Possible roles for the vacuolar hydrogen ion-ATPase and F(1)F(0)-ATPase in aluminum resistance

Crop yields are significantly reduced under acidic soil conditions, where the speciation of aluminum (Al) compounds changes so that Al becomes soluble and/or toxic. Recent research has focused on developing crop cultivars resistant to soluble Al in the soil. Previous research in our laboratory identified two 51 kD, Al-induced proteins from an Al-resistant cultivar of wheat ( Triticum aestivum L.).; This thesis describes the purification and identification of the 51 kD, aluminum-induced proteins from wheat. Peptide microsequence analysis revealed that the purified peptides were homologous to subunits of the vacuolar H +-ATPase (V-ATPase) and the mitochondrial ATP synthase (F₁F ₀-ATPase). Both V-ATPase and F₁F₀-ATPase activities were induced by Al in the Al-resistant cultivar, PT741, and not in other cultivars tested. Based on these results, I hypothesized that increased V-ATPase activity could contribute to Al resistance by counteracting cytoplasmic acidosis caused by Al-inhibition of the plasma membrane ATPase. Vacuolar ATPase activity could maintain cytoplasmic pH homeostasis by transporting protons into the vacuole. Alternatively, it may be required to allow tonoplast transmembrane transport of other molecules, such as organic acids. The ATP required to fuel this process could be provided by the mitochondrial ATP synthase.; This hypothesis was tested in the yeast and Arabidopsis model systems. Yeast V-ATPase mutant strains were hypersensitive to Al, while F₁F₀-ATPase mutant strains exhibited wild-type growth. Since the tonoplast membrane potential generated by V-ATPase activity is also thought to be required for NaCl tolerance in yeast, the scope of my research was expanded to include analysis of the yeast response to NaCl. As was observed with Al, V-ATPase mutant strains were hypersensitive to NaCl while F ₁F₀-ATPase mutant strains exhibited wild-type growth. Transgenic Arabidopsis expressing a cDNA encoding the B subunit of the V-ATPase in the antisense orientation were also hypersensitive to Al and, to a lesser extent, NaCl.; In conclusion, this research describes the development and testing of a novel hypothesis that describes potential roles for the vacuolar H +-ATPase and the F₁F₀-ATPase in Al and NaCl tolerance. Determination of the precise mechanism by which increased V-ATPase activity mediates Al and NaCl tolerance awaits future investigation.