Inhibitors of Carotenoid Biosynthesis
Carotenoids play a role in photosynthesis by harvesting light and transferring the captured energy to chlorophyll molecules within the photosynthetic apparatus. However, carotenoids play three essential protective roles in the photosynthetic apparatus. They do this because they are highly effective quenchers, having the ability to absorb excitation energy and dissipate it harmlessly as heat. The first protective role is the ability to quench triplet chlorophyll molecules back to the ground state. The second is to quench singlet oxygen molecules (which are destructive) back to the normal triplet state (oxygen is unusual in that its triplet state is more stable than its singlet state). The third role is in quenching the photosystem reaction centers when overexcited in very bright light. For this last role, zeaxanthin, a specific carotenoid, is produced from violaxanthin that is normally present in the chloroplast. Inhibitors of carotenoid biosynthesis cause a general bleaching of the plant. This is because each time a chlorophyll molecule absorbs the energy from a photon there is a small, but finite chance that it will generate a triplet state. Without the presence of carotenoids to quench triplet chlorophyll, active oxygen species are generated and destroy the photosynthetic apparatus within the thylakoid membrane. Destruction of chlorophyll causes a bleaching of the leaf.
Herbicides that inhibit the biosynthesis of carotenoids may do so early in the isoprenoid biosynthetic pathway. An example is clomazone (Command) which inhibits isoprenoid biosynthesis at the level of isopentylpyrophosphate, at the very start of the pathway for carotenoid biosynthesis.
The effects are not specific to the production of carotenoids, but the mode of action results from photodynamic damage due to the inhibition of carotenoid biosynthesis. Photodynamic damage is cellular harm caused by absorption of light energy by a molecule unable to safely dissipate the energy. The pictures shown are corn and velvetleaf plants sprayed with Command herbicide.
Notice the lack of pigmentation in the leaves. The inhibitor blocks production of carotenoids. In the absence of carotenoids, chlorophyll molecules are much more susceptible to bleaching in sunlight. With no pigments, the plants cannot carry out photosynthesis and will die once reserves of energy in the seed are depleted.
Components in the carotenoid biosynthetic pathway that have proven to be effective sites for herbicides are the desaturase enzymes. These desaturase enzymes are dehydrogenases that remove hydrogen atoms and electrons from molecules, forming double bonds. In the carotenoid biosynthetic pathway, there are three successive desaturase steps between phytoene and lycopene. The addition of these additional double bonds is critical for the ability of carotenoids to quench triplet chlorophyll molecules and singlet oxygen species. Herbicides such as norflurazone inhibit the desaturase enzymes and block the biosynthesis of carotenoids; treated plants are then very sensitive to photodynamic damage. Other herbicides also affect the desaturation process in carotenoid biosynthesis, but do so indirectly. Examples are the isoxazole and triketone herbicides which inhibit the enzyme p-hydroxyphenylpyruvate dioxygenase. This enzyme is located in the biosynthetic pathway for plastoquinone biosynthesis and plastoquinone is a cofactor for the desaturase enzymes. By inhibiting p-hydroxyphenylpyruvate dioxygenase, the biosynthesis of carotenoids is also inhibited.
These are pictures of corn and velvetleaf plants sprayed with Balance herbicide, an isoxazole inhibitor of the carotenoid biosynthesis pathway. Again notice that inhibiting carotenoid biosynthesis causes photobleaching of the chlorophyll and destruction of the photosynthetic apparatus.
The third site for herbicides that inhibit carotenoid biosynthesis is cyclization. Lycopene is a linear intermediate that is cyclized (6-atom rings) at both ends to form the carotenes, which can in turn be hydroxylated to form the xanthophylls. Xanthophyll carotenoids are important for quenching over-excited reaction centers if leaves encounter very high light intensities. An example of a herbicide with a mode of action that inhibits carotenoid cyclization is Amitrole.