INTERMEDIATES OF SALICYLIC-ACID BIOSYNTHESIS IN TOBACCO

Salicylic acid (SA) is an important component of systemic-acquired res istance in plants. It is synthesized from benzoic acid (BA) as part of the phenylpropanoid pathway. Benzaldehyde (BD), a potential intermedi ate of this pathway, was found in healthy and tobacco mosaic virus (TM V)-inoculated tobacco (Nicotiana tabacom L. cv Xanthi-nc) leaf tissue at 100 ng/g fresh weight concentrations as measured by gas chromatogra phy-mass spectrometry. ED was also emitted as a volatile organic compo und from tobacco tissues. Application of gaseous ED to plants enclosed in jars caused a 13-fold increase in SA concentration, induced the ac cumulation of the pathogenesis-related transcript PR-1, and increased the resistance of tobacco to TMV inoculation. [C-13(6)]BD and [H-2(5)] benzyl alcohol were converted to BA and SA. Labeling experiments using [C-13(1)]Phe in temperature-shifted plants inoculated with the TMV sh owed high enrichment of cinnamic acids (72%), BA (34%), and SA (55%). The endogenous ED, however, contained nondetectable enrichment, sugges ting that ED was not the intermediate between cinnamic acid and BA. Th ese results show that ED and benzyl alcohol promote SA accumulation an d expression of defense responses in tobacco, and provide insight into the early steps of SA biosynthesis.