Linking Jasmonic Acid to Grapevine Resistance against the Biotrophic Oomycete Plasmopara viticola

Abstract

Plant resistance to biotrophic pathogens is classically believed to be mediated through salicylic acid (SA) signaling leading to hypersensitive response followed by the establishment of Systemic Acquired Resistance. Jasmonic acid (JA) signaling has extensively been associated to the defense against necrotrophic pathogens and insects inducing the accumulation of secondary metabolites and PR proteins. Moreover, it is believed that plants infected with biotrophic fungi suppress JA-mediated responses. However, recent evidences have shown that certain biotrophic fungal species also trigger the activation of JA-mediated responses, suggesting a new role for JA in the defense against fungal biotrophs. Plasmopara viticola is a biotrophic oomycete responsible for the grapevine downy mildew, one of the most important diseases in viticulture. In this perspective, we show recent evidences of JA participation in grapevine resistance against P. viticola, outlining the hypothesis of JA involvement in the establishment of an incompatible interaction with this biotroph. We also show that in the first hours after P. viticola inoculation the levels of OPDA, JA, JA-Ile, and SA increase together with an increase of expression of genes associated to JA and SA signaling pathways. Our data suggests that, on the first hours after P. viticola inoculation, JA signaling pathway is activated and the outcomes of JA-SA interactions may be tailored in the defense response against this biotrophic pathogen.

Keywords: Vitis vinifera; biotroph; downy mildew; jasmonic acid; salicylic acid.

FIGURE 1

On the first hours after Plasmopara viticola inoculation, the levels of SA, JA, and JA-Ile increased in V. vinifera cv. Regent relatively to mock inoculated control plants. Rapid accumulation of bioactive JA-Ile promotes SCF-COI1 mediated ubiquitination and subsequent degradation of JAZ proteins and corepressors TLP and NINJA via the 26S proteasome, relieving the transcription factors such as MYC2 and promoting the expression of JA-responsive genes such as PR10. High levels of SA mediate a change in the cellular redox potential, resulting in the reduction of the NPR1 oligomer to its active monomeric form. Monomeric NPR1 is then translocated into the nucleus where it functions as a transcriptional co-activator of SA-responsive genes, such as PR-1. Both the SA and JA signaling pathways seem to be simultaneously activated on the first hours of interaction. See text for details on the molecular processes underlying both JA- and SA-signaling. Solid lines indicate established accumulation and dashed lines suggested activities.

FIGURE 2

Vitis vinifera cv. Regent plants were inoculated with P. viticola (Ri) as described in Figueiredo et al. (2012). Plant material was harvested at 6, 12, and 24 hpi. Mock inoculated samples (Rm) were done for each time-point. (A) Determination of the endogenous levels (ng g^-1 FW) of OPDA, JA, JA-ILE, and SA. Briefly, 50 mg of lyophilized samples were used for phytohormone quantification in a 4000 QTRAP LC/MS/MS system (AB Sciex) at the Proteomics & Mass Spectrometry Facility at the Danforth Plant Science Center (USA). Phytohormone levels are represented as the mean and standard deviation of three biological replicates. (B) qPCR expression analysis of JA- and SA-signaling associated genes. Total RNA extraction, cDNA synthesis, and qPCR experiments were done according to Monteiro et al. (2013). Primer sequences, amplicon size, amplification efficiency, annealing and melting temperatures for each gene studied are given in Supplementary Table 1. To normalize expression data, ubiquitin conjugating enzyme (UBQ), Elongation factor 1α (EF1α) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were used (Monteiro et al., 2013). Transcript abundance of inoculated samples relative to mock inoculated controls at each time point is represented as the mean and standard deviation of five biological replicates. Expression between 0 and 1 represents a down-regulation. Asterisks (*) represent significant difference (p ≤ 0.05) between inoculated and control samples at the same time point (Mann–Whitney U test; SPSS Inc., USA, V20).