Influence of Culture Medium pH on Growth, Aggregation, and Biofilm Formation of Xylella fastidiosa

Non-Targeted Spectranomics for the Early Detection of Xylella fastidiosa Infection in Asymptomatic Olive Trees, cv. Cellina di Nardò

Olive quick decline syndrome (OQDS) is a disease that has been seriously affecting olive trees in southern Italy since around 2009. During the disease, caused by Xylella fastidiosa subsp. pauca sequence type ST53 (Xf), the flow of water and nutrients within the trees is significantly compromised. Initially, infected trees may not show any symptoms, making early detection challenging. In this study, young artificially infected plants of the susceptible cultivar Cellina di Nardò were grown in a controlled environment and co-inoculated with additional xylem-inhabiting fungi. Asymptomatic leaves of olive plants at an early stage of infection were collected and analyzed using nuclear magnetic resonance (NMR), hyperspectral reflectance (HSR), and chemometrics. The application of a spectranomic approach contributed to shedding light on the relationship between the presence of specific hydrosoluble metabolites and the optical properties of both asymptomatic Xf-infected and non-infected olive leaves. Significant correlations between wavebands located in the range of 530-560 nm and 1380-1470 nm, and the following metabolites were found to be indicative of Xf infection: malic acid, fructose, sucrose, oleuropein derivatives, and formic acid. This information is the key to the development of HSR-based sensors capable of early detection of Xf infections in olive trees.

Titanium Ions Inhibit the Bacteria in Vase Solutions of Freshly Cut Gerbera jamesonii and Extend the Flower Longevity

Titanium ions significantly promote plant growth, but the mechanism is still unclear. Cut flowers are ideal materials for the study of plant growth and senescence. In this study, freshly cut Gerbera jamesonii were used to study the effects of titanium ions (8 mg/L) on the flower longevity. Flowering observation showed that the gerbera vase life was significantly prolonged in the presence of titanium ions. Plate colony counts showed that the amounts of bacteria in the vase solution of the control group were approximately 1700 times more than that of titanium ion treatment group. High-throughput sequencing was used to determine the sequences of 16S rRNA gene V3-V4 variable regions of the vase solutions to analyze bacterial species, their average proportions, and absolute abundance. The results showed that the titanium ions reduced the entire bacterial counts as well as altered the absolute abundance of different bacterial species in the vase solution. The most prevalent bacteria were mainly Enterobacteriaceae, Pseudomonas veronii, Pseudomonas sp., Delftia sp., Agrobacterium sp., Sphingobacterium multivorum, Acinetobacter johnsonii, and Clostridiaceae. In combination with plate colony counts, we demonstrated that all the bacterial growths were significantly inhibited by titanium ions, regardless of their average proportions increased or decreased. These results showed that titanium ions could extend effectively the longevity of gerberas and possess the broad-spectrum antibacterial properties. This study provides a basis for further mechanism exploration of titanium ions action and its applications in cut flower preservation and agricultural production.

Effect of oxygen on the growth and biofilm formation of Xylella fastidiosa in liquid media

Xylella fastidiosa is a xylem-limited bacterial pathogen, and is the causative agent of Pierce's disease of grapevines and scorch diseases of many other plant species. The disease symptoms are putatively due to blocking of the transpiration stream by bacterial-induced biofilm formation and/or by the formation of plant-generated tylosis. Xylella fastidiosa has been classified as an obligate aerobe, which appears unusual given that dissolved O2 levels in the xylem during the growing season are often hypoxic (20-60 μmol L(-1)). We examined the growth and biofilm formation of three strains of X. fastidiosa under variable O2 conditions (21, 2.1, 0.21 and 0 % O2), in comparison to that of Pseudomonas syringae (obligate aerobe) and Erwinia carotovora (facultative anaerobe) under similar conditions. The growth of X. fastidiosa more closely resembled that of the facultative anaerobe, and not the obligate aerobe. Xanthomonas campestris, the closest genetic relative of X. fastidiosa, exhibited no growth in an N2 environment, whereas X. fastidiosa was capable of growing in an N2 environment in PW(+), CHARDS, and XDM2-PR media. The magnitude of growth and biofilm formation in the N2 (0 % O2) treatment was dependent on the specific medium. Additional studies involving the metabolism of X. fastidiosa in response to low O2 are warranted. Whether X. fastidiosa is classified as an obligate aerobe or a facultative anaerobe should be confirmed by gene activation and/or the quantification of the metabolic profiles under hypoxic conditions.

Grapevine xylem sap enhances biofilm development by Xylella fastidiosa

Xylella fastidiosa is able to form biofilms within xylem vessels of many economically important crops. Vessel blockage is believed to be a major contributor to disease development caused by this bacterium. This report shows that Vitis riparia xylem sap increases growth rate and induces a characteristic biofilm architecture as compared with biofilms formed in PD2 and PW media. In addition, stable cultures could be maintained, frozen and reestablished in xylem sap. These findings are important as xylem sap provides a natural medium that facilitates the identification of virulence determinants of Pierce's disease.