Plants are in constant contact with soil bacteria, some of which are pathogenic, some symbiotic. Certain soil bacteria invade plant roots and form nodules in which they fix nitrogen, providing the plant with free biological fertilizer. So far it is not clear how the bacteria alter plant development to make a now root organ. We have been investigating whether these bacteria manipulate plant development by interfering with the plant's own growth hormones, in particular with auxin.

Our findings showed that nitrogen fixing bacteria altered the transport of auxin not just locally, where they invade the root, but even over long distances in the leaves of the plant. By modifying how much of the growth hormones flow from the leaves to the roots, the numbers of nodules on the root system can be modified. In search for a secondary signal that the bacteria use to modify auxin transport, we used RNA interference to silence the flavonoid pathway in a legume plant. Flavonoids have been suggested as potential auxin transport inhibitors. Our study provides the first genetic evidence that flavonoids in the plant are necessary for the regulation of auxin transport during the development of a nodule.

Prayitno J, Rolfe B.G., Mathesius U. (2006) The ethylene insensitive sickle mutant of Medicago truncatula shows altered auxin transport regulation during nodulation. Plant Physiology , in press (published online 14/07/2006 ).

Wasson, A.P., Pellerone, F.I. and Mathesius U. (2006) Silencing the flavonoid pathway in Medicago truncatula inhibits root nodule formation and prevents auxin transport regulation by rhizobia . Plant Cell 18, 1617-1629. (This paper is accompanied by an editorial "In this issue" Plant Cell 18: 1539-1540.)

Van Noorden, G.E., Ross, J.J, Reid, J.B., Rolfe, B.G. and U. Mathesius (2006) Defective long distance auxin transport regulation in the Medicago truncatula super numeric nodulation mutant. Plant Physiology 140: 1494-1506