Convergent evolution of starch metabolism in cyanobacteria and archaeplastida

Abstract

It is widely accepted that Archaeplastida phylum comprising Glaucophyta, Rhodophyta, and Chloroplastida originates from a unique endosymbiosis event, called primary plastid endosymbiosis, between a cyanobacterium and a eukaryotic cell. In addition to acquiring oxygenic photosynthesis, the three sister lineages gained the ability to synthesize a novel semi-crystalline storage polysaccharide: starch. In Archaeplastida, several lines of evidence reveal that the transition from glycogen synthesis to starch accumulation results in the recruitment of an isoamylase (ISA)-type debranching enzyme. The latter removes short-branched glucan chains, which prevent amylopectin crystallization. Recently, a small group of unicellular diazotrophic cyanobacteria, possibly the closest relative of the ancestral plastid, have been reported accumulating starch-like granules composed of both amylose and amylopectin fractions instead of glycogen particles. In order to understand starch metabolism in this particular group of cyanobacteria, a random mutagenesis was carried out on the unicellular starch-accumulating Cyanobacterium sp. CLg1. Throughout iodine crystal vapors screening, fourteen mutant strains have substituted starch granules by that of glycogen particles. Interestingly, such as in plants, all mutant strains were impaired in an isoamylase-type debranching enzyme activity. However, phylogenetic analyses point out that the critical step for starch crystallization in Archaeplastida did not evolve from the cyanobacterial isoamylase/glgX gene, but from another pathogenic bacteria. Based on this work, it appears that the transition from glycogen to starch has evolved independently in both cyanobacteria and Archaeplastida by following a common glucan trimming mechanism.