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.

Molecular cloning and functional analysis of a biphenyl phytoalexin-specific O-methyltransferase from apple cell suspension cultures

Abstract: This manuscript describes the cloning and functional characterization of a biphenyl phytoalexin biosynthetic gene, 3,5-dihydroxybiphenyl O-methyltransferase from elicitor-treated cell cultures of scab resistant apple cultivar ‘Florina’. Apples belong to the subtribe Malinae of the Rosaceae family. Biphenyls and dibenzofurans are the specialized phytoalexins of Malinae, of which aucuparin is the most widely distributed biphenyl. The precursor of aucuparin, 3,5-dihydroxybiphenyl, is a benzoate-derived polyketide, which is formed by the sequential condensation of three molecules of malonyl-CoA and one molecule of benzoyl-CoA in a reaction catalyzed by biphenyl synthase (BIS). This 3,5-dihydroxybiphenyl then undergoes sequential 5-O-methylation, 4-hydroxylation, and finally 3-O-methylation to form aucuparin. A cDNA encoding O-methyltransferase (OMT) was isolated and functionally characterized from the cell cultures of scab-resistant apple cultivar ‘Florina’ (Malus domestica cultivar ‘Florina’; MdOMT) after treatment with elicitor prepared from the apple scab causing fungus Venturia inaequalis. MdOMT catalyzed the regiospecific O-methylation of 3,5-dihydroxybiphenyl at the 5-position to form 3-hydroxy-5-methoxybiphenyl. The enzyme showed absolute substrate preference for 3,5-dihydroxybiphenyl. The elicitor-treated apple cell cultures showed transient increases in the MdOMT (GenBank ID MF740747) and MdBIS3 (GenBank ID JQ390523) transcript levels followed by the accumulation of biphenyls (aucuparin and noraucuparin) and dibenzofuran (eriobofuran) phytoalexins. MdOMT fused with N- and C-terminal yellow fluorescent protein showed cytoplasmic localization in the epidermis of Nicotiana benthamiana leaves. In scab inoculated greenhouse-grown ‘Florina’ plants, the expression of MdOMT was transiently induced in the stem followed by the accumulation of biphenyl phytoalexins.

The Phytoalexins Desoxyhemigossypol and Hemigossypol Are Elicited by Xanthomonas in Gossypium cotyledons.

Abstract: Gossypium (cotton) spp. produce an array of sesquiterpenoid defense compounds, some of which accumulate in pigmof dHG and HG.ent glands and in root epidermis of healthy plants, and others which function as phytoalexins. Of the phytoalexins which accumulate in stem stele of Verticillium wilt-resistant cotton in response to Verticillium dahliae infection, Mace, Stipanovic and Bell (1985) Physiological Plant Pathology, 26, 209, have shown that desoxyhemigossypol (dHG) has the highest antifungal activity. Of the phytoalexins previously observed in foliar tissue of bacterial blight-resistant Gossypoium hirsutum in response to Xanthomonas campestris pv. malvacearum (Xcm) infection [2,7-dihydroxycadalene (DHC), lacinilene C, lacinilene C 7-methyl ether, and 2-hydroxy-7-methoxycadalene], DHC has the highest antibacterial activity. Both groups of phytoalexins have cadinane carbon skeletons, but they differ in the positions of oxygen-containing functional groups. dHG and its oxidation product hemigossypol (HG) have now been identified as part of the foliar resistance response to Xcm. A time course study showed that the bacterial blight-resistant, pigment-glandless G. hirsutum line WbMgl accumulated dHG and HG more quickly than the cadalene and lacinilene phytoalexins and to similar peak amounts (1–5 μmol/g fr. wt). Bioassays on logarithmically growing cultures of Xcm in defined liquid medium in the dark revealed that both dHG and HG have phytoalexin activity toward this pathogen, but are less potent than DHC. Whether dHG and HG contribute to resistance toward the infection by Xcm or play a different role depends on where these phytoalexins accumulate in inoculated cotyledons, which has yet to be determined.

Transcriptional Regulation of Phytoalexin Biosynthetic Genes

Abstract: In legumes, isoflavonoid derivatives function as antimicrobial phytoalexins, whereas phytoalexins of solanaceous species are of terpenoid origin. The phenylpropanoid and isoprenoid pathways leading to these phytoalexins are involved in the synthesis of a wide range of secondary metabolites with important functions in plant growth, development and responses to the environment. Elicitation of phytoalexin biosynthesis involves transcriptional activation of the genes encoding enzymes of general phenylpropanoid/terpenoid biosynthesis, and of the genes for the specific branch pathways leading to antimicrobial compounds. In order to understand the molecular controls determining the developmental and environmental regulation of the general and specific enzymes of phytoalexin synthesis, we are studying the promoter regions of three elicitor inducible genes, chalcone synthase (chs, isoflavonoid pathway, general), isoflavone reductase (ifr,isoflavonoid pathway, specific) and 3-hydroxy-3-methylglutaryl CoA reductase (hmgr,terpenoid pathway, general). We describe cis-elements and trans-factors involved in the expression of these genes in relation to their tissue specific expression and response to biotic stress. Two elements, the G-box and H-box, located within 50 bp of the TATA box, are important for regulation of expression of chs and probably hmgr, but are not present in the alfalfa ifr promoter.

A Further Investigation of Phytoalexin Formation in the Genus Trifolium

Abstract: An earlier study of phytoalexin formation in the genus Trifolium has now been extended to include a further 61 species and subspecies. Using the drop-diffusate method, isoflavonoid phytoalexins were isolated from the fungus-inoculated leaflets of 55 accessions, whilst four others produced the stilbene derivative resveratrol. Phytoalexins could not be obtained from the leaflets of two species, T. billardieri and T. grandiflorum. The pterocarpan medicarpin was the most commonly encountered phytoalexin, occurring alone or in various combinations with the known Trifolium isofjavonoids maackiain (pterocarpan), vestitol, isovestitol, sativan, isosativan and arvensan (all isoflavans). Two additional pterocarpans, 4-methoxymedicarpin from T. cherleri and T. pallescens, and 4-hydroxyhomopterocarpin also from T. pallescens, were recognized for the first time as Trifolium phytoalexins. Re-examination of T. aureum re- vealed that this species produced resveratrol as a phytoalexin and not the isoflavan vestitol as previously described.