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Atrazine absorption and effects on photosynthesis in …

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.

Like the majority of herbicides, atrazine acts in the chIoroplasts whereit disrupts photosynthesis.

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 (oxygen is unusual in that its triplet state is more stable than its singlet state). The third role is in the 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 . Inhibitors of carotenoid biosynthesis cause a general bleaching of the plant. This is because each time a chlorophyll molecule absorbs the energy from a 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.

Atrazine inhibits photosynthesis and other metabolic processes ..

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 (oxygen is unusual in that its triplet state is more stable than its singlet state). The third role is in the 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 . Inhibitors of carotenoid biosynthesis cause a general bleaching of the plant. This is because each time a chlorophyll molecule absorbs the energy from a 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.

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.

Plant and Soil Sciences eLibrary

It has been estimated that perhaps as many as half of available herbicides have a mode of action that involves interaction with a few components in the chain of II. Remember that transfer of electrons from Photosystem II to Photosystem I is essential for the production of photosynthetic energy. ( for animation review--Link currently broken, please come back after 12-31-12). A key step in this electron transfer chain is the of plastoquinone by the D1 protein in the thylakoid membrane. Herbicides with a mode of action involving this site act as inhibitors of plastoquinone binding. These herbicides bind to the D1 protein and block the binding of PQ. By inhibiting the binding of PQ, the process of photosynthetic electron transfer is interrupted, and the synthesis of ATP and NADPH in the is compromised. This results in an inability to fix CO2 and produce the nutrients needed for the plant to survive. The block in electron transfer also causes an oxidative stress and the generation of radicals which cause rapid cellular damage.

Much effort has gone into the design of this class of inhibitors. Since the mode of action involves competition for a binding site within a membrane environment, the effectiveness of the herbicide will be greatly affected by small changes in its structure. Thus, small modifications in the molecular structure of a herbicide may cause differential sensitivity in different species of plants. Also, because the D1 protein in different plant species will have slightly different sequences of amino acid residues, differential effectiveness is possible with the same herbicide molecule. Plants also have detoxification systems that may greatly affect the response of different crops to herbicides. For example, corn is relatively insensitive to atrazine because of an efficient detoxification system involving reaction with glutathione, a protective tripeptide, and transport to the cell’s vacuole. This is a velvetleaf plant that was sprayed with Atrazine (Figure: Atrazine). Notice that damage is starting to become apparent around the leaf periphery.

It has been estimated that perhaps as many as half of available herbicides have a mode of action that involves interaction with a few components in the chain of II. Remember that transfer of electrons from Photosystem II to Photosystem I is essential for the production of photosynthetic energy. ( for animation review--Link currently broken, please come back after 12-31-12). A key step in this electron transfer chain is the of plastoquinone by the D1 protein in the thylakoid membrane. Herbicides with a mode of action involving this site act as inhibitors of plastoquinone binding. These herbicides bind to the D1 protein and block the binding of PQ. By inhibiting the binding of PQ, the process of photosynthetic electron transfer is interrupted, and the synthesis of ATP and NADPH in the is compromised. This results in an inability to fix CO2 and produce the nutrients needed for the plant to survive. The block in electron transfer also causes an oxidative stress and the generation of radicals which cause rapid cellular damage.

Much effort has gone into the design of this class of inhibitors. Since the mode of action involves competition for a binding site within a membrane environment, the effectiveness of the herbicide will be greatly affected by small changes in its structure. Thus, small modifications in the molecular structure of a herbicide may cause differential sensitivity in different species of plants. Also, because the D1 protein in different plant species will have slightly different sequences of amino acid residues, differential effectiveness is possible with the same herbicide molecule. Plants also have detoxification systems that may greatly affect the response of different crops to herbicides.

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Herbicides That Act Through Photosynthesis

Protoporphyrinogen oxidase (PPO) is an enzyme in the of the plant cell that oxidizes protoporphyrinogen to produce IX. This product is important because it is a precursor molecule for both chlorophyll (needed for photosynthesis) and heme (needed for chains) (Figure: Protoporphyrin lX).

PubMed - National Center for Biotechnology Information

The majority of available herbicides interact with a plant cell in a manner that causes damage from the energy in sunlight. Herbicides that inhibit the normal production of IX, a photosensitizing molecule, cause severe photodynamic damage. Herbicides that inhibit biosynthesis of carotenoids deprive plant cells of the photoprotection given by these molecules, permitting damage from chlorophyll mediated photosensitization. Inhibitors of from II block and starve the cell of the energy normally produced by photosynthesis. And finally, some herbicides act by diverting high-energy electrons from Photosystem I to generate damaging superoxide and other free radicals. Although each of these four classes of herbicides has a distinct mode of action, each interferes with the plant's ability to safely handle the high energy present in sunlight.

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