Team:Wageningen UR/Background
A world, where upon waking up, you cannot get that daily dose of caffeine. A world,
where you cannot sit in the sun while enjoying a nice glass of wine. Where you do not get enough
vitamins, because you cannot drink a nice glass of orange juice. This scenario might seem a bit
farfetched, but these crops are all being threatened by one single pathogen: Xylella
fastidiosa. In 1892, Newton Pierce was researching an epidemic grapevine disease in Southern California,
which had huge consequences for the grape industry in that region [1]. Later, this disease
became known as Pierce’s disease. At first, it was believed that the causal agent of Pierce’s
disease was a virus. It was not until 1978 that they successfully isolated and cultivated this
causal agent: a bacterium. They inoculated new grapevine plants which then developed the
symptoms of Pierce’s disease. Besides this, they discovered that this pathogenic bacterium was
indistinguishable from the bacterial causal agent of almond leaf scorch disease in almonds [2].
In 1987, John Wells isolated 25 strains of this pathogenic bacterium from 10 different diseased
plant species: Pierce’s disease of grapevine, periwinkle wilt, phony disease of peach and leaf
scorch of almond, plum, elm, sycamore, oak and mulberry. The bacterium was named
Xylella
fastidiosa [3]. In 2000, X. fastidiosa was the first bacterial plant pathogen to have
its complete genome sequenced [4]. Currently, X. fastidiosa is present all over the globe, as can be seen on the map in
Figure 1. In South America, Argentina, Brazil and Venezuela economically important crops are
infected with X. fastidiosa, while in North- and Central America the pathogen is
present in the United States, Canada, Mexico, Puerto Rico and Costa Rica. In Asia, the
bacterium has been found in Taiwan and Iran. Since 2013, the bacterium is present in Europe,
mainly the Puglia region of
Italy is infected. Since then, X. fastidiosa also has been found in parts of
France, Spain and Portugal [5]. The spread of X. fastidiosa in Southern Europe can
be seen in the map in Figure 2. X. fastidiosa infections can have big consequences. When X. fastidiosa is
detected in a plant, all possible host plants are eradicated in a 100-meter radius. On top
of this, a restricted zone with a 5 km radius is instated, in which no host plants when
eradication is needed can be traded for 5 years. This means that a farmer cannot sell or
trade any of his crops for five years, which often results in bankruptcy. More
importantly, when containment measures are taken, this buffer zone can be expanded up to 10
km in radius [6]. The X. fastidiosa strain currently present in Europe is X.
fastidiosa pauca, which can infect olive trees, but not grapevines. That is why the most
measures and regulations against X. fastidiosa are being taken towards olive trees and not
towards grapevines. However, if grapevines in Europe also get infected by X. fastidiosa, the
same measures will be taken towards grapevines. Infections with X. fastidiosa can result in big losses. In 2014 alone, X. fastidiosa
cost the State of California approximately 104 million dollar, both due to research into X.
fastidiosa and as a result of the disease itself [7]. In Italy, the farmers' organization
Coldiretti has reported that 21 million trees are infected by to X. fastidiosa, which has led
to damages for the Italian olive-oil producers of up to €1.2 billion. The Puglia region in Italy
accounts for 40% of the Italian olive oil production. Moreover, according to this organization, the
border of the X. fastidiosa infected region moves at a speed of 2 km a month [8,9,10]. In
Italy, the disease is mainly present in the Puglia region, which accounts for 40% of Italy’s olive oil
production [11]. In Brazil, X. fastidiosa has devastated the citrus industry. In citruses, the
pathogen causes citrus variegated chlorosis, which affects almost all
sweet orange cultivars in the country that is responsible for 30% of the worldwide sweet orange supply
[12]. In fact, in 2012, X. fastidiosa caused 120 million dollar in damages to the Brazilian
citrus tree industry [13]. However, there are not only economic damages. In Italy, a lot of olive trees are protected as part of the
cultural heritage. Some of these trees are over a thousand years old and have become a part of the
landscape and the local culture. A lot of olive trees and farms have been providing an income for
families for hundreds of years and the farmers often have the olives as their sole source of income
[14]. In 2014, the European Union dictated
that, to save the Italian olive trees, diseased plants needed to be removed, as well as not infected
neighboring trees [15]. However, farmers did not follow these rules. Reason for this was their beliefs
in conventional disease treatment methods and their distrust of the government, scientists and the
European Union [16]. This lack of action created an opportunity for X. fastidiosa to establish
itself as a dominant pathogen in Italy. Xylella fastidiosa is phytopathogenic Gram-negative bacterium. It has a thin rod shape that
lacks any flagella, but has fimbriae that allows for motility [1]. X. fastidiosa has a very
unusual lifestyle, being able to live both inside a plant host and in an insect vector, making it a very
successful pathogen. Inside of the plant, X. fastidiosa is limited to the xylem, the plant’s
vascular system, that transports water and nutrients from the roots to the leaves. X.
fastidiosa is known to have over 350 different plant hosts [2]. About two-thirds of these plant
species do not experience a large negative impact from this interaction and will never show symptoms of
X. fastidiosa infection. To the remaining one-third, infection has deadly consequences, as
large populations of X. fastidiosa block the sap-flow in the xylem. This prevents water from
reaching the upper part of plants, causing the plants to dry out and perish. The xylem is a very inaccessible part of the plant, making it hard for any pathogen to spread to
new plant hosts. This is where a unique feature of X. fastidiosa’s lifestyle comes in.
By expressing adhesin proteins, the pathogen can adhere to the chitin surfaces of the
piercing-sucking mouthparts of insects feeding on X. fastidiosa infected plants [3].
Chitin is an integral building block of the insect’s exoskeleton that is commonly exposed on
these mouthparts. Once inside the insect, X. fastidiosa colonizes the insect by
establishing a biofilm in the foregut. Using this strategy, it is can remain inside the insect
vector for its entire life [4]. Once the insect vector feeds on new, still healthy plants, a
part of the established colony dissociates from the biofilm and infects the healthy plant.
Transmission does not seem to suffer from a latency period, as transmission can already occur
directly after the insect acquires the pathogen [5]. In stark contrast to most insect
transmitted pathogens, like plant viruses, X. fastidiosa is not specialized to a
specific insect species. This is because the strategy of adhesion and biofilm formation used for
colonization, makes use of very general features of these insects, such as the chitin molecules
of the exoskeleton. This means that in theory, X. fastidiosa is able colonize most of
the xylem feeding insects [6, 7], but insect population dynamics and feeding behaviour have a
large impact on the insects ability to efficiently spread the pathogen [8]. While insect vectors provide an important reservoir for (re)infection, the plant xylem is the
preferred environment of the pathogen. Which is quite remarkable as the xylem is a harsh
environment, in which few microbes can survive, because it is very nutrient poor, containing
only water, minerals and some proteins [9]. This provides X. fastidiosa with a niche in
which it faces very little competition. Adapted to this habitat, X. fastidiosa is known
to grow very slowly, with doubling times between 9 hours and 2.3 days [1]. In order to grow
inside the plant undisturbed, X. fastidiosa cleverly avoids detection by the plant’s
immune system. Generally, the plant’s immune system recognizes conserved structures found on the
outside of invading bacteria, called lipopolysaccharides (LPS). X. fastidiosa also has
these features, but disguises them with an O-antigen, shielding the LPS sites from being
recognized by plant’s innate immune system [10]. This allows X. fastidiosa to go
unnoticed during the first phase of infection, enabling the pathogen to firmly establishes
itself inside the plant. Once established inside the plant, X. fastidiosa spreads aggressively throughout the
xylem’s vascular system [11]. The pathogen can travel with and against the sap flow of the xylem
using his fimbriae, that allow it to "crawl" over the xylem’s inner lining [12]. For most of the
infection period, the plant shows no symptoms, depending on the plant species, this asymptomatic
period can take anywhere from a few weeks up to two years. But all this time X.
fastidiosa is multiplying and spreading throughout the plant. X. fastidiosa
reinforces itself by forming a complex biofilm on the walls of the xylem [13]. At one point the
overabundance of the pathogen triggers the plant’s immune system. The plant host starts to form
vascular inclusions inside the xylem, in an attempt to block the spread of X.
fastidiosa throughout the plant [14]. However, at this point the plant is already too
late to fight off the pathogen and the large amount of accumulated X. fastidiosa cells
together with the vascular inclusions start to block the vascular flow of the xylem [15]. This
prevents water from reaching the upper part of the plant, causing the plant to dry out and
perish. Because symptoms show so late in the pathogen’s life cycle, the disease appears to
progress very quickly, even though X. fastidiosa is a very slow growing organism. It is shown that the insect vectors have a preference for asymptomatic plants [16, 17]. This gives rise
to a delicate balance: more X. fastidiosa cells inside plants, gives a larger chance of insect
uptake and thus a more successful pathogen. However if X. fastidiosa grows too much, the plant
will show symptoms, greatly reducing the chance of insect feeding on the tree, lowering the pathogens
success. To maximize insect transmission efficiency, X. fastidiosa monitors cell density in
order to ensure it doesn’t grow so much as to cause symptoms [15]. To achieve this, X.
fastidiosa uses a specific kind of quorum sensing molecule. This quorum sensing molecule,
called Diffusible Signalling Factor (DSF), is both synthesized and detected by the pathogen itself. At a
low cell density, X. fastidiosa exhibits a relatively high growth rate and is actively
spreading throughout the vascular system. But once the cell density and thus the DSF concentration,
surpasses a certain threshold, X. fastidiosa switches behaviour slowing down its growth. It
enters what is called "vector acquisition mode" and starts expressing adhesin proteins that allow it to
colonize xylem feeding insects [18]. It was shown that X. fastidiosa colony sizes peak before
the plant starts to show symptoms. This means that hosts are most effective at spreading the disease
while still asymptomatic [17]. This, together with the quick death of plants after symptoms do show,
underlines the importance of early detection and intervention.Background
Imagine...
Where It All Began
Small Organism,
Big Consequences
Economic and Cultural Impact
Scientific Background
Know Your Enemy
-Sun Tzu
Dissemination Using The Insect Vector
Inside the Plant Host
Advanced Survival Strategies