Naturally produced phytotoxins

Glufosinate (GA) is a post-emergence and nonselective herbicide [91, 92], The L-enantiomer of this herbicide, which is also called phos-phinothricin, is a natural microbial phytotoxin, produced by Strepto-myces viridochromogenes and Streptomyces hygroscopicus, and acts as an inhibitor of glutamine synthetase, while the D-enantiomer shows no activity on this enzyme. The herbicidal action involves a rapid accumulation of ammonia, a deficiency in several amino acids, an inhibition of photosynthesis and, finally, the death of the plant cell [91, 93]. 

Some plant pathogenic bacteria and their phytotoxins have been screened in bioassays that monitor the effects of their toxins (antibiotic and phytotoxic) on other sensitive bacteria. For example, several fluorescent Pseudomonas syringae pvs. produce extracellular phytotoxins.76,106,116 Tabtoxin is produced by P. syringae pv. tabaci and pv. coronafacines, and this natural product inhibits glutamine synthetase.34,46,116 Phaseolotoxin, produced by P. syringae pv. phaseolicola... 

The first really molecular study of phytotoxins was done at the Rockefeller Institute, starting in the late 1940 s by a physiologist-biochemist team, Braun and Woolley, who worked with Pseudomonas syringae pv. tabaci. This bacterium produces the so called wildfire toxin, the name coming from the rapidity with which chlorosis is induced by the pathogen producing toxin in the leaves of the natural host, tobacco. They proposed a structure for this toxin, and because of its presumed structural and physiological resemblances to methionine sulfoximine, a recently discovered compound that interfered with methionine metabolism, they hypothesized that wildfire toxin acted in a similar, if not identical, manner. 

Triketides are relatively rare. Triacetic acid lactone (4.2) has been detected in Penicillium patulum. It is also produced by fatty acid synthase in the absence of the reductant NADPH. Radicinin (4.3) is a major phytotoxin isolated from Ahernaria radicina (Stemphyllium radicinum) which causes a black rot of carrots. It is also formed by other Ahernaria species. Its pyrano[4,3- ]pyran structure, the identification of which had eluded purely chemical degradative studies, was established in one of the earlier applications of NMR spectroscopy to natural product structure elucidation. The biosynthesis of radicinin from acetate units was studied in 1970 by both radio-isotope methods using carbon-14 and by carbon-13 enrichment studies with NMR methods of detection. This was one of the first applications of this NMR technique to biosynthetic problems. These results established the labelling pattern for radicinin shown in 4.3. 

Allelopathy may be invoked with confidence when soil concentrations of an allelochemical are at or above levels applied to soil that cause phytotoxicity. When concentrations are below such levels, allelopathy may still be taking place due to the continuous input of the compounds by the producing plant. This would be very difficult to imitate by exogenous application of the compounds. Furthermore, some have invoked synergism between natural phytotoxins to support the view that weakly phytotoxic phytochemicals are allelochemicals (e.g. [13]), but others have disputed such claims (e.g. [14,15]). This topic is discussed in more detail in the Section on Common Phenolic Acids. 

Molecular biology is having profound effects on the pesticide industry. In addition to its use in producing herbicide-resistant crops, in roved biocontrol agents, and more allelopathic or conq)etitive crops (23), it can be used in both the search for new molecular target sites and the elucidation of modes of action of natural phytotoxins. 

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