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.

Taxonomy

Phylum: Proteobacteria; class: Gammaproteobacteria; order: Xanthomonadales; family: ; genus: ; species: . Subspecies: fastidiosa*, multiplex*, pauca*, sandyii, morus. *Formally described in bacterial taxonomy.

Properties

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 pili-mediated twitching. lacks the type III secretion system typical of biotrophic plant pathogenic bacteria [1].

Genome

The genome sequence of 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 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 strains from multiple crop and ornamental hosts, and multiple countries.

Phylogeny and genomic population structure

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 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 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, 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 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 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 , 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 strain–host plant combinations have resulted in devastating epidemics, in the majority of plant hosts 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 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 .

Multiple insect species can transmit 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– interactions lack specificity, with multiple insect vectors capable of transmitting to many hosts, and more than one strain of identified in the same insect [10]. Once acquired, adheres to the insect foregut, forming biofilms, and can persist for the life of the insect unless shed during moulting. Although multiplies within the insect vector, it does not enter the circulatory system and cannot be transmitted vertically to offspring [10]. As insect vectors of 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.

What determines the host range of genetic lineages and specific strains of ?

Where does strain recombination occur (plant, insect)?

How do secreted proteins and effectors influence –plant interactions?

Do naturally occurring plasmids convey fitness benefits?

To what extent does environmental adaptation of play a role in pathogen range and disease?