Epidermal Patterning and Silica Phytoliths in Grasses: An Evolutionary History

Due to the immense ecological and economic significance of grasses, their highly characteristic long-short epidermal patterning and associated silica phytoliths represent significant diagnostic markers in studies of ancient climate change and agriculture. We explore the link between epidermal cell patterning and phytolith development and review the evolutionary history of phytoliths in the context of recent well-resolved phylogenetic analyses of grasses and allied Poales, focusing on early-divergent grasses and the subfamilies that constitute the BEP group (the bamboos and their allies). Dimorphic epidermal patterning is a common feature of Poaceae and the related family Joinvilleaceae, where phytoliths are located primarily in the short cells. However, 'Joinvillea' lacks the short-cell pairs that occur in many grasses. The costal rows of phytoliths that characterize some grasses could represent loss of long-short cell patterning over the veins. Unlobed phytoliths probably represent the ancestral condition in grasses, though bilobate phytoliths evolved at an early stage. Either transverse-unlobed or transverse-bilobate phytoliths predominate in the early-divergent lineages, whereas axial-bilobates (or polylobates) primarily characterize the PACMAD clade and the BEP subfamily Pooideae.