THE ROLE OF GROWTH SUPPRESSION IN LEAFY LIVERWORT MORPHOGENESIS AND PHYLOGENY

Abstract

  Three important roles are proposed for the place-dependent suppression of growth in leafy liverwort morphogenesis and phylogeny. The first is in the setting apart of apical meristematic regions during the transition from diffuse growth of the “protonemal” stage to the strictly polar, apical growth of the leafy gametophytic stage. The second is to stop further development of organ primordia as soon as they are initiated. The failure of primordia to give rise to mature organs is responsible for the recurrent trend in leafy liverwort families toward “recessive” or “reductive” evolution, by the “loss” in the numbers (i.e. leaves) and/or kinds (i.e. branch-types) of parts. The third role is the modulation of the size and shape of leaves. Our research indicates that the chemoregulation of all three aforementioned manifestations of place-dependent suppression are accomplished by the interrelated actions of auxin, ethylene, and some cell surface-associated hydroyproline-containing protein; probably an arabinogalactan protein (AGP).