Evolution of plant GSK3 function

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GSK3 genes may have played an instrumental role in the acquisition of stress tolerance mechanisms during the adaptation to life outside water because land plants are unusual in possessing large GSK3 gene families. GSK3 gene diversification has occurred more than once during land plant evolution [36,50]. In seed plants, three GSK3 clades are represented in gymnosperms, with divergence into four clades occurring before (or early in) angiosperm evolution [36,50]. The moss Physcomitrella patens diverged early from the seed plant lineage around 470 million years ago, shortly after plants appeared on land. The five Physcomitrella GSK3 genes discovered by cDNA library screening [50] fit into a separate subclade that is most similar to flowering plant clades I and IV [36,50]. Physcomitrella GSK3s share 95% sequence identity across the whole protein [50], suggesting a large degree of redundancy and possibly recent gene duplications in moss. The complete sequencing of the Physcomitrella [61] and the lycophyte Selaginella moellendorffii [62] genomes has made it possible to assess the full complement of GSK3 gene sequences in the early land plants. We can detect seven putative GSK3 homologues in Physcomitrella, and two in Selaginella (J.C. Coates, unpublished). Our tentative phylogenies suggest that the Selaginella GSK3 genes duplicated after the divergence of Selaginella and Physcomitrella.

The functions of GSK3s in Physcomitrella and Selaginella are unknown. However, as in Arabidopsis and rice, two Physcomitrella genes show increased transcript abundance when exposed to sorbitol and polyethylene glycol [50] (Table 1), suggesting a conserved role in osmotic stress responses. Interestingly, sequenced genomes of green and red algae (Chlamydomonas reinhardtii, Volvox carteri, Porphyra umbilicalis and Cyanidioschyzon merolae) each contain only one GSK3 gene, further suggesting that all the 'green' GSK3 gene duplications occurred after the evolution of land plants.

Within the heterokont lineage, we can detect a single GSK3 homologue in the sequenced genomes of each of: Ectocarpus siliculosus (multicellular brown alga), Aureococcus anophagefferens (unicellular brown alga), Thalassiosira pseudonana (diatom), Phaeodactylum tricornutum (diatom), and Phytophthora infestans (oomycete). The situation in alveolates is more complex. Single GSK3 homologues exist in Plasmodium sp. and Toxoplasma sp. with larger gene families in Paramecium tetraurelia and Tetrahymena thermophila. Within excavates a single GSK3 homologue is present in Leishmania sp. with two possible homologues in Trypanosoma sp. Together, these data suggest a single common ancestor for GSK3. Whether the GSK3s identified in algae and other protists function in osmotic stress tolerance is unknown.