Abstract
Novel molecular pinball machines of the plasma membrane control cytosolic Ca2+ levels that regulate plant metabolism. The essential components involve: 1. an auxin-activated proton pump; 2. arabinogalactan glycoproteins (AGPs); 3. Ca2+ channels; 4. auxin-efflux “PIN” proteins. Typical pinball machines release pinballs that trigger various sound and visual effects. However, in plants, “proton pinballs” eject Ca2+ bound by paired glucuronic acid residues of numerous glycomodules in periplasmic AGP-Ca2+. Freed Ca2+ ions flow down the electrostatic gradient through open Ca2+ channels into the cytosol, thus activating numerous Ca2+-dependent activities. Clearly, cytosolic Ca2+ levels depend on the activity of the proton pump, the state of Ca2+ channels and the size of the periplasmic AGP-Ca2+ 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 Ca2+ channels and their activation by reactive oxygen species (ROS) are yet another factor regulating cytosolic Ca2+. 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 Ca2+ homeostasis of plants depends on cell surface external storage as a source of dynamic Ca2+, 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.
Highlights
Brief Historical PerspectiveSixty years ago [1], the discovery of hydroxyproline (Hyp) firmly bound to the cell wall was the “founder event” for a new field in plant biology
Nobel Prize for protein chemistry in 1958 inspired cell wall protein analyses. Those showed most of the hydroxyproline was localised in the primary cell walls isolated and purified from the first cell suspension cultures generated from cambial explants of sycamore
A tree reverts to its algal ancestors, provoking Robin Hill’s remark “Wouldn’t it be wonderful to turn an alga into a tree!”
Summary
Sixty years ago [1], the discovery of hydroxyproline (Hyp) firmly bound to the cell wall was the “founder event” for a new field in plant biology. The amino acid molar ratios were normalized to 30 moles of hydroxyproline compared with the tomato cell wall Such proteins, later named arabinogalactan proteins (AGPs), located mainly between the plasma membrane and cell wall, were periplasmic, analogous to Peter Mitchell’s bacterial periplasm [10]. The identification of the glycopeptide link between protein and polysaccharide depended, again, on a classical Sanger strategy, which involved exploiting the differing stability of various covalent bonds to chemical and enzymic attack Both peptide and glycosidic linkages are acid labile. Hyp-AGs [16] led to the discovery of their essential role in Ca2+ homeostasis, validated by recent direct evidence [17,18,19] and as described in subsequent sections that show how the structure of Hyp-arabinogalactan glycomodules leads to Ca2+ homeostasis and plasma membrane Ca2+ ion influx that regulates plant growth
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