the diketopiperazine, cyclo (L-Pro-L-Tyr) is a host-specific phytotoxin
produced by Alternaria alternata on spotted knapweed (Centaurea
maculosa) (Stierle et al 1988). Its unique selectivity, apparent safety,
and simple structure, make maculosin an ideal chemical lead for developing a
safe and an environmentally friendly antiknapweed herbicide (Bobylev et al
1996). Even though this fungus produces numerous, non-toxic
diketopiperazines, only one, maculosin cyclo(L-Pro-L-Tyr), causes chlorotic
spots developing into black necrotic lesions on the leaves of knapweed, three-five
days after treatment (Park, 1992; Stierle et al 1988). Cultivars of
spotted knapweed vary dramatically in their reaction to maculosin, suggesting
that modification of the toxin may be occurring to yield bioinactive
metabolites (Park et al

         In examining the diketopiperazinesit,
is apparent that certain functional groups are necessary for activity (Stierle et al
1988). Maculosin, the most active compound, possesses a phenolic moiety,
not uncommon in phytotoxins (Sakamura, 1981). The
minor activity of compound 2, cyclo (-L-Pro-L-Phe-), compared to the inactivity
of its diastereomer, suggests the importance of conformation to bioactivity. Studies
on diketopiperazines containing proline indicated that L,L compounds assume an extended conformation, while
the LD diastereomers would have a more folded conformation (Young et al 1975). Stierle et al (Stierle et al 1988) isolated and identified phytotoxic activity of seven
diketopiperazines. The compounds were first applied to knapweed leaves and
hypocotyls that induced lesions at 10-3, lo-4, and 10-5
M. The compounds cyclo(-L-Pro-L-Phe-) and cyclo (-L-ProD-Phe-), differed in
phytotoxicity: the L,L diastereomer induced necrotic lesions on knapweed leaves
at 10-3 M, but the L,D isomer was not active, even at 10-3
M. Maculosin (cyclo-Pro-Tyr) is an ideal prototype for creating a safe and an
environmentally friendly antiknapweed herbicide. To evaluate this possibility, Bobylev et al
1996 synthesized and tested a series of 18 maculosin analogs in the
greenhouse on whole knapweed plants by spray or brush application. Interestingly,
many of the maculosin analogs have significant potential as natural herbicides
against spotted knapweed. Even the simplest analog, cyclo-Pro-Phe, destroys two
thirds of the spotted knapweed foliage at a concentration of 6 × 10-2 mol/L within
15 days.

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