Abstract
Bioinformatics analyses of caleosin/peroxygenases (CLO/PXG) demonstrated that these genes are present in the vast majority of Viridiplantae taxa for which sequence data are available. Functionally active CLO/PXG proteins with roles in abiotic stress tolerance and lipid droplet storage are present in some Trebouxiophycean and Chlorophycean green algae but are absent from the small number of sequenced Prasinophyceaen genomes. CLO/PXG-like genes are expressed during dehydration stress in Charophyte algae, a sister clade of the land plants (Embryophyta). CLO/PXG-like sequences are also present in all of the >300 sequenced Embryophyte genomes, where some species contain as many as 10–12 genes that have arisen via selective gene duplication. Angiosperm genomes harbour at least one copy each of two distinct CLO/PX isoforms, termed H (high) and L (low), where H-forms contain an additional C-terminal motif of about 30–50 residues that is absent from L-forms. In contrast, species in other Viridiplantae taxa, including green algae, non-vascular plants, ferns and gymnosperms, contain only one (or occasionally both) of these isoforms per genome. Transcriptome and biochemical data show that CLO/PXG-like genes have complex patterns of developmental and tissue-specific expression. CLO/PXG proteins can associate with cytosolic lipid droplets and/or bilayer membranes. Many of the analysed isoforms also have peroxygenase activity and are involved in oxylipin metabolism. The distribution of CLO/PXG-like genes is consistent with an origin >1 billion years ago in at least two of the earliest diverging groups of the Viridiplantae, namely the Chlorophyta and the Streptophyta, after the Viridiplantae had already diverged from other Archaeplastidal groups such as the Rhodophyta and Glaucophyta. While algal CLO/PXGs have roles in lipid packaging and stress responses, the Embryophyte proteins have a much wider spectrum of roles and may have been instrumental in the colonisation of terrestrial habitats and the subsequent diversification as the major land flora.
Highlights
Comparative genomic and functional analyses of individual gene families can shed considerable light on the process of plant evolution and on the physiological role(s) of particular groups of proteins[1, 2]
The Viridiplantae, which include all of the land plants and green algae are divided into two groups, namely the Streptophyta (Charophyta + Embryophyta) and the Chlorophyta[4,5,6]
As of November 2017 we found >1310 sequence hits from the Viridiplantae that were classified in public databases, such as NCBI, as being members of the ‘caleosin superfamily’
Summary
Comparative genomic and functional analyses of individual gene families can shed considerable light on the process of plant evolution and on the physiological role(s) of particular groups of proteins[1, 2]. In this study we have analysed a well-conserved gene family, which is normally annotated in databases as ‘caleosin’ and/or ‘peroxygenase’, with the aim of tracing the evolution, expression patterns and functional roles of the encoded proteins in plants. In terms of their functional description, the caleosin/peroxygenases (CLO/PXG) are members of the EC:1.11.2.3 class of oxidoreductases, Pfam reference PF05042. While CLO/PXGs appear to be ubiquitous in all land plant (Embryophyta) genomes, they are present in some, but by no means all, algal taxa within the Charophyta and Chlorophyta[7]. CLO/PXGs are found in many fungal taxa but are absent from other the major Opisthokont clades, including animals [8, 9]
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