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THE GREEN BODY BEAUTIFUL: APPROACHES TO UNDERSTANDING THE ORIGIN AND EARLY EVOLUTION OF PLANT BODY SYMMETRY AND DEVELOPMENTAL RESPONSES TO GRAVITY.  L.E. Graham¹ and M.E. Cook².  ¹Dept of Botany, Univ of Wisconsin, Madison. ²Dept of Biological Sciences, Illinois State Univ, Normal.

     Plants, defined here as the monophyletic embryophytes (bryophytes and vascular plants) achieved multicellularity independently of animals and fungi, and display a variety of body forms for which symmetry modifications form a basis. Insight into the origin and early diversification of plant bodies can be obtained by comparing early-divergent groups of modern plants (bryophytes andl lycophytes) with the modern green algae closest to plant ancestry, charophyceans.  Mapping structural, physiological, reproductive, and genetic traits onto increasingly robust phylogenies provides insight into the order and mechanism of early  plant evolution.

     Early structural innovations  that arose during charophycean diversification include: cellulose-rich cell wall produced by rosette-shaped cellulose synthesizing complexes, cytokinetic phragmoplasts, intercellular communication channels (plasmodesmata), cell division in more than one direction, simple apical meristems, asymmetric cell divisions, and cell specialization.  Charophyceans have been useful model systems for experimental analysis of gravity responses and cytoskeletal dynamics.

     Innovations linked with divergence of earliest embryophytes include sporophytes, histogenetic meristems, and tissue differentiation. Bryophytes offer the benefit of efficient gene targeting systems, and are also used to explore gravisensing, the origins of hormonal control of plant development, and homeobox gene diversification.