Two distinct biosynthetic pathways have been proposed for plant and fungal production of abscisic acid. The oxidative cleavage of xanthophylls is believed to be the key step in the plant C40 pathway (carotenoid pathway), while the direct cyclization of a derivative of farnesyl pyrophosphate is presumed to be in the fungal C15 pathway (direct pathway). Recently extensive research has tended to validate the existence of each pathway independently. In this review, we are going to introduce our recent evidence that supports the existence of each pathway from perspectives of organic chemistry. Xanthoxin has been recognized as a stress-induced intermediate characteristic for plant biosynthesis which results from cis-xanthophyll selective cleavage in Lycopersicon esculentum, while analogues of γ-ionylideneacetic acid derived from γ-ionylideneethanol have been identified as the fungal biosynthetic intermediates in Cercospora cruenta.
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