Organizations of root apical meristem in extant lycophytes

Abstract

The independent origin of roots in lycophytes and euphyllophytes has been proposed based on evidence from fossilized plants. I previously reported how roots evolved within these lineages by analyzing the organization of the root apical meristem (RAM) in extant lycophytes, which is key to understanding the early evolution of roots. The RAM in lycophytes (Lycopodiaceae, Isoetaceae, and Selaginellaceae) has been classified into four types based on cell division activities and anatomy. Each of these four types is comparable to the RAM organization in euphyllophytes. The type I RAM found in Lycopodium is characterized by the presence of the common initial zone, a region with a very low frequency of cell division, reminiscent of the quiescent center (QC) in angiosperm roots. Although types II and III and the apical cell type in lycophytes are structurally similar to the RAM types in euphyllophytes, they have no QC-like area. The organization of the RAM in extant lycophytes is more diverse than expected. These results support the paleobotanical hypothesis that roots evolved several times in lycophytes and euphyllophytes.