One day of Xylella fastidiosa in a host plant

Always interested to know what is the day like of the killer bacterium Xylella fastidiosa? Here you are:

The bacteria enter the xylem at the insect feeding site, i.e. where the insect vector bites, and moves thereafter passively and actively through the vessels. Type-4 pili-mediated twitching motility allows the bacteria to migrate to distal tissues, systemically infecting the plant. X. fastidiosa thrives in dead xylem vessels.

Once in the xylem vessel, X. fastidiosa needs to overcome the host immune system, compete with the respective microbiomes and gain access to nutrients. Inversely, early and successful detection of pathogen in dead xylem vessels is a question of life and death and a major challenge for infected plants.

Xylella fastidiosa life cycle.

Xylem-limited microorganisms are able to thrive in an environment, which has a highly fluctuating negative pressure, and low oxygen and nutrient content. X. fastidiosa is well adapted to these conditions.

The bacterium produces cell-surface adhesins, which increase surface attachment, cell aggregation, and biofilm formation. The ability to switch between motile cells and attachment-driven biofilm formation benefits the multiplication and establishment of a critical bacterial load in the vessel, movement within the xylem and thus colonization of plants, and acquisition by insect vectors.

To multiply, X. fastidiosa needs to obtain all factors required for growth and survival from the xylem. Xylem sap is extremely dilute, i.e. > 95% water. Beyond water and minerals, xylem sap contains amino acids, alcohols, organic acids, sugars and vitamins, potentially satisfying nutritional requirements for microbial growth. The metabolic network of X. fastidiosa has been shown to be complete, but strikingly minimalist, with a metabolism mostly relying on pathways that cannot ensure a fast proliferation. Various media to grow X. fastidiosa in vitro are available, containing at least citrate and succinate, three amino acids (L-glutamine, L-asparagine, and L-cysteine), hemin chloride, potato starch, and mineral salts.

Clogging of xylem by Xylella fastidiosa.

The bacterium multiplies in vitro in xylem sap, showing differences in planktonic and biofilm growth depending on the xylem sap from different plant hosts. In addition, X. fastidiosa secretes cell wall degrading enzymes. These enzymes depolymerize cellulose and pectin, major constituents of pit membranes, porous structures interconnecting adjacent vessels, allowing X. fastidiosa not only moving from vessel to vessel but exploiting plant polysaccharides as carbon sources.

X. fastidiosa also has a complete chitin-processing machinery including a chitinase. The bacterium can thus grow in medium with chitin as the sole carbon source, important for colonization of insect vectors. It is possible that X. fastidiosa uses the chitinase to inhibit the growth of endophytic fungi, outcompeting other microbes in xylem vessels.

The establishment of a pathogen lifestyle depends on interactions with the host microbiome and the host immune system. Understanding these interactions provides us with opportunities to affect them, e.g. with introduction of proper endophytes, and thus to control the growth and proliferation of the pathogen.

This blog is based on the recent scientific articles co-authored by our scientists from CNR Institute of Sustainable Plant Protection,
Xylella fastidiosa’s relationships: the bacterium, the host plants and the plant microbiome.

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