Phytotoxins from various plants

In relation with resistance of weeds to herbicides, Duke et al. (2000) mentioned that new mechanisms of action for herbicides are highly desirable to fight evolution of resistance in weeds, to create or exploit unique market niches, and to cope with new regulatory legislation. Comparison of the known molecular target sites of synthetic herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively little effort has been expended on determination of the sites of action of phytotoxins from natural sources, suggesting that intensive study of these molecules will reveal many more novel mechanisms of action. These authors gave some examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways AAL-toxin, hydantocidin, and various plant-derived terpenoids. 

There are relatively few reported diterpenoid phytotoxins produced by plants, although there are several potent diterpenes from fungi. The duvatriene-diol diterpenoids from tobacco leaf surfaces are phytotoxic to the serious weed Echinocloa crus-galli.26 Macias et al.32 summarized the effects of various... 

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... 

Rhizobitoxine. Various Rhizoblum. laponlcum strains produce rhizobitoxine [75], whose structure was reported In 1972 (2691. Although the host of this bacterium Is soybean, this compound Is also phytotoxic to many other plant species. This phytotoxin Is an analog of cystathionine and acts as an Irreversible Inhibitor of -cystathlonase which catalyzes production of homoserlne from cystathionine (2701. Rhizobitoxine also Inhibits ethylene production from methionine (2711. as does a similar phytotoxin, 2-am1no-4-methoxy-3-eno1c acid (28). 

---