It is not clear whether nitrate per se is involved in the activation of PEPcase and inactivation of SPS. These changes may be due to metabolites formed during nitrate assimilation. SPS is inactivated when ammonium is supplied to detached leaves (Champigny et al, 1992), and glutamine promotes the light activation of PEPcase kinase and subsequent phosphorylation and activation of PEPcase in tobacco leaves (Li et al, 1996).

In tobacco transformants with very low nitrate reductase activity it has been shown that nitrate acts as a signal to induce organic acid metabolism and repress starch metabolism (Scheible et al, 1997). These transformants resemble nitrate-limited wildtype plants except they accumulate large amounts of nitrate. Nitrate induces genes that encode NR, NiR, Fd-GOGAT, a cytosolic GS, PEPcase, cytosolic pyruvate kinase, citrate synthase, and NADP-isocitrate dehydrogenase. Nitrate decreased ADP-glucose pyrophosphorylase transcript abundance and activity, and decreased starch in leaves and roots. Thus, nitrate acts as a signal to initiate coordinated changes in carbon and nitrogen metabolism (Scheible et al, 1997).

Champigny M-L, Foyer C 1992 Nitrate activation of cytosolic protein kinases diverts photosynthetic carbon from sucrose to amino acid biosynthesis. Plant Physiol. 100: 7-12.

Li B, Zhang X-Q, Chollet R 1996 Phosphoenolpyruvate carboxylase kinase in tobacco leaves is activated by light in a similar but not identical way as in maize. Plant Physiol. 111: 497-505.

Scheible W-R, Gonzalez-Fontes A, Lauerer M, Muller-Rober B, Caboch M, Stitt M 1997 Nitrate acts as a signal to induce organic acid metabolism and repress starch metabolism in tobacco. Plant Cell 9: 783-798.