The phytoalexins desoxyhemigossypol and hemigossypol are elicited by Xanthomonas in Gossypium cotyledons

Gossypium (cotton) spp. produce an array of sesquiterpenoid defense compoun ds, some of which accumulate in pigment glands and in root epidermis of hea lthy plants, and others which function as phytoalexins. Of the phytoalexins which accumulate in stem stele of Verticillium wilt-resistant cotton in re sponse 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 o bserved 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-h ydroxy-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 r esistance response to Xcm. A time course study showed that the bacterial bl ight-resistant, pigment-glandless G. hirsutum line WbMgl accumulated dHG an d HG more quickly than the cadalene and lacinilene phytoalexins and to simi lar peak amounts (1-5 mu mol/g fr. wt). Bioassays on logarithmically growin g cultures of Xcm in defined liquid medium in the dark revealed that both d HG and HG have phytoalexin activity toward this pathogen, but are less pote nt than DHC. Whether dHG and HG contribute to resistance toward the infecti on by Xcm or play a different role depends on where these phytoalexins accu mulate in inoculated cotyledons, which has yet to be determined. (C) 1999 E lsevier Science Ltd. All rights reserved.