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Effect of Fusicoccin on the Early Infection Process of Legume Roots by Rhizobium spp

Nod factors, the first detectable signals produced by Rhizobium spp., were reported to induce cytosolic pH changes and plasma membrane depolarization in root hairs as specific responses. In this study, it has been found that fusicoccin inhibits nodulation of alfalfa roots. This inhibition was only observed when fusicoccin was applied in the earlier steps of the bacteria-plant interaction. The observed effect is similar to that caused by the undissociated permeant acetic acid, which decreases the cytoplasmic pH and, like fusicoccin, significantly stimulates net H⁺ efflux. These results suggest that the fusicoccin-induced plasma membrane H⁺-ATPase activity and membrane hyperpolarization could be responsible for the nodulation inhibition observed. Moreover, it was found that nodulation was inhibited by removing external calcium with EGTA. When fusicoccin is present, a lower concentration of EGTA is necessary to inhibit nodulation. Furthermore, the addition of Ca²⁺ ionophore A23187 was found to inhibit H⁺ efflux by roots. These observations support the idea that the Nod factor-triggered calcium signal modulates the activity of the proton pump.