Photodynamic herbicides

Rebeiz and co-workers have attempted to develop the use of ALA and chlorophyll biosynthesis modulators as photodynamic herbicides/ This novel approach makes use of the fact that the biosynthesis of ALA appears to be the rate-limiting step in tetrapyrrole biosynthesis. The application of ALA circumvents this rate-limiting step, and treated plant tissues accumulate tetrapyrroles, which can act as photodynamic herbicides and induce phytotoxicity similar to that now established for NDPEs. The susceptibility of treated plants is dependent on the nature of the tetrapyrroles that accumulate.  

The dual application of ALA with one or more chlorophyll biosynthesis modulators can result in selective herbicide action due to the four different greening patterns within the multibranched chlorophyll pathway that occur in plants. The chlorophyll biosynthesis modulators are classified into three major groups depending on their mode of action enhancers of ALA conversion to tetrapyrroles (e.g., 2-pyridine aldehyde, picolinic acid, 4,4 -dipyridyl, phenanthridine, 2,2 -pyridylamine) inducers of tetrapyrrole accumulation (e.g., 1,10-phenanthroline, 2,2 -dipyridyl) and inhibitors of mono vinyl protochlorophyllide accumulation (1,7- and 4,7-phenanthroline, 2,3-dipyridyl, 2,4-dipyridyl).  

Inhibition of Isopentenyl Pyrophosphate Isomerase and Prenyl Transferase 

Dimethazone or clomazone, 2-[(2-chlorophenyl)methyl]-4,4-dimethyl-3-isoxazolidinone, was introduced in 1984 as a selective preemergence/ 

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Photodynamic herbicides themselves do not contain metallotetrapyrroles but they are responsible for their formation and, subsequently, for their photodamage action. These herbicides have been successfully applied to more than 10 weed species. 

Physiol 48, 316 (1971). Use of labelled acid in porphyrin biosynthesis studies C. A. Rebciz et al. ibid. 46. 543 (1970). Enhancement of chlorophyll formation S. Ochiai, E. Hase, Plant Cell Physiol 11, 663 (1970), Proposed use as photodynamic herbicide C. A. Rebei2 et al. Enzyme Mt eroh. 

Sensitizers can be employed for agricultural purposes as herbicides and insecticides, or for medical purposes as antibacterial and antiviral agents. Moreover, sensitizer-based methods serve as tools for the analysis of the interaction faces of polymer complejKs and the sequence-selective photocleavage of double-stranded DNA. The ways in which photosensitized reactions are utilized are illustrated by the following typical examples. The first case relates to the photochemotherapy of cancer cells in superficial solid tumors [48]. The sophotodynamic therapy, PDT, is based on the selective incorporation of a photosensitizer into tumor cells, followed by exposure to light (commonly at A=600 nm). Cytotoxic products, namely singlet oxygen, O, and superoxide radical anions,... 

However, a problem with these chemicals is their indiscriminate toxicity in the presence of light and molecular oxygen. This toxicity precludes their use as pesticides. Safe alternatives are to treat target organisms with chemicals that either are selectively metabolized to photodynamic compounds or cause the target organism to produce toxic levels of natural photodynamic compounds with its own biochemical machinery. This review examines one aspect of the latter alternative - treatment of plants with compounds that cause the accumulation of herbicidal levels of photodynamic porphyrins. 

Although the results of these extensive experiments are not easily interpreted, the hypotheses were made (a) that PChlide is the most important and ubiquitous photodynamic species caused to accumulate by ALA-based treatments, (b) that MV PChlide is a more effective photodynamic pigment than DV PChlide in DDV/LDV and DMV/LDV species, and (c) that both DDV/LDV and DMV/LDV species are highly susceptible to a mixture of Mg-PPIX and Mg-PPIXME (14) The results are difficult to interpret because equimolar levels of different porphyrins were not produced and the combinations of porphyrins produced by different modulators varied with species. Potential differences in tolerance to toxic oxygen species between species were not considered. Others have attempted to explain differential sensitivity to porphyrin-generating herbicides between species (15) and between herbicide-sensitive biotypes within species (16) by differences in ability to detoxify toxic oxygen species. As with other herbicides, penetration of the leaf cuticle by ALA and/or DP can also play a role in differences in efficacy of this herbicide combination (17). 

Although ALA In combination with various Chi synthesis modulators has been patented for herbicide use, none of the combinations Is presently commercially available. However, a large number of synthetic herbicides that act by causing the accumulation of photodynamic porphyrins are sold throughout the world. 

Despite investigations by many laboratories, the nature of the photoreceptor for the photodynamic damage remained an enigma for more than two decades. Studies demonstrating that there was a metabolic requirement before the herbicide could cause effects like a photodynamic dye (24 28, 30> 43) should have provided a clue to the actual mechanism - the induction of the accumulation of a natural photodynamic compound. 

Mg-PPIX, or Mg-PPIXME. Furthermore, neither of the photodynamic precursors of PPIX, coproporphyrinogen nor uroporpophyrinogen (extracted as coproporphyrin III and uroporphyrin III) accumulate in diphenyl ether herbicide-treated plant tissues (Table IV). All of our data are consistent with the view that PPIX is the primary photodynamic pigment involved in the mechanism of action of these herbicides. 

In conclusion, the 2-aryl-1,2,4-triazine-3,5-diones are a new class of membrane disrupting herbicides. We have demonstrated preemergence tolerance toward soybean and postemergence tolerance toward corn. The mechanism of action has been found to involve inhibition of protoporphyrinogen oxidase which results in the build-up of a photodynamic toxicant, protoporphyrin IX. 

A second very large potential market for levulinic acid is in the production of the photodynamic pesticide delta aminolevulinic acid (DALA). DALA is the active ingredient in a range of environmentally benign broad-spectrum herbicides and insecticides under development at the University of Illinois, Urbana Champaign (14). Biofine, in conjunction with the National Renewable Energy Laboratory (NREL), Golden, CO has developed and patented a process for conversion of levulinic acid to DALA in reasonable yield (75). 

D-aminolevulinic acid Herbicide, active component in photodynamic cancer treatment... 

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