INDUCTION OF BENZOIC-ACID 2-HYDROXYLASE IN VIRUS-INOCULATED TOBACCO

Salicylic acid (SA) plays an important role in the induction of plant resistance to pathogens. An accompanying article (N. Yalpani, J. Leon, M.A. Lawton, I. Raskin [1993] Plant Physiol 103: 315-321) shows that SA is synthesized via the decarboxylation of cinnamic acid to benzoic acid (BA), which is, in turn, hydroxylated to SA. Leaf extracts of tob acco (Nicotiana tabacum L. cv Xanthi-nc) catalyze the 2-hydroxylation of BA to SA. The monooxygenase catalyzing this reaction, benzoic acid 2-hydroxylase (BA2H), required NAD(P)H or reduced methyl viologen as a n electron donor. BA2H activity was detected in healthy tobacco leaf e xtracts (1-2 nmol h-1 g-1 fresh weight) and was significantly increase d upon inoculation with tobacco mosaic virus (TMV). This increase para lleled the levels of free SA in the leaves. Induction of BA2H activity was restricted to tissue expressing a hypersensitive response at 24-d egrees-C. TMV induction of BA2H activity and SA accumulation were inhi bited when inoculated tobacco plants were incubated at 32-degrees-C. H owever, when inoculated plants were incubated for 4 d at 32-degrees-C and then transferred to 24-degrees-C, they showed a 15-fold increase i n BA2H activity and a 65-fold increase in free SA content compared wit h healthy plants incubated at 24-degrees-C. Treatment of leaf tissue w ith the protein synthesis inhibitor cycloheximide blocked the inductio n of BA2H activity by TMV. The effect of TMV inoculation on BA2H could be duplicated by infiltrating leaf discs of healthy plants with BA. T his response was observed even when applied levels of BA were much low er than the levels observed in vivo after virus inoculation. Feeding t obacco leaves with phenylalanine, cinnamic acid, or o-coumaric acid (p utative precursors of SA) failed to trigger the induction of BA2H acti vity. BA2H appears to be a pathogen-inducible protein with an importan t regulatory role in SA accumulation during the development of induced resistance to TMV in tobacco.