Engineering Disease Resistance in Plants: An Overview Page: 257
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Figure 2 Differential metabolism of pterocarpan phytoalexin stereoisomers by Nectria haematococca.
have evolved extremely specific detoxification enzymes, and the ability to detoxify phytoalexins appears
to play an important role in determining the virulence of pathogens.' In one example, isolates of the
red clover pathogen Nectria haematococca could readily degrade (-) maackiain, the pterocarpan
phytoalexin isomer found in red clover, but could not degrade (+) maackiain at all (Figure 2),98 and
were much more sensitive to (+) than (-) maackiain in bioassays.99 Many legumes contain pterocarpan
phytoalexins, either (-) (alfalfa, red clover, chickpea), (+) (certain cultivars of peanut), or both isomers
(Sophora japonica).10 Van Etten and others9 have proposed that by moving the appropriate genes from
a (+) pterocarpan-producing legume to a (-) pterocarpan-producing legume, the recipient plant could
be engineered to make phytoalexins of the opposite stereochemistry.9 In general, pathogens with stereo-
specific detoxification (such as the above mentioned Nectria) would be unable to degrade the "unnatural"
isomer and may therefore be unable to infect. It is thought that only two enzymes control which isomer
forms in the legume, isoflavone reductase and pterocarpan synthase; isoflavone reductase has been cloned
from alfalfa'0' and chickpea'02 and pterocarpan synthase genes should be available in the near future.103104
Several authors have proposed structure-activity relationships based on bioassays with various plant
pathogens.'00'5 Striking increases in bioactivity were correlated with prenylation of isoflavonoids,
presumably due to the increase in lipophilicity. For example, wighteone, kievitone, and phaseollidin
are all much more antifungal than their unprenylated precursors (Figure 3).10-108 Moving prenyltransfer-
ases into legumes which currently accumulate unprenylated isoflavonoids may result in the production
of novel antimicrobial compounds. For example, a prenyltransferase (dimethylallyl pyrophosphate: 3,9-
dihydroxypterocarpan 10-dimethylallyl transferase) involved in the biosynthesis of the bean phytoalexin
phaseollin has been purified from bean cell cultures and found to act also on the alfalfa phytoalexin
medicarpin."" The product has not yet been identified, but expression of this enzyme in alfalfa may
greatly increase the antifungal activity of the resulting phytoalexins.
Methylation of free hydroxyls has also been shown to increase the antifungal activity of isoflavonoids,
again presumably by increasing lipophilicity, and may also help protect hydroxyl groups from oxidative
detoxification reactions. Methyltransferases for isoflavones and pterocarpans have been partially charac-
terized from alfalfa"" and pea"' and, once cloned, may prove useful in modifying phytoalexins. The
substrate specificity of such biosynthetic enzymes can be very high; the O-methyltransferase which
carries out the final methylation to produce pisatin (pea pterocarpan phytoalexin) is totally inactive on
the pterocarpan (-) medicarpin and therefore could not be used to directly methylate this alfalfa
phytoalexin. In contrast, the purified alfalfa O-methyltransferase was active on a number of isoflavo-
Another way in which phytoalexin modification may increase resistance is by the production of
phytoalexin analogs which act as inhibitors of detoxification enzymes, even if they have no antimicrobial
activity of their own. A well-characterized example of this comes from studies of (3-lactam antibiotics.
Clavulanic acid alone is not toxic to Escherichia coli, but is a powerful inhibitor of @-lactamases; the
addition of a small amount of clavulanic acid to penicillin or other 13-lactam antibiotics can prevent
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Dixon, R. A.; Paiva, Nancy L. & Bhattacharyya, Madan Kumar. Engineering Disease Resistance in Plants: An Overview, chapter, 1995; [Boca Raton, Florida]. (https://digital.library.unt.edu/ark:/67531/metadc674022/m1/11/: accessed June 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.