The evolution of plant actins

Why plants have evolved large families of highly similar actin isoforms, and what functional distinctions sustain their retention, remains an unresolved problem in cytoskeletal biology. Actin is among the most ancient and conserved proteins in eukaryotes, yet plant genomes encode an unusually high number of closely related paralogs whose specialization is poorly understood. Here, we reconstruct the evolutionary history of plant actins across the green lineage by combining sequence retrieval, phylogenetics, comparative sequence analysis, and structural mapping. We find that plant actin diversification arose multiple times and, strikingly, relatively late in the history of this ancient protein family. A major duplication produced two deeply conserved seed-plant actin clades, corresponding to the previously described vegetative and reproductive actins; here, we refer to these as Type I and Type II to reflect their phylogenetic relationships. Across the green lineage, increases in actin copy number track key transitions in plant complexity, particularly in tracheophytes. Despite this diversification, only a limited set of conserved substitutions distinguishes major actin lineages. Mapping sequence variation onto actin structures reveals a prominent surface patch that appears permissive to mutation, suggesting relaxed functional constraint, whereas isoform-specific changes cluster at sites likely to influence filament stability, turnover, and treadmilling. A finer-scale analysis in Brassicales pinpoints recent substitutions predicted to alter hydrogen-bonding patterns within monomeric and filamentous actin. Together, our results argue that plant actin diversification is not redundant drift, but a recurrent evolutionary strategy that preserves the actin core while tuning the intrinsic biophysical behavior of f-actin.