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

Gossypium (cotton) spp. produce an array of sesquiterpenoid defense compounds, some of which accumulate in pigment 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 mu mol/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. (C) 1999 Elsevier Science Ltd. All rights reserved.