Microbe Profile: Xylella fastidiosa – a devastating agricultural pathogen with an endophytic lifestyle

Xylella fastidiosa is a vector-borne plant vascular pathogen that has caused devastating disease outbreaks in diverse agricultural crops worldwide. A major global quarantine pathogen, X. fastidiosa can infect hundreds of plant species and can be transmitted by many different xylem sap-feeding insects. Several decades of research have revealed a complex lifestyle dependent on adaptation to the xylem and insect environments and interactions with host plant tissues.


PROPERTIES
X. fastidiosa is a Gram-negative rod. The cells are typically 0.25-0.35 µm in diameter and 0.9-3.5 µm in length. The bacterium is non-flagellate, but motile via type IV pilimediated twitching. X. fastidiosa lacks the type III secretion system typical of biotrophic plant pathogenic bacteria [1].

GENOME
The genome sequence of X. fastidiosa citrus-pathogenic strain 9a5c was published in 2000, making it the first genome sequence for a plant pathogenic bacterial species [2]. Complete genome assemblies for X. fastidiosa range from 2.5 to 2.7 Mb and many strains carry one-three plasmids of 1.2 to 51 kb. Expansion of available genome sequences to include a wide range of strains from different host plants has provided a tool for tracing new introductions worldwide. Currently there are publicly available genome assemblies for X. fastidiosa strains from multiple crop and ornamental hosts, and multiple countries.

PHYLOGENY AND GENOMIC POPULATION STRUCTURE
X. fastidiosa is divided into multiple subspecies groups, based on genetic differences. Subspecies fastidiosa, pauca and multiplex are formally described in bacterial taxonomy and represent genetic lineages originally associated with Pierce's disease of grapevine, citrus variegated chlorosis, and various leaf scorch diseases, respectively [3]. The formally classified lineages are now also associated with additional diseases, such as almond leaf scorch (fastidiosa) and olive quick decline syndrome (OQDS, pauca) [4]. Other subspecies have been proposed for strains that infect oleander (subspecies sandyi) and mulberry (subspecies morus), but these have not been formally described in bacterial taxonomy [5]. In addition to disease events, strains of all subspecies can be found in asymptomatic hosts, or causing mild scorching symptoms that do not persist from 1 year to the next. To further define phylogenetic relationships of X. fastidiosa strains, a multi-locus sequence typing (MLST) system based on sequences of 7 housekeeping genes was widely adopted and 87 sequence types have been described to date [6]. More recently, whole-genome sequencing has emerged as the gold standard for strain classification.

KEY FEATURES AND DISCOVERIES
Leaf scorching and die-back symptoms characteristic of X. fastidiosa infection are a combined result of bacterial growth within the vascular tissue and host defence responses that limit water flow in the plant. On initial infection, X. fastidiosa moves systemically through the xylem vessels facilitated by the secretion of cell wall-degrading enzymes and twitching motility using type IV pili. During the early stages of infection, plant response to X. fastidiosa invasion is delayed by the outer O-antigen portion of bacterial lipopolysaccharide surface structures that mask recognition by the innate immune system [7]. Outer-membrane vesicles (OMVs) are produced from the bacterial surface, which modulate adhesion to xylem cell walls and delay the formation of biofilms. As infection progresses, a diffusible signal factor (DSF) represses the production of OMVs once bacterial populations have reached higher densities, at which point X. fastidiosa attaches tightly to xylem vessel walls and forms biofilm structures dependent on the production of exopolysaccharide and fimbrial and afimbrial adhesins [1].
Horizontal gene transfer has been observed between strains and subspecies of X. fastidiosa, likely due to a high degree of natural competence for acquiring genetic material from the environment [5]. Natural transformation and homologous recombination occur at high frequency under in vitro conditions [8].
Despite the fact that specific X. fastidiosa strain-host plant combinations have resulted in devastating epidemics, in the Abstract Xylella fastidiosa is a vector-borne plant vascular pathogen that has caused devastating disease outbreaks in diverse agricultural crops worldwide. A major global quarantine pathogen, X. fastidiosa can infect hundreds of plant species and can be transmitted by many different xylem sap-feeding insects. Several decades of research have revealed a complex lifestyle dependent on adaptation to the xylem and insect environments and interactions with host plant tissues. majority of plant hosts X. fastidiosa does not cause severe disease. Specific factors that lead to parasitic rather than commensal interactions are not known, but some of the traits exhibited by X. fastidiosa during plant colonization contribute to virulence attenuation [7]. However, phenotypes necessary for insect transmission, such as the adhesive biofilm stage, also likely influence overall adaptation of X. fastidiosa.
Multiple insect species can transmit X. fastidiosa between plants through feeding on xylem sap. Insects of the leafhopper (Hemiptera: Cicadellidae) and spittlebug (Hemiptera: Aphrophoridae) families are the most common vectors associated with disease outbreaks [9,10]. Insect-X. fastidiosa interactions lack specificity, with multiple insect vectors capable of transmitting X. fastidiosa to many hosts, and more than one strain of X. fastidiosa identified in the same insect [10]. Once acquired, X. fastidiosa adheres to the insect foregut, forming biofilms, and can persist for the life of the insect unless shed during moulting. Although X. fastidiosa multiplies within the insect vector, it does not enter the circulatory system and cannot be transmitted vertically to offspring [10]. As insect vectors of X. fastidiosa often have multiple feeding and reproductive host plants, the general nature of acquisition and transmission interactions contributes to the complexity of tracing and managing epidemics.

OPEN QUESTIONS
• What determines the host range of genetic lineages and specific strains of X. fastidiosa? Service appropriated project 2034-22000-012-00D. We apologize to our colleagues whose work we were unable to cite owing to page limitations.

Conflicts of interest
The authors declare that there are no conflicts of interest.