Stress upregulates periplasmic arabinogalactan-proteins

Stress upregulates AGPs typified by gum exudates, a well-known wound stress response. Salt-stress also dramatically upregulates AGP release; recently we showed that four species, adapted to growth as suspension cultures in high salt, gave much higher AGP yields (mg AGP/g cells fresh weight) and AGP release rates than control cells. This discovery has wide ramifications. For example, quantifying the AGP flux through the cell surface pools [membrane-bound, soluble periplasmic, wall and growth medium (M -> S -> W -> G([sink]))])] implied that AGPs migrate through the wall, mainly by `plug flow' extrusion in rapidly growing unadapted tobacco BY-2 cells, but largely by diffusion in salt-adapted cells. We propose that AGPs may act as plasticizers by decreasing pectin cross-linking; this increases pectin porosity and enhances wall rheology. Thus we interpret the rapid inhibition of pollen tip growth by the Yariv reagent as a loss of monomeric AGP plasticizers when they form cross-linked Yariv-AGP complexes in muro.