Xylella fastidiosa is a xylem-limited bacterium causing the Olive Quick Decline Syndrome, which is currently devastating the agricultural landscape of Southern Italy. The bacterium is injected into the xylem vessels of leaf petioles after the penetration of the insect vector’s stylet. From here, it is supposed to colonize the xylem vasculature moving against water flow inside conductive vessels. Widespread vessel clogging following the bacterial infection and causing the failure of water transport seemed not to fully supported by the recent empirical xylem anatomical observations in infected olive trees. We tested the hypothesis that the higher susceptibility to the X. fastidiosa’s infection in Cellina di Nardò compared with Leccino is associated to the higher vulnerability to air embolism of its larger vessels. Such hypothesis is motivated by the recognized ability of X. fastidiosa in degrading pit membranes and also because air embolism would possibly provide microenvironmental conditions more favourable to its more efficient aerobic metabolism. We revised the relevant literature on bacterium growth and xylem physiology, and carried out empirical field, mid-summer measurements of xylem anatomy and native embolism in olive cultivars with high (Cellina di Nardò) and low susceptibility (Leccino) to the infection by X. fastidiosa. Both cultivars had similar shoot mass traits and vessel length (~80 cm), but the highly susceptible one had larger vessels and a lower number of vessels supplying a given leaf mass. Native air embolism reduced mean xylem hydraulic conductance by ~58 % (Cellina di Nardò) and ~38 % (Leccino). The higher air-embolism vulnerability of the larger vessels in Cellina di Nardò possibly facilitates the X. fastidiosa’s infection compared to Leccino. Some important characteristics of the vector–pathogen–plant interactions still require deep investigations acknowledging both the pathogen metabolic pathways and the biophysical principles of xylem hydraulics.

中文翻译：

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油橄榄对木杆菌的易感性和功能性木质部解剖结构：重新审视植物与病原体相互作用的故事


苛养木杆菌（Xylella fastidiosa）是一种限制木质部的细菌，会引起橄榄快速衰退综合症，该综合症目前正在破坏意大利南部的农业景观。在昆虫媒介的管心针穿透后，细菌被注射到叶柄的木质部导管中。从这里开始，它应该定居在木质部脉管系统中，逆着传导性血管内的水流移动。最近对受感染橄榄树的木质部解剖观察结果似乎并不完全支持细菌感染后广泛的血管堵塞并导致水运输失败。我们测试了这样的假设：与 Leccino 相比，Cellina di Nardò 对 X. fastidiosa 感染的敏感性更高，这与其较大血管更容易发生空气栓塞有关。这种假设的动机是由于 X. fastidiosa 具有降解凹坑膜的公认能力，而且空气栓塞可能会为其更有效的有氧代谢提供更有利的微环境条件。我们修订了有关细菌生长和木质部生理学的相关文献，并对对 X 感染具有高敏感性 (Cellina di Nardò) 和低敏感性 (Leccino) 的橄榄品种进行了木质部解剖结构和天然栓塞的实证现场、仲夏测量。苛养。两个品种都具有相似的枝条质量性状和导管长度（~80厘米），但高度敏感的品种具有较大的导管和较少数量的导管，提供给定的叶量。天然空气栓塞使平均木质部水力传导率降低约 58% (Cellina di Nardò) 和约 38% (Leccino)。与 Leccino 相比，Cellina di Nardò 中较大血管的空气栓塞脆弱性更高，这可能促进了 X. fastidiosa 的感染。 媒介-病原体-植物相互作用的一些重要特征仍然需要深入研究，承认病原体代谢途径和木质部水力的生物物理原理。