Observations on Rhamnus alaternus Naturally Infected by Xylella fastidiosa subsp. multiplex ST87 in the Mediterranean Maquis of Monte Argentario (Tuscany, Italy)

The diseases caused by the xylem-inhabiting bacterium Xylella fastidiosa (Xf) are considered a consequence of the drastic reduction in water flow that this endophyte causes in some of its hosts. To date, within the EU, Xf has been found infecting several taxa that are common members of the Mediterranean maquis, a resource-poor environment with very hot and dry summers. To adapt, native plants possess several means to cope with water paucity and derived gas voids, including a high density of small lumen vessels in the wood. We describe for the first time the anatomy of the anisohydric shrub Rhamnus alaternus spontaneously growing within the Xf demarcated area of Monte Argentario (Italy), when naturally infected by X. fastidiosa subsp. multiplex (Xfm) ST87. By comparing infected plants using quantitative PCR and histological analyses, we could not find a clear relationship between the symptoms of scattered leaf chlorosis, premature leaf drop and branch dieback, and the presence and abundance of ST87 in branches, or the fraction of occluded vessels (p > 0.05). In contrast, in leaves, there was a highly significant relationship (p < 0.001) between the presence of ST87 in their midribs and chlorosis of their lamina. We observed here that this host accumulates phenolics-rich mucilage inside vessels in a way that appears to be independent from the physical presence of ST87 in their lumen. Further experiments in a controlled environment are necessary to prove the ability of Xfm ST87 to cause the symptoms of dehydration that are recurrently observed on R. alaternus at Monte Argentario.