Difference between revisions of "Team:Bordeaux/Problem"

The Problem

This year, iGEM Bordeaux’s project is focused on Downy Mildew

This disease, caused by an oomycete (fungus-like eukaryotic microorganism) called Plasmopara viticola , is unfortunately famous in the Aquitaine region because it affects tens of hectares of Bordeaux vineyards every year and threatens wine production (How much?) . It was originally observed in the United States of America in 1834 and has been most abundantly found in the northern and midwestern areas of the United States. Shortly after, the pathogen was introduced in European countries where it played a devastating role in the yield and production of their wine. In 1878, the first cases of Downy mildew were observed in France (in the region of Lyon) and also in Swizerland and Italy. Common symptoms include necrosis of the stem or shoot, discoloration, brown spotting and yellowish-green tips of the leaves and mycelium invasion of the grapes. While some North American species have become resistant to this parasite, European species such as Vitis vinifera (the grapevine used for wine) are extremely sensitive. Depending on the year, production of grapes in France has been estimated to be at a loss of 50% or more ref and the Aquitaine region is particularly affected due to the favorable climate and the economic importance of the wine industry. Thus, Downy mildew has been considered the most devastating disease caused by a filamentous pathogen to affect European vineyards and this has lead vineyards to search for effective measures to protect their vines. Unfortunately, most of these mesures have a bad environmental impact and pollute the surrounding soils.

In 2015, vineyards are still threatened by the disease

Our iGEM team has been following this year's effect of mildew closely reading the official vineyard mildew bulletins available on the vinopole website. It is clearly shown that there is a significant increase of mildew infection on parcels that haven't been treated with copper sulfate compared to those that have been treated. Furthermore, the infection of mildew on treated parcels appears to be much more easily controled on parcels treated with copper sulfate. Evidently, without any alternative treatment, wine production in the region would be affected and this shows just how important our project is!

Different models (Caffi model, Potential systems model) take into account pluviometry, temperature, relative humidity and plant morphology to decide when are the best moments to apply the fungicides. However, even if these models have allowed vinyards to drastically reduce the quantities of fungicides used, they still cause environmental and sanitary problems in the surrounding regions.

Since repairing damaged tissues infected by downy mildew is impossible, the main solutions available to vinyards are preventive solutions, mainly through preventing primary infections. This is mainly done by spraying fungicides on the organs that are most infected: leaves and stems. The most efficient preventive treatment was discovered at the end of the 19th century: a solution made of copper sulfate also known as "Bouillie Bordelaise", the only treatment used until the end of the 20th century. Recently, synthetic fungicides have replaced this chemical treatment and more and more research is being done on alternative eco-friendly preventive treatments.

Natural defenses of plants

Immunization of plants is based on the same principle than humans: activate the natural defenses before contamination by the infectious agent. The concept is simple; it is to put the plant in contact with a molecule able to activate plant's natural defenses: an elicitor . In nature there are many elicitors produced by micro-organisms (exogenous elicitors) or by the plant itself when it is attacked (endogenous elicitors). The presence of an elicitor in the plant triggers a series of cellular reactions including the production of molecules to strengthen the resistance of walls , but also the production of plant antibiotics such as phytoalexins or defense proteins. These compounds have antifungal and antibacterial properties. The external application of a natural elicitor or a similar synthetic molecule thus results in the production of phytoalexins or defense proteins in the absence of any pathogen. The plant "immunized" is ready to fight back if attacked. First, the cell wall forms a physical barrier which prevents the penetration of most microbes. Then, if some pathogens are able to cross this wall, the infection depends on the ability of the plant to perceive and to trigger defense reactions that would prevent the development of the disease. This recognition is done with certain compounds, known as elicitors from the pathogen or plant. The fixing of an elicitor to a plant cell receptor initiates a cascade of events that leads to the synthesis of defense compounds . The best known are antimicrobial compounds such as phytoalexins and Pathogenesis Related proteins .

There are two types of defenses: "passive" and "active". Natural defense reactions of plants are passive when these components were preformed prior to infection. They remain active until the pathogen penetration. Active defenses are induced upon recognition of the pathogen and remain active during infection.

Consequences of the infection

Delta-viniferin is a grapevine phytoalexin produced during infection by Plasmopara viticola . Phytoalexins are antimicrobial and often antioxidative substances synthesized by plants that accumulate rapidly at areas of pathogen infection. Phytoalexins produced in plants act as toxins to the organism. They damage the cell wall, delay maturation, disrupt metabolism or prevent reproduction of the pathogen.

When phytoalexin biosynthesis is inhibited, the susceptibility of infection in plant tissue increases showing its importance in defense mechanism . While, when a plant cell recognizes particles from damaged cells or from the pathogen, the plant launches a two-pronged resistance: a general short-term response and a delayed long-term specific response.

During the short-term response , the plant deploys reactive oxygen species such as superoxide and hydrogen peroxide to neutralize invading cells. The short-term response corresponds to the hypersensitive response, in which cells surrounding the site of infection are signaled to undergo apoptosis (programmed cell death), in order to prevent the spread of the pathogen to the rest of the plant.

The long-term response , also called systemic acquired resistance (SAR), permits a communication of damaged tissues with the rest of the plant using plant hormones such as jasmonic acid, ethylene, abscisic acid or salicylic acid. The reception of the signal leads to changes within the plant inducing genes that protect from more pathogen intrusion like genes coding enzymes involved in the production of phytoalexins. Also, if jasmonates or ethylene is released from the wounded tissue, neighboring plants also synthesize phytoalexins in response.

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