Plant Sensing and Communication by Richard Karban;

Author:Richard Karban; [Karban, Richard]
Language: eng
Format: epub
ISBN: 9780226264844
Publisher: University of Chicago Press
Published: 2015-05-18T00:00:00+00:00

8.4 Plants communicate with mycorrhizal fungi

Roots are the plant organs that absorb water and nutrients from the soil, although roots are not particularly good at these tasks. The capacity of roots to absorb nutrients is enhanced by associations with mycorrhizal fungi because fungal hyphae are finer than plant roots and often extend beyond the limits of the root zone, greatly enlarging the soil volume for absorption (Clarkson 1985). Some authors have argued that mycorrhizae, and not roots, are the chief organs of nutrient acquisition for land plants (Smith and Read 2008). Approximately 80% of plant species are associated with mycorrhizae—either endomycorrhizae whose hyphae penetrate plant cells or ectomycorrhizae that do not penetrate cells but are abundant enough to be comparable in mass to their host roots. Mycorrhizae are particularly good at taking up phosphorus, other minerals such as zinc and copper, and water. In exchange for nutrients, plants supply the mycorrhizal fungi with carbohydrates.

Plant-mycorrhizal relationships may be context dependent and facultative (Smith et al. 2009). Phosphorus-deficient situations make mycorrhizae particularly valuable and cause plants to allow or encourage the relationship by supplying more carbon. In fertilized soils, mycorrhizae have less to contribute and may become parasitic on their hosts. Under these conditions, plants may treat mycorrhizae as they would other pathogenic microbes and restrict their interaction (Brundrett 1991).

There are far fewer species of mycorrhizae than of plants; most mycorrhizae are generalists, capable of associating with many different plant species to a greater or lesser extent (van der Heijden et al. 1998, Smith and Read 2008). Extensive mycorrhizal networks develop that exchange carbon and other nutrients, linking numerous plant individuals and species (Simard et al. 2012). Mycorrhizal networks transfer water between plants along with any other compounds that are water-soluble. These include amino acids, allelochemicals, and informative chemical signals (He et al. 2005, Barto et al. 2011).

Plants may communicate with one another (or eavesdrop) using information transferred via mycorrhizal networks. Some plants sense and respond to cues that indicate the experiences of their neighbors in this manner. For example, uninfested tomato plants that were linked by mycorrhizal networks responded to pathogen infection of connected neighbors; the uninfested plants elevated levels of resistance to pathogenic fungi before being attacked themselves (Song et al. 2010). Since mycorrhizal networks may cover meters, communication through these networks may be possible over greater distances than have been detected in research involving airborne cues. Bean plants that were linked by mycorrhizae to neighbors infested with aphids emitted volatiles that repelled aphids and attracted their parasitoids (Babikova et al. 2013). In this case the bean plants prepared for aphid attack despite no direct contact with these insects.

Colonization of new plant host individuals by mycorrhizae is highly conditional. Plants can signal their location to recently germinated fungal spores (Giovannetti et al. 1996, Akiyama et al. 2005). Plant roots release a sesquiterpene recognized by the mycorrhizal fungi that causes increased hyphal branching, leading to contact with the root and ultimately root penetration. Plants mount an SAR response that involves SA when they perceive most fungi.

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