Xylella fastidiosa: Plant-Pathogen or Misunderstood Member Of The Plant Microbiome?
Updated: Jun 12, 2021
Xylella fastidiosa possesses a dual personality, cleverly equipped to live in two vastly different environments—insects and xylem, the water transport system of plants. It is a member of a number of plant microbiomes without ever causing disease. So is it a threatening plant pathogen or in the wrong place at the wrong time?
Artist Credit: @organizedmike
Transmission: From Insect Microbiome To Plant Pathogen
X. fastidiosa possesses a dual personality, cleverly equipped to live in two vastly different environments—insects and xylem, the water transport system of plants. Inside the insect, this plant pathogen must combat extreme conditions. As the insect feeds on a plant it can create an intense flow that can violently dislodge X. fastidiosa. So how does this little microbe hold on? To maintain its location within the foregut of the insect, X. fastidiosa creates a biofilm. A biofilm is a sticky structure that can help the bacteria hold their position as the influx of liquid passes through the insect house. I often think of it as a warm blanket or a shield. In our Society of Symbionts, their campus buildings are constructed from biofilm! Biofilms help bacteria stick to surfaces and protect them from adverse conditions. It helps them create a nice safe space to thrive and survive.
Sharpshooters are the predominant insect that transmits X. fastidiosa. The insects are fairly common. They like to reside near citrus orchards and vineyards. In California, X. fastidiosa is a huge problem to the wine industry causes a disease called Pierce's Disease. In vineyards that are adjacent to orange orchards, the disease is more severe as the vector population can grow to much larger quantities and thus transmit X. fastidiosa throughout a vineyard. When the insect feeds on an infected plant, X. fastidiosa can be sucked up. It will then shift its lifestyle to live inside the insect. This is probably a pretty cushy life for the microbe, it forms its nice safe space, the insect is constantly supplying new food that the microbe can eat and survive off from. But eventually, the insect will feed on another plant. The pressure and current of the feeding event can shoot X. fastidiosa into the plant host where it must adapt to the new environment. Sometimes this microbe is just a casual member of the plant microbiome, but other times it becomes a plant pathogen!
Xylella fastidioa Symptoms and Diagnosis as a Plant Pathogen:
Xylella’s presence leads to xylem clogging, causing water stress symptoms including leaf scorch and dehydrated fruit. The clogging may even be due to the plant sabotaging its own water transport system! You see one of the major defense mechanisms in plants is tyloses. Tyloses are balloon-like structures that shoot out from the plant cells. The hope is the tyloses will create a barrier, essentially blocking the movement of the plant pathogen from spreading all over the plant. However, if it blocks Xyellea from moving throughout the xylem, it also blocks water transport. If a plant throws out a few of these tyloses to stop a threat, this is usually ok. The xylem structure and function are still maintained. However, this is not the case for X. fastidiosa in grapevines. Between the tyloses that are released, and the survival strategy of X. fastidiosa the grapevine does not adequately clear the infection. In these situations, X. fastidiosa is a plant pathogen that can kill its host.
While the disease can look a lot like water stress in the grapevine, there are distinct Pierce's Disease symptoms. Other symptoms include “matchstick petioles” which are when the leaves fall off the plant but leave behind the tissue that connects it to the stem. This tissue that connects the leave to the stem is called a petiole. Burnt leaf tips are also seen. It creates this very pretty-looking leaf, with orange almost sunburst tips and green and yellow centers. The final big clue that it is Pierce's Disease and not water stress is stunted growth, uneven maturation, and weak-looking canopies. Every year the plant is infected with this plant pathogen it grows weaker. The shoots will be fewer in quantity and shorter over time as the plant doesn't have the resources to grow optimally. Due to this, the canopy will also shrink, there will be fewer leaves, fewer vines, and the whole plant will look sickly. When X. fastidiosa becomes a plant pathogen to grapevines, it can take 12-18 weeks before these symptoms arise.
Researchers are able to locate this microbe within the plant in a number of molecular ways. However, Xylella can be very elusive, often leading to inaccurate results and false negatives.
Life Cycle of Xylella fastidiosa:
X. fastidiosa uses biofilm, a sticky microbial tent, for protection. For most pathogens, biofilms enhance their arsenal for war, but when X. fastidiosa is unable to produce biofilm it becomes more virulent. Most plant pathogens also have a T3SS (type III secretion system), a missile-launching control center. Xylella does not possess such artillery, although it does have some clever means of exploring and adapting to new environments. Xylella must be a survivor and adapt to overcome the constant kidnapping by flying giants who aggressively strip them away from their homes and vomit them into a foreign and hostile new land.
Once deposited into this foreign land, Xylella doesn’t attack, but explores, using its pili to twitch its way across the xylem currents. When it meets a membrane barrier, X. fastidiosa sends out its cell wall degrading enzymes to pulverize that blockade. In response, the plant will create tyloses, balloon barriers. But the joke is on the plant—those balloons are so effective, not even water gets past them. Xylem vessel diameter and water conductivity can be drastically different from one grape variety to the next. For instance, Thompson Seedless which produces more tyloses, and has wider xylem vessel diameters, is more susceptible to the disease than Merlot. The plant response to the microbe enhances disease progression. Most people quickly categorize a microbe as the villain without ever looking to see how the host provokes and exacerbates the situation.
Treatment For X. fastidiosa as a Plant Pathogen:
There is no known cure for this disease. Management strategies include altering cultural practices (irrigation, pruning, vine removal) and insect control (pesticide application). Researchers have also tried to breed resistant varieties. More and more research is showing that the innate microbial community of the plant, the plant's microbiome may play a role in disease progression either by changing the behavior of the plant, insect, or pathogen or by directly attacking the pathogen. Another exciting therapeutic strategy is injecting bacteriophages, tiny viruses that hunt bacteria.
X. fastidiosa As A Member Of The Plant Microbiome:
We've primarily discussed X. fastidiosa as a pathogen of grapevines, the destroyer of wine, and the causal agent of Pierce's Disease. However, this is not the whole story of X. fastidiosa. While we predominately research this microbe as a plant pathogen this is not its only role just the only role we humans really care about. X. fastidiosa finds homes in over 350 different plant species across a broad range of diverse plant families. Most of these plants are not bothered by harboring this little guy and do so with no complaints. Xylella fastidiosa can live in the xylem of many plants but only causes diseases in a few. Unfortunately for Xylella fastidiosa the plants it does cause disease in our plants we really care about like citrus, grapevine, almonds, and coffee. Because of this X. fastidiosa is often labeled as a plant pathogen not a member of the plant microbiome. In my opinion, it has a lot of bad PR and I hope this little post shows sometimes plant pathogens are just misunderstood members of the plant microbiome.