Abstract
Infection of the stem base of sunflower ( Helianthus annuus L.) plants by Sclerotini sclerotiorum (Lib.) de Bary induces the accumulation of at least four RNA species in stems and leaves (2.5, 1.6, 0.56 and 0.135 kb) which hybridize to a genomic clone of hydroxyproline-rich glycoproteins (HRGPs) from carrot. This induction is observed as early as 2 days after infection in a tolerant line (PS3B), while it occurred only after 3 days in a susceptible one (HA89). It appears that the tolerance is associated to an early increase of extensin hybridizable RNA; steady state levels of these transcripts could be used as a molecular marker of sunflower tolerance to the stem base white mold. Oxalic acid, the major toxin produced by Sclerotinia sclerotiorum, induces a differential accumulation of the same HRGP transcripts in the two lines, in an organo-specific way. Only the 1.6- and the 0.135-kb RNA species are representative of the tolerance of PS3B to both the toxin and the fungus. Besides its role as toxin in this host-pathogen interaction, oxalic acid also appears to play the role of an elicitor as inducer of the accumulation of HRGP transcripts.
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