Xylem cavitation susceptibility and refilling mechanisms in olive trees infected by Xylella fastidiosa

Abstract

In olive trees, Xylella fastidiosa colonizes xylem vessels and compromises water transport causing the olive quick decline syndrome (OQDS). The loss of hydraulic conductivity could be attributed to vessel occlusions induced both by the bacteria biofilm and by plant responses (tyloses, gums, etc.) that could trigger embolism. The ability of the infected plants to detect embolism and to respond, by activating mechanisms to restore the hydraulic conductivity, can influence the severity of the disease symptomatology. In order to investigate these mechanisms in the X . fastidiosa -resistant olive cultivar Leccino and in the susceptible Cellina di Nardò, sections of healthy olive stems were analysed by laser scanning microscope to calculate the cavitation vulnerability index. Findings indicated that the cultivar Leccino seems to be constitutively less susceptible to cavitation than the susceptible one. Among the vascular refilling mechanisms, starch hydrolysis is a well-known strategy to refill xylem vessels that suffered cavitation and it is characterized by a dense accumulation of starch grains in the xylem parenchima; SEM-EDX analysis of stem cross-sections of infected plants revealed an aggregation of starch grains in the Leccino xylem vessels. These observations could indicate that this cultivar, as well as being anatomically less susceptible to cavitation, it also could be able to activate more efficient refilling mechanisms, restoring vessel’s hydraulic conductivity. In order to verify this hypothesis, we analysed the expression levels of some genes belonging to families involved in embolism sensing and refilling mechanisms: aquaporins, sucrose transporters, carbohydrate metabolism and enzymes related to starch breakdown, alpha and beta-amylase. The obtained genes expression patterns suggested that the infected plants of the cultivar Leccino strongly modulates the genes involved in embolism sensing and refilling.

Molecular Effects of Xylella fastidiosa and Drought Combined Stress in Olive Trees

“…The severity of symptoms of DED associated with water stress was increased in plants with large vessels more incline to cavitation. Vessel cavitation is considered a determinant process of the wilting of the plant in stress conditions [41]. Again, as reported by Sabella et al (2018) [24], Xf resistance of olive trees cv Leccino could be influenced by lignin amount in the xylem vessels that limit the bacteria movement and the host invasion by slowing down the disease progression.…”

Secondary Metabolites in Xylella fastidiosa–Plant Interaction

“…These two cultivars are characterized by high susceptibility to the pathogen and, due to high incidence of infection and severity of the symptoms, these olive trees very often face desiccation and death. Conversely, few are the other cultivar, such as Leccino, considered resistant to Xf , due to the low bacterial titer in the plant tissues [ 71 , 72 , 73 , 74 ]. Studies have indicated that infected field tree or potted inoculated plants of cultivar Leccino have a lower bacterial titer (4 × 10 4 CFU∙mL −1 ) than Ogliarola salentina or Cellina di Nardò (2 × 10 6 CFU∙mL −1 ) [ 15 ], underlying the resistant behavior of this cultivar.…”

Verticillium dahliaeinoculation andin vitropropagation modify the xylem microbiome and disease reaction to Verticillium wilt in a wild olive genotype

“…In olive, several defense mechanisms, including both biochemical responses and plant structural characteristics, have been proposed as factors contributing to the resistance shown by different genotypes against its vascular pathogens Vd and/or X. fastidiosa . Those mechanisms should operate within the xylem tissues contributing to reduce systemic colonization by the pathogen, and may include build-up of vessel occlusions by gums, gels or tyloses, phenolics and lignin content or their accumulation, induction of pathogenesis-related proteins, antioxidant-related enzymes, and ionome content (Báidez et al, 2007; Markakis et al, 2010; Jiménez-Fernández et al, 2016; Gharbi et al, 2017a, 2017b; Leyva-Pérez et al, 2017; Luvisi et al, 2017; Sabella et al, 2017, 2019; D’Attoma et al, 2019).…”