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How leaves talk to roots
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Upper panel: Wild type roots form nodules whether or not they are transgenic (the latter are marked by green fluorescence). Lower panel: Downregulation of miR2111 in transgenic roots (marked by green fluorescence) leads to reduced …more 
 New findings show that a micro RNA from the shoot keeps legume roots susceptible to symbiotic infection by downregulating a gene that would otherwise hinder root responses to symbiotic bacteria. These findings reveal what it takes to make nitrogen-fixing symbiosis efficient, and how to exploit it agronomically.

An international research team reports that a small RNA sequence, micro RNA miR2111, travels from leaves to roots in legume plants that can form a nitrogen-fixing symbiosis with root bacteria. Once in the root, miR2111 ensures that the plant is susceptible to symbiotic infection. Roots express a gene that will normally hinder root responses to symbiotic bacteria, and miR2111 interferes with the activity of this gene. The findings also explain why roots without shoots are unable to embark on nitrogen-fixing symbiosis, and have just been published in the international journal Science.

The new findings overturn the previous understanding of how plants control symbiosis, as the results reveal that mechanisms regulating infection are already in place before the plant comes into contact with bacteria. This resource-costly solution ensures a rapid plant response once the right bacteria are present, emphasizing that continuous control is vital for a host plant to prevent abuse.

Beans, peas and soy, like other members of the legume family, have a lot of proteins, and are in high demand by vegetarians and agriculture. Unlike other plants such as cereals, potatoes or apple trees, legumes have evolved the ability to host nitrogen-fixing bacteria within special organs in their roots. These bacteria, rhizobia, provide the plant host with reduced nitrogen in the form of ammonia and in return receive a full supply of mineral and carbohydrate nutrients from the plant. Legume plants therefore grow well without chemical nitrogen fertilizer, which enhances their value as crop plants, especially in regions with nitrogen-poor soil.
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