Phytoalexin

About: Phytoalexin is a research topic. Over the lifetime, 1161 publications have been published within this topic receiving 63405 citations. The topic is also known as: phytoalexins.

Biosynthesis of cabbage phytoalexins from indole glucosinolate

Abstract: Brassica crop species are prolific producers of indole–sulfur phytoalexins that are thought to have an important role in plant disease resistance. These molecules are conspicuously absent in the model plant Arabidopsis thaliana, and little is known about the enzymatic steps that assemble the key precursor brassinin. Here, we report the minimum set of biosynthetic genes required to generate cruciferous phytoalexins starting from the well-studied glucosinolate pathway. In vitro biochemical characterization revealed an additional role for the previously described carbon–sulfur lyase SUR1 in processing cysteine–isothiocyanate conjugates, as well as the S-methyltransferase DTCMT that methylates the resulting dithiocarbamate, together completing a pathway to brassinin. Additionally, the β-glucosidase BABG that is present in Brassica rapa but absent in Arabidopsis was shown to act as a myrosinase and may be a determinant of plants that synthesize phytoalexins from indole glucosinolate. Transient expression of the entire pathway in Nicotiana benthamiana yields brassinin, demonstrating that the biosynthesis of indole–sulfur phytoalexins can be engineered into noncruciferous plants. The identification of these biosynthetic enzymes and the heterologous reconstitution of the indole–sulfur phytoalexin pathway sheds light on an important pathway in an edible plant and opens the door to using metabolic engineering to systematically quantify the impact of cruciferous phytoalexins on plant disease resistance and human health.

GmPep914, an Eight-Amino Acid Peptide Isolated from Soybean Leaves, Activates Defense-Related Genes

Abstract: Only a handful of endogenous peptide defense signals have been isolated from plants. Herein, we report a novel peptide from soybean (Glycine max) leaves that is capable of alkalinizing the media of soybean suspension cells, a response that is generally associated with defense peptides. The peptide, DHPRGGNY, was synthesized and found to be active at 0.25 nM and requiring only 5 to 10 min to obtain a maximal pH change. The peptide is located on the carboxy-terminal end of a 52-amino acid precursor protein (Glyma12g00990) deduced from the soybean genome project. A search of the soybean databank revealed a homolog (Glyma09g36370) that contained a similar peptide, DLPRGGNY, which was synthesized and shown to have identical activity. The peptides, designated GmPep914 (DHPRGGNY) and GmPep890 (DLPRGGNY), were capable of inducing the expression of both Glyma12g00990 (GmPROPEP914) and Glyma09g36370 (GmPROPEP890) in cultured soybean suspension cells within 1 h. Both peptides induced the expression of defense genes, including CYP93A1, a cytochrome P450 gene involved in phytoalexin synthesis, chitinaseb1-1, a chitinase involved in pathogen defense, and Glycine max chalcone synthase1 (Gmachs1), chalcone synthase, involved in phytoalexin production. Both GmPROPEP914 and GmPROPEP890 were highly expressed in the roots, relative to the aerial portions of the plant. However, treatment of the aerial portion of soybean plants with hormones involved in elicitation of defense responses revealed a significant increase in expression levels of GmPROPEP914 and GmPROPEP890. A search of gene databases revealed homologous sequences in other members of the Fabales and also in the closely related Cucurbitales but not in any other order of plants.

Production of Stilbenoids and Phenolic Acids by the Peanut Plant at Early Stages of Growth

Abstract: The peanut plant (Arachis hypogaea) is known to produce stilbene phytoalexins as a defensive response to fungal invasion; however, the distribution of phytoalexins among different organs of the peanut plant at early stages of growth under axenic conditions has not been studied. Axenic plants produced a stilbenoid, resveratrol, as well as soluble bound and free phenolic acids, including 4-methoxycinnamic acid, which is reported in peanuts for the first time. Neither resveratrol nor phenolic acids were found in the root mucilage; the prenylated stilbenes were restricted to the mucilage and were not found in other organs of the peanut plant. These findings may lead to a better understanding of the defensive role of peanut stilbenes and phenolic acids.

Increased susceptibility and reduced phytoalexin accumulation in drought-stressed peanut kernels challenged with Aspergillus flavus.

Abstract: Three genotypes of peanut (Arachis hypogaea L.), with ICG numbers 221, 1104, and 1326, were grown in three replicate plots and drought stressed during the last 58 days before harvest by withholding irrigation water. Within each plot there were eight levels of stress ranging from 1.1 to 25.9 cm of water. Kernels harvested from the plots were hydrated to 20% moisture and challenged with Aspergillus flavus. Fungal colonization, aflatoxin content, and phytoalexin accumulation were measured. Fungal colonization of non-drought-stressed kernels virtually ceased by 3 days after inoculation, when the phytoalexin concentration exceeded 50 micrograms/g (fresh weight) of kernels, but the aflatoxin concentration continued to rise exponentially for an additional day. When fungal colonization, aflatoxin production, and phytoalexin accumulation were measured 3 days after drought-stressed material was challenged, the following relationships were apparent. Fungal colonization was inversely related to water supply (r varied from -0.848 to -0.904, according to genotype), as was aflatoxin production (r varied from -0.876 to -0.912, according to genotype); the phytoalexin concentration was correlated with water supply when this exceeded 11 cm (r varied from 0.696 to 0.917, according to genotype). The results are discussed in terms of the critical role played by drought stress in predisposing peanuts to infection by A. flavus and the role of the impaired phytoalexin response in mediating this increased susceptibility.

Rapid induction of phenylalanine ammonia-lyase and chalcone synthase mRNAs during fungus infection of soybean (Glycine max L.) roots or elicitor treatment of soybean cell cultures at the onset of phytoalexin synthesis.

Abstract: The differential regulation of the activities and amounts of mRNAs for two enzymes involved in isoflavonoid phytoalexin biosynthesis in soybean was studied during the early stages after inoculation of primary roots with zoospores from either race 1 (incompatible, host resistant) or race 3 (compatible, host susceptible) of Phytophthora megasperma fsp glycinea, the causal fungus of root rot disease In the incompatible interaction, cloned cDNAs were used to demonstrate that the amounts of phenylalanine ammonia-lyase and chalcone synthase mRNAs increased rapidly at the time of penetration of fungal germ tubes into epidermal cell layers (1–2 h after inoculation) concomitant with the onset of phytoalxxin accumulation; highest levels were reached after about 7 h In the compatible interaction, only a slight early enhancement of mRNA levels was found and no further increase occurred until about 9 h after inoculation The time course for changes in the activity of chalcone synthase mRNA also showed major differences between the incompatible and compatible interaction The observed kinetics for the stimulation of mRNA expression related to phytoalexin synthesis in soybean roots lends further support to the hypothesis that phytoalexin production is an early defense response in the incompatible plant-fungus interaction The kinetics for the enhancement of mRNA expression after treatment of soybean cell suspension cultures with a glucan elicitor derived from P megasperma cell walls was similar to that measured during the early stages of the resistant response of soybean roots