P.J.J. Hooykaas, M.A. Hall, K.R. Libbenga (Eds.), Biochemistry and Molecular Biology of Plant Hormones 0 1999 Elsevier Science B.V. All rights reserved CHAPTER 13 Salicylic acid biosynthesis Marianne C. Verberne", Retno A. Budi Muljono" and Robert Verpoorte Division of Pharmacognosy, LeidedAnsterdam Center for Drug Research, PO Box 9502, 2300 RA Leiden, The Netherlands E-mail: VERPOORT@LACDR.LeidenUniv.NL 1. Introduction Salicylic acid (SA) is probably one of the best known natural products, because of its acetyl-derivative, the widely used drug, aspirin [I]. Surprisingly, until recently there was very little known about the biosynthesis of SA in plants. In 1952 Geissmann and Hinreiner [21 suggested that C&, class compounds arise from degradation of phenylpropanoid compounds (Fig. 1). Later Gross and Schutte [3] reported that the benzoic acid moiety of cocaine was radio-active after the administration of (P-''C) phenylalanine to Erythroxylon novogranatense showing the involvement of phenylalanine in the biosynthesis of benzoic acid derivatives. The first evidence for the involvement of the phenylpropanoid pathway in the SA-biosynthesis was the detection of radioactivity in SA after administration of (~-14C)-tran~-~innami~ acid to Gaultheria pvocumbens plants [4]. For many years no further research was done on the pathway, until in 1979 when White [5] reported that the application of exogenous SA or its acetyl-derivative induces pathogenesis-related genes and causes partial resistance to plant diseases. Thus, the biosynthesis of SA became a hot topic because of its important signaling role in plant defence against pathogens (for reviews, see references [6,7]). Salicylic acid also has a number of other functions in plants, such as stimulation of flowering in Lemnaceae [%lo], inhibition of the biosynthesis of the plant hormone ethylene [ll], regulation of stomata1 closure [12] and root ion uptake [13]. Moreover, endogenous SA regulates heat production in the inflorescences of Arum lilies [ 141. However, despite the growing interest in SA as a natural signal in systemic acquired resistance in plants, the complete biosynthetic pathway has still not yet been resolved. Besides the phenylpropanoid pathway, which includes trans-cinnamic acid (CA) as a putative intermediate, SA can also be formed along a completely different biosynthetic pathway, via isochorismate, which is directly derived from chorismate [15,16] (Fig. 1). The latter pathway is found in microorganisms and its occurrence in plants cannot yet be excluded, though in the few plants studied more extensively so far, SA seems to be derived from the phenylpropanoid pathway. Here we will review both pathways. The identified intermediates and the enzymes involved will be described. The biosynthesis of the closely related 2,3-dihydroxybenzoic * Both these authors should be considered as first author. MCV having a major contribution on the chorismate/ isochorismate pathway, RABM on the phenylpropanoid pathway. 295