Histological analysis of <i>Xylella fastidiosa</i> infection in <i>Quercus pyrenaica</i> in Northern Portugal

<i>Quercus pyrenaica</i> Willd thrives in the intermediate zone between the Mediterranean sclerophyllous and the temperate deciduous forest. In December 2022, the presence of the bacteria <i>Xylella fastidiosa (Xf)</i> was confirmed in samples collected from a <i>Quercus pyrenaica</i> located in Sabrosa, Vila Real, Portugal. Following <i>Xf</i> infection, the transport of water and nutrients is hindered due to the occlusion of xylem vessels. This loss of hydraulic conductivity may lead to vessel blockage and subsequent embolism formation. The objective of this study was to investigate the interaction between <i>Xf</i> and <i>Quercus pyrenaica</i> tissues, as well as the mechanism by which the bacteria can spread through the plant's xylem vessels, ultimately resulting in the formation of vascular plugs. At the time of the sample collection (10 months post-detection), symptoms of Bacterial Leaf Scorch (BLS) began to appear. Examination of xylem vessels using both light and scanning electron microscopy (SEM) revealed the presence of various types of occlusions, predominantly tyloses. Additionally, fibrillar networks, gums, starch grains, and crystals were observed. The stem vessels exhibited significantly more occlusions compared to the leaves. Furthermore, individual bacterial cells were observed to be attached to the vessel wall. This implies that occlusions were primarily induced by tyloses and gums as a defensive response to the invasion of vascular pathogens, in addition to the pathogen itself. This study highlights the presence of starch grains in stems, which may function as a refilling mechanism, thereby preventing the loss of hydraulic conductivity in plants and potentially acting as a means to entrap the bacteria. These mechanisms exemplify the constitutive defense systems of the plant against <i>Xf</i>. Understanding the interaction between <i>Xylella fastidiosa</i> and <i>Quercus pyrenaica</i> is crucial, given that the latter species occupies nearly 95% of the natural distribution area of Portugal.