Intermediates of salicylic acid biosynthesis in tobacco

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