Xylella fastidiosa causes transcriptional shifts that precede tylose formation and starch depletion in xylem
Pierce's disease (PD) in grapevine (Vitis vinifera) is caused by the bacterial pathogen Xylella fastidiosa. X. fastidiosa is limited to the xylem tissue and following infection induces extensive plant-derived xylem blockages, primarily in the form of tyloses. Tylose-mediated vessel occlusions are a hallmark of PD, particularly in susceptible V. vinifera. We temporally monitored tylose development over the course of the disease to link symptom severity to the level of tylose occlusion and the presence/absence of the bacterial pathogen at fine-scale resolution. The majority of vessels containing tyloses were devoid of bacterial cells, indicating that direct, localized perception of X. fastidiosa was not a primary cause of tylose formation. In addition, we used X-ray computed microtomography and machine-learning to determine that X. fastidiosa induces significant starch depletion in xylem ray parenchyma cells. This suggests that a signalling mechanism emanating from the vessels colonized by bacteria enables a systemic response to X. fastidiosa infection. To understand the transcriptional changes underlying these phenotypes, we integrated global transcriptomics into the phenotypes we tracked over the disease spectrum. Differential gene expression analysis revealed that considerable transcriptomic reprogramming occurred during early PD before symptom appearance. Specifically, we determined that many genes associated with tylose formation (ethylene signalling and cell wall biogenesis) and drought stress were up-regulated during both Phase I and Phase II of PD. On the contrary, several genes related to photosynthesis and carbon fixation were down-regulated during both phases. These responses correlate with significant starch depletion observed in ray cells and tylose synthesis in vessels. Overall design: Potted V. vinifera cv. Cabernet Sauvignon were mechanically inoculated with Xylella fastidiosa subsp. fastidiosa or 1X PBS (negative controls) on the sixth internode of the stem. Stems were collected at two time points: Phase I (symptom score = 1-2) and Phase II (symptom score = 2-3) of Pierce's Disease symptom development. Symptom scores were based on the published Pierce's Disease Rating Index (Guilhabert and Kirkpatrick, MPMI, 2005). Three biological replicates, corresponding to individual stems (internode directly above the point of inoculation) were used for RNA extraction.