A Molecular Pinball Machine of the Plasma Membrane Regulates Plant Growth-A New Paradigm.

Novel molecular pinball machines of the plasma membrane control cytosolic Ca levels that regulate plant metabolism. The essential components involve: 1. an auxin-activated proton pump; 2. arabinogalactan glycoproteins (AGPs); 3. Ca channels; 4. auxin-efflux "PIN" proteins. Typical pinball machines release pinballs that trigger various sound and visual effects. However, in plants, "proton pinballs" eject Ca bound by paired glucuronic acid residues of numerous glycomodules in periplasmic AGP-Ca. Freed Ca ions flow down the electrostatic gradient through open Ca channels into the cytosol, thus activating numerous Ca-dependent activities. Clearly, cytosolic Ca levels depend on the activity of the proton pump, the state of Ca channels and the size of the periplasmic AGP-Ca capacitor; proton pump activation is a major regulatory focal point tightly controlled by the supply of auxin. Auxin efflux carriers conveniently known as "PIN" proteins (null mutants are pin-shaped) pump auxin from cell to cell. Mechanosensitive Ca channels and their activation by reactive oxygen species (ROS) are yet another factor regulating cytosolic Ca. Cell expansion also triggers proton pump/pinball activity by the mechanotransduction of wall stress via Hechtian adhesion, thus forming a Hechtian oscillator that underlies cycles of wall plasticity and oscillatory growth. Finally, the Ca homeostasis of plants depends on cell surface external storage as a source of dynamic Ca, unlike the internal ER storage source of animals, where the added regulatory complexities ranging from vitamin D to parathormone contrast with the elegant simplicity of plant life. This paper summarizes a sixty-year Odyssey.