Apple is typically stored under low temperature and controlled atmospheric conditions to ensure a year round supply of high quality fruit for the consumer. During storage, losses in quality and quantity occur due to spoilage by postharvest pathogens. One important postharvest pathogen of apple is Botrytis cinerea. The fungus is a broad host necrotroph with a large arsenal of infection strategies able to infect over 1,400 different plant species. We studied the apple-B. cinerea interaction to get a better understanding of the defense response in apple. We conducted an RNAseq experiment in which the transcriptome of inoculated and non-inoculated (control and mock) apples was analyzed at 0, 1, 12, and 28 h post inoculation. Our results show extensive reprogramming of the apple’s transcriptome with about 28.9% of expressed genes exhibiting significant differential regulation in the inoculated samples. We demonstrate the transcriptional activation of pathogen-triggered immunity and a reprogramming of the fruit’s metabolism. We demonstrate a clear transcriptional activation of secondary metabolism and a correlation between the early transcriptional activation of the mevalonate pathway and reduced susceptibility, expressed as a reduction in resulting lesion diameters. This pathway produces the building blocks for terpenoids, a large class of compounds with diverging functions including defense. 1-MCP and hot water dip treatment are used to further evidence the key role of terpenoids in the defense and demonstrate that ethylene modulates this response.
Apple is a seasonal product that is stored for long periods of time, during which fungal‐caused decay can occur. Previous infection experiments of intact Jonagored apple with Botrytis cinerea displayed apparent differences in lesion expansion rate with respect to inoculation position on the fruit (on, above or below the equator). The goal of this study was to investigate whether these differences are consistent or not and if so, to relate them to fruit characteristics. The study involved measuring the hue angle of the intended inoculation spots prior to inoculation, and firmness and total soluble solids content of fruit from the same batch. Results showed that firmness correlated somewhat (−0.55 and −0.72 for shadow and sun side, respectively) with lesion diameter expansion rate. In a subsequent step, X‐ray imaging was carried out for samples from each position. Analysis of 3D reconstructions by microcomputed tomography of these diffraction images showed that cell size was strongly correlated (0.996) to lesion diameter expansion rate. Finally, it was investigated if cell size could also be used to rank different apple cultivars for their susceptibility to B. cinerea. The result shows that there is a clear distinction between Jonagold and Golden Delicious (non‐blushing variety), which have a smaller cell size, and Braeburn and Kanzi, which have a larger cell size (overall correlation of 0.87). This indicates that cell size may also be an important factor in determining susceptibility across cultivars.
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