BackgroundOne of the main sunflower diseases is the white mold Sclerotinia sclerotiorum. The oxalic acid (OA), which is one of the main pathogenicity factors of this fungus, beside the direct toxicity on the host, has other functions such as the disruption of the cell wall and chelating out the calcium ions.ObjectivesRegarding the importance of this disease, it is important to study the reactions of the plant against OA which is a nonspecific toxin of many other necrotrophic fungi.Materials and MethodsIn this study, two susceptible and moderately resistant sunflower lines were inoculated with OA and samples at the first leaf stage were collected within the intervals of 2, 6, 12 and 24 hours post inoculation. The expression of five genes related to tricarboxylic acid cycle, including citrate synthase, fumarase, iso-citrate lyase, malate synthase and malate dehydrogenase was studied under OA treatment.ResultsTwo hours after the inoculation, no significant change was observed in the expression of the five studied genes in the moderately resistant line. The iso-citrate lyase gene, which is related to glyoxylate cycle (a variation of the tricarboxylic acid cycle), showed no change in the moderately resistant line; however, it showed an increase in the susceptible line. The increase in fumarase gene expression in moderately resistant line was higher than the susceptible line. The result showed the activation of glyoxylate cycle and destruction of fatty acids in the susceptible line.ConclusionsActivation of glyoxylate cycle indicated induction of senescent symptoms by OA in susceptible line. Increasing in H2O2 leads to oxidative burst and cell death. Cell death has an apparent benefit for development and growth of necrotrophic pathogens in the plant cells. The study of resistance mechanisms in response to the pathogen is useful for breeding programs to provide lines with higher resistance to this pathogen.

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Oxalic acid (OA) is found naturally in many plants and animals: it plays diverse roles in nature. It is an important pathogenicity determinant of many necrotrophic pathogens including Sclerotinia sclerotiorum (Lib.) de Bary. In order to understand the resistance mechanisms in Helianthus annuus L., a proteomic study was conducted on sunflower 12 h after inoculation by OA. A total of 17 differentially expressed proteins (either OA-induced or -suppressed proteins) were identified as a result of OA treatment. The candidate proteins were classified into two groups depending on their up/downregulation. The first group – upregulated proteins – included 13 proteins identified as being involved in the Calvin cycle, photosynthesis, programmed cell death (PCD) pathway, heat shock proteins, proteins with antioxidant activities and flavonol synthase (FLS). The second group – downregulated proteins – included those from the cupin family, ATP synthase subunit β, ketol-acid reducto-isomerase, and actin. Studying the biological significance of proteins responsive to OA might ultimately convey us to improve sunflower lines with higher levels of resistance to Sclerotinia and help to control this devastating necrotrophic plant pathogen.

The time of antioxidant system activation in host is important in order to contribute to defence responses. To date, the changes in the expression of PR1 and PDF 1.2 and contribution of (1,4)-β-glucanase enzyme in sunflower defence responses were not reported in previous studies.