Early events prior to visual symptoms in the apoplectic form of grapevine esca disease

Plant infection by pathogens generates various forms of symptoms. Most of them have been described as soon as they become visible, whereas preceding, discrete signs during incubation are poorly or not understood. In Vitis vinifera, esca-related pathogenic fungi inhabit living trunk wood and induce the so-called apoplexy, a sudden wilting of leaves within a few days. To further understand the apoplexy expression, the period preceding symptom appearance was investigated by following physiological and molecular markers associated with photosynthetic mechanisms and stress responses. Within the week preceding symptoms, drastic physiological alterations of photosynthesis were registered in pre-apoplectic vines, as revealed by a decrease in gas exchange, changes in chlorophyll fluorescence, and repression of photosynthesis-related genes. In the meantime, expression of defense-related genes was induced and amplified during symptom expression. Water-stress-related genes were specifically investigated because water transport may be impeded by clogging xylem vessels due to esca-causing fungi. Neither of the tested water-stress-related genes was affected in pre-apoplectic grapevine leaves whereas these genes were downregulated in drying leaves. Our results suggest that, during incubation of esca apoplexy, grapevine perceives some signals (likely fungi-originated toxins) and reacts by reducing photosynthesis and triggering defense mechanisms.