Phytotoxins host-selective

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. 

The Alternaria black spot fungi A. brassicae and A. brassicicola produce host-selective toxins as well. While A. brassicicola produces brassicicolin A (11) as the major host-selective phytotoxin [28], A. brassicae produces destruxin B (12) (Fig. 9.4) [29]. Consistent with the virulence of these phytopathogens, both brassicicolin A (11) and destruxin B (12) appeared to be more phytotoxic to the susceptible cruciferous species B.juncea than to the tolerant B. napus. 

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... 

Wang H, Li J, Bostock RM, Gilchrist DG (1996) Apoptosis A Functional Paradigm for Programmed Plant Cell Death Induced by a Host-selective Phytotoxin and Invoked During Development. Plant Cell 8 375... 

In 2008, Pardeshi and Ward reported the synthesis of lairdinol A, a component of the host-selective phytotoxin depsilairdin on the basis of the Diels-Alder reaction of (R)-carvone with 3-trimethylsilyloxy-l,3-pentadiene. The key step of this synthesis established the trans ring fusion by the preferential epoxidation of a tra 5-fused enone into the corresponding epoxide through an equilibrating mixture of the c -fused and trans-fnsed diaster-eomers. This DKR is depicted in Scheme 1.58. This chiral epoxide was further converted into the expected lairdinol A in six steps with 18% overall yield from (R)-carvone. 

Recently, we have found that pathogens with a restricted host range and unrelated to common crop pathogens do produce novel phytotoxins. These novel phytotoxins have both unusual chemical structures and surprising, in some cases unprecedented, bioiogical activities. We will summarize these studies below, beginning with the ieast selective phytotoxins and progressing to phytotoxins with some host selectivity. 

The parent quinone may well be the ultimate phytotoxin. Alteichin does not show host selectivity in either the whole leaf or protoplast assays. In all test plants it causes necrotic flecks in leaf puncture wounds (2.7 mM 2% ethanol)) within 12 h of application. 

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. 

In the past several years a number of investigations on the chemistry of phytotoxins from weed pathogens has uncovered novel structures. The biological activity of these compounds varied from non-selective to narrow host range phytotoxicity (2,... 

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