Phylogenetic Distribution and Evolution of Root Apical Meristem Organization in Dicotyledonous Angiosperms

Abstract

Root apical meristem (RAM) organization was characterized in a wide range of dicotyledonous angiosperms and interpreted in light of recent phylogenies based on DNA sequence data. RAM organization was classified using the open and closed system of Guttenberg. In longitudinal section, closed RAMs have clonally distinct tiers of initials, but in open RAMs the initials are shared among the primary tissues. For open RAMs, we distinguished between intermediate-open (fate of derivatives easily identified) and basic-open (fate of derivatives not immediately evident). The ancestral state of angiosperm RAMs was intermediate-open, with closed organization first appearing near the base of the core eudicots. Basic-open was derived and was found in the eurosid I orders Fabales and Cucurbitales; therefore, at least two new RAM organizations appeared during the radiation of angiosperms. In the core eudicots, there is evidence for multiple independent reversions, from closed to intermediate-open. In all the species we observed, T divisions of the root cap protoderm initial delimit protoderm from the innermost cell layer of the peripheral root cap. Changes in RAM organization appear to be a normal part of the life cycle of the root; therefore, the choice of root age is important in studies of RAM organization.