Independent origins of endodermis in lycopsid and euphyllophyte roots

The endodermis is a vital barrier tissue in plants, but its origin, evolution, and homologies across land plants remain ambiguous. Here, we implement a multifaceted investigation of the endodermis in lycopsids to shed light on its evolution in roots. We confirmed the presence of an endodermis with Casparian strips (CSs) in two of the three living lineages of lycopsids, while endodermoid layers with polar lignin caps were identified in the Lycopodiaceae. Although lignified tissues, including CSs, can be preserved in fossils, we did not find evidence for either an endodermis or an endodermoid in early fossils spanning the origin of the vascular plant lineage, the divergence of lycophytes and euphyllophytes, or the split of the early lycophytes and lycopsids. Together with evidence from extant lineages, these observations are most consistent with the endodermis having evolved at least twice independently in vascular plants. Expression analysis of homologous genes of the known seed plant regulatory genes SHORT-ROOT (SHR) and SCARECROW (SCR) suggests an ancestral role in defining the boundary between vascular and non-vascular tissues and the independent recruitment in lycopsids and seed plants.