Plum pox virus (PPV, sharka) is a limiting factor for peach production, and no natural sources of resistance have been described. Recent studies, however, have demonstrated that grafting the almond cultivar “Garrigues” onto the “GF305” peach infected with Dideron-type (PPV-D) isolates progressively reduces disease symptoms and virus accumulation. Furthermore, grafting “Garrigues” onto “GF305” prior to PPV-D inoculation has been found to completely prevent virus infection, showing that resistance is constitutive and not induced by the virus. To unravel the phytohormone signaling of this mechanism, we analyzed the following phytohormones belonging to the principal hormone classes: the growth-related phytohormones cytokinin trans-zeatin (tZ) and the gibberellins GA3 and GA4; and the stress-related phytohormones ethylene acid precursor 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA). PPV inoculation produced a significant increase in GA3 and ABA in peach, and these imbalances were related to the presence of chlorosis symptoms. However, grafting “Garrigues” almond onto the PPV-inoculated “GF305” peach produced the opposite effect, reducing GA3 and ABA contents in parallel to the elimination of symptoms. Our results showed the significant implication of SA in this induced resistance in peach with an additional effect on tZ and JA concentrations. This SA-induced resistance based in the decrease in symptoms seems to be different from Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR), which are based in other reactions producing necrosis. Further studies are necessary, however, to validate these results against PPV-D isolates in the more aggressive Marcus-type (PPV-M) isolates.

No natural sources of resistance to Plum pox virus (PPV, sharka disease) have been identified in peach. However, previous studies have demonstrated that grafting a “Garrigues” almond scion onto “GF305” peach rootstock seedlings heavily infected with PPV can progressively reduce disease symptoms and virus accumulation. Furthermore, grafting a “Garrigues” scion onto the “GF305” rootstock has been shown to completely prevent virus infection. This study aims to analyse the rewiring of gene expression associated with this resistance to PPV transmitted by grafting through the phloem using RNA-Seq and RT-qPCR analysis. A total of 18 candidate genes were differentially expressed after grafting “Garrigues” almond onto healthy “GF305” peach. Among the up-regulated genes, a HEN1 homolog stands out, which, together with the differential expression of RDR- and DCL2-homologs, suggests that the RNA silencing machinery is activated by PPV infection and can contribute to the resistance induced by “Garrigues” almond. Glucan endo-1,3-beta D-glucosidase could be also relevant for the “Garrigues”-induced response, since its expression is much higher in “Garrigues” than in “GF305”. We also discuss the potential relevance of the following in PPV infection and “Garrigues”-induced resistance: several pathogenesis-related proteins; no apical meristem proteins; the transcription initiation factor, TFIIB; the speckle-type POZ protein; in addition to a number of proteins involved in phytohormone signalling.

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