Poster: Water relations

Abs # 150: 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 Phys lab, Faculty of Civil and Environmental Eng.,Technion IIT

The hypothesis that changes in apoplastic acidification are involved in the spatially variable distribution of cell elongation in roots under water deficit was investigated. Primary roots of whole maize seedlings (Zea mays L) were grown in aerated nutrient solution ± PEG 6000 at -0.5 MPa water potential to induce water deficits. Distribution of segmental elongation in the root apex was determined by marking experiments. Proton flux was determined by analysis of images of proton diffusion zones around roots contacted for 5 min with pH indicator gels. Apoplastic pH was determined by confocal laser scanning microscopy using a fluorescent pH indicator dye. Instantaneous elongation of roots was determined by tip tracking at 20 s intervals. Profiles of segmental elongation rate in the primary roots of whole maize seedlings subjected to water deficit for 48 h correlated well with profiles of proton flux along the surface of the elongation zone. Elongation and proton flux were maintained in the region of accelerating growth 0 to 3 mm behind the root tip and were inhibited, by comparison with control roots, in the region of transition and decelerating growth extending 3 to 10 mm behind the root tip. Concurrent changes in apoplastic pH inside expanding epidermal cell walls were revealed by confocal laser scanning microscopy. Finally, exogenous acidification of roots under water deficit, by addition of 2 mM succinate buffer at pH 4.5 or of 10 µM fusicoccin (to activate plasma membrane bound H+ pumping ATPase) induced temporary acceleration of root growth. We conclude that water deficit induces a spatially selective inhibition of wall acidification and hence elongation, in the cell expansion zone of growing maize roots.