Induction of a sunflower CC-NBS-LRR resistance gene analogue during incompatible interaction with Plasmopara halstedii.

Downy mildew caused by Plasmopara halstedii is one of the main diseases causing economic losses in cultivated sunflower. Resistance in this host is conferred by major genes denoted Pl. The inbred sunflower line QIR8, which contains the Pl8 locus and is resistant to all known downy mildew races, was used to isolate a full-length resistance gene analogue (RGA) belonging to the CC-NBC-LRR class of plant resistance genes. The genetically incompatible combination involving downy mildew races 300 and sunflower line QIR8 was characterized by a hypersensitive-like reaction. Semi-quantitative RT-PCR analysis showed that the transcript of Ha-NTIR11g RGA was specifically induced during the incompatible reaction. The transcript was induced approximately 3 d post-infection (dpi), and then decreased by 9 dpi. The high level of transcriptional expression of this RGA coincides with a transcript accumulation of the hsr203J gene which is a marker of the hypersensitive reaction. Treatment with signalling molecules, including salicylic acid and methyl jasmonate, did not activate transcription of the Ha-NTIR11g gene, indicating that Ha-NTIR11g expression is not regulated by defence signalling pathways triggered by these molecules. Ha-NTIR11g was not induced by treatment with hydrogen peroxide or wounding. These results suggest that Ha-NTIR11g RGA may play a critical role in protecting sunflower cells against P. halstedii. The transcript accumulation of R gene-mediated signalling components was also examined.