Mode of Action of Herbicides Affecting Carotenogenesis

Abstract

Carotenoids (carotenes and xanthophylls) are obligate pigments in the photosynthetic apparatus. They transfer absorbed light to chlorophylls and quench singlet oxygen which otherwise may degrade chlorophyll. Absence of colored carotenoids will lead to bleaching of leaves and death of the plants. Carotenoids have a C40 isoprenoid structure with conjugated double bonds. These are formed by processing phytoene via lycopene into beta-carotene. Inhibition of H-abstraction along this pathway produces non-functional, non-colored precursors. Many of such inhibitors have been developed as herbicides, like norflurazon, diflufenican or sulcotrione. Research on the genetic and molecular level has been addressed to phytoene desaturase. The plant-type gene has been sequenced and mutagenized to produce a herbicide-resistant enzyme. A Japanese group has isolated a gene from the bacterium Erwinia uredovora whose enzyme exhibits a striking resistance to many bleaching herbicides. Transgenic resistant tobacco lines have been engineered.