Allelochemicals Biologica... - Name

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DAVID E. WEDGE
AND BARBARA J. SMITH
Antibiotics, antineoplastics, herbicides, and insecticides often originate from plant
and microbial chemical defense mechanisms (Wedge and Camper, 2000). Secondary
metabolites, once considered unimportant products, are now thought to mediate chemical
defense mechanisms by providing chemical barriers against animal and microbial
predators (Agrios, 1997; Wedge and Camper, 2000). Plants produce numerous chemicals
for defense and communication, and can elicit their own form of offensive chemical
warfare by targeting the proliferation of pathogens. These chemicals may have
general or specific activity against key target sites in bacteria, fungi, and viruses.
Exploiting the chemical warfare that occurs between plants and their pathogens shows
promise in providing new natural products for new anti-infectives for human, plant
and animal health. The successful development of strobilurin fungicides and spinosad
insecticides has continued the interest in natural products as crop protectants. The
importance and future of natural product agrochemistry is emphasized by the fact
that 21 companies have filed 255 patent applications primarily for use of the strobilurin
class of fungicides (Qo I MET complex 3 inhibitors).
1.1. Direct Acting Defense Chemicals
Since the discovery of the vinca alkaloids in 1963, many of the known antitubulin
agents used in today’s cancer chemotherapy arsenal are products of plant and fungal
secondary metabolism. Since 1991, 16 of 43 new pharmaceuticals were derived from
natural products. In certain therapeutic areas 78% of the antibacterials and 74% of
the anticancer compounds are natural products or have been derived from natural
products (Roughi, 2003). These “natural products” are probably defense chemicals
that target and inhibit cell division in invading pathogens (Wedge and Camper, 2000).
Therefore, it is reasonable to hypothesize that plants and certain fungi can produce
chemicals, such as resveratrol and strobilurin, that act directly in their defense by
inhibiting pathogen proliferation, or indirectly by disrupting chemical signal processes
related to growth and development of pathogens or herbivores (Wedge and
Camper 2000).
1.2. Indirect Acting Defense Chemicals
Plant resistance to pathogens is considered to be systemically induced by some
endogenous signal molecule produced at the infection site that is then translocated to
other parts of the plant (Oku, 1994). The search for, and identification of, the putative
signal molecule(s) is of great interest to many plant scientists because such moieties
have possible uses as “natural product” disease control agents. However, research
indicates that no single compound is involved, but rather a complex signal transduction
pathway, which, in plants, can be mediated by a number of compounds that appears to
influence octadecanoid metabolism. In response to wounding or pathogen attack,
fatty acids of the jasmonate cascade are formed from membrane-bound α-linolenic
acid by lipoxygenase-mediated peroxidation (Vick and Zimmerman, 1984). Analogous
to the prostaglandin cascade in mammals, α-linolenic acid is thought to participate