The Impact of Xylem Geometry on Olive Cultivar Resistance to Xylella fastidiosa: An Image-based Study

Xylella fastidiosa is a xylem-limited plant pathogen infecting many crops globally and is the cause of the recent olive disease epidemic in Italy. One strategy proposed to mitigate losses is to replant susceptible crops with resistant varieties. Several genetic, biochemical and biophysical traits are associated to X. fastidiosa disease resistance. However, mechanisms underpinning resistance are poorly understood. We hypothesize olive cultivars' susceptibility to infection will correlate to xylem vessel diameters, with narrower vessels being resistant to air embolisms and having slower flow rates limiting pathogen spread. To test this, we scanned stems from four olive cultivars of varying susceptibility to X. fastidiosa using X-ray Computed Tomography. Scans were processed via a bespoke methodology that segmented vessels, facilitating diameter measurements. Though significant differences were not found comparing stem-average vessel section diameters among cultivars, they were comparing diameter distributions. Moreover, the measurements indicated that though vessel diameter distributions may play a role regarding the resistance of Leccino, it is unlikely they do for FS17®. Considering Young-Laplace and Hagen-Poiseuille equations, we inferred differences in embolism susceptibility and hydraulic conductivity of the vasculature. Our results suggest susceptible cultivars, having a greater proportion of larger vessels, are more vulnerable to air embolisms. In addition, results suggest that under certain pressure conditions, functional vasculature in susceptible cultivars could be subject to greater stresses than in resistant cultivars. These results support investigation into xylem morphological screening to help inform olive re-planting. Furthermore, our framework could test the relevance of xylem geometry to disease resistance in other crops.