DAG: Diacylglycerol; IP₃: Inositol-1,4,5- Trisphosphate; IP₆: Inositol-1,2,3,4,5,6-Hexakisphosphate, PI-PLC: Phosphoinositide-Specific Phospholipase C, PKC: Protein Kinase C.

Inositol-1,4,5-trisphosphate [Ins(1,4,5)P3, IP₃] is a second messenger involved in transient release of Ca²⁺ from the ER that activates cytosolic Ca²⁺ signalling cascades in response to extracellular and intracellular stimuli [1,2]. Phosphatidylinositol-4,5-bisphosphate is cleaved by phosphoinositide-specific phospholipase C (PI-PLC) into the second messengers diacylglycerol (DAG) and IP₃ [3,4]. These second messengers then activate protein kinase C (PKC) and the ER-localised IP₃ receptor, respectively, in animal cells [1,2]. However, although the PI-PLC signaling cascade is present in plants [5-7], genes encoding PKC and the IP₃ receptor have not been found in terrestrial plant genomes, suggesting differences in second messenger systems between animals and plants. To date, the genomes of a variety of unicellular and multicellular algae have been sequenced [8-23] as shown in (Table 1). In addition, large-scale EST information for the red seaweeds Porphyra umbilicalis and Porphyra purpurea has been accumulated [24-26]. Such rich gene information enables us to identify the genes encoding IP₃ receptor gene homologues in algae to hypothesize the evolutionary route of the loss of the IP₃ gene in plant lineages.

Table 1: List of algal species whose genome sequences have been analyzed.

The origin of the IP₃ receptor-dependent transient Ca²⁺ release system predates the divergence of animals and fungi [27,28]. Indeed, homologues of genes encoding the IP₃ receptor have been identified in protozoa such as the choanoflagellate Monosiga brevicollis [29], the myxomycete Dictyostelium discoideum [30], the ciliate Paramecium tetraurelia [31], and the parasite Trypanosoma brucei [32]. Thus, it is plausible that an ancient eukaryotic cell containing an IP₃ receptor gene was the target of endosymbiosis with an ancient cyanobacterium to produce plant cells, after which the IP₃ gene was lost from plant lineages. At present, IP₃ receptor homologues have been found in green algae, such as Chlamydomonas reinhardtii [10] and Volvox carteri [33,34], and in heterokont algae including Aureococcus anophagefferrens [21] and Ectocarpus siliculosus [22], but have not been identified in red algae or streptophytes (land plants and charophytic algae) (Figure 1). These findings have led to proposals that the IP₃ receptor gene homologue was lost on multiple occasions during plant evolution. Because an ancestor of both green and red photosynthetic algal cells appeared after the primary endosymbiosis of a cyanobacterium into an ancient non-photosynthetic eukaryotic cell [35], the IP₃ receptor homologue was probably lost from lineages of red algae and green algae except for Volvocales (Figure 1). In fact, the genomes of unicellular Aureococcus anophagefferrens and multicellular Ectocarpus siliculosus carry an IP₃ receptor gene homologue (Figure 1). Because both photosynthetic algae arose from secondary endosymbiosis of a red algal cell into an ancient non-photosynthetic eukaryotic cell [35], it appears that red algae subsequently lost the IP₃ receptor gene homologue during their evolution, although some of Heterokontophyta that evolved by secondary symbiosis retain an ancient progenitor of the IP₃ receptor gene to this date. Moreover, in the green plant lineage, streptophytes have an impaired IP₃ receptor that is structurally similar to that in animals, Volvocales of chlorophytes, and brown seaweed (Figure 1). Thus, the loss of the IP₃ receptor may also occurred after the divergence of chlorophytes and streptophytes. Accordingly, there have been multiple occasions upon which the IP₃ receptor was lost from plant lineages. In contrast to the above conclusions drawn from genomic sequence information, there is evidence of IP₃-dependent Ca²⁺ release in terrestrial plants [36-42], which suggests the presence of a Ca²⁺ channel functionally resembling the IP₃ receptor in streptophytes. However, IP₃-dependent Ca²⁺ release has been reported only in green algae among plants [43,44]. Because the major intracellular store of Ca²⁺ in plant cells is the vacuole [45,46], IP₃ receptor activity is thought to be localised to vacuolar membranes in green algae and streptophytes. Such is the case in the fungus Neurospora crassa, in which IP₃-mediated Ca²⁺ release occurs from vacuoles [47], as it also does in protozoan ciliates and trypanosomes, in which the IP₃ receptor has been visualized on vacuolar membranes [27,28]. Thus, the green plant lineage has maintained an ancient system for transient release of Ca²⁺ from vacuoles, which is distinct from ER-mediated Ca²⁺ release in animal cells that do not possess vacuoles.

Figure 1: Conservation of the IP3 receptor genes in algae and terrestrial plants. The presence of the gene is indicated by “+” with highlighting order names with yellow.

IP₃-mediated Ca²⁺ release has been observed at the ER membrane that predominates in the perinuclear and apex regions of cells of the brown seaweed Fucus serratus [48]. Because an IP₃ receptor homologue was found in the Ectocarpus siliculosus genome [22], it is possible that the location of the IP₃-sensitive Ca²⁺ store shifted from the vacuole to the ER in brown seaweeds, where it is currently found in animal cells. Thus, the brown seaweeds might possess a PI-PLC signaling system more similar to that in animals.

Although IP₃-mediated Ca²⁺ release has not yet been shown in red algae, an inhibitor of the IP₃-receptor, 2-APB, prevented establishment of cell polarity for the migration and germination of monospores in the red seaweed Pyropia yezoensis [49], which suggests the presence of an IP₃ receptor-mediated Ca²⁺ release system in red seaweeds. However, an IP₃-receptor homologue has not yet been identified in the Pyropia yezoensis genome. As there is currently no evidence indicating the presence of IP₃ in Pyropia yezoensis, biochemical determinations of this inositol derivative will be necessary to elucidate Ca²⁺ release upon PIPLC action in red algae.

In plant cells, DAG is usually phosphorylated by DAG kinase [50,51] to produce phosphatidic acid, and IP₃ is phosphorylated by inositol phosphate kinases, IPK1 and IPK2 [52,53] to produce inositol- 1,3,4,5,6-pentakisphate and inositol-1,2,3,4,5,6-hexakisphosphate [Ins(1,2,3,4,5,6)P6; phytate; IP₆], a high-abundance molecule that is considered important for phosphorus storage in plant cells. To date, PA and IP₆ are thought to act as major second messengers in plant cells [7,54], although the function of IP₃ as a second messenger in plants has not been ruled out [42,47]. For instance, Munnik and Vermeer [54] have proposed that IP₆, which is rapidly converted from IP₃, is a major second messenger involved in abscisic acid-dependent inhibition of stomatal opening. They have also proposed a parallel between the IP₃ and IP₆ signalling systems because these two molecules are both produced by the action of PI-PLC. Although neither an IP₃ nor an IP₆ receptor have yet been identified in terrestrial plants, it is possible that an IP₃ receptor or an IP₆ receptor of unknown structure is present in streptophytes. Taken together, comparative genomic information clearly demonstrates the loss of the IP₃ receptor gene in red algae, green algae except for Volvocales and streptophytes during plant evolution. However, IP₃-dependent transient Ca²⁺ release from intracellular stores has been shown in these organisms by physiological experiments, although whether plants lacking the IP₃ receptor might both possess a common system for such transient Ca²⁺ release is uncertain. Therefore, the identification and characterization of genes encoding putative IP₃ or IP₆ receptors of unknown structure is of the highest priority for elucidating and comparing the regulation of the PI-PLC signalling cascade between IP₃ receptor-carrying and -lacking algae.