Origin and Evolution of the Main Starch Biosynthetic Enzymes

Abstract

Starch is a special semi-crystalline energy storage form mainly existing in plastids of plants, which plays an important role in food or no-food usages. To date, the main enzymes involved in starch biosynthetic pathway have been characterized, while their origin and evolution have long been controversy. Here, the phylogenetic and structural analysis of three types of starch biosynthetic enzymes, including starch synthase (SS), starch branching enzyme (SBE) and isoamylase-type debranching enzyme (ISA) from 51,151 annotated genomes, suggested a possible scenario for the origin and evolution of starch biosynthetic pathway. Firstly, the ancestor of SBE originated from an unknown bacterium before the last common ancestor of eukaryotes (LECA) by horizontal gene transfer (HGT), and the transfer event likely helped the eukaryote ancestor obtain the ability to synthesize glycogen. Secondly, during the formation of Archaeplastida, one clade of SS was transferred from Deltaproteobacteria by HGT, ISA and the other clade of SS originated from Chlamydiae by an endosymbiosis gene transfer (EGT), both of which help to form the original starch biosynthetic pathway. Thirdly, after Viridiplantae diverged from Rhodophyta, all three enzymes generated functionally specialized isoforms through multiple duplications and extension domains of N-terminus, which finally formed the complete starch biosynthetic pathway. Our study sheds light on the evolutionary origins of the key enzymes involved in starch biosynthetic pathway, providing important insights into the evolution events of plants.

Keywords: Origin, Evolution, Starch synthase, Starch branching enzyme, Isoamylase-type debranching enzyme；Starch biosynthesis