The sucrose signalling route controls Flavescence dorée phytoplasma load in grapevine leaves

Flavescence dorée (FD) is a phytoplasma disease transmitted by insects, causing severe damage in vineyards across Europe. Since there is no effective treatment, infected plants must be removed to prevent further spread. There is variation in susceptibility to FD among different grapevine cultivars, and some exhibit symptom remission, known as recovery, although the mechanisms behind this are unclear. Diseased plants accumulate soluble sugars, including sucrose, which influences the concentration of trehalose-6-phosphate (T6P), a signalling molecule affecting plant growth and stress responses. It is hypothesized that sucrose-mediated signalling via T6P could trigger defence mechanisms, reducing FD pathogen load and increasing plant recovery. To test this hypothesis, two grapevine genotypes with different susceptibility to FD were compared, revealing increased sucrose level and trehalose-6-phosphate synthase (TPS) activity in the more tolerant cultivar. However, FD-infected plants showed inhibited sucrose-cleaving enzymes and no activation of TPS expression. Attempts to enhance sucrose levels through trunk infusion and girdling promoted sucrose metabolism, T6P biosynthesis, and defence gene expression, facilitating symptom recovery. Girdling particularly enhanced T6P biosynthesis and expression of defence genes above the treatment point, reducing FD pathogen presence and promoting recovery. These findings indicate that elevated sucrose levels, possibly signalling through T6P, may limit FD pathogen spread, aiding in plant recovery.