Plants are susceptible to attack by bacteria, viruses, fungi and insects. These pests limit their growth or flowering, damage seeds, fruits and vegetables. But despite their immobility and lack of a “real” immune system, plant disease remains the exception. Because plants have developed effective and original defense systems, which we are beginning to understand in all their finesse and which offer serious alternatives to phytosanitary products … Thus, natural substances are currently being tested to provoke and reinforce the active defense of plants, whether on apple, potato or wheat.

The lignified rigid walls ensure the wearing of plants, but also an effective defense. Impermeable to pathogens, difficult to consume by insects, they sometimes also contain antibacterial chemical molecules. But if the pests still manage to pass this first protection, cellular surveillance systems detect the intrusion. Plant cells recognize molecules, the “elicitors”, that are emitted by the pathogen or resulting from the degradation of the walls due to the action of the pathogen.

New leads for biocontrol

These elicitors will activate plant defense systems, which aim to stop the spread of pathogens. Some will lead to rapid death of the cells in the area attacked, others will activate the cell wall strengthening and production of antimicrobial compounds and insecticides around this area. Finally, some will emit warning signals (salicylic acid, jasmonic acid, ethylene) from this region to the whole plant. 

Researchers have identified substances of natural origin that function as elicitors. These are sugars, lipids, proteinsor even minerals like sulfur – historically used on the vine. This strategy, called “stimulation of plant defenses” (SDP) is part of the biocontrol tracks. These alternative and complementary solutions to phytosanitary treatments are currently being tested and some products are already approved for commercialization.

Research on the apple tree

Marie-Noëlle Brisset, head of the SDP program on the Apple tree at INRA, seeks for example to evaluate the natural products really capable of stimulating the apple tree’s defenses: “The screening tests start in the laboratory to reduce the difficult and expensive experiments for a perennial crop like the apple tree. “. A molecular kit has been created. It quantifies the level of 28 defense markers in plants previously treated with SDP products. It allows to select the best defense activators. 

The products can also be tested, always in the laboratory, to know their protection capacity: the plants are then treated and then inoculated with this or that pathogen of the apple tree.“The protection responses are never the same level as that obtained with pesticides, but can exceed 50%,” says Marie-Noelle Brisset. Then, it’s the test in real conditions. For three years, the researcher has been coordinating a project funded by the Ministry of Agriculture and bringing together INRA, technical institutes and regional stations to study 5 SDP products in orchards after screening 30 products in the laboratory. “The difficulty with SDP is that there is still little control over the methods of application and integration in technical itineraries. Environmental conditions, varietiesor the application of certain inputs can strongly influence the results. Our work must make it possible to specify the recommendations of use to maximize the effect of SDP in culture conditions. “ .

Other plants concerned by the SDP

Septoria is the fungal disease (fungus) most commonly encountered on the soft wheat species . Choosing a variety of wheat tolerant to septoria makes it possible to lower the pressure of the parasite on the plant and thus to reduce its nuisance capacity. However, the effectiveness is only partial … Arvalis-Institut du végétal is looking for elicitors that can strengthen the fight against this fungus. “The goal is to gain a semi-chemical treatment on wheat , says Claude Maumené, cereals disease expert, in charge of biocontrol at Arvalis-Institut du végétal.Our job is to detect an activity among the prospective elitists and to value them in a treatment program. For example, we study sulfur, which has effects of stimulating plant defenses but also a direct action on the pathogen. “. 

The complexity of the immune response is very large and also depends on the overall health of the plant in relation to its environment. The response is often partial but the range of activated defenses is wide and the probability of resistance selection in the pathogen is low. “So, SDPs help plants to heal themselves in a sustainable way. “ . 

Arvalis-Institut du végétal is working on elicitors for corn, potatoes, flax and tobacco.“Progress will be strongly correlated with the regulatory perimeters that are being put in place. According to their classification, some elicitors can not (or more) be marketed as biocontrol products , “ warns Claude Maumené. 

Today, many international companies have developed SDP alongside small businesses. The market is booming, particularly in response to plans to reduce the use of pesticides. It is all the more important that it is aimed at gardeners, local authorities and farmers in organic and conventional …