Expression map for genes involved in nitrogen and carbon metabolism in alfalfa root nodules

During root nodule development several key genes involved in nitrogen fixat ion and assimilation exhibit enhanced levels of expression. However, little is known about the temporal and spatial distribution patterns of these tra nscripts. In a systematic study the transcripts for 13 of the essential enz ymes involved in alfalfa (Medicago sativa) root nodule nitrogen and carbon metabolism were localized by in situ hybridization, A serial section approa ch allowed the construction of a map that reflects the relative distributio n of these transcripts. In 33-day-old root nodules, the expression of nifH, NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14) and a cytosoli c isoform of glutamine synthetase (GS13; GS; EC 6.3.1.2) were localized pre dominantly in a 5- to 15-cell-wide region in the distal part of the nitroge n-fixing zone. This zone was also the region of high expression for leghemo globin, a second cytosolic glutamine synthetase isoform (GS100), aspartate aminotransferase-2 (AAT-2; EC 2.6.1.1), asparagine synthetase (AS; 6.3.5.4) , phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), and sucrose synthase (SuSy; EC 2.4.1.13). This suggests that, in 33-day-old alfalfa root nodule s, nitrogen fixation is restricted to this 5- to 15-cell-wide area. The con tinued significant expression of the GS100 subclass of GS and AS in the pro ximal part of the nitrogen-fixing zone implicates these gene products in ni trogen remobilization. A low constitutive expression of NADH-dependent glut amate dehydrogenase (NADH-GDH; EC 1.4.1.2) was observed throughout the nodu le, The transcript distribution map will be used as a navigational tool to assist in developing strategies for the genetic engineering of alfalfa root nodules for enhanced nitrogen assimilation.