Minisymposium 22: Salinity

Abs # 43001: Reduced maize leaf growth under salinity is associated with decreased reactive oxygen species concentration in the elongation zone

Presenter:	Lascano, Hernán R., hrlascano@correo.inta.gov.ar
Authors	Rodríguez, Andrés A. (A)   Lascano, Hernán R. (A)   Córdoba, Alicia R (A)   Taleisnik, Edith L. (A)
Affiliations:	(A): Instituto de Fitopatología y Fisiología Vegetal-INTA

In grasses, stress conditions that decrease leaf elongation rates affect the blade elongation zone, reducing its length and elemental growth rates within it. Reactive oxygen species (ROS) in the apoplast of cells in the growing zone of grass leaves are required for elongation growth. This work evaluates whether salinity-induced reductions in leaf elongation are related to altered ROS production. Studies were performed in actively growing leaf segments (SEZ) obtained from maize seedlings gradually salinized to 150 mM NaCl. Salinity reduced elongation rates and the length of the leaf growth zone. When SEZ obtained from the elongation zone of salinized plants (SEZs) were incubated in 100 mM NaCl, concentration where growth inhibition was approximately 50%, O^.₂^- presence was lower than in similar segments obtained from control plants. The NaCl effect was salt-specific, and not osmotic, as incubation in 200 mM sorbitol did not reduce formazan staining intensity. SEZs elongation rates were higher in 200 mM sorbitol than in 100 mM NaCl, but the difference could be cancelled by scavenging or inhibiting O^.₂^-production (with 10 mM MgCl₂ or 200 µM diphenyleneiodonium (DPI), respectively). SEZs elongation in 100 mM NaCl could be stimulated by a ^.OH-generating medium, and 1 mM EGTA, added to the salinized ^.OH-generating medium, completely reverted the growth stimulation and depressed elongation to values even lower than those attained with 100 mM NaCl only, suggesting that the stimulating action of ROS on SEZs elongation requires an external source of Ca²⁺. Changes caused by salinity in apoplastic O^.₂^- concentration in the growing zone of leaf blades could have resulted from either an inhibition of ROS-generating mechanisms, or from the activation of antioxidant defence elements or both. Preliminary results from SOD and peroxidase activity measurements in apoplastic fluid do not indicate increased activities of these enzymes in salinized plants. NADPH-dependent O^.₂^-production by plasma membrane fractions was also inhibited by the presence of NaCl, DPI and by Ca²⁺ scavengers, as measured both spectrophotometrically and in native gels, however, it was not inhibited by the 1 mM KCN, an inhibitor of peroxidase activity. The activity bands were sectioned and run in SDS-PAGE, and were recognized by an antibody against human gp91^phox, suggesting that plasmalemma NADPH oxidase activity may be responsible for apoplastic O^.₂^- presence, and that NaCl could negatively affect it.