Physiological and molecular aspects of the melon-Fusarium oxysporum f. sp. melonis race 1.2 pathosystem

Fusarium oxysporum f.sp. melonis race 1.2 (FOM1.2) is the most virulent and yield-limiting pathogen of melon (Cucumis melo L.) cultivation worldwide. Colonization of plants by FOM leads to necrosis of the infected tissues, collapse of vascular vessels and decay of the plant. Resistance to FOM1.2 appears to be controlled by multiple recessive genes and strongly affected by environment. A RNA-Sequencing approach was used to investigate the transcriptome dynamic during incompatible and compatible interactions for the identification of candidate resistance genes in the melon-FOM1.2 pathosystem. The doubled-haploid (DH) resistant line NAD and the susceptible cultivar Charentais-T (CHT), both inoculated with FOM1.2, were analyzed at 24 and 48 hours post-inoculation (hpi). The landscape of differentially expressed genes (DEGs) diverged significantly in the two genotypes. Transcriptome analysis of NAD identified 2461 and 821 DEGs (82% up-regulated at 24 hpi and 69% at 48 hpi), while in CHT 882 and 2237 DEGs were recovered at 24 hpi and at 48 hpi, respectively (29% up-regulated at 24 hpi and 81% at 48 hpi). Several unannotated transcripts were found to be modulated, providing a basis for further exploration of plant defense-related genes. NAD, unlike CHT, modulates a higher proportion of up-regulated genes at 24 hpi, suggesting a more prompt response. Gene ontology (GO) enrichment of DEGs highlighted that ″defence response, incompatible interaction″ and ″response to stress″ GO groups emerged as major effectors of NAD resistance to FOM1.2. Both constitutive and inducible defense responses contribute to reduced FOM1.2 vascular colonization of melon resistant genotype. Of particular interest were transcripts involved in the cell wall reinforcement and disease resistance genes including FMO1, E3 Ubiquitin protein ligase and pathogenesis-related thaumatin encoding genes. Although the early expression of ankyrin-repeat containing genes, probably related to salicylic acid (SA) pathway, in NAD FOM1.2 resistance seems mainly mediated by the crosstalk among jasmonate/ethylene (JA/ET), auxin and abscissic acid (ABA) hormone signaling networks. Fungal transcripts expressed in planta were also detected for identifying potential virulence effectors. The achievement of candidate resistance genes and the identification of unique melon sequences with unknown functions required the establishment of efficient, genotype-specific melon regeneration and transformation protocols for future functional genomics studies. Three cultivars (CHT, Vedrantais and Isabelle) and three DH melon lines (NAD, DH-L2 and DH-L6) were tested for their potential to regenerate under in vitro conditions. The results confirmed that the hormonal requirement and the genotype strongly influence plant regeneration in melon. In order to confirm genetic stability of the regenerated melon plants a PCR-based Random Amplified Polymorphic DNA (RAPD) analysis was carried out using 20 decamer-primers.