The evolution of plant cell walls was critical for land colonization, yet its genetic basis remains incompletely understood. Charophyte green algae are the closest relatives of land plants and provide key insights into the origins of cell wall biosynthesis. Here, we conducted a genome-wide analysis of the CesA/Csl superfamily in the charophyte alga Chara braunii to investigate the evolutionary origins of plant cell wall formation. Thirty genes were identified and classified into three major clades: CesA, CslA, and CslC. CesA genes were exclusively associated with primary cell wall lineages, with no homologs of secondary wall-specific CesA4/CesA7/CesA8, indicating an early origin of cellulose biosynthesis followed by later functional specialization. In contrast, Csl genes were restricted to the CslA and CslC subfamilies, suggesting that diversification of hemicellulose biosynthesis occurred after the divergence of charophytes and land plants. Expression and promoter analyses revealed clade-specific regulatory patterns and stress-responsive expression, and the quantitative RT-PCR validated that CesA, CslA, and CslC genes respond differentially to NaCl and ABA treatments, with CslA genes strongly induced by ABA and CslC genes preferentially by salt. Together, these findings are consistent with a stepwise model of cell wall evolution and suggest that C. braunii may represent an important intermediate stage in the transition to land plants.
Zhou et al. (Mon,) studied this question.