Abstract
BackgroundExtensin deposition is considered important for the correct assembly and biophysical properties of primary cell walls, with consequences to plant resistance to pathogens, tissue morphology, cell adhesion and extension growth. However, evidence for a direct and causal role for the extensin network formation in changes to cell wall properties has been lacking.ResultsHydrogen peroxide treatment of grapevine (Vitis vinifera cv. Touriga) callus cell walls was seen to induce a marked reduction in their hydration and thickness. An analysis of matrix proteins demonstrated this occurs with the insolubilisation of an abundant protein, GvP1, which displays a primary structure and post-translational modifications typical of dicotyledon extensins. The hydration of callus cell walls free from saline-soluble proteins did not change in response to H2O2, but fully regained this capacity after addition of extensin-rich saline extracts. To assay the specific contribution of GvP1 cross-linking and other wall matrix proteins to the reduction in hydration, GvP1 levels in cell walls were manipulated in vitro by binding selected fractions of extracellular proteins and their effect on wall hydration during H2O2 incubation assayed.ConclusionsThis approach allowed us to conclude that a peroxidase-mediated formation of a covalently linked network of GvP1 is essential and causal in the reduction of grapevine callus wall hydration in response to H2O2. Importantly, this approach also indicated that extensin network effects on hydration was only partially irreversible and remained sensitive to changes in matrix charge. We discuss this mechanism and the importance of these changes to primary wall properties in the light of extensin distribution in dicotyledons.
Highlights
Extensin deposition is considered important for the correct assembly and biophysical properties of primary cell walls, with consequences to plant resistance to pathogens, tissue morphology, cell adhesion and extension growth
H2O2-mediated effects on cell wall hydration and thickness The swelling behaviour of isolated cell walls from grapevine callus in solution with 0-100 mM KCl at pH 4.5 is typical of a weak polyelectrolyte (Figure 1, closed circles)
H2O2-induced reduction in cell wall hydration is accompanied by GvP1 network formation We have previously reported that grapevine callus cells contained high levels of a single monomeric extensin, GvP1 [22], which is uniformly distributed as a monomer in the lateral walls [29]
Summary
Extensin deposition is considered important for the correct assembly and biophysical properties of primary cell walls, with consequences to plant resistance to pathogens, tissue morphology, cell adhesion and extension growth. Most current primary cell wall models agree that the major wall polymers are bound to each other largely non-covalently, physically intertwined [1,2]. In these models, hemicellulose is associated with cellulose through. The regulation of extension growth is thought to involve processes leading to a loss of wall plasticity, rather than a loss of turgor pressure [13] Such processes include processive pectin methyl esterases which demethylate homogalacturonans (HGs) to promote Ca2+ bridging and rigidification [14]. Class III peroxidases are regarded as potentially important cell wall stiffening enzymes [18], since peroxidase/H2O2-driven reactions may fix the viscoelastically extended cell wall through phenolic cross-linking [19], which can occur between feruloylated pectins [20] or extensins [21,22]
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