Cotton phytoalexins

Chen, X.Y., Chen, Y, Heinstein, P. and Davisson, V.J. (1995) Cloning, expression and characterization of (-l-)-delta-cadinene synthase a catalyst for cotton phytoalexin biosynthesis. Arch. Biochem. Biophys., 324, 255-66. 

Research reported herein was initiated to determine the relative toxicity of the cotton phytoalexins and address specific questions which have plagued the study of phytoalexins ) solubility in an aqueous medium, 2) location in vivo at the site of infection, 3)... 

The limited solubility of most phytoalexins in an aqueous medium has raised concerns as to their ability to act as effective fungicides (11). With the exception of dHG, the cotton phytoalexins are relatively insoluble in water. At pH 6.3 (the -pH of infected xylem vessels) 50 pg/ml of dHG will dissolve in phosphate buffer the solubility of the other three phytoalexins range from 2 to 4 pg/ml at pH 6.3 (10). 

Figure 1. Structures of cotton phytoalexins, dHG -desoxyhemigossypol dMHG — desoxyhemigossypol-6-methyl ether HG - hemigossypol MHG - hemigoss)rpol-6-methyl ether.

Recent studies have confirmed that the phytoalexin isolated from species of cotton (Go55ypmw) infected with the fungus Verticillium dahliae is hemigossypol (161) (cf. Vol. 6, p. 66) and not, as previously reported,isohemigossypol (162). A related compound, p-hemigossypolone (163) has been identified as one of the compounds which inhibits the growth of tobacco budworm (Heliothis virescens)in cotton buds. ... 

The major phytoalexins formed by green tissues (cotyledons, leaves, bracts, and the epicarp of bolls) are the cadal.enes and lacinilines shown in Fig. 2 (12, 18). About 75% of these compounds are methylated in the cultivate 6. hirsutum (19), whereas methylation is either greatly restricted or absenfln the other cultivated cottons (G. arboreum, . herbaceum and Gi. barbadense). These compounds have been demonstFated Tn palisade parenchyma cel Is... 

Concentrations of constitutive terpenoids in the root epidermis of cotton are unrelated to differences in resistance. But concentrations of terpenoid aldehydes formed in the vicinity of the pericycle, near the head of the animal, act as phytoalexins and are closely correlated with levels of resistance (11, 48). Little or no phytoalexin is formed in the pericycle of susceptible cultivars. Mixtures of terpenoid phytoalexins are more toxic to the nematode than gossypol alone, and mixtures containing methylated terpenoid phytoalexins (from 6. hirsutum) are more toxic than those that contain only nonmethyTated phytoalexins (from 6. arboreum) (49). Thus, the structure of terpenoid phytoalexins also is importan for resistance to root knot nematode. 

The importance of terpenoid phytoalexins in resistance to wilts was further demonstrated in studies of temperature effects on resistance and in studies of induced resistance. Increase in temperature from 25 to 30 C causes a marked increase in resistance. This temperature change also slows the rate of sporulation of the fungus and increases the rate of phytoalexin formation by cotton (56). Likewise, treatments that induce resistance also induce pEy toalexin synthesis (57). Phytoalexin synthesis, therefore, is also important to explain environmental effects on disease resistance. 

We have examined these lethal reactions biochemically (93, unpublished). In all cases spontaneous defense reactions (phytoalexin synthesis, tannin synthesis and tannin oxidation) occur concurrent with the onset of symptoms. The quantities of terpenoid aldehydes and tannins found in tissues of severely affected plants are usually very similar to those found in severely diseased plants dying from microbial infections. Thus, it is possible that many of the disease symptoms seen in cotton are due to the toxic terpenoids and tannins formed in response to infections. 

Aflatoxin Bj is the most potent carcinogenic known substance occurring naturally. Interestingly enough 3-methyl-2-butanol is one of the main characteristic odors of living colonies of A. flavus. As an illustration of the complexity of these interactions it may be mentioned that treating cotton leaves with cell-free extracts of A. flavus induces five phytoalexins (lacinilene C, lacinilene C 7-methyl ether, scopoletin, 2-hydroxy-7-methoxycadalene, and 2,7-dihydroxycadalene) in the leaves (135). 

Scheme 7. Five phytoalexins, lacinilene C [1], lacinilene C 7-methyl ether [2], scopoletin [3], 2-hydroxy-7-methoxycadalene [4], and 2,7 dihydroxycadalene [5]) induced by A. flavus in cotton leaves. The antibiotic cortalcerone [6] is induced in the fungus Corticium caeruleum by various stimuli.

Phytoalexins and Phototoxins in Plant Pest Control presents a model approach to the binding of a phytoalexin elicator to DNA, studies of phytoalexins in cotton and peanuts, chapters on phototoxic metabolites of tropical plants and on photosensitizing porphyrins as herbicides. 

Another compound bordering between preformed and induced defenses is gossypol. This compound accumulates within epidermal glands on the surface of cotton plants and is very toxic to some insects [ .]. Gossypol, and a series of structurally related compounds on the same biosynthetic pathway, accumulate to substantially increased levels in plants that have been challenged by potential pathogens. For this reason they have been considered phytoalexins by some plant pathologists. Phytoalexins are inducible antibiotics and will be discussed later. 

Desoxyhemigossypol (dHG), a phytoalexin produced by cotton in response to infection, has been shown to be highly toxic to the pathogen,... 

A number of sesquiterpenoid phytoalexins related to gos-sypol (29, Fig. 21.14) have been isolated from members of the genus Gossypium (Malvaceae) (Coxon, 1982 Gershen-zon and Croteau, 1991). These compounds inhibit germination of conidia of the fungus responsible for verticillium wilt in cotton at levels well below that of fungicidal activity (Mace et al., 1985). The principal phytoalexin produced is hemigossypol (39) (Brooks and Watson, 1985). 

Mace, M. E., R. D. Stipanovic, and A. A. Bell, Toxicity and role of terpenoid phytoalexins in verticillium wilt resistance in cotton. Physiol. Plant Path., 26, 209-218 (1985). 

Pierce, M.L., Cover, E.C., Richardson, P.E., Scholes, VE. and Essenberg, M. (1996) Adequacy of cellular phytoalexin concentrations in hypersensi-tively responding cotton leaves. Physiol Mol. Plant Pathol. 48, 305-324 Pieterse, C.M.J., van Loon, L.C. (1999) Salicylic acid independent plant defense pathways. Trends Plant Sci., 4, 52-58... 

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