The effect of hydrogen peroxide on the viability of tomato cells and of the fungal pathogen Cladosporium fulvum

An oxidative burst was previously demonstrated to be induced in tomato plan ts by race specific elicitors of the fungal pathogen Cladosporium fulvum. T he in planta levels of H2O2 estimated to occur during elicitor treatment, w ere compared with the levels required to show toxicity to host cells and to the fungal pathogen. Injection of Cf-9 tomato leaves with 100 mM H2O2 caus ed an insignificant degree of necrosis and 1 M H2O2 was required to cause c omplete leaf necrosis comparable to that induced by the AVR9 elicitor. Assa ys with Cf-5 tomato cell suspensions confirmed the low toxicity of H2O2 to tomato cells but, as expected, the addition of Fe2+ with H2O2 (or with inte rcellular fluids containing AVR5 elicitor) enhanced cell death as determine d by the Evans Blue assay. Germination and germ tube growth of conidia of C . fulvum were significantly retarded by 4-5 mM H2O2, and at higher concentr ations, death of germ tubes was observed (ED50 = 22 mM), as determined by t he fluorescein diacetate assay. The addition of Fe2+ with H2O2 had little e ffect on fungal growth or viability in vitro. These results suggest that th e amount of H2O2 accumulating during an elicitor-induced response in leaves may be sufficient to affect fungal colonization but not to affect viabilit y of host cells unless the Fe2+ status in the apoplast is in some way alter ed by the elicitor to facilitate OH. production via the Fenton reaction. (C ) 1999 Academic Press.