The ‘wattle-blossom’ model depicts ‘the AGP as a whole is spheroidal’: GlcA, glucuronic acid Rha, rhamnose Gal, galactose Ara, arabinose p, pyranose f, furanose. Each Hyp residue of about 24 is attached to an arabinogalactan chain that contains from one to 15 repeats of a β-(l-3)-linked galactose (Gal) oligosaccharide with a degree of polymerization (d.p.) of 12. ĪGP models: wattle blossom, hairy rope, necklace. Insets: M s, S and W as a percentage of T d: (A) salt-adapted (B) control cells. Note similar values for AGPs in protoplasts, plasma membrane and the pectolyase-insoluble residue of non-adapted control cells, but significantly lower values for plasma membrane-associated AGPs in 2 % salt-adapted cells (340 m m NaCl). Each data point represents a minimum of five separate experiments using 7-d cultures of salt-adapted cells and 7-d cultures for controls. Intact cells ( T d), AGPs that remain bound to washed cells. PL-released, soluble AGPs released by pectolyase treatment of intact cells reflect soluble periplasmic AGPs plus AGPs in muro ( S + W). PL-insoluble, AGPs remaining bound to cells after treatment with pectolyase also reflect PM-bound AGPs, hence M PLdirect. Sonic-released ( S), soluble AGPs released by ultrasonic cell disruption. Cell walls ( W), AGPs assayed in the isolated wall fraction. Plasma membrane ( M s), PM-bound AGPs calculated from the relation M = T d − ( S + W). Protoplasts ( M pr), AGPs remaining bound after approx. Solid columns, salt-adapted empty columns, non-adapted controls. For Permissions, please email: reagent assay of AGP distribution in tobacco BY-2 cells adapted to growth in 2 % NaCl versus non-adapted controls. Published by Oxford University Press on behalf of the Annals of Botany Company. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.ĪGP–Ca2+ flux capacitor Arabinogalactan proteins calcium signalling hydroxyproline-rich glycoproteins ion currents plant cell wall protein. The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain.ĪGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5♵ but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3) the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. ![]() Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). Arabinogalactan proteins (AGPs) are ubiquitous in green plants.
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