Difference between revisions of "Team:Bordeaux/Problem"

The Problem

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

Downy mildew is a disease caused by an oomycete (fungus-like eukaryotic microorganism) called Plasmopara viticola . It is unfortunately famous in the Aquitaine region because it affects tens of hectares of Bordeaux vineyards every year and threatens wine production . For example, this year, in the Aquitaine region, between 5% and 60% of merlot parcels are contaminated [4] and a large quantity of the grapes won't be able to be used for wine . Until the middle of the 20th century, research efforts were mainly concentrated on optimizing the application of cooper fungicides such as "Bouillie Bordelaise" and developing new molecules and formulations for controlling the disease. Now, more and more alternative treatments are being tested and effective models which take into account meteorologic conditions are being developed. This could drastically change the environmental impact of winemaking.

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.

Recently, more models and devices designed to be used by individual growers measure temperature, humidity and leaf wetness and provide treatment recommendations based on algorithms similar to the Goidànich model. This general algorithm defined as the 3–10 spray strategy, as the first treatment is prescribed when the average temperature is above 10°C, more than 10 mm of rain have fallen within 24 h and shoot length in the vineyard is at least 10 cm. This plans out the following treatments at regular intervals. Despite evident imprecision due to the strict parameters, this general model can reliably predict the first risk period and recommend thereafter a treatment schedule that will allow growers to prevent development of severe downy mildew in vineyards. The weakness of this type of model is that the number of recommended pesticide sprays is usually greater than what is needed to avoid an epidemic, particularly at the beginning of the season (Gherardi et al., 1999).

Applying a concept based on a tolerance threshold for downy mildew under the particular climatic conditions of southern Switzerland in a vineyard with cv. Merlot trained in Guyot over several years, Jermini and colleagues (Jermini et al., 2006) were able to eliminate half of the recommended treatments. This paradigm change, from a focus on the pathogen and the disease toward a threshold concept, requires detailed knowledge of the host and its relationship with the environment and human activities, such as pruning and thinning. Plant compensation for foliar damage (Jermini et al., 2010b), and the relation to the reserves in the overwintering wood (Jermini et al., 2010a) may allow treatments which protect only the grape bunches in later season. These are highly complex interactions and there is little available data describing them (Giuntoli and Orlandini, 2000; Orlandini et al., 2001). The information that is available is heavily biased by site, year and cultivar factors, and so cannot be readily used for simulation and modeling activities. [1]

Natural defenses of plants

Plant Immunization is based on the same principle as human immunization: activating it's natural defenses before contamination by an infectious agent. The concept is simple; it is to put the plant in contact with a molecule able to activate it'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 cell 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 "immunized" plant 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.

Literature Cited: A METTRE

[6] Les leçons du mildiou en 1930 Progres Agricole et Viticole (1931) 95, 187-188

Figure 3: Schematic representation of the plant's imune system (?)

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.

Literature Cited: A METTRE

[6] Les leçons du mildiou en 1930 Progres Agricole et Viticole (1931) 95, 187-188

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.

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