An Arabidopsis Mutant Over-Expressing Subtilase SBT4.13 Uncovers the Role of Oxidative Stress in the Inhibition of Growth by Intracellular Acidification

Intracellular acid stress inhibits plant growth by unknown mechanisms and it occurs in acidic soils and as consequence of other stresses. In order to identify mechanisms of acid toxicity, we screened activation-tagging lines of for tolerance to intracellular acidification induced by organic acids. A dominant mutant, , was isolated twice and shown to over-express subtilase SBT4.13, a protease secreted into endoplasmic reticulum. Activity measurements and immuno-detection indicate that the mutant contains less plasma membrane H -ATPase (PMA) than wild type, explaining the small size, electrical depolarization and decreased cytosolic pH of the mutant but not organic acid tolerance. Addition of acetic acid to wild-type plantlets induces production of ROS (Reactive Oxygen Species) measured by dichlorodihydrofluorescein diacetate. Acid-induced ROS production is greatly decreased in and mutants. The latter is deficient in two major NADPH oxidases (NOXs) and is tolerant to organic acids. These results suggest that intracellular acidification activates NOXs and the resulting oxidative stress is important for inhibition of growth. The inhibition of acid-activated NOXs in the mutant compensates inhibition of PMA to increase acid tolerance.