Association between Ion Fluxes and Defense Responses in Pea and Cowpea Tissues

The glycopeptide elicitor from a pea pathogen, Mycosphaerella pinodes, induced rapid alkalinization and increases in levels of Na⁺ and K⁺ ions in the extracellular solution upon contact with pea and cowpea tissues. The presence of monensin, nigericin, lidocaine, quinidine or phenytoin together with the elicitor markedly inhibited these changes, whereas the presence of valinomycin, gramicidin D, tetraethylammonium, CsCl and aminopyridine did not. The production of phytoalexins in pea and cowpea tissues was also strongly inhibited by the simultaneous presence of the former reagents but not of the latter reagents. Inhibitory effects on the production of phytoalexins were diminished when monensin, nigericin or a Na⁺-channel blocker was applied 3 h after the start of treatment with elicitor. Furthermore, orthovanadate and neomycin, which suppress defense responses in both tissues, also inhibited the above mentioned changes. By contrast, the species-specific suppressor from M. pinodes inhibited the elicitor-induced release of Na⁺ and K⁺ ions from pea tissues, but, conversely, by itself it elicited either the defense response or the release of Na⁺ and K⁺ ions from cowpea tissues. The results indicate that these ion-related changes, in particular the efflux of Na⁺ and K⁺ ions, might be closely associated with the signal transduction system for defense responses at the tissue level.