Genetic responses induced in aerial olive tissues during the interaction in roots with the defoliating pathotype of Verticillium dahliae

Abstract

Verticillium dahliae Kleb is the causal agent of Verticillium wilt of olive (Olea europaea L.) (VWO), one of the most threatening diseases in many areas where this tree is cultivated, particularly in the Mediterranean basin. The current spread of the disease and the severity of its attacks need of an integrated disease management strategy to achieve effective control. The use of resistant olive cultivars is one of the most plausible measures for controlling VWO within such framework. While diverse studies aimed to identify VWO resistant or tolerant genotypes under diverse inoculation conditions have provided some promising results, knowledge on the genetics of resistance to VWO is still very limited. One of our current lines of research aims to shed light on the genetic responses taking place during V. dahliae-olive interaction. In the present study, we have conducted a functional genomics analysis to unravel responses that could be induced in aerial tissues of the resistant olive cultivar Frantoio. We particularly aimed to elucidate whether early systemic responses can be differentially triggered in above-ground tissues upon artificial inoculation of V. dahliae in the root system. A suppression subtractive hybridization cDNA library, enriched in up-regulated genes, was generated from aerial olive tissues of ‘Frantoio’ nursery-produced plants, sampled at different time points along 21 days after root-dip inoculation with a representative isolate of the defoliating pathotype. This strategy has enabled the identification of 612 ESTs (71 contigs and 541 singlets) expressed in aerial tissues during the interaction of V. dahliae with roots. Querying (Blastx) the non-redundant NCBI database allowed the attribution of homologous hits for transcripts (approximately 43.8%) with coding sequences present in genomes of woody plants such as Vitis vinifera, Populus trichocarpa and Ricinus communis. Moreover, 2.3% of up-regulated genes matched to genes already identified in olive. Finally, nearly 24% of detected olive transcripts corresponded to unidentified genes. Computational analysis showed a number of transcripts involved, among other processes, in plant defense response to biotic and abiotic stresses (i.e., formamidase, phosphatase 2c, PR proteins such as thaumatin-like protein belonging to PR-5 family and lipoxygenases), phenylpropanoid biosynthesis (i.e., caffeoyl- O-methyltransferase) or terpenoids and hormones biosynthesis (i.e., acetone-cyanohydrin lyase and malate dehydrogenase). Similarly, different classes of transcription factors such as GRAS1 and WRKYs were shown to be up-regulated in aerial tissues after V. dahliae inoculation in roots. Thus, a broad range of defensive responses seems to be induced in tissues located far away from where the pathogen colonization process is taking place in a resistant olive cultivar.