JXB Advance Access published online on April 11, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern034
RESEARCH PAPER |
Evidence for sugar signalling in the regulation of asparagine synthetase gene expressed in Phaseolus vulgaris roots and nodules
1Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Av. Universidad 2001, Colonia Chamilpa, Cuernavaca, CP 62210, Morelos, México
2Instituto de Biotecnología, Universidad Nacional Autónoma de México, Av. Universidad 2001, Colonia Chamilpa, Cuernavaca, CP 62210, Morelos, México
To whom correspondence should be addressed. E-mail: lara{at}ccg.unam.mx
A cDNA clone, designated as PvNAS2, encoding asparagine amidotransferase (asparagine synthetase) was isolated from nodule tissue of common bean (Phaseolus vulgaris cv. Negro Jamapa). Southern blot analysis indicated that asparagine synthetase in bean is encoded by a small gene family. Northern analysis of RNAs from various plant organs demonstrated that PvNAS2 is highly expressed in roots, followed by nodules in which it is mainly induced during the early days of nitrogen fixation. Investigations with the PvNAS2 promoter gusA fusion revealed that the expression of PvNAS2 in roots is confined to vascular bundles and meristematic tissues, while in root nodules its expression is solely localized to vascular traces and outer cortical cells encompassing the central nitrogen-fixing zone, but never detected in either infected or non-infected cells located in the central region of the nodule. PvNAS2 is down-regulated when carbon availability is reduced in nodules, and the addition of sugars to the plants, mainly glucose, boosted its induction, leading to the increased asparagine production. In contrast to PvNAS2 expression and the concomitant asparagine synthesis, glucose supplement resulted in the reduction of ureide content in nodules. Studies with glucose analogues as well as hexokinase inhibitors suggested a role for hexokinase in the sugar-sensing mechanism that regulates PvNAS2 expression in roots. In light of the above results, it is proposed that, in bean, low carbon availability in nodules prompts the down-regulation of the asparagine synthetase enzyme and concomitantly asparagine production. Thereby a favourable environment is created for the efficient transfer of the amido group of glutamine for the synthesis of purines, and then ureide generation.
Key words: Asparagine and ureide synthesis, asparagine synthetase, nodules, Phaseolus vulgaris, sugar signalling
* These authors contributed equally to this work.
Received 21 September 2007; Revised 22 January 2008 Accepted 23 January 2008