Engineering Disease Resistance in Plants: An Overview Page: 255
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V. MANIPULATION OF PHYTOALEXINS TO INCREASE DISEASE RESISTANCE
Phytoalexins (antimicrobial secondary metabolites) are thought to contribute to the resistance of plants
to disease. Phytoalexins have been identified in many different plant species and are structurally diverse,
being synthesized from a wide range of precursors."' In many cases it has been shown that they quickly
accumulate to very high levels around the site of pathogen attack, but not to a high degree in the
surrounding uninfected tissue.82-84 In some cases, before infection, significant amounts of the phytoalexin
may be constitutively accumulated, but usually in special cells or organelles, or in a conjugated,
inactive form."5"7 During infection, the "stored" phytoalexins are mobilized, while genes for biosynthetic
pathways are induced and the synthesis of more phytoalexin begins. Little is known about the turnover
or degradation of phytoalexins by the whole plant following accumulation; studies with elicited cell
cultures indicate that plant peroxidases may cause degradation of phytoalexins.".89 Much more significant
can be the degradation of phytoalexins by plant pathogens.9 Successful pathogens either have very
effective detoxification machinery, are not sufficiently sensitive to the phytoalexins of the host plant,
or infect without inducing phytoalexin synthesis.
Given an understanding of the interaction between a host plant and a particular pathogen, several
strategies can be outlined for improving plant disease resistance by modifying phytoalexin production.
These strategies fall into the three general categories of (1) introducing an entirely new class of
phytoalexins, (2) modifying the structure(s) of the phytoalexin of the host, and (3) altering the level
and/or timing of phytoalexin synthesis. Specific examples of published, on-going, or proposed/possible
manipulations are described below, followed by a potential "checklist" of concerns that should be
addressed before undertaking such projects.
A. INTRODUCING NEW PHYTOALEXINS
A successful pathogen may have evolved to detoxify or avoid the natural phytoalexins of its host plant,
but might be sensitive to phytoalexins from other plants. Two groups have succeeded in transferring
single enzyme genes into tobacco, resulting in the production of novel secondary metabolites. First,
introduction of a stilbene synthase gene from peanut into tobacco resulted in the measurable production
of the peanut stilbene resveratrol.91 Various types of stilbenes are important phytoalexins in peanut
(Arachis hypogaea), grape (Vitis sp.), and conifers such as pine (Pinus sp.) and spruce (Picea sitchensis),9'
but stilbenes are not normally made in tobacco. Stilbene synthase converts p-coumaroyl-CoA and
malonyl-CoA (1:3 ratio) to a C14 molecule in much the same way that chalcone synthase converts the
same precursors to flavonoids (Figure l1a).92 Results of any pathogen challenges on these transgenic
tobacco have not yet been reported.
Second, introduction of a fungal gene for a sesquiterpene cyclase, trichodiene synthase, resulted in
the accumulation of low levels of trichodiene, the precursor of many fungal mycotoxins.34 Solanaceous
plants do accumulate sesquiterpenoid phytoalexins, but these contain carbon skeletons unlike tricho-
diene."' Sesquiterpene cyclases are found in many plants and fungi; all use farnesyl pyrophosphate
(FPP) as their substrate, but fold and cyclize the molecule in a number of different ways. Further
modification results in the hundreds of known sesquiterpenoids, including the phytoalexins of cotton"195
and sweet potato (Figure 1 b).' Successful production of trichodiene demonstrates that a wide variety
of sesquiterpenoid skeletons may be introduced into plants, but the initial cyclization products are not
as antimicrobial as the final modified phytoalexins. Geranylgeranylpyrophosphate, produced by the
addition of a five carbon unit to FPP, is an intermediate in diterpene biosynthesis found in many plants.
Expression of the casbene synthase gene recently cloned from castor bean97 may likely lead to the
accumulation of the diterpene phytoalexin casbene, which is directly antifungal.
Initial metabolite and enzyme accumulation was very low in both of the above examples, but this
was possibly due to lack of optimization of the expression vectors used. These two cases represent rare
examples where introduction of one gene can produce a relatively new molecule. To introduce other
new phytoalexins could require the cloning and introduction of several genes. For example, to generate
tobacco plants that could make pisatin, the first characterized phytoalexin, would require the introduction
of at least nine enzymatic steps, and most of these enzymes/genes have not yet been cloned.
B. MODIFYING EXISTING PHYTOALEXINS
There is much evidence that small modifications in the structure of an existing phytoalexin might greatly
alter its toxicity to pathogens and/or its rate of degradation by detoxifying enzymes. Certain pathogens
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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/m1/9/: accessed June 17, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.