Regulation of Ethylene Biosynthesis in Virus-Infected Tobacco Leaves : II. TIME COURSE OF LEVELS OF INTERMEDIATES AND IN VIVO CONVERSION RATES.

Ethylene production was stimulated severalfold during the hypersensitive reaction of Samsun NN tobacco to tobacco mosaic virus (TMV). Exogenous methionine or S-adenosylmethionine (SAM) did not increase ethylene evolution from healthy or TMV-infected leaf discs, although both precursors were directly available for ethylene production. This indicates that ethylene production is not controlled at the level of methionine concentration or availability, nor at the level of SAM production or concentration. In contrast, 1-aminocyclopropane-1-carboxylic acid (ACC) stimulated ethylene production considerably. Thus, ethylene production is primarily limited at the level of ACC production.The regulation of ethylene production during the hypersensitive reaction to TMV was further studied by determining the time course of the concentrations of methionine, SAM, and ACC, as well as the course of their in vivo conversion rates. Endogenous concentrations of methionine and SAM remained unaffected until late in infection. On the contrary, the peak in ethylene production near the time of local lesion development was preceded by a large increase in ACC production. As a result of this increase, ACC accumulated in the leaf tissue. Only after local lesions became visible, the capacity to convert ACC into ethylene increased severalfold, associated with a sharp decrease in ACC content and a large increase in ethylene production.Ethylene production in tobacco leaves reacting hypersensitively to TMV is thus regulated at the level of both the production of ACC and its conversion to ethylene.