Evolution of the Cellulose Synthase (CesA) Gene Family: Insights from Green Algae and Seedless Plants

Abstract

The structure of cellulose microfibrils has been correlated with the organization of the arrays of integral plasma membrane protein particles that synthesize them. These “terminal complexes” (TCs) are composed in part of cellulose synthase catalytic subunits (CesAs), which not only catalyze the polymerization of glucan chains, but also play a role in TC assembly. The catalytic domains of CesA proteins from prokaryotes and eukaryotes are conserved. However, differences in CesA structure between plants and bacteria presumably contribute to variation in TC and microfibril structure between these organisms. The genetic basis for this variation may be revealed by examining the CesA genes of green algae, a group of related organisms that nonetheless have different types of TCs. Vascular plants and their closest green algal relatives share rosette TCs and highly similar CesAs. This demonstrates a congruence of TC and CesA structure over deep time and provides a basis for analyzing the CesA genes of green algae with different types of TCs. In seed plants, the members of large CesA gene families are differentially expressed during primary and secondary cell wall deposition, particularly in vascular tissue. Phylogenetic analysis of the CesA gene families from vascular plants and the nonvascular plant Physcomitrella patens is consistent with independent CesA diversification in the moss and vascular plant lineages. Characterization of CesA genes from P. patens, which is uniquely suited for targeted mutagenesis and analysis of TC structure by freeze-fracture electron microscopy, also provides a convenient model to manipulate and test the functions of domains potentially involved in TC assembly.