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Transcriptome analyses suggest a disturbance of iron homeostasis in soybean leaves during white mould disease establishment

Calla, Bernarda, Blahut-Beatty, Laureen, Koziol, Lisa, Simmonds, Daina H., Clough, Steven J.
Molecular plant pathology 2014 v.15 no.6 pp. 576
Sclerotinia sclerotiorum, cell death, crops, cytochrome P-450, ferritin, gene expression, gene expression regulation, genes, genotype, homeostasis, iron, leaves, microarray technology, mutants, oxalate oxidase, oxalic acid, pH, pathogens, plant proteins, soybeans, transcriptome, transgenic plants, virulence
Sclerotinia sclerotiorum is a serious pathogen of numerous crops around the world. The major virulence factor of this pathogen is oxalic acid (OA). Mutants that cannot produce OA do not cause disease, and plants that express enzymes that degrade OA, such as oxalate oxidase (OxO) are very resistant to S. sclerotiorum. To look at the effect that OA has on plants, we infiltrated soybean leaves with 5 mM OA and examined gene expression changes 2 hours post infiltration. By comparing gene expression levels between leaves of a transgenic soybean carrying an OxO gene (OxO) and its parent AC Colibri (AC) infiltrated with OA (pH 2.4) or water (pH 2.4 or 5.5), we were able to compare the effects of OA dependent or independent of its pH. Gene expression by microarray analysis identified 2,390 genes that were changing in expression as determined by using overall F-tests p-value cut off of 0.001. The additional requirement that at least one pairwise t-test fdr corrected p-value be less than 0.001 reduced the list of most highly significant differentially expressed genes to 1054. Independent of pH, OA altered expression levels of 78 genes, with ferritin showing the strongest induction by OA. The combination of OA plus its low pH caused 1,045 genes (99% of all the significant genes) to be differentially expressed, with many of the up-regulated genes being related to basal defense, such as genes of the phenylpropanoid pathway and various cytochrome P450s. RNA-seq was also conducted on four samples: OxO or AC Colbri genotypes infiltrated with either OA pH 2.4 or water pH 2.4. The RNA-seq analysis also identified ferritin paralogs as strongly induced by OA. As expression of ferritin, a gene that encodes for an iron-storage protein, is induced by free iron, these results suggest that S. sclerotiorum is benefitting from the ability of OA to free iron from plant proteins, as this would induce host cell death, and to also uptake and assimilation the iron for its own metabolic needs.