In vivo and in vitro evidence for the inhibition of homogentisate solanesyltransferase by cyclopyrimorate.

BACKGROUND: Cyclopyrimorate is a highly effective bleaching herbicide discovered by Mitsui Chemicals Agro, Inc. The target site was recently reported to be homogentisate solanesyltransferase (HST) in the plastoquinone (PQ) biosynthesis pathway on the basis of the number of intermediates in cyclopyrimorate-treated plants and in vitro HST assays. Here, the target site of cyclopyrimorate was further explored using both in vivo and in vitro experiments. RESULTS: The cyclopyrimorate-dependent bleaching effect on Arabidopsis thaliana was reversed by decyl PQ, suggesting that this symptom is attributable to the inhibition of PQ biosynthesis. Furthermore, homogentisate (HGA), a substrate of HST, weakly reversed the bleaching effect of cyclopyrimorate in a dose-dependent manner. We expected that the weak reversal by HGA was due to competitive inhibition by cyclopyrimorate or des-morpholinocarbonyl cyclopyrimorate (DMC), a metabolite of cyclopyrimorate in plants that exhibit higher HST-inhibitory activity as compared to cyclopyrimorate. Kinetic analysis was therefore conducted using DMC. DMC inhibited HST competitively with respect to HGA, and was a mixed non-competitive inhibitor with respect to the other substrate, farnesyl diphosphate. Moreover, neither cyclopyrimorate nor DMC inhibited 2-methyl-6-phytyl-1,4-benzoquinone/2-methyl-6-solanesyl-1,4-benzoquinone methyltransferase, which is located downstream of HST in the PQ biosynthesis pathway. CONCLUSIONS: The target site of cyclopyrimorate and DMC is HST, which is a novel target site for commercial herbicides. © 2019 Society of Chemical Industry.

Action mechanism of bleaching herbicide cyclopyrimorate, a novel homogentisate solanesyltransferase inhibitor.

The action mechanism of cyclopyrimorate, a novel herbicide for weed control in rice fields, was investigated. Cyclopyrimorate caused bleaching symptoms in Arabidopsis thaliana similar to those caused by existing carotenoid biosynthesis inhibitors, mesotrione and norflurazon. However, cyclopyrimorate treatment resulted in significant accumulation of homogentisate and a reduction in the level of plastoquinone. A metabolite of cyclopyrimorate, des-morpholinocarbonyl cyclopyrimorate (DMC), was detected in plants. These data suggested that cyclopyrimorate and/or DMC inhibit homogentisate solanesyltransferase (HST), a downstream enzyme of 4-hydroxyphenylpyruvate dioxygenase in the plastoquinone biosynthesis pathway. In vitro assays showed that A. thaliana HST was strongly inhibited by DMC and weakly by cyclopyrimorate, whereas other commercial bleaching herbicides did not inhibit HST. DMC derivatives showed a positive correlation between HST inhibition and in vivo bleaching activities. These results indicate that the target site of cyclopyrimorate and DMC is HST, a novel target site of commercial herbicides.