Our diet largely relies on plants: we use crops to feed ourselves as well as the animals we farm. Microbial diseases and herbivore damages are major factors of crop loss around the world. Reducing these damages represents then an important challenge. Because plants are sessile organisms, meaning they cannot move and run away when threatened, they have developed a range of sophisticated strategies to face attacks from herbivores like ruminants, insects or pests, but also from microbes. By uncovering and understanding how plants can defend themselves against these different attackers, scientists and researchers aim to decrease crop losses. The knowledge of the different mechanisms that are involved in the context of plant defense enables scientists to reinforce plants capacity to defend themselves, using different strategies.

For instance, attacked plant cells alert the whole plant using phytohormones and volatiles to prepare for a potential attack. This powerful mechanism is known as Systemic Acquired Resistance. It results in plants that are resistant against a wide spectrum of pathogens for a few days to several weeks. Scientists are investigating the possibility of activating plant immune systems to preemptively prevent infection, but this strategy could be limited because immune system activation happens at the expense of growth, reducing crop yield.

Scientists are also trying to breed plants with more efficient defenses. First, species or individual plants that have an interesting trait have to be identified. This trait can be anything from capacity to recognize a specific pathogen, to more solid spines and prickles, to increased production of antifungal or insecticide molecules. The trait is then introduced into crop species by successive crossings to improve their defense mechanisms against would-be predators. The whole process of breeding is clearly explained in this TED-ED lesson. Unfortunately, this process takes a long time. To circumvent this, in coffee plants for instance, grafting appears to be an efficient compromise. For example, the cultivar Arabica is known to produce the tastiest berries, while Robusta is known to be the most resistant to nematodes and also robust to climate changes. Grafting shoots of Arabica on root stem of Robusta results in plants that carry both beneficial traits.

Scientists can also characterize the selected mechanisms at the genetic level, which means knowing the genes that are involved. This knowledge enables approaches aiming to select plants that have the best genetic background to have performant defense systems. This knowledge also enables scientists to modify the genes of plants into more efficient versions, an approach called Directed Mutagenesis. Last but not least, plant defense performance can be enhanced using the very controversial approach of genetically transforming plants with defense genes. One of the most striking example is the introduction of new plant receptor to enlarge the spectrum of pathogen recognition. For instance, FLS2 is the plant receptor that recognizes bacterial flagellin. FLS2 is present is most land plants. In contrast, the plant receptor EFR, that recognizes the bacterial elongation factor EF-Tu, is specific to brassicaceae plants, a plant family that includes broccoli, mustard, turnips, rapeseeds, or the research plant Arabidospsis. EFR is absent from non-brassicaceae plants. By introducing EFR into non-brassicaceae plants that lack EFR but do have FLS2, such as tomato and tobacco, scientists managed to make these plants more efficient to mount anti-bacterial defenses. This is explained in more detail here.

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