Phytotoxin production

Microbial biocatalytic systems, 76 398 Microbial biocontrol agents, 73 347-348 foliar application of, 73 349 phytotoxin production and, 73 351 problems associated with, 73 348-349 shelf life and storage of, 73 350 Microbial biomass, 26 471 474 substrates for, 26 473-474 Microbial catalysts, rapid screening of, 76 405... 

Pedras MSC, Chumala PB, Jin W, Islam MS, Hauck DW (2009) The phytopathogenic fungus Alternaria brassicicola phytotoxin production and phytoalexin elicitation. Phytochemistry 70 394-402... 

While much research on the nature and impact of phytotoxins has been conducted, many questions remain unanswered. Little is known about the rate of phytotoxin production and accumulation, localized concentration, threshold soil concentration for expression of bioactivity, duration of bioactivity, stability in soil, and environmental redistribution. Although the fate of specific chemicals under defined experimental conditions is discussed, it is hoped that the principles involved can be extended to provide a basic understanding of the fate of the wide range of allelochemical substances in the soil. 

Each weed species is a host to an array of plant pathogenic fungi and bacteria. Many of these organisms induce disease symptoms by virtue of phytotoxin production (1). By focusing on these plant pathogens and on their phytotoxins we hope to find novel, environmentally compatible, host-selective chemical agents for weed control. 

We have also observed that the production of these phytotoxins by A. alternata is controlled by metabolites from the host plant (9> ) The fungus is always grown with an effusion of the host plant added to the culture medium. Elimination of the extract from the broth results in little or no phytotoxin production, in spite of abundant iqycelial growth. Other fungi also seem to be dependent upon host plant products to activate phytotoxin production (IZ,... 

The early literature on naturally occurring plant growth inhibitors and the influence one plant might exert on another by chemical means (allelopathy) is considered comprehensively in various reviews (12, 15, 36, 37, 48, 49, 61, 67, 94, 121, 162). Reviews of studies concerned with the influence of plant exudates on root-infecting fungi (130) and the effects of phytotoxins which arise as decomposition products (113) have appeared recently. Because of the excellent coverage of the topics by others, no attempt is made here to review the early literature exhaustively. Instead, consideration is restricted essentially to specific compounds and to some of the more recent literature. 

Regarding the discovery of novel metabolites from fungal species previously investigated, it is of interest to point out that several fungal genome sequences currently available in public databases have revealed the existence of many orphan biosynthetic pathways for which the encoded natural products remain unknown [30]. Therefore, it is likely that further new metabolites and/or phytotoxins will be isolated from fungal species such as L. maculans and A. brassicicola when grown under different conditions such as culture medium or temperature. 

A recent search for general and specific elicitors from L. maculans demonstrated that the phytotoxins sirodesmin PL (1) and deacetylsirodesmin PL (2) are general elicitors since both induced the production of phytoalexins in resistant brown mustard and in susceptible canola [31]. Furthermore, two specific elicitors, a mixture of cerebrosides C (13) and D (14), were reported from mycelia of liquid cultures of L. maculans virulent on canola (Fig. 9.5) [19]. Previously, cerebrosides C (13) and D(14) were reported from a number of phytopathogenic fungi and were reported to induce the production of phytoalexins in rice plants and disease resistance to the rice blast fungus [32]. 

Pedras MSC Biesenthal CJ (1998) Production of the host-selective phytotoxin phomalide by isolates of Leptosphaeria maculans and its correlation with sirodesmin PL production. Can J Microbiol 44 547-553... 

The first fermentation was accomplished in dark conditions at 25°C. The resulting active organic extract (mycelial and broth) (IC50 = 195-0 Xg/ml against A. hypochondriacus) yielded 1-hydroxy-2-oxoeremo-phil-l(lO), 7(11), 8(9)-trien-12(8)-olide (58), a novel natural product and penicillic acid (59), a known phytotoxin (Fig. 18). ... 

Phenolic acid metabolism. During the last 30 years, much has been learned about the production of phytotoxins during plant residue decomposition in the laboratory and in the field ( -, 18). However, little is known about the fate of these molecules in the soil. 

Phenolic compounds have also been oxidatively polymerized to humic substances by clay minerals (29) and by the mineral fraction of a latasol (66). After a 10-day equilibration period, montmoril-lonite and illite clay minerals yielded 44 to 47% of the total added phenolic acids as humic substances whereas quartz gave only 9%. Samples of a latasol yielded over 63% of the total amount, from mixtures in varied proportion, of mono-, di- and trihydroxy phenolic compounds as humic substances (66). Extractions of the reaction products yielded humic, fulvic, and humin fractions that resembled soil natural fractions in color, in acid-base solubility, and in infrared absorption spectra. Wang and co-workers (67) further showed that the catalytic polymerization of catechol to humic substances was, enhanced by the presence of A1 oxide and increased with pH in the 5.0 to 7.0 range. Thus the normally very reactive products of Itgnin degradation can be linked into very stable humic acid polymers which will maintain a pool of potentially reactive phytotoxins in the soil. 

Dihydropyrenophorin, from Drechslera avenae, is a leaf pathogen of both wild and cultivated oats. It causes reddish brown lesions with a necrotic sunken center. At least one compound isolated from broth cultures of this fungus caused comparable lesions on oats and a variety of other plants at 3.2 x 10" M (15). The phytotoxin was characterized by spectrometric analyses and chemical conversion as (-)-dihydropyrenophorin (Vl), an important di lactone macrolide (15). However, the major product obtained in our extraction procedure used to isolate (-)-dihydropyrenophorin was the diol VII (j 6), which was not active in our bioassay tests. 

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