Inside Job

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Science  16 Sep 2011:
Vol. 333, Issue 6049, pp. 1551-1553
DOI: 10.1126/science.333.6049.1551-d

Most plants only thrive in intimate associations with fungi—or arbuscular mycorrhizal (AM) fungi—and engage in mutualistic nutrient exchange with the plant. Some AM fungi have recently been discovered to have their own bacterial symbionts. Ghignone et al. have explored the genome of a newly discovered species of rod-shaped endobacterium that lives within vacuoles of the Gigaspora margarita AM fungus. The genome sequence of the endobacterium shows that it is unable to degrade starch or sugars, unlike the fungus, and indeed has a limited ability to import sugars from its host. What it can do, though, is obtain energy by breaking down amino acids extracted from fungal oligopeptides—and to alleviate this parasitism, the endobacterium has the capacity to synthesize vitamin B12 and thus potentially to donate this nutrient to its fungal host. This is not the symbiont's only trick, because, like the pathogen Salmonella, the bacterium possesses the genes for the syringe-like type III secretion system, by which means it may inject various effectors across the vacuole wall. This genome offers another glimpse into the nested interdependencies we are beginning to expect to see when we observe microorganisms closely.

ISME J. 5, 10.1038/ismej.2011.110 (2011).

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