Underground and in the dark, there is a subtle chemistry that brings bacteria and plants together in the ultimate marriage of convenience. Legumes like rice and wheat possess the ability to form bulbous nodules -- stuffed with nitrogen-fixing bacteria -- on their roots. These nodules help improve plant growth. But how do these structures actually form? Researcher Sharon Long of Stanford University, Palo Alto, California, US, says that this nodule formation is initiated by the plant itself. Molecules called flavanoids seep out from the plant's roots, enter the soil bacterium and interact with a protein inside it. This protein, a gene regulator, is normally present in the microbe's genetic material and the flavanoid activates certain key genes in the bacterium, called nod ('nodulation') genes. In turn, the enzymes encoded by the nod genes send a signal back to the plant. It is this signal that tells the plant to make a nitrogen-fixing nodule ( New Scientist , Vol 153, No 2064).
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