Biochemical characterization and differential expression of genes associated with apple tolerance to Penicillium expansum

Abstract

The so-called blue mold is one of the most important fungal pathologies causing major pome fruits postharvest losses. To understand apple defense mechanisms, great interest has been put on aspects such as changes in fruit tissue gene expression or biochemical composition during infections. In the present study, the expression patterns of 15 genes and isoforms previously reported to be involved in the defense response of apple against Penicillium expansum were analyzed. Thus, 28 apple accessions from the germplasm bank of the Experimental Station of Aula Dei (EEAD-CSIC) (NE Spain) were wounded and artificially inoculated with P. expansum. At 14 days post inoculation (dpi), lesion diameter and lesion extension were measured, ranking apple accessions according to their susceptibility/tolerance. In addition, several biochemical components were assessed in healthy fruits at the moment of inoculation (0 dpi) and after two weeks of incubation (14 dpi). Sugar and organic acid concentrations, total flavonoids content and ascorbic acid levels were measured. Enzyme activities, including phenylalanine ammonia-lyase (PAL), peroxidase (POX), and polyphenol oxidase (PPO), were also assessed. In summary, some parameters exhibited significant negative correlations with susceptibility, such as PPO enzymatic activity at 0 dpi and total sugars at harvest. Furthermore, positive correlations were identified, including total sugars at 0 dpi and 14 dpi, α-ketoglutaric acid at 14 dpi, total organic acids, and quinic acid at harvest. Notably, the variation in some biochemical components during infection, such as PPO activity, significantly correlated with ‘lesion diameter’. Concerning gene expression, 10 accessions were studied. The PAL1 and the allergen Mal d 1.03 were found to be induced by the inoculation with P. expansum. Furthermore, LDOX and Mal d 1.03 were differentially expressed between tolerant and susceptible cultivars at 14 dpi. In addition, their expression level was positively correlated with tolerance, making them potential biomarkers for future selection programs.