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

Abstract

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.

Keywords: bleaching herbicide; carotenoid biosynthesis inhibitor; cyclopyrimorate; homogentisate solanesyltransferase; plastoquinone biosynthesis inhibitor.

Fig. 1. Carotenoids and plastoquinone biosynthesis pathways. Target enzymes of inhibitors are indicated in bold. Two consecutive arrows indicate multi-step reactions. DOXPS, 1-deoxy-D-xylulose-5-phosphate synthase; PDS, phytoene desaturase; 4-HPPD, 4-hydroxyphenylpyruvate dioxygenase; HST, homogentisate solanesyltransferase.

Fig. 2. Structures of cyclopyrimorate and its metabolite, des-morpholinocarbonyl cyclopyrimorate (DMC).

Fig. 3. Effect of herbicide treatment (500 ppm) on Arabidopsis seedlings. (A) Control, (B) cyclopyrimorate, (C) mesotrione, and (D) norflurazon. Scale bar=1 mm. The upper panel shows whole seedlings, whereas the lower panel shows the expanded leaves of each seedling. Images were taken 5 days after sowing.

Fig. 4. The amount of plastoquinone (PQ) and homogentisate (HGA) in various plants 5 days after herbicide treatment. (A) A. thaliana seedlings, (B) B. juncea leaves, and (C) S. juncoides shoots. The concentration of each herbicide (ppm) is indicated along the X-axes. Data for PQ are normalized relative to the control, and data for HGA are expressed on a fresh weight basis. Values represent the mean±SD (n=3).

Fig. 5. Effect of various herbicides on A. thaliana HST. ●, cyclopyrimorate; ■, DMC; △, haloxydine; ▽, mesotrione; ◇, norflurazon. Values represent the mean±SD (n=3).

Fig. 6. Correlation between HST inhibitory activities and in vivo bleaching activities of herbicides. pIC₅₀=log (1/IC₅₀ (μM)) and pEC₅₀=log (1/EC₅₀ (μM)). ◆, DMC and its derivatives; □, cyclopyrimorate.