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.