A mutant of Arabidopsis thaliana partially resistant to fusicoccin has reduced plasma membrane H+-ATPase

5-2 is a mutant of Arabidopsis thaliana which is partially resistant to fusicoccin in vivo, We have analysed fusicoccin binding acid the activity and amount of H+-ATPase in plasma membrane isolated from mature leaves of the wild type and of mutant 5-2, Fusicoccin binding was similar in plasma membrane from the two genotypes, while H+-ATPase activity was markedly (c. 50%) lower in plasma membrane from mutant 5-2 than in that from the wild type, The H+-ATPase of mutant 5-2 was activated by fusicoccin as much as that of the wild type. In plasma membrane from mutant 5-2, the amount of immunodetectable H+-ATPase, quantified by densitometry of Western blots, was about half that in the wild type. These results indicate that the major defect of mutant 5-2 detectable at the plasma membrane level is a reduction in the amount of H+-ATPase.