MODULATION OF ETHYLENE SYNTHESIS IN ACOTYLEDONOUS SOYBEAN AND WHEAT SEEDLINGS

The characteristics of ethylene production and ACC conversion in 8-day -old soybean seedlings were examined and a relationship between cytoch rome P-450 activity and ethylene-forming enzyme (EFE) activity was fou nd. An atmosphere containing 10% carbon monoxide (CO) significantly in hibited ethylene production and ACC conversion in control soybean seed lings, but had only a slight effect on soybean seedlings treated with uniconazole. Foliar application of triclopyr, a pyridine analogue of t he phenoxy herbicides, significantly increased ethylene production and ACC conversion in control, but not in uniconazole-treated seedlings. Triclopyr treatment also resulted in a three-fold increase in extracta ble cytochrome P-450 of 5-day-old etiolated soybeans. At equimolar con centrations tetcyclacis was more effective than uniconazole in reducin g shoot elongation and endogenous ethylene production. Although unicon azole and tetcyclacis did not inhibit ACC conversion in nonherbicide-t reated soybean seedlings, they did prevent the observed increase in AC C-dependent EFE activity following triclopyr application. However, the rate of ACC conversion in etiolated soybean segments was sensitive to uniconazole, and tetcyclacis inhibited the rate of ACC conversion by 2.6-fold in etiolated soybean segments within 4 h after treatment. Mic rosomal membranes were isolated from 5-day-old naphthalic anhydride-tr eated etiolated wheat shoots as this tissue contains much higher cytoc hrome P-450 levels than soybean shoots. Optical difference spectroscop y demonstrated that ACC generated binding spectrum characteristic of a reverse-type-I cytochrome P-450 substrate when combined with reduced microsomes. In vitro conversion of ACC to ethylene by microsomal membr anes was NADPH-dependent, inhibited by CO, and had an apparent K(m) an d V(max) of 45-mu-M and 0.345 nl/mg protein/h, respectively. These res ults suggest that cytochrome P-450-mediated monooxygenase reactions ma y be intimately involved in the conversion of ACC to ethylene in young soybean and wheat seedlings.