Genetic control of α-farnesene production in apple fruit and its role in fungal pathogenesis

Terpenes are important compounds in plant trophic interactions. A meta-analysis of GC-MS data from a diverse range of apple (Malus × domestica) genotypes revealed that apple fruit produces a range of terpene volatiles, with the predominant terpene being the acyclic branched sesquiterpene (E,E)-α-farnesene. Four quantitative trait loci (QTLs) for α-farnesene production in ripe fruit were identified in a segregating 'Royal Gala' (RG) × 'Granny Smith' (GS) population with one major QTL on linkage group 10 co-locating with the MdAFS1 (α-farnesene synthase-1) gene. Three of the four QTLs were derived from the GS parent, which was consistent with GC-MS analysis of headspace and solvent-extracted terpenes showing that cold-treated GS apples produced higher levels of (E,E)-α-farnesene than RG. Transgenic RG fruit downregulated for MdAFS1 expression produced significantly lower levels of (E,E)-α-farnesene. To evaluate the role of (E,E)-α-farnesene in fungal pathogenesis, MdAFS1 RNA interference transgenic fruit and RG controls were inoculated with three important apple post-harvest pathogens [Colletotrichum acutatum, Penicillium expansum and Neofabraea alba (synonym Phlyctema vagabunda)]. From results obtained over four seasons, we demonstrate that reduced (E,E)-α-farnesene is associated with decreased disease initiation rates of all three pathogens. In each case, the infection rate was significantly reduced 7 days post-inoculation, although the size of successful lesions was comparable with infections on control fruit. These results indicate that (E,E)-α-farnesene production is likely to be an important factor involved in fungal pathogenesis in apple fruit.

Fungal attack and host defence pathways unveiled in near-avirulent interactions of Penicillium expansum creA mutants on apples

Amongst the universal diseases affecting apples, blue mould caused by Penicillium expansum is a major concern, resulting in yield and quality losses as a result of the production of the mycotoxin patulin. Despite the characterization of the patulin biosynthetic gene cluster at both the molecular and chemical levels, the underlying regulation of patulin biosynthesis in P. expansum and the mechanisms of apple colonization remain largely obscure. Recent work has indicated that sucrose, a carbon catabolite repressive metabolite, is a critical factor in the regulation of patulin synthesis. Here, CreA, the global carbon catabolite regulator, was assessed for virulence both in vitro and in vivo. We showed that loss-of-function creA strains were nearly avirulent and did not produce patulin in apples. On the basis of RNA-sequencing (RNA-seq) analysis and physiological experimentation, these mutants were unable to successfully colonize apples for a multitude of potential mechanisms including, on the pathogen side, a decreased ability to produce proteolytic enzymes and to acidify the environment and impaired carbon/nitrogen metabolism and, on the host side, an increase in the oxidative defence pathways. Our study defines CreA and its downstream signalling pathways as promising targets for the development of strategies to fight against the development and virulence of this post-harvest pathogen.

Identification of differentially expressed genes involved in spore germination of Penicillium expansum by comparative transcriptome and proteome approaches

In this study, Penicillium expansum, a common destructive phytopathogen and patulin producer was isolated from naturally infected apple fruits and identified by morphological observation and rDNA-internal transcribed spacer analysis. Subsequently, a global view of the transcriptome and proteome alteration of P. expansum spores during germination was evaluated by RNA-seq (RNA sequencing) and iTRAQ (isobaric tags for relative and absolute quantitation) approaches. A total of 3,026 differentially expressed genes (DEGs), 77 differentially expressed predicted transcription factors and 489 differentially expressed proteins (DEPs) were identified. The next step involved screening out 130 overlapped candidates through correlation analysis between the RNA-seq and iTRAQ datasets. Part of them showed a different expression trend in the mRNA and protein levels, and most of them were involved in metabolism and genetic information processing. These results not only highlighted a set of genes and proteins that were important in deciphering the molecular processes of P. expansum germination but also laid the foundation to develop effective control methods and adequate environmental conditions.