Variations on a theme: synthesis and modification of plant benzoic acids Mary C Wildermuth Plant benzoic acids (BAs) are critical regulators of a plant’s species of 30 different families and function as pollinator interaction with its environment. In addition, innumerable plant- attractants [3]. In addition, MeSA is involved in tri- derived pharmacological agents contain benzoyl moieties. trophic interactions; MeSA is emitted in response to Despite the prevalence and import of plant BAs, their herbivore damage (e.g. that caused by aphids or mites), biosynthetic pathways are not well-defined. Mounting attracting the predator of the herbivore [3,4]. As shown evidence suggests that BAs are synthesized both directly from in Figure 1, BAs are also incorporated into numerous plant shikimate/chorismate and from phenylalanine in plants; secondary metabolites that have established roles in however, few genes in these pathways have been identified. plant–herbivore or plant–pathogen interactions, such as Exciting progress has been made in elucidating genes that BA glucosinolate esters (e.g. in A. thaliana [5]), salicin (the modify BAs via methylation, glucosylation, or activation with major phenolic glycoside in willow [6]), xanthones (e.g. in Coenzyme A. As these modifications alter the stability, Hypericum androsaemum [7]), cocaine (in Erythroxylum coca solubility, and activity of the BAs, they impact the functional [8]), and taxol (in Taxus cuspidate [9]). Many of these BA roles of these molecules. The combination of multiple BA and benzoyl compounds are also important pharmacolo- biosynthetic routes with a variety of chemical modifications gical agents; for example, salicylate is the active ingre- probably facilitates precise temporal and spatial control over dient in aspirin, taxol is a potent anti-cancer drug, and active forms, as well as the channeling of intermediates to cocaine is a local analgesic. particular benzoate products. Despite the importance and prevalence of plant benzo- Addresses ates, the biosynthesis of BAs is not well defined. It is University of California, Department of Plant and Microbial Biology, 221 likely that several BA biosynthetic pathways exist in a Koshland Hall, Berkeley, California 94720-3102, USA given species, providing fine control over spatial and Corresponding author: Wildermuth, Mary C temporal synthesis and channeling intermediates to par- ([email protected]) ticular benzoate products. A now classic example is the synthesis of the active C5-unit isopentenyl diphosphate (IPP), the building block of all isoprenoids. A cytosolic Current Opinion in Plant Biology 2006, 9:288–296 mevalonate pathway synthesizes IPP, which is used in the This review comes from a themed issue on biosynthesis of sterols, sesquiterpenes, and triterpenoids. Physiology and metabolism By contrast, the plastidic, non-mevalonate 1-deoxy-D- Edited by Eran Pichersky and Krishna Niyogi xylulose-5-phosphate pathway (similar to that used by bacteria) is involved in the biosynthesis of plastidic iso- Available online 4th April 2006 prenoids, including carotenoids, phytol (the side chain of 1369-5266/$ – see front matter chorophylls), plastoquinone-9, and isoprene [10]. Modi- # 2006 Elsevier Ltd. All rights reserved. fications of BA (and SA) that influence their volatility, DOI 10.1016/j.pbi.2006.03.006 membrane permeability, solubility, and activity are also crucial to their transport and function. Thus, understand- ing the biosynthesis and modification of BAs in plants is a crucial first step to understanding the regulation and Introduction function of these important molecules. Plant benzoic acids (BAs) and their derivatives are com- mon and widespread mediators of plant responses to Biosynthesis of BAs in plants biotic and abiotic stress. For example, salicylic acid Plant BAs have been reported to originate either from (SA, 2-hydroxybenzoic acid) is a key signaling molecule phenylalanine (Phe) or directly from a shikimate-derived that mediates plant defense against a variety of pathogens product such as isochorismate, in which the carboxyl in a number of species, including tobacco and Arabidopsis carbon of shikimate is retained in the BA (Figure 2). thaliana. Its accumulation is required for the establish- By contrast, the carboxyl carbon of BAs that are synthe- ment of local and systemic required resistance (SAR) sized from Phe originates from the b-carbon of the Phe responses [1]. SA is also synthesized in response to side chain. Predominant early studies supported BA oxidative stressors, such as ozone, and is in part respon- synthesis from Phe, and thus most work has focused on sible for induced plant protective responses [2]. The Phe-derived pathways. As is readily apparent from volatiles methyl benzoate (MeBA) and methyl salicylate Figure 2, few of the plant genes that encode the enzymes (MeSA) are present in the floral scents of more than 100 of these pathways have been cloned; thus, much work is Current Opinion in Plant Biology 2006, 9:288–296 www.sciencedirect.com Benzoic acids Wildermuth 289 Figure 1 Representative plant BAs and secondary compounds that incorporate or are biosynthesized from a benzoyl moiety (shown in blue). needed to validate, define, and refine the biosynthetic [13] and 3-hydroxybenzoate (3HBA), which is incorpo- pathways for BAs. rated into amarogentin in Swertia chirata ([14]; Figure 1). In A. thaliana, the bulk of the SA that is produced in Biosynthesis of BAs directly from shikimate/chorismate response to pathogens is synthesized from isochorismate Bacteria provided the first evidence for BA biosynthesis and not from Phe, as confirmed by the lack of induced SA directly from shikimate/chorismate. For example, SA and accumulation in isochorismate synthase (ics1) mutants 2,3-dihydroxybenzoic acid (DHBA) are synthesized from [15]. This study provided the first genetic evidence for chorismate via isochorismate, and are precursors for side- a shikimate/chorismate direct pathway for the synthesis of rophores such as pyochelin in Pseudomonas aeruginosa [11] BAs in plants. Additional studies supporting, but not and enterobactin in Escherichia coli [12]. In plants, evi- providing direct evidence for, this pathway include: first, dence for the shikimate/chorismate direct pathway of BA the correlation of DHBA accumulation with ICS activity synthesis includes two 13C NMR studies in which the in elicted Catharanthus roseus cell cultures [16,17], and 13C-labeling of the carboxyl carbon is consistent with second, the incorporation of radiolabeled 3HBA (but not synthesis directly from shikimate/chorismate for gallic of Phe, trans-cinnamic acid [t-CA], or BA) into the ben- acid (3,4,5-trihydroxybenzoic acid) in the tree Rhus typina zoyl moiety of xanthone coupled with lack of induction of www.sciencedirect.com Current Opinion in Plant Biology 2006, 9:288–296 290 Physiology and metabolism Figure 2 Biosynthesis of BAs in plants by the direct shikimate/chorismate pathway and via phenylalanine. The carboxyl carbon of shikimate is labeled (13C) as is the b-carbon of phenylalanine (*). The plant enzymes involved in BA biosynthesis for which genes have been cloned are indicated, as are the chorismate-utilizing enzymes anthranilate synthase (AS) and aminodeoxychorismate synthase (ADCS). Pathways from trans-cinnamic acid alone are shown for simplicity; similar pathways from precursors in which either hydroxyl or methoxyl functionalities decorate the benzene ring (e.g. p-coumaric acid) have been reported. In addition, possible glucosylated precursors are not shown. C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate:CoA ligase; CM, chorismate mutase; ICS, isochorismate synthase; PAL, phenylalanine ammonia lyase. phenylalanine ammonia lyase (PAL) in elicited cells of confirmation of the non-oxidative route and a require- Centaurium erythraea [18]. The term shikimate/chorismate ment for CoA and the enzyme 4-coumarate:CoA ligase direct pathway is used to allow for the possibility that (4CL) in support of the CoA-dependent b-oxidation other intermediates (such as shikimate-3-phosphate or pathway. Support for the more recently proposed third anthranilate), in addition to isochorismate, might be route (Figure 2c) includes a requirement for CoA and utilized (Figure 2). Interconversion of the BA products 4CL coupled with detection of the benzaldehyde inter- (e.g. SA, DHBA, 3HBA, and BA) could then occur using mediate. There is an additional possibility that BA (or free or activated BAs (e.g. benzoyl-Coenzyme A [CoA] or BA-CoA) could be produced from Phe via phenylpyru- BA glucose conjugate). vate in a manner similar to that used by bacteria to catabolize Phe [19]. As limited conclusive evidence exists Biosynthesis of BAs from phenylalanine for the reported plant pathways from Phe to BA, alter- BA biosynthesis from Phe has been shown to proceed native possibilities such as this should be explored. from t-CA, which is produced from Phe via PAL. The conversion of t-CA to BA (or benzoyl-CoA) requires the The CoA-dependent b-oxidation pathway mirrors fatty cleavage of two carbons from the C3 side-chain of the acid b-oxidation ([20]; Figure 2a). In this pathway, t-CA is precursor. As shown in Figure 2, three possible routes activated by formation of the cinnamoyl-CoA ester, which have been reported for this production of BA from t-CA: is then hydrated to form 3-hydroxy-3-phenylpropionyl- CoA-dependent b-oxidation (Figure 2a), a CoA-indepen- CoA. This hydroxyl group is oxidized to a ketone, and the dent non-oxidative route (Figure 2b), and a CoA-depen- b-keto thioester is cleaved by a reverse Claisen conden- dent, non-oxidative pathway (Figure 2c). To distinguish sation to form benzoyl-CoA. 4CL catalyzes the activation between these pathways, researchers have used the iden- of t-CA but the subsequent enzymes have not been tification of the intermediate benzaldehyde as identified. In support of this pathway, radiolabeled Phe Current Opinion in Plant Biology 2006, 9:288–296 www.sciencedirect.com Benzoic acids Wildermuth 291 was not incorporated into benzaldeyde in tobacco structurally related molecules as a substrate with varied mosaic virus (TMV)-inoculated tobacco (Nicotiana taba- affinity or catalytic efficiency.