Analysis of Differential Transcript Expression Reveals Time-Dependent Leaf Responses to Sclerotinia seclerotiorum in Wild and Cultivated Sunflower

The necrotrophic pathogen Sclerotinia sclerotiorum is a causal agent of rot diseases in sunflower and is described as one of the most damaging pathogens of cultivated sunflower. Resistance to this pathogen is found in some genotypes of wild sunflower in particular characterised by significantly reduced lesion lengths in capitulum, stems and leaves. This study was conducted to characterise transcriptomic alterations during interaction of host and pathogen in lesion-surrounding areas of the leaf using differential display RT-PCR and to compare molecular responses between a resistant and a susceptible genotype. Leaves were examined during the first stages of pathogenesis (dpi 2, 3 and 4) after inoculation with S. sclerotiorum. By means of computational analysis of fluorescently labelled expression data, expression patterns were evaluated and significant differentially expressed transcripts were selected. The expression profile revealed that a response measured by the number of significant differentially expressed transcripts differed between the resistant and susceptible genotype in timing. Nine differentially expressed transcripts were successfully sequenced of which two transcripts originated from the mRNA population of the pathogen, two transcripts were derived from the susceptible cultivar of Helianthus annuus and five transcripts were isolated from the resistant genotype of Helianthus maximiliani. Semi-quantitative real-time PCR was accomplished to verify the significant differential expression of the potentially resistance-associated transcripts coumarate-CoA-ligase and cysteine protease transcript in the resistant H. maximiliani accession and differential expression of a chlorophyll-a/b-binding-protein and an S-adenosyl-methionine-synthetase transcript originating from the susceptible H. annuus cultivar