Verticillium wilt resistant and susceptible olive cultivars express a very different basal set of genes in roots

Abstract

Background: Olive orchards are threatened by a wide range of pathogens. Of these, Verticillium dahliae has been in the spotlight for its high incidence, the difficulty to control it and the few cultivars that has increased tolerance to the pathogen. Disease resistance not only depends on detection of pathogen invasion and induction of responses by the plant, but also on barriers to avoid the invasion and active resistance mechanisms constitutively expressed in the absence of the pathogen. In a previous work we found that two healthy non-infected plants from cultivars that differ in V. dahliae resistance such as 'Frantoio' (resistant) and 'Picual' (susceptible) had a different root morphology and gene expression pattern. In this work, we have addressed the issue of basal differences in the roots between Resistant and Susceptible cultivars.

Results: The gene expression pattern of roots from 29 olive cultivars with different degree of resistance/susceptibility to V. dahliae was analyzed by RNA-Seq. However, only the Highly Resistant and Extremely Susceptible cultivars showed significant differences in gene expression among various groups of cultivars. A set of 421 genes showing an inverse differential expression level between the Highly Resistant to Extremely Susceptible cultivars was found and analyzed. The main differences involved higher expression of a series of transcription factors and genes involved in processes of molecules importation to nucleus, plant defense genes and lower expression of root growth and development genes in Highly Resistant cultivars, while a reverse pattern in Moderately Susceptible and more pronounced in Extremely Susceptible cultivars were observed.

Conclusion: According to the different gene expression patterns, it seems that the roots of the Extremely Susceptible cultivars focus more on growth and development, while some other functions, such as defense against pathogens, have a higher expression level in roots of Highly Resistant cultivars. Therefore, it seems that there are constitutive differences in the roots between Resistant and Susceptible cultivars, and that susceptible roots seem to provide a more suitable environment for the pathogen than the resistant ones.

Keywords: Olea europaea L.; RNA-Seq; Roots; Transcriptome; Verticillium dahliae.

Fig. 1

Top 20 Biological Processes at level 7 related with genes up regulated (a) and down regulated (b) in cultivars HR. Red boxes highlight terms that differ between both groups

Fig. 2

Enriched GO terms of the up-regulated genes in cultivars HR versus the rest of the groups

Fig. 3

Top 20 Biological Processes at level 7 related with genes up regulated (a) and down regulated (b) in cultivars ES. Red boxes highlight terms that differ between both groups

Fig. 4

Top 15 Enriched GO terms of the up-regulated genes in cultivars ES versus the rest of the groups

Fig. 5

Enriched GO terms of the down-regulated genes in cultivars ES versus the rest of the groups

Fig. 6

Genes that show an inverse expression profile from HR to ES cultivars. (a) Heat map. The gradient from red to blue represents the higher or lower gene expression value, respectively. (b) Trend line of genes overexpressed in HR cultivars and (c) overexpressed in ES cultivars. HR = Highly resistant; R = Resistant; MS = Moderately susceptible; S = Susceptible and ES = Extremely susceptible cultivars

Fig. 7

Proposed model summarizing the main gene expression findings in olive roots and the V. dahliae resistance phenotype