Resisting Xylella fastidiosa: xylem anatomical changes in the susceptible olive cultivar Cellina di Nardò after long-term infection.

The Xylella fastidiosa subsp. pauca (Xfp) disease has destroyed the olive cultivations of Salento, a peninsula in the extreme south-east of Italy, and is dangerously moving northwards. Cellina di Nardò is among the cultivars most susceptible to the pathogen. However, in recent years, spontaneous canopy restoration was observed in some naturally infected plants. In this work, we investigated xylem morphological changes, resembling a response to drought stress, in plants of the susceptible cultivar Cellina di Nardò more than 10 years after the appearance of Xfp in Apulia. Results showed a significant reduction in xylem vessel diameter and frequency in 1-year-old branches, two anatomical traits already associated to Xfp resistance in several species, as well as a change in vessel placement and in the number of stand-alone (or solitary) xylem vessels. In fact, confocal microscope studies demonstrated a rise in stand-alone vessels while decreasing the number of clustered vessels. Reduced contact points in the xylem network can slow down the pathogen movement from one xylem vessel to another in host plants. The same set of observations reports the absence of bacterial vessel occlusions in new branches of Cellina di Nardò plants with an asymptomatic canopy. In summary, these findings provide novel insights into the olive trees' adaptive response to Xfp.