SALICYLIC-ACID IN RICE - BIOSYNTHESIS, CONJUGATION, AND POSSIBLE ROLE

Salicylic acid (SA) is a natural inducer of disease resistance in some dicotyledonous plants. Rice seedlings (Oryza sativa L.) had the highe st levels of SA among all plants tested for SA content (between 0.01 a nd 37.19 mu g/g fresh weight). The second leaf of rice seedlings had s lightly lower SA levels than any younger leaves. To investigate the ro le of SA in rice disease resistance, we examined the levels of SA in r ice (cv M-201) after inoculation with bacterial and fungal pathogens. SA levels did not increase after inoculation with either the avirulent pathogen Pseudomonas syringae D20 or with the rice pathogens Magnapor the grisea, the causal agent of rice blast, and Rhizoctonia solani, th e causal agent of sheath blight. However, leaf SA levels in 28 rice va rieties showed a correlation with generalized blast resistance, indica ting that SA may play a role as a constitutive defense compound. Biosy nthesis and metabolism of SA in rice was studied and compared to that of tobacco. Rice shoots converted [C-14]cinnamic acid to SA and the li gnin precursors p-coumaric and ferulic acids, whereas [C-14]benzoic ac id was readily converted to SA. The data suggest that in rice, as in t obacco, SA is synthesized from cinnamic acid via benzoic acid. In rice shoots, SA is largely present as a free acid; however, exogenously su pplied SA was converted to beta-O-D-glucosylSA by an SA-inducible gluc osyltransferase (SA-GTase). A 7-fold induction of SA-GTase activity wa s observed after 6 h of feeding 1 mM SA. Both rice roots and shoots sh owed similar patterns of SA-GTase induction by SA, with maximal induct ion after feeding with 1 mM SA.