditions, less potential for environmental degradation and lower costs. Crop rotation and intercropping of legumes with cereals are known to contribute to soil fertility [15,16]. In addition, legumes themselves provide a major source of food for humans and domestic animals.

The above paragraphs document the agricultural meaning of plant symbioses and indicate how important it is to understand these plantmicrobe associations in detail. Two leguminous plants, Lotus japonicus [17] and Medicago truncatula [18], have emerged as equal-ranking model organisms to study both arbuscular mycorrhiza and root nodule symbiosis. Research in these model legumes has furthermore disclosed some striking similarities in cellular signaling of symbiotic bacterial invasion and pathogen attack [19]. Genome sequence data of L. japonicus [20] and M. truncatula [21] are available.

The most important food legume, soybean Glycine max, is now also fully sequenced [22]. Comprehensive bioinformatic resources (Table 2) will assist functional genomics studies in L. japonicus, M. truncatula and G. max. These tools can be employed intensively for collaborative analyses to direct identification of e.g. conserved features of proteins and protein families of interest (specific amino acid residues, peptide motifs, calculated three-dimensional structures). Ultimately, potentially promising traits, such as enhanced stress tolerance, or higher nutritional value can be optimised prior to introduction into soybean, in a very targeted manner.

The ability of L. japonicus (Sato et al. [20]) and M. truncatula to engage in the two main types of symbiosis has allowed to define overlaps and unique features in the plant’s response to arbuscular mycorrhizal fungi and nitrogen-fixing rhizobacteria [12]. The establishment of these symbioses involves calcium oscillations in plant root cells. A distinct set of genes encoding host factors non-specifically required for the formation of Arbuscular mycorrhiza and root nodules could be identified.

This provided insight into the evolution of plant-microbe interactions. Using forward genetic screens in legumes, a signaling pathway, the common symbiosis pathway, was uncovered (reviewed by Parniske [23]). As mentioned above, AM symbiosis is widely distributed in the plant kingdom. The evolutionary much younger legume root nodule symbiosis has likely evolved from the ancient AM symbiosis. Common symbiosis pathway genes may therefore be functionally conserved between legumes and non-nodulating plants plants [23]. In fact, the orthologs of three common symbiosis genes from model legumes were also found in rice, OsPollux, OsCastor and OsCCaMK [24].

Castor and Pollux encode nuclear porins. In legumes, these proteins likely act as ion channels to modulate the nuclear membrane potential, thereby contributing to the perinuclear calcium spiking induced during legume symbioses [25]. CCaMK encodes a calcium/ calmodulin-dependent protein kinase, which is essential in the interpretation of calcium oscillations in plant root cells for the establishment of AM and root nodule symbiosis [26-28]. OsPollux and OsCCaMK were shown to be indispensable for rice AM symbiosis [24]. What is more, overexpression of either OsCastor or OsCCaMK could fully complement AM and RN symbiosis defects in the corresponding Lotus japonicus mutant lines [24].

From a biotechnological perspective, the straight-forward question is: “What is the “minimal legume code?” Can RN symbiosis be transferred to other crops, thus minimizing the need for exogenous nitrogen supply?” A recent break-through technology for multiple gene transfer via Agrobacterium tumefaciens-mediated transformation (see below), was the development of the binary vector pHUGE-Red. In a case study, eight genes essential for Medicago truncatula to establish a symbiosis with rhizobia bacteria were transferred as one 74 kb TDNA into four non-leguminous species (strawberry, poplar, tomato and tobacco) [29]. Intriguingly, the vector was also equipped with an inducible recombination system allowing subsequent removal of the selection markers in the newly generated transgenic plants. It appears feasible to imprint not only single functions but entire pathways onto plants.

Model monocots

The above-described model plants are all dicots and thus only distantly related to the most important plants for human subsistence, the domesticated grass crops rice and wheat.

Oryza sativa: Oryza sativa (rice), whose genome was the second plant genome to be fully sequenced [4], has served as reference monocot for many years. Rice is the cereal with the smallest genome, consisting of 420-466 Mb across 12 chromosomes (Table 1). The two major subspecies, japonica and indica differ in their growth requirements and grain texture. While ssp. japonica, which has short and sticky grains, is grown in dry fields, ssp. indica produces non-sticky long grains and is grown mostly submerged. Rice molecular biologists have benefited from the fact that both subspecies are fully sequenced. A comparison between ten varieties of japonica and indica revealed a higher salinity tolerance in the indica varieties [30].

Furthermore, in viral infection experiments with japonica and indica cells, followed by transcriptome and proteome analysis a certain overlap between virus-induced and abiotic stress-related responses was noted. A number of transcripts/proteins differentially expressed in the partially resistant japonica cultivar were identified [31,32]. Altered transcriptome profiles as well as substantial sequence divergence between indica and japonica subspecies have also been observed in a more recent large-scale study [33]. The rice research community has provided resource to numerous bioinformatic tools and data sets. GRAMENE and GRASIUS furthermore facilitate cross-species comparisons within the monocot group (Table 2).

Brachypodium distachyon: Unfortunately, unlike Arabidopsis (petite, rapid growth, small genome, self-compatible), rice has less convenient characteristics. It is demanding in its growth requirements; the rice genome is approximately four times larger than that of Arabidopsis. An alternative species was proposed as model plant for functional genomics in monocots: Brachypodium distachyon [34]. In terms of plant size (15-20 cm), generation time (8-12 weeks) and growth requirements (simple), Brachypodium distachyon is similar to Arabidopsis (Table 1).

These desirable features were the driving force for establishing Brachypodium distachyon as grass Arabidopsis [35]. As a result, key resources including genetic stocks, EST collections, BAC libraries and transformation protocols have become available [36] (Table 2). Synergistic efforts of the International Brachypodium Initiative allowed accomplishing full sequencing and annotation of the Brachypodium genome (International Brachypodium Initiative, 2010). A large collection of T-DNA insertion lines can be employed for reverse genetics approaches. Thus, in terms of research progress, Brachypodium will keep pace with Arabidopsis. Both species are excellent models for monocots and dicots, respectively. Their value further correlates with the availability of comprehensive bioinformatics resources (Table 2).

Since its introduction as model monocot, the value of Brachypodium for addressing diverse areas of plant biology has been amply substantiated. To name a few, these reports include studies on vernalisation [37] and flowering time pathways [38] and seed storage proteins [39], as well as responses to drought stress [40,41] and pathogens [42,43]. The roles of microRNAs involved in drought and cold stress responses have also been addressed [44,45]. The number of reports on comparative genetic analysis of Brachypodium and cultivated crop species is growing [38,46-48].

One highlight demonstrating the biotechnological potential of Brachypodium is found in a very recent report [49]. The authors characterized a cinnamoyl alcohol dehydrogenase gene, BdCAD1, involved in lignification. Disruption of the gene alters the branching of lignin; saccharification yields are improved in Bdcad1 mutant lines, while biomass yield is not compromised. With respect to optimizing the cellulose-to-ethanol conversion process, knocking out or silencing BdCAD1 homologs in other grasses is a promising approach for the fuel industry. An equally promising strategy is the overexpression of recombinant cellulolytic enzymes in sugarcane [50].

The simplicity of A. thaliana and B. distachyon on the one hand and their taxonomic distance on the other hand shall assist identification of universal factors. If a particular Arabidopsis gene (characterised, documented positive effect on e.g. stress resistance) has similar effects when heterologously overexpressed in B. distachyon (and vice versa), it may be functional in most plants (monocots and dicots). Reciprocally, a B. distachyon gene that confers desired characteristics to transgenic A. thaliana may also act universally upon heterologous overexpression in other target species (Figure 1).

advancements-genetic-engineering-taxonomic-distance-Arabidopsis

Figure 1: Employing simplicity and taxonomic distance of Arabidopsis and brachypodium genetics to test candidates “universal” genes. Strategy 1 (top) starts with a plant over expressing an advantageous gene (of known sequence). Strategy 2 (bottom) starts with a mutant lacking a disadvantageous gene (of known sequence).

Model trees and biofuel: Forest trees have tremendous economic and ecological value. Wood is an important source for fuel. It also represents the major form of structural material in industry. Forests are unique ecosystems; they prevent soil erosion and play an irreplaceable role in carbon sequestration. A number of anatomical and physiological features solely occur in trees. These include e.g. their perennial growth habit, extensive formation of secondary xylem (i.e. wood), seasonal reallocation of nutrients, cold hardiness and longevity [51].

Research in very large and long-lived organisms is a difficult and challenging undertaking. From these considerations, the demand for a model system for forest trees became apparent. In 2000, poplar (Populus) was proposed as model forest tree [51]. Now, black cottonwood (Populus trichocarpa) is the established model organism in tree research. Poplar combines several advantages, including rapid growth, prolific sexual reproduction, ease of cloning and accessibility to genetic transformation (Table 3). The Populus trichocarpa genome is comparatively small (550 Mb); it was the first tree genome to be fully sequenced [5] (Table 1). Poplars are bred on detached female branches with pollen that can be stored for several years Seeds germinate (within 24 h) and grow rapidly (1-2 m in the first year) [51].

Organism Transformation principle, time required (co-cultivation to the planting of transformants into pots) Protocol references
Arabidopsis thaliana floral dip (Weigel and Glazebrook, 2006)
Arabidopsis lyrata root explants, 4 months (Fobis-Loisy et al., 2007)
Arabidopsis halleri root explants (Hanikenne et al., 2008)
Brassica napus (rapeseed) seedling explants, 2.5-3.5 months (Bhalla and Singh, 2008)
Lotus japonicus root explants, 4 months (Lombari et al., 2003)
Medicago truncatula root explants, 4 months (Crane et al., 2006)
alternative:                 Agrobacterium
rhizogenes-mediated  transformation via hairy roots
(Deng  et  al.,  2011)  (Crane  et  al.,
2006)
Glycine max (soybean) hypocotyls (Wang and Xu, 2008)
Brachypodium distachyon immature embryos, 8 months (Pacurar  et  al.,  2008;  Vain  et  al.,
2008; Vogel and Hill, 2008; Alves et al., 2009; Thole and Vain, 2012)
Oryza sativa ssp. indica explants   from   mature   seeds,   2
months
(Sahoo et al., 2011)
Oryza sativa ssp. japonica embryos (scutellum), 2-3 months (Duan et al., 2012)
additional advance:
non-antibiotic selectable agent
Triticum (wheat) floral dip (pre-anthesis spikes with
clipped florets)
(Agarwal et  al.,  2009;  Zale  et  al.,
2009)
in-planta   transformation   of   germ cells (agrobacterial incubation of pistil filaments during artificial pollination) (Mamontova et al., 2010)
Triticosecale ssp. (triticale) in     vitro-prepared     nano-complex consisting of transferred DNA, agrobacterial virulence  protein  D2, and recombination protein A delivered to triticale microspores, 2 months (Ziemienowicz et al., 2012)
Zea mays (maize) embryos, 3 months (Ishida  et  al.,  2007;  Sidorov  and
Duncan,  2009;  Mamontova  et  al.,
2010)
Populus trichocarpa (black cotton
wood)
stem explants, 5 months (Song et al., 2006)
Transient expression systems
Nicotiana benthamiana NIcotiana tabacum leaf infiltration (Cevik and Kazan, 2013)
(Persak and Pitzschke, 2013)
Tropaeolum majus leaf infiltration (Pitzschke, 2013)

Table 3: Model plants and current transgenesis protocols via Agrobacterium-mediated transformation. For an overview on alternative transformation strategies the reader is referred to a recent review (Barampuram and Zhang, 2011) and references therein.

Unlike many other model organisms, poplar is of direct economic and ecologic importance: Besides its value as source of wood and fuel, poplars can be utilised for phytoremediation of environmental toxins [52-54] as well as for long-term biomonitoring of soil contamination [55]. Poplar is also considered suitable for large-scale carbon sequestration [56-58]. Poplar plantations might thus be used to partially offset increasing atmospheric carbon dioxide concentrations.

The accessibility of poplar to transgenesis (Table 3) on the one hand and the rich pool of bioinformatic resources on the other hand (Table 3) renders this tree a prime candidate for the development of transgenic energy crops. A recent study exemplifies the value of combinatorial bioinformatic and transgenic approaches: Transgenic poplar, in which the RabG3bCA gene (encoding a GTPase) from Arabidopsis was overexpressed, were taller and thicker and had a higher cellulose content and enhanced enzymatic digestibility (saccharification) compared to wild- type poplar [59]. Furthermore, fiber cell length, which is the most important factor determining paper and pulp quality, was increased in RAbG3b oxerexpressing poplar. This transgenic line and its use were patented in 2013.

Transgenesis: A plant’s value as a model organisms and its usability for functional genomics strongly depends on its accessibility to genetic manipulation, i.e., transformation with tailor-made DNA constructs. The major plant transgenesis technologies include agrobacteriummediated transformation, microprojectile bombardment, electroporation, pollen tube pathway transformation and silicon-carbide-whiskersmediated transformation. A comprehensive overview of these methods has recently been published by Barampuram and Zhang [60]. Agrobacterium-mediated transformation, the most predominantly employed strategy, will be addressed in the following paragraphs. A contemporary review on novel nanotechnology-based strategies can be found in Rafsanjani et al. [61].

The transfer of designed DNA cassettes into plant cells with the aid of Agrobacterium tumefaciens, is a standard method for plant transformation [62-64]. An overview about alternative, less often employed strategies can be found in Ziemienowicz [65]. The soil-borne pathogen A. tumefaciens is the only organism capable of inter-kingdom DNA transfer. Under natural conditions, it causes tumor growth in infected host plants. However, so-called “disarmed” agrobacterial strains, deprived of their tumorigenicity, can be employed as “DNA shuttles” for the delivery of engineered transfer DNA (T-DNA) into plant cells [62-64].

Depending on whether the nuclear-targeted T-DNA is degraded or incorporated into the host’s chromosomal DNA, agrobacterial infection leads to transient (lasting several days) or stable (inherited) transgene expression [66]. In the latter case, whole transgenic plants can be obtained. The regeneration of stable transgenic plants mostly involves the rather tedious procedure of tissue culturing. Briefly, explants from various organs are incubated with agrobacteria, resulting in the formation of non- differentiated cells (calli), from which whole plants can be regenerated.

Protocols are being developed and modified frequently to simplify and shorten the transformation procedure and/or to enhance transformation rates. Table 3 provides an overview on current transformation protocols, with a focus on model plants and selected crop species. Table 4 lists a number of recent examples of transgenic crops with improved stress tolerance. This list is not saturating but shall provide a glimpse on the diverse possibilities. Arabidopsis thaliana stable transformants are easily obtained via the “floral dip” method [67]. Plants at the early-flowering stage are dipped into agrobacterial suspension and allowed to set seeds. Normally, 0.1-1% of seeds, representing the next generation, harbor one or multiple copies of the transgene [68]. Floral dipping may also be applicable to transgenesis of wheat [69,70].

Source of transgene Constitutive expression,
promoter desription
Stress-related
phenotype in transgenic crops
Reference
TaNAC69, Stress-responsive
transcription factor from wheat
ectopic expression (drought-
inducible HvDhn4s promoter)
enhanced tolerance to
moderate salt and
drought stress in barley
(Xue et al., 2011)
OsNAC1, stress-responsive
transcription factor from rice
overexpression (maize
ubiquitin promoter)
enhanced tolerance to
salt and drought stress in wheat
(Saad et al., 2013)
ZmPTF1, low-phosphate-
inducible bHLH-type transcription factor from maize
overexpression enhanced tolerance to
phosphate deficiency
in maize
(Li et al., 2011)
OsNAC5, NAC transcription
factor from rice
root-specific promoter RCc3:OsNAC5 enhanced drought
tolerance and increased
grain yield in rice
(Jeong et al., 2013)
LOS5/ABA3 (molybdenum
cofactor sulphurase) indirectly controls abscisic acid biosynthesis
in Arabidopsis
constitutive overexpression enhanced drought
tolerance, antixodative enzyme activities and proline accumulation in soybean
(Li et al., 2013)
TsVP, proton-translocating
PPase from Thellungiella halophila
constitutive overexpression
(CaMV35S promoter)
enhanced osmotic +
drought stress tolerance and higher seed yield in cotton
(Lv et al., 2009)
TsVP, proton-translocating
PPase from Thellungiella halophila
constitutive overexpression
(ubiquitin promoter)
enhanced tolerance to
phosphate deficiency
in maize
(Pei et al., 2012)
AtNDPK2, Nucleoside
diphosphate kinase 2 from Arabidopsis
oxidative stress-inducible
SWPA2 promoter from sweet potato
enhanced tolerance to
cold, salinity and oxidative stress in potato and sweetpotato
(Tang et al., 2008; Kim
et al., 2009)
enhanced tolerance to
oxidative stress, antioxidant enzyme activities and growth in poplar
(Kim et al., 2011)

Table 4: Examples of transgenic crops exhibiting enhanced stress tolerance. It should be noted that in many cases the transgene employed for crop transformation originates from a model plant.

The simplicity of Arabidopsis floral dipping and its versatility as heterologous expression system has facilitated characterization of genes from various plant species. For biotechnological applications, a fast and informative insight into factors from Arabidopsis’ close relative, rapeseed (Brassica napus), is particularly interesting. Integrative molecular biology studies involving Arabidopsis and rapeseed aim at enhancing bio-diesel production. For example, wri1 (encoding an AP2/ EREBP transcription factor) was found to be differentially expressed in B. napus lines with different oil content. Constitutive overexpression of Bnwri1 in A. thaliana resulted in highly elevated seed oil content and enlarged seed size and mass [71]. Similar effects were observed in transgenic Arabidopsis overexpressing BnGRF2 (growth-regulating factor2, a transcription factor) [72].

The authors concluded that Bnwri1 and BnGRF2 have potential applications in food and oil industries and in rapeseed breeding. Reciprocally, overexpression of AtNHX1, a vacuolar Na(+)/H(+) antiport protein from Arabidopsis, conferred enhanced salt tolerance to rapeseed [73]. These examples show the interchangeability of genetic factors between Arabidopsis and rapeseed and suggest that desired properties in crops can be achieved through the modification of a single trait.

Agrobacterium-mediated transformation for transient expression studies

Stable transformation is the only means to maintain transgene expression in the next generations. However, for various questions in molecular biology, transient expression systems are the method of choice. They provide rapid answers on e.g. subcellular localization, stability and posttranslational modification of recombinant proteins. Agrobacterial infiltration into Nicotiana leaves (N. benthamiana, N. tabacum) is the most popular system for transient expression studies in plant tissue [74].

Chimeric promoter-reporter gene constructs can be utilized to assess promoter activities and their responsiveness to environmental stimuli. This approach can also be employed for identification of regulatory cis-elements. Moreover, transient expression directed by heterologous promoters provides some (limited) information on gene regulation in other species. Recent advances in that direction include the identification of a novel moderate constitutive promoter from poplar [75] and the characterization of an AP2-family transcription factor-activated promoter from chickpea involved in growth regulation and stress tolerance [76].

Because multiple transgenes can be delivered at once, transient expression in Nicotiana is a convenient system for studying proteinprotein interactions and promoter/effector (transcription factor) relations–a comparatively simple approach towards the deciphering of signaling pathways [77]. Agrobacterial infiltration of Nicotiana yields transformation efficiencies (i.e. fraction of transgene-expressing cells) of up to 100% (own data). However, recombinant protein accumulation may vary significantly depending on leaf age and leaf position, thus complicating quantitative comparisons. A recently developed protocol circumvents this problem: Randomised pools of leaf discs rather than leaves of intact N. tabacum plants are subjected to agrobacterial treatment [78].

Unfortunately, most plants are poorly, or not at all, accessible to transient transformation by Agrobacteria. In Arabidopsis, transformation efficiency is low; it strictly depends on growth conditions and has yielded satisfactory results only in young seedlings [79]. Since Nicotiana (order Solanales) is only distantly related to Arabidopsis (order Brassicales), data from heterologous expression in Nicotiana may not fully reflect the situation in Arabidopsis. Clearly, an expression system combining the simplicity of leaf infiltration and the detailed knowledge of Arabidopsis genomics would be desirable.

The development of such expression system is part of my current research. As reported very recently, nasturtium (Tropaeolum majus), which belongs to the same order as Arabidopsis, i.e., Brassicales, is easily accessible to agroinfiltration, yielding high transformation efficiencies. This new expression system may provide the Arabidopsis community with a tool to study subcellular localisation, protein-protein interactions and reporter gene activities in a genetic background that is closely related to their model organism [80]. An additional plus is the fact that Tropaeolum majus can be colonized by arbuscular mycorrhizal fungi.

Fine-tuning gene expression for improving stress tolerance

Transcription factors (TFs) are key components of signal transduction pathways. TFs translate information received from their upstream regulators (e.g. stress-activated kinases) into altered expression of target genes. Since TFs usually have numerous target genes, they can initiate substantial re-programming of a plant’s transcriptome. TFs are particularly promising candidates for genetic engineering of stress-tolerant crops. However, improved stress resistance through overexpression of TFs often comes at the expense of retarded growth or other developmental abnormalities [81,82].

The main reasons for these undesired side effects are: i) the introduced transcription factor regulates additional genes besides the desired target(s); and ii) permanent hyper-activation of stress response pathways is costly; and energy resources are withdrawn from “maintenance programmes”. These side effects may be circumvented or reduced, by placing candidate transcription factors under the control of stress-responsive promoters. Transcriptome analysis (employing bioinformatic resources) to identify genes displaying low basal but strongly stress-induced expression levels. Isolate promoter of such gene. Select a transcription factor. Criteria: Documented positive regulator of stress in at least one species, well-conserved protein sequence.

This can for instance be a TF from Arabidopsis or another model plant. Place this transcription factor under control of above-mentioned stress-responsive promoter and introduce the resulting construct into the species of interest. Combining the knowledge about conserved stress-responsive promoters plus functional characterization of candidate TF proteins will allow activation of self-defense mechanisms in engineered plants on demand (Figure 2). Ideally, only as long as a certain stressor is present, will a TF be produced, thereby minimizing undesired side effects.

advancements-genetic-engineering-Fine-tuning-gene

Figure 2: Fine-tuning gene expression for improving stress tolerance in crops. A chimeric DNA construct, consisting of a stress-responsive promoter from a crop species and a functionally-conserved well- characterised transcription factor gene from a model plant is employed. See main manuscript for details.

Acknowledgement

I gratefully acknowledge the Austrian Science Foundation (FWF) for funding (Elise-Richter-Project V167-B09).

References

  1. Fields S, Johnston M (2005) Cell biology. Whither model organism research? Science 307: 1885-1886.
  2. Pitzschke A, Hirt H (2010) Bioinformatic and systems biology tools to generate testable models of signaling pathways and their targets. Plant Physiol 152: 460-469.
  3. Koornneef M, Meinke D (2010) The development of Arabidopsis as a model plant. Plant J 61: 909-921.
  4. Yu J, Hu S, Wang J, Wong GKS, Li S, et al. (2002) A Draft Sequence of the Rice Genome (Oryza sativa L. ssp. indica). Science 296: 79-92
  5. Tuskan GA, Difazio S, Jansson S, Bohlmann J, Grigoriev I, et al. (2006) The genome of black cottonwood, Populus trichocarpa (Torr. & Gray). Science 313: 1596-1604.
  6. Hu TT, Pattyn P, Bakker EG, Cao J, Cheng JF, et al. (2011) The Arabidopsis lyrata genome sequence and the basis of rapid genome size change. Nat Genet 43: 476-481.
  7. Claire-Lise M, Nathalie V (2012) The use of the model species Arabidopsis halleri towards phytoextraction of cadmium polluted soils. N Biotechnol 30: 9-14.
  8. Barabasz A, Krämer U, Hanikenne M, Rudzka J, Antosiewicz DM (2010) Metal accumulation in tobacco expressing Arabidopsis halleri metal hyperaccumulation gene depends on external supply. J Exp Bot 61: 3057-3067.
  9. Barabasz A, Wilkowska A, RuszczyÅ, Ska A, Bulska E, Hanikenne M, et al. (2012) Metal response of transgenic tomato plants expressing P(1B) -ATPase. Physiol Plant 145: 315-331.
  10. Harrier LA, Watson CA (2004) The potential role of arbuscular mycorrhizal (AM) fungi in the bioprotection of plants against soil-borne pathogens in organic and/or other sustainable farming systems. Pest Manag Sci 60: 149-157.
  11. Gosling P, Hodge A, Goodlass G, Bending GD (2006) Arbuscular mycorrhizal fungi and organic farming. Agriculture, Ecosystems & Environment 113: 17-35
  12. Parniske M (2000) Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease? Curr Opin Plant Biol 3: 320-328.
  13. Brewin NJ, Legocki AB (1996) Biological nitrogen fixation for sustainable agriculture. Trends Microbiol 4: 476-477.
  14. Bloem JF, Trytsman G, Smith HJ (2009) Biological nitrogen fixation in resource-poor agriculture in South Africa. Symbiosis 48: 18-24
  15. Hesterman OB, Sheaffer CC, Fuller EI (1986) Economic Comparisons of Crop Rotations Including Alfalfa, Soybean, and Corn. Agronomy Journal 78: 24-28
  16. Davis AS, Hill JD, Chase CA, Johanns AM, Liebman M (2012) Increasing cropping system diversity balances productivity, profitability and environmental health. PLoS One 7: e47149.
  17. Sato S, Tabata S (2006) Lotus japonicus as a platform for legume research. Curr Opin Plant Biol 9: 128-132.
  18. Cook DR (1999) Medicago truncatula--a model in the making! Curr Opin Plant Biol 2: 301-304.
  19. Hentschel U, Steinert M, Hacker J (2000) Common molecular mechanisms of symbiosis and pathogenesis. Trends Microbiol 8: 226-231.
  20. Sato S, Nakamura Y, Kaneko T, Asamizu E, Kato T, et al. (2008) Genome structure of the legume, Lotus japonicus. DNA Res 15: 227-239.
  21. Young ND, Debelle F, Oldroyd GED, Geurts R, Cannon SB, et al. (2011) The Medicago genome provides insight into the evolution of rhizobial symbioses. Nature 480: 520-524
  22. Schmutz J, Cannon SB, Schlueter J, Ma JX, Mitros T, et al. (2010) Genome sequence of the palaeopolyploid soybean. 463: 178.
  23. Parniske M (2008) Arbuscular mycorrhiza: the mother of plant root endosymbioses. Nat Rev Microbiol 6: 763-775.
  24. Banba M, Gutjahr C, Miyao A, Hirochika H, Paszkowski U, et al. (2008) Divergence of evolutionary ways among common sym genes: CASTOR and CCaMK show functional conservation between two symbiosis systems and constitute the root of a common signaling pathway. Plant Cell Physiol 49: 1659-1671.
  25. Charpentier M, Bredemeier R, Wanner G, Takeda N, Schleiff E, et al. (2008) Lotus japonicus CASTOR and POLLUX are ion channels essential for perinuclear calcium spiking in legume root endosymbiosis. Plant Cell 20: 3467-3479.
  26. Lévy J, Bres C, Geurts R, Chalhoub B, Kulikova O, et al. (2004) A putative Ca2+ and calmodulin-dependent protein kinase required for bacterial and fungal symbioses. Science 303: 1361-1364.
  27. Mitra RM, Gleason CA, Edwards A, Hadfield J, Downie JA, et al. (2004) A Ca2+/calmodulin-dependent protein kinase required for symbiotic nodule development: Gene identification by transcript-based cloning. Proceedings of the National Academy of Sciences of the United States of America 101: 4701-4705
  28. Miwa H, Sun J, Oldroyd GE, Downie JA (2006) Analysis of Nod-factor-induced calcium signaling in root hairs of symbiotically defective mutants of Lotus japonicus. Mol Plant Microbe Interact 19: 914-923.
  29. Untergasser A, Bijl GJ, Liu W, Bisseling T, Schaart JG, et al. (2012) One-step Agrobacterium mediated transformation of eight genes essential for rhizobium symbiotic signaling using the novel binary vector system pHUGE. PLoS One 7: e47885.
  30. Lee KS, Choi WY, Ko JC, Kim TS, Gregorio GB (2003) Salinity tolerance of japonica and indica rice (Oryza sativa L.) at the seedling stage. Planta 216: 1043-1046.
  31. Ventelon DM, Nguyen TT, Wissocq A, Berger C, Laudie M, et al. (2003) Analysis of the transcriptional response to Rice Yellow Mottle Virus infection in Oryza sativa indica and japonica cultivars. Mol Genet Genomics 270: 253-262.
  32. Ventelon-Debout M, Delalande F, Brizard JP, Diemer H, Van Dorsselaer A, et al. (2004) Proteome analysis of cultivar-specific deregulations of Oryza sativa indica and O. sativa japonica cellular suspensions undergoing rice yellow mottle virus infection. Proteomics 4: 216-225.
  33. Liu X, Lu T, Yu S, Li Y, Huang Y, et al (2007) A collection of 10,096 indica rice full-length cDNAs reveals highly expressed sequence divergence between Oryza sativa indica and japonica subspecies. Plant Mol Biol 65: 403-415
  34. Draper J, Mur LA, Jenkins G, Ghosh-Biswas GC, Bablak P, et al. (2001) Brachypodium distachyon. A new model system for functional genomics in grasses. Plant Physiol 127: 1539-1555.
  35. Bevan MW, Garvin DF, Vogel JP (2010) Brachypodium distachyon genomics for sustainable food and fuel production. Curr Opin Biotechnol 21: 211-217.
  36. Garvin DF, Gu YQ, Hasterok R, Hazen SP, Jenkins G, et al. (2008) Development of genetic and genomic research resources for Brachypodium distachyon, a new model system for grass crop research. Crop Science 48: S69-S84.
  37. Huan Q, Mao Z, Zhang J, Xu Y, Chong K (2013) Transcriptome-wide Analysis Of Vernalization Reveals Conserved and Species-specific Mechanisms in Brachypodium. J Integr Plant Biol.
  38. Higgins JA, Bailey PC, Laurie DA (2010) Comparative genomics of flowering time pathways using Brachypodium distachyon as a model for the temperate grasses. PLoS One 5: e10065.
  39. Laudencia CDL, Vensel WH (2008) Globulins are the main seed storage proteins in Brachypodium distachyon. Theor Appl Genet 117: 555-563.
  40. Bertolini E, Verelst W, Horner DS, Gianfranceschi L, Piccolo V, et al. (2013) Addressing the role of microRNAs in reprogramming leaf growth during drought stress in Brachypodium distachyon. Mol Plant 6: 423-443.
  41. Verelst W, Bertolini E, De Bodt S, Vandepoele K, Demeulenaere M, et al. (2013) Molecular and physiological analysis of growth-limiting drought stress in Brachypodium distachyon leaves. Mol Plant 6: 311-322.
  42. Lee MY, Yan L, Gorter FA, Kim BY, Cui Y, et al. (2012) Brachypodium distachyon line Bd3-1 resistance is elicited by the barley stripe mosaic virus triple gene block 1 movement protein. J Gen Virol 93: 2729-2739.
  43. Figueroa M, Alderman S, Garvin DF, Pfender WF (2013) Infection of Brachypodium distachyon by formae speciales of Puccinia graminis: early infection events and host-pathogen incompatibility. PLoS One 8: e56857.
  44. Zhang J, Xu Y, Huan Q, Chong K (2009) Deep sequencing of Brachypodium small RNAs at the global genome level identifies microRNAs involved in cold stress response. BMC Genomics 10: 449.
  45. Luo N, Liu J, Yu X, Jiang Y (2011) Natural variation of drought response in Brachypodium distachyon. Physiol Plant 141: 19-29.
  46. Tyler L, Bragg JN, Wu J, Yang X, Tuskan GA, et al. (2010) Annotation and comparative analysis of the glycoside hydrolase genes in Brachypodium distachyon. BMC Genomics 11: 600.
  47. Somyong S, Munkvold JD, Tanaka J, Benscher D, Sorrells ME (2011) Comparative genetic analysis of a wheat seed dormancy QTL with rice and Brachypodium identifies candidate genes for ABA perception and calcium signaling. Funct Integr Genomics 11: 479-490.
  48. Li C, Rudi H, Stockinger EJ, Cheng H, Cao M, et al. (2012) Comparative analyses reveal potential uses of Brachypodium distachyon as a model for cold stress responses in temperate grasses. BMC Plant Biol 12: 65
  49. d'Yvoire MB, Bouchabke CO, Voorend W, Antelme S, Cezard L, et al. (2013) Disrupting the cinnamyl alcohol dehydrogenase 1 gene (BdCAD1) leads to altered lignification and improved saccharification in Brachypodium distachyon. Plant Journal 73: 496-508
  50. Harrison MD, Geijskes J, Coleman HD, Shand K, Kinkema M, et al. (2011) Accumulation of recombinant cellobiohydrolase and endoglucanase in the leaves of mature transgenic sugar cane. Plant Biotechnol J 9: 884-896.
  51. Bradshaw HD, Ceulemans R, Davis J, Stettler R (2000) Emerging model systems in plant biology: Poplar (Populus) as a model forest tree. J Plant Growth Regulation 19: 306-313
  52. Chang SW, Lee SJ, Je CH (2005) Phytoremediation of atrazine by poplar trees: toxicity, uptake, and transformation. J Environ Sci Health B 40: 801-811.
  53. Dai HP, Jia GL, Feng SJ, Wei AZ, Song H, et al. (2011) Phytoremediation with transgenic poplar. Journal of Food Agriculture & Environment 9: 710-713
  54. Pilipovic A, Orlovic S, Nikolic N, Borisev M, Krstic B, et al. S (2012) Growth and Plant Physiological Parameters as Markers for Selection of Poplar Clones for Crude Oil Phytoremediation. Sumarski List 136: 273-281
  55. Madejón P, Ciadamidaro L, Marañón T, Murillo JM (2013) Long-term biomonitoring of soil contamination using poplar trees: accumulation of trace elements in leaves and fruits. Int J Phytoremediation 15: 602-614.
  56. Jansson C, Wullschleger SD, Kalluri UC, Tuskan GA (2010) Phytosequestration: Carbon Biosequestration by Plants and the Prospects of Genetic Engineering. Bioscience 60: 685-696
  57. Mao R, Zeng DH, Li LJ, Hu YL (2012) Changes in labile soil organic matter fractions following land use change from monocropping to poplar-based agroforestry systems in a semiarid region of Northeast China. Environ Monit Assess 184: 6845-6853.
  58. Rytter RM (2012) The potential of willow and poplar plantations as carbon sinks in Sweden. Biomass & Bioenergy 36: 86-95
  59. Jung YH, Cho HJ, Lee JS, Noh EW, Park OK, et al. (2013) Evaluation of a transgenic poplar as a potential biomass crop for biofuel production. Bioresour Technol 129: 639-641.
  60. Barampuram S, Zhang ZJ (2011) Recent advances in plant transformation. Methods Mol Biol 701: 1-35.
  61. Rafsanjani MS, Alvari A, Samim M, Hejazi MA, Abdin MZ (2012) Application of novel nanotechnology strategies in plant biotransformation: a contemporary overview. Recent Pat Biotechnol 6: 69-79.
  62. Gelvin SB (2010) Plant proteins involved in Agrobacterium-mediated genetic transformation. Annu Rev Phytopathol 48: 45-68.
  63. Pitzschke A, Hirt H (2010) New insights into an old story: Agrobacterium-induced tumour formation in plants by plant transformation. EMBO J 29: 1021-1032.
  64. Gelvin SB (2012) Traversing the Cell: Agrobacterium T-DNA's Journey to the Host Genome. Front Plant Sci 3: 52.
  65. Ziemienowicz A (2010) Plant transgenesis. Methods Mol Biol 631: 253-268.
  66. Li J, Krichevsky A, Vaidya M, Tzfira T, Citovsky V (2005) Uncoupling of the functions of the Arabidopsis VIP1 protein in transient and stable plant genetic transformation by Agrobacterium. Proc Natl Acad Sci U S A 102: 5733-5738.
  67. Clough SJ, Bent AF (1998) Floral dip: a simplified method for Agrobacterium-mediated transformation of Arabidopsis thaliana. Plant J 16: 735-743.
  68. Weigel D, Glazebrook J (2006) In Planta Transformation of Arabidopsis. Cold Spring Harbor Protocols 2006: pdb.prot4668
  69. Agarwal S, Loar S, Steber C, Zale J (2009) Floral transformation of wheat. Methods Mol Biol 478: 105-113.
  70. Zale JM, Agarwal S, Loar S, Steber CM (2009) Evidence for stable transformation of wheat by floral dip in Agrobacterium tumefaciens. Plant Cell Rep 28: 903-913.
  71. Liu J, Hua W, Zhan G, Wei F, Wang X, et al. (2010) Increasing seed mass and oil content in transgenic Arabidopsis by the overexpression of wri1-like gene from Brassica napus. Plant Physiol Biochem 48: 9-15.
  72. Liu J, Hua W, Yang HL, Zhan GM, Li RJ, et al. (2012) The BnGRF2 gene (GRF2-like gene from Brassica napus) enhances seed oil production through regulating cell number and plant photosynthesis. J Exp Bot 63: 3727-3740.
  73. Zhang HX, Hodson JN, Williams JP, Blumwald E (2001) Engineering salt-tolerant Brassica plants: characterization of yield and seed oil quality in transgenic plants with increased vacuolar sodium accumulation. Proc Natl Acad Sci U S A 98: 12832-12836.
  74. Matoba N, Davis KR, Palmer KE (2011) Recombinant protein expression in Nicotiana. Methods Mol Biol 701: 199-219.
  75. Chen Z, Wang J, Ye MX, Li H, Ji LX, et al. (2013) A Novel Moderate Constitutive Promoter Derived from Poplar (Populus tomentosa Carrière). Int J Mol Sci 14: 6187-6204.
  76. Jain D, Chattopadhyay D (2013) Promoter of CaZF, a chickpea gene that positively regulates growth and stress tolerance, is activated by an AP2-family transcription factor CAP2. PLoS One 8: e56737.
  77. Persak H, Pitzschke A (2013) Tight interconnection and multi-level control of Arabidopsis MYB44 in MAPK cascade signalling. PLoS One 8: e57547.
  78. Piotrzkowski N, Schillberg S, Rasche S (2012) Tackling heterogeneity: a leaf disc-based assay for the high-throughput screening of transient gene expression in tobacco. PLoS One 7: e45803.
  79. Li JF, Park E, von Arnim AG, Nebenführ A (2009) The FAST technique: a simplified Agrobacterium-based transformation method for transient gene expression analysis in seedlings of Arabidopsis and other plant species. Plant Methods 5: 6.
  80. Pitzschke A (2013) Tropaeolum tops tobacco-simple and efficient transgene expression in the order brassicales. PLoS One, 8, e73355.
  81. Jin H, Cominelli E, Bailey P, Parr A, Mehrtens F, et al. (2000) Transcriptional repression by AtMYB4 controls production of UV-protecting sunscreens in Arabidopsis. EMBO J 19: 6150-6161.
  82. Park MR, Yun KY, Mohanty B, Herath V, Xu F, et al. (2010) Supra-optimal expression of the cold-regulated OsMyb4 transcription factor in transgenic rice changes the complexity of transcriptional network with major effects on stress tolerance and panicle development. Plant Cell Environ 33: 2209-2230.
Citation: Pitzschke A (2013) From Bench to Barn: Plant Model Research and its Applications in Agriculture Adv Genet Eng 2:110.

Copyright: © 2013 Pitzschke A. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Top