Minisymposium 17: Water

Abs # 32004: Spatially selective growth responses to water deficit in the maize root cell-elongation zone are regulated by changes in apoplastic pH

Presenter:	Neumann, Peter M., agpetern@tx.technion.ac.il
Authors	Neumann, Peter M. (A)   Fan, Ling  (A)
Affiliations:	(A): Plant Physiology lab., Faculty of civil and environmental eng., Technion, Haifa 32000, Israel

The ongoing growth of roots facilitates the efficient mining by higher-plants of essential supplies of water and minerals from the soil and is inhibited by water deficits. Interestingly, this inhibition is spatially variable. Thus, growth in the zone of accelerating elongation at 0-3 mm behind the tip of maize (Zea mays L.) roots is remarkably maintained while growth in adjacent zones situated a few millimeters further behind the tip is inhibited. We investigated the hypothesis that changes in wall acidification are involved in regulating the different responses of adjacent root tissues to water deficit. Primary roots of whole maize seedlings were grown in aerated nutrient solution ± PEG 6000 at -0.5 MPa water potential to induce water deficits. Segmental elongation in the root apex was determined by marking experiments and instantaneous elongation of roots by tip tracking. A spatially selective inhibition of proton flux along the root elongation zone, after 48 h of water deficit, was quantified by high-resolution analysis of proton diffusion images produced after root contact for 5 min with pH indicator gel. Image posterization clearly revealed similar qualitative changes in the distribution of proton diffusion along the root surface. Apoplastic pH inside epidermal cell walls was independently determined by confocal laser scanning microscopy using a ratiometric method and fluorescent pH indicator dye. Changes in profiles of proton flux induced by water deficit were consistent with measured changes in wall pH at different distances behind the root tip and correlated with altered profiles of segmental elongation rate (r²=0.74). Importantly, both elongation and proton flux were well maintained in the region of accelerating growth 0 to 3 mm behind the root tip and were inhibited, by comparison with control roots, in adjacent regions of decelerating growth. Exogenous acidification of roots under water deficit, by addition of 2 mM succinate buffer at pH 4.5 or 10 µM fusicoccin (to activate plasma-membrane H+ ATPase) rapidly induced a temporary acceleration of growth. On the basis of these new findings and the known loosening effects of acid pH on wall mechanical properties, we propose that the apparently adaptive maintenance of growth 0 to 3 mm behind the tips of roots under water deficit and inhibition of growth 3 to 10 mm behind the tips, are related to a spatially variable, stress-induced inhibition of proton-pumping into the expanding cell walls.