ROLE OF AMMONIUM ACCUMULATION IN BACTERIA-INDUCED HYPERSENSITIVE AND COMPATIBLE REACTIONS OF TOBACCO AND COTTON PLANTS

Leaves of 12-week-old tobacco plants (nicotiana tobacum L. cv. Samsun NN) were infiltrated with suspensions of Pseudomonas syringae pv. pisi (DSM 50291) to induce hypersensitive reaction (HR). Cotyledons of 2-w eek-old cotton plants (Gossypium hirsutum L. cv. Acala 442 and Coker B R) were infiltrated with Xanthomonas campestris pv. malvacearum (race 10) to induce the disease. In tobacco, HR-related increases in NH4+ le vels started within 2 h after infection and continued up to the time o f tissue decay. Increase of NH4+ and especially K+ efflux were detecte d in intercellular washing fluids (IWF). Antibiotics stopped and later reverted NH4+ production and K+ efflux, but only if applied within 2 h after infection. When 10 mM NH4+ was injected into leaves, it was ra pidly consumed from the IWF, and also, although more slowly, within th e leaf cells. The concomitant K+ efflux was strong but delayed, and mo st of the K+ was reabsorbed after 2 h. Bacterial cell multiplication i n HR stopped before the appearance of HR symptoms and cell necrosis. I n the compatible reaction in cotton cotyledons, both NH4+ accumulation and K+ efflux proceeded much more slowly than in the HR with tobacco, and bacteria continued to multiply until general cell necrosis occurr ed. The compatible reaction developed faster in constant darkness than in a light/dark rhythm. Bacterial enzymes produced NH4+, mainly from proteins of host cells, in both light and darkness. Continuous light d elayed the main peak of both NH4+ production and K+ efflux. High CO2 c oncentration inhibited both processes, thus indicating that photorespi ration plays a role in enhancing the release of free ammonium during b acterial pathogenesis. This is supported by shifts in the pattern of a mino acids. The results demonstrate the accelerating and aggravating e ffect of ammonium in pathogenesis and HR, though ammonium is not the p rimary agent.