The Peroxisomal β-Oxidative Pathway and Benzyl Alcohol O-Benzoyltransferase HSR201 Cooperatively Contribute to the Biosynthesis of Salicylic Acid.

The phytohormone salicylic acid (SA) regulates plant defense responses against pathogens. Previous studies have suggested that SA is mainly produced from trans-cinnamic acid (CA) in tobacco, but the underlying mechanisms remain largely unknown. SA synthesis is activated by wounding in tobacco plants in which the expression of WIPK and SIPK, two stress-related mitogen-activated protein kinases, is suppressed. Using this phenomenon, we previously revealed that HSR201 encoding benzyl alcohol O-benzoyltransferase is required for pathogen signal-induced SA synthesis. In this study, we further analyzed the transcriptomes of wounded WIPK-/SIPK-suppressed plants and found that the expression of NtCNL, NtCHD and NtKAT1, homologous to cinnamate-coenzyme A (CoA) ligase (CNL), cinnamoyl-CoA hydratase/dehydrogenase (CHD) and 3-ketoacyl-CoA thiolase (KAT), respectively, is associated with SA biosynthesis. CNL, CHD and KAT constitute a β-oxidative pathway in the peroxisomes and produce benzoyl-CoA, a precursor of benzenoid compounds in petunia flowers. Subcellular localization analysis showed that NtCNL, NtCHD and NtKAT1 localize in the peroxisomes. Recombinant NtCNL catalyzed the formation of CoA esters of CA, whereas recombinant NtCHD and NtKAT1 proteins converted cinnamoyl-CoA to benzoyl-CoA, a substrate of HSR201. Virus-induced gene silencing of any one of NtCNL, NtCHD and NtKAT1 homologs compromised SA accumulation induced by a pathogen-derived elicitor in Nicotiana benthamiana leaves. Transient overexpression of NtCNL in N. benthamiana leaves resulted in SA accumulation, which was enhanced by co-expression of HSR201, although overexpression of HSR201 alone did not cause SA accumulation. These results suggested that the peroxisomal β-oxidative pathway and HSR201 cooperatively contribute to SA biosynthesis in tobacco and N. benthamiana.