Corrigendum: Transcriptome Analysis of the Melon-Fusarium oxysporum f. sp. melonis Race 1.2 Pathosystem in Susceptible and Resistant Plants.

[This corrects the article on p. 362 in vol. 8, PMID: 28367157.].

Transcriptome Analysis of the Melon-Fusarium oxysporum f. sp. melonis Race 1.2 Pathosystem in Susceptible and Resistant Plants.

Fusarium oxysporum f. sp. melonis Snyd. & Hans race 1.2 (FOM1.2) is the most virulent and yield-limiting pathogen of melon (Cucumis melo L.) worldwide. Current information suggest that the resistance to race 1.2 is controlled by multiple recessive genes and strongly affected by the environment. RNA-Seq analysis was used to identify candidate resistance genes and to dissect the early molecular processes deployed during melon-FOM1.2 interaction in the resistant doubled haploid line NAD and in the susceptible genotype Charentais-T (CHT) at 24 and 48 h post-inoculation (hpi). The transcriptome analysis of the NAD-FOM1.2 interaction identified 2,461 and 821 differentially expressed genes (DEGs) at 24 hpi and at 48 hpi, respectively, while in susceptible combination CHT-FOM1.2, 882 and 2,237 DEGs were recovered at 24 hpi and at 48 hpi, respectively. The overall expression profile suggests a prompt activation of the defense responses in NAD due to its basal defense-related machinery that allows an early pathogen recognition. Gene Ontology (GO) enrichment analyses revealed a total of 57 GO terms shared by both genotypes and consistent with response to fungal infection. GO classes named "chitinase activity," "cellulase activity," "defense response, incompatible interaction," "auxin polar transport" emerged as major factors of resistance to FOM1.2. The data indicated that NAD reacts to FOM1.2 with a fine regulation of Ca2+-mediated signaling pathways, cell wall reorganization, and hormone crosstalk (jasmonate and ethylene, auxin and abscissic acid). Several unannotated transcripts were recovered providing a basis for a further exploration of the melon resistance genes. DEGs belonging to the FOM1.2 genome were also detected in planta as a resource for the identification of potential pathogenicity factors. This work provides a broader view of the dynamic changes of the melon transcriptome triggered by FOM1.2 and highlights that the resistance response of NAD is mainly signaled by jasmonic acid and ethylene pathways mediated by ABA and auxin. The role of candidate plant and fungal responsive genes involved in the resistance is discussed.

A newly developed real-time PCR assay for detection and quantification of Fusarium oxysporum and its use in compatible and incompatible interactions with grafted melon genotypes.

A reliable and species-specific real-time quantitative polymerase chain reaction (qPCR) assay was developed for detection of the complex soilborne anamorphic fungus Fusarium oxysporum. The new primer pair, designed on the translation elongation factor 1-α gene with an amplicon of 142 bp, was highly specific to F. oxysporum without cross reactions with other Fusarium spp. The protocol was applied to grafted melon plants for the detection and quantification of F. oxysporum f. sp. melonis, a devastating pathogen of this cucurbit. Grafting technologies are widely used in melon to confer resistance against new virulent races of F. oxysporum f. sp. melonis, while maintaining the properties of valuable commercial varieties. However, the effects on the vascular pathogen colonization have not been fully investigated. Analyses were performed on 'Charentais-T' (susceptible) and 'Nad-1' (resistant) melon cultivars, both used either as rootstock and scion, and inoculated with F. oxysporum f. sp. melonis race 1 and race 1,2. Pathogen development was compared using qPCR and isolations from stem tissues. Early asymptomatic melon infections were detected with a quantification limit of 1 pg of fungal DNA. The qPCR protocol clearly showed that fungal development was highly affected by host-pathogen interaction (compatible or incompatible) and time (days postinoculation). The principal significant effect (P ≤ 0.01) on fungal development was due to the melon genotype used as rootstock, and this effect had a significant interaction with time and F. oxysporum f. sp. melonis race. In particular, the amount of race 1,2 DNA was significantly higher compared with that estimated for race 1 in the incompatible interaction at 18 days postinoculation. The two fungal races were always present in both the rootstock and scion of grafted plants in either the compatible or incompatible interaction.

Distinct colonization patterns and cDNA-AFLP transcriptome profiles in compatible and incompatible interactions between melon and different races of Fusarium oxysporum f. sp. melonis.

BACKGROUND: Fusarium oxysporum f. sp. melonis Snyd. & Hans. (FOM) causes Fusarium wilt, the most important infectious disease of melon (Cucumis melo L.). The four known races of this pathogen can be distinguished only by infection on appropriate cultivars. No molecular tools are available that can discriminate among the races, and the molecular basis of compatibility and disease progression are poorly understood. Resistance to races 1 and 2 is controlled by a single dominant gene, whereas only partial polygenic resistance to race 1,2 has been described. We carried out a large-scale cDNA-AFLP analysis to identify host genes potentially related to resistance and susceptibility as well as fungal genes associated with the infection process. At the same time, a systematic reisolation procedure on infected stems allowed us to monitor fungal colonization in compatible and incompatible host-pathogen combinations. RESULTS: Melon plants (cv. Charentais Fom-2), which are susceptible to race 1,2 and resistant to race 1, were artificially infected with a race 1 strain of FOM or one of two race 1,2 w strains. Host colonization of stems was assessed at 1, 2, 4, 8, 14, 16, 18 and 21 days post inoculation (dpi), and the fungus was reisolated from infected plants. Markedly different colonization patterns were observed in compatible and incompatible host-pathogen combinations. Five time points from the symptomless early stage (2 dpi) to obvious wilting symptoms (21 dpi) were considered for cDNA-AFLP analysis. After successful sequencing of 627 transcript-derived fragments (TDFs) differentially expressed in infected plants, homology searching retrieved 305 melon transcripts, 195 FOM transcripts expressed in planta and 127 orphan TDFs. RNA samples from FOM colonies of the three strains grown in vitro were also included in the analysis to facilitate the detection of in planta-specific transcripts and to identify TDFs differentially expressed among races/strains. CONCLUSION: Our data suggest that resistance against FOM in melon involves only limited transcriptional changes, and that wilting symptoms could derive, at least partially, from an active plant response.We discuss the pathogen-derived transcripts expressed in planta during the infection process and potentially related to virulence functions, as well as transcripts that are differentially expressed between the two FOM races grown in vitro. These transcripts provide candidate sequences that can be further tested for their ability to distinguish between races.Sequence data from this article have been deposited in GenBank, Accession Numbers: HO867279-HO867981.