Pathogen and Predator Resistance Plants are subject to many potential biological enemies, including bacterial, fungal and viral pathogens that cause disease as well as parasites, insects, birds, and grazing animals that feed on plants. Partnerships with microbes can help plants resist these threats. In the simplest case, microbial partners may simply occupy niches that otherwise might be vulnerable to pathogens. When bacteria form a biofilm around the roots of a plant, microbial pathogens and soil–dwelling parasites cannot gain access. The beneficial microbes in the rhizosphere may be doing more than just getting in the way; they may also be producing any one of a number of chemicals that act directly against pathogens. Many of the antibiotics humans use to treat infections are derived from bacteria or fungi that produce them to kill or inhibit competing microbes. Other microbially-produced chemicals may serve to mask the presence of the plant from would be parasites or predators, attract beneficial organisms, or stimulate the plants’ immune system. Bacteriophage — viruses that infect bacteria — may kill pathogenic bacteria directly. There is even evidence that microbes can generate electrical fields that can attract or deter other microbes and soil invertebrates like nematodes. Above ground as below, microbes can serve the plant simply by occupying space that otherwise might be an entry point for pathogens. Surface and endophytic microbes also make a variety of potentially helpful compounds including toxins that deter grazers, volatile compounds that alert neighboring plants to the presence of a threat, and small molecules that trigger protective responses like the closing of stomata. There is crosstalk between the “conversations” going on between plants and the microbial communities above and below ground; events in the soil can trigger responses in leaves and vice versa. For example, when tomato plants are attacked by the early blight fungus, their activated immune system produces stress signals that are perceived by fungi that live around the roots. Through their hyphae, the beneficial root fungi then transmit the stress signal to neighboring plants warning them to up-regulate their stress responses. Microbes produce a wide array of compounds that inhibit or kill competing microbes. If a bacterium, virus or fungus can deter another microbe that is harmful to a plant, it may well be in the plant’s interests to provide the helpful microbe with shelter and nourishment. It is likely that many plant-microbe partnerships have evolved on this basis: the plant supplies carbohydrates to certain microbes in return for the deterrence of pathogens or predators or other benefits. It has been estimated that up to 30% of a plant’s primary production (that is, the amount of carbon the plant turns into organic matter through photosynthesis) actually leaves the plant as exudate into the soil; the microbes must be making a fairly substantial contribution to earn such a high investment of the plant’s resources. Read whole article here. sciencedaily/releases/2013/08/130827204536.htm
Posted on: Mon, 02 Sep 2013 13:06:20 +0000
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