Phytotoxins defined

A recent review of the metabolites of L. maculans and L biglobosa produced in diverse culture conditions [19] emphasized that both species biosynthesize host-selective and non-selective phytotoxins. Importantly, it was shown that the composition of metabolite profiles of L. maculans depended on the composition of the culture medium. In a chemically defined liquid medium, isolates virulent on canola produced mainly sirodesmin PL (1), a non-host-selective phytotoxin, minor sirodesmins with one, three, or four sulfurs bridging the dioxopiperazine ring (sirodesmin H (3) [20], sirodesmin J (4) and K (5) [21]) and phomalirazine (6) (Fig. 9.1). The various sirodesmins 1-5 and phomalirazine (6) caused necrotic lesions of different intensities on leaves of both resistant and susceptible plants. Phomalide (7), the first host-selective phytotoxin isolated from virulent isolates of L. maculans, caused disease symptoms (necrotic, chlorotic, and reddish lesions) on canola (susceptible to L maculans) but not on brown mustard or white mustard... 

Fig. 9.1 Chemical structures of non-selective phytotoxins 1-6 and host-selective phytotoxin 7 produced by canola virulent isolates of Leptosphaeria maculans. Phytotoxins 1-7 are produced in a chemically defined medium...

L. maculans isolates Laird 2 and Mayfair 2 (virulent on brown mustard but not on canola) produced in a chemically defined medium the host-selective phytotoxin depsilairdin (8) (Fig. 9.2), containing a novel amino acid residue ((25,35,45)-3,4-dihydroxy-3-methylprolyl) and a sesquiterpene moiety (lairdinol A, synthesized recently [25,26]). Depsilairdin (8) caused disease symptoms similar to those caused by the pathogen on brown mustard, that is, strong necrotic and chlorotic lesions, but no lesions on canola. 

Fig. 9.4 Chemical structures of the selective phytotojdns 11 and 12 produced by mustard virulent isolates of Alternaria brassicicola and A. brassicae, respectively. Phytotoxins 11 and 12 are produced in a chemically defined medium...

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. 

Veno-occlusive disease (VOD) describes the occlusion of small hepatic veins and is defined as a radicular form of the Budd-Chiari syndrome. A variety of endotheliotoxic noxae, particularly phytotoxins, are responsible for this clinical picture. In 1951 reports were simultaneously published for the first time both in South Africa (G. Selzer et al.) and Jamaica (K. R. Hill) dealing with this disease of the small venous branches, which results from chronic intoxication with pyrrolizidine alkaloids, (s. pp 548, 571) Similar morphological and clinical effects can also be caused by cytostatic agents (6-mercaptopurine, dacarbazine, thioguanine), azathioprine, contraceptives and exposure to X-rays. Since 1957, the term Stuart-Bras syndrome has also been used to describe the occlusion of the small hepatic veins, (s. p. 832)... 

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