Tenuazonic acid from Stemphylium loti inhibits the plant plasma membrane H -ATPase by a mechanism involving the C-terminal regulatory domain.

Pathogenic fungi often target the plant plasma membrane (PM) H -ATPase during infection. To identify pathogenic compounds targeting plant H -ATPases, we screened extracts from 10 Stemphylium species for their effect on H -ATPase activity. We identified Stemphylium loti extracts as potential H -ATPase inhibitors, and through chemical separation and analysis, tenuazonic acid (TeA) as a potent H -ATPase inhibitor. By assaying ATP hydrolysis and H pumping, we confirmed TeA as a H -ATPase inhibitor both in vitro and in vivo. To visualize in planta inhibition of the H -ATPase, we treated pH-sensing Arabidopsis thaliana seedlings with TeA and quantified apoplastic alkalization. TeA affected both ATPase hydrolysis and H pumping, supporting a direct effect on the H -ATPase. We demonstrated apoplastic alkalization of A. thaliana seedlings after short-term TeA treatment, indicating that TeA effectively inhibits plant PM H -ATPase in planta. TeA-induced inhibition was highly dependent on the regulatory C-terminal domain of the plant H -ATPase. Stemphylium loti is a phytopathogenic fungus. Inhibiting the plant PM H -ATPase results in membrane potential depolarization and eventually necrosis. The corresponding fungal H -ATPase, PMA1, is less affected by TeA when comparing native preparations. Fungi are thus able to target an essential plant enzyme without causing self-toxicity.