Extended Summary New Perspectives in Mechanisms of Herbicide Action. Inhibition of 4-Hydroxyphenylpyruvate Dioxygenase: the Mode of Action of the Herbicide RPA 201772 (Isoxaflutole)

Abstract

The benzoyl isoxazole herbicide RPA 201772, common name isoxaÑutole ( Fig. 1), is a novel product being developed for pre-and early post-emergence weed control in maize and sugarcane.1 In plants and soil the isoxazole ring opens, forming a diketonitrile derivative (Fig. 1).2 This is likely to be the active herbicidal principle of isoxaÑutole, as it is a potent inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD) in plants. Furthermore, the subsequent metabolic degradation of the diketonitrile occurs more rapidly in tolerant species such as maize and this appears to be the basis for herbicidal selectivity.2IsoxaÑutole causes a bleaching symptomology in susceptible species similar to that seen with herbicidal inhibitors of carotenoid biosynthesis, e.g. deÑufenican and other phytoene desaturase (PDS) inhibitors. Coincident with decreases in carotenoid levels following isoxaÑutole treatment is an accumulation of the PDS substrate, phytoene. IsoxaÑutole and its diketonitrile derivative were tested for their ability to inhibit PDS isolated from cultured carrot cell microsomes. At concentrations up to 100 kM neither compound signiÐ-cantly inhibited PDS activity, whereas standards such as diÑufenican and Ñurtamone had values of 100 IC 50 and 400 nM respectively. Therefore, the accumulation of phytoene in treated leaves and bleaching symptoms appears to be due to an indirect e †ect on PDS.HPPD catalyses the oxidative decarboxylation of 4-hydroxyphenylpyruvate forming homogentisate. The reaction mechanism, which is still not fully understood, involves ring peroxidation, leading to ring hydroxylation and side chain migration.3,4 Homogentisate then undergoes prenylation and methylation forming isoprenoid quinones required in biological redox reactions, such as plastoquinone. In bleached leaves levels of plastoquinone are depleted in advance of carotenoids. For example, HPLC analysis of Brassica kaber Wheeler seedlings revealed 40 and 75% decrease in plastoquinone 24 and 48 h after treatment with 63 g ha~1 isoxaÑutole. Carotenoid levels were identical to untreated controls after 24 h and were decreased by 35% after 48 h when bleaching became visible. Furthermore, accumulation of phytoene became apparent after 48 h. It is suggested that inhibition of HPPD results in an indirect e †ect on carotenoid biosynthesis due to the depletion of plastoquinone, a proposed cofactor of PDS.HPPD is a low-abundance enzyme in plants but it has now been puriÐed and characterized from cultured carrot cells.5 An assay has been developed involving the 83 Pestic. Sci. 0031-613X/97/$17.50 1997 SCI. Printed in Great Britain (

The Effects of Temperature and Soil Moisture on Chickpea (Cicer arietinum L.) Genotype Sensitivity to Isoxaflutole

“…In this study, shoot height measurement was taken as a bioassay parameter to assess isoxaflutole phytotoxicity. Isoxaflutole inhibits the activity of the enzyme HPPD which converts 4-hydroxyphenylpyruvate to homogentisate (Pallett et al 1997). This results in the inhibition of meristematic tissue, which is responsible for the reduction in shoot height.…”

A specific role for tocopherol and of chemical singlet oxygen quenchers in the maintenance of photosystem II structure and function inChlamydomonas reinhardtii

“…Inhibitors and quenchers were added at zero time. Optimal concentrations of oxygenase inhibitors (not documented here) block growth of the algae and induce chlorophyll bleaching as in the systems quoted above [8–13]. Therefore, and as discussed below, the oxygenase inhibitors were used at suboptimal concentration to allow for sufficient plastoquinone biosynthesis for photosynthesis and growth at low light.…”

Phytotoxicity of Whole Crop Forage Rice to Benzobicyclon

“…Benzobicyclon은 prenylquinone 생합성 경로의 phydroxyphenylpyruvate dehydrogenase (HPPD) 효소를 억제 하여 plastoquinone 및 alpha-tocoperol의 생합성을 저해하 며, 결과적으로 신엽의 백화현상을 일으켜 식물체를 고사 시킨다 (Barta and Böger, 1996;Garcia et al, 1999;Lee et al, 1997;Pallett et al, 1997). …”

“…시킨다 (Barta and Böger, 1996;Garcia et al, 1999;Lee et al, 1997;Pallett et al, 1997). (Han et al, 2009;Im et al, 2007;Im et al, 2008), 약해 정도도 일반형보다 통일형 품종에서 심한 것 으로 보고되었다 (Kim et al, 2012;Kwon et al, 2012).…”