The origin of plants: body plan changes contributing to a major evolutionary radiation.

Abstract

Plants (land plants, embryophytes) are of monophyletic origin from a freshwater ancestor that, if still extant, would be classified among the charophycean green algae. Plants, but not charophyceans, possess a life history involving alternation of two morphologically distinct developmentally associated bodies, sporophyte and gametophyte. Body plan evolution in plants has involved fundamental changes in the forms of both gametophyte and sporophyte and the evolutionary origin of regulatory systems that generate different body plans in sporophytes and gametophytes of the same species. Comparative analysis, based on molecular phylogenetic information, identifies fundamental body plan features that originated during radiation of charophycean algae and were inherited by plants. These include, in probable evolutionary order: cellulosic cell wall, multicellular body, cytokinetic phragmoplast, plasmodesmata, apical meristematic cell, apical cell proliferation (branching), three-dimensional tissues, asymmetric cell division, cell specialization capacity, zygote retention, and placenta. Body plan features whose origin is linked to the dawn of plants include: multicellular sporophyte body, histogenetic apical meristem in the gametophyte body, and capacity for tissue differentiation in both sporophyte and gametophyte. Origin of a well-defined sporophytic apical stem cell and a system for its proliferation, correlated with capacity for organ production and branching, occurred sometime between the divergence of modern bryophytes and vascular plant lineages. Roots and their meristem and a multilayered tunica-corpus shoot apical meristem arose later. Regulatory genes affecting shoot meristems, which have been detected by analysis of higher plant mutants, may be relevant to understanding early plant body plan transitions. Fundamental aspects of the plant body plan are remarkably consistent within the plant kingdom and are different from metazoans. All plants exhibit at least one form of apical meristem consisting of one or more cells that are …