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Differential Effects of Combined N Sources on Early Steps of the Nod Factor--Dependent Transduction Pathway in Lotus japonicus

August 2007 , Volume 20 , Number  8
Pages  994 - 1,003

Ani Barbulova , Alessandra Rogato , Enrica D'Apuzzo , Selim Omrane , and Maurizio Chiurazzi

Institute of Genetics and Biophysics A, Buzzati Traverso, Via P. Castellino 111, 80131, Napoli, Italy

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Accepted 2 April 2007.

The development of nitrogen-fixing nodules in legumes is induced by perception of lipochitin-oligosaccharide signals secreted by a bacterial symbiont. Nitrogen (N) starvation is a prerequisite for the formation, development, and function of root nodules, and high levels of combined N in the form of nitrate or ammonium can completely abolish nodule formation. We distinguished between nitrate and ammonium inhibitory effects by identifying when and where these combined N sources interfere with the Nod-factor-induced pathway. Furthermore, we present a small-scale analysis of the expression profile, under different N conditions, of recently identified genes involved in the Nod-factor-induced pathway. In the presence of high levels of nitrate or ammonium, the NIN gene fails to be induced 24 h after the addition of Nod factor compared with plants grown under N-free conditions. This induction is restored in the hypernodulating nitrate-tolerant har1-3 mutant only in the presence of 10 and 20 mM KNO3. These results were confirmed in Lotus plants inoculated with Mesorhizobium loti. NIN plays a key role in the nodule organogenesis program and its downregulation may represent a crucial event in the nitrate-dependent pathway leading to the inhibition of nodule organogenesis.

© 2007 The American Phytopathological Society