Pretreatment with bleaching herbicides alleviates the light-induced inhibition of maize 3-hydroxy-3-methylgutaryl-coenzyme A reductase activity

Abstract

Etiolated seedlings of maize ( Zea mays L., var. ‘Dekalb XL72AA’) contain high levels of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, EC 1.1.1.34).The influence of pretreatments with several herbicides known to interfere with plant isoprenoid synthesis on the light-induced inhibition of maize microsomal HMGR activity was examined. Pretreatment with the bleaching herbicides clomazone, norflurazon, fluridone, and acifluorfen (Na-salt) stimulated by four- to sevenfold the in vivo activity of microsomal HMGR extracted from 5-day-old maize seedlings grown in a 24-hr photoperiod with strong light (600 μmol/m 2/sec). Pretreatment with the growth-retarding herbicides EPTC and metolachlor did not affect the in vivo activity of microsomal HMGR extracted from maize seedlings grown under the same conditions. The in vivo activity of maize microsomal HMGR was not stimulated by any of the four bleaching herbicides when herbicide-treated maize seedlings were grown under continuous dark conditions. When herbicidetreated maize seedlings were germinated and grown in a 6-hr photoperiod with weak light (40 μmol/m 2/sec), the only herbicide that bleached maize seedlings and stimulated the in vivo activity of their microsomal HMGR was clomazone. None of the bleaching herbicides examined in this study exhibited any direct effects on the in vitro activity of microsomal HMGR extracted from etiolated maize seedlings. Mevinolin, a highly competitive inhibitor of HMGR, inhibited the in vitro activity of microsomal HMGR extracted from etiolated maize seedlings. These results suggest that the light-induced inhibition of maize microsomal HMGR activity may be indirect, resulting from the direct influence of light on chloroplast morphogenesis, a differentiation process disrupted by bleaching herbicides.