Of all the essential nutrients, the rate of photosynthesis is most critically limited by the availability of N. A major symptom of N limitation is the loss of chlorophyll. It is frequently accompanied by alterations in photosynthetic capacity (Terashima and Evans, 1988) and energy transduction (Rhiel et al., 1986; Saux et al., 1987; Plumley and Schmidt, 1989). N limitation also decreases the gas exchange capacity of plants that is indicative of decreased carboxylation capacity and decreased ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity and/or protein (Brown, 1978; Stitt and Krapp, 1999). Allocation ofN into Rubisco appears to be different in C3 and C4 plants(Fig. 1)(Sugiyama et al., 1984). Differences in growth strategies are reflected in the ways that in C3 and C4 plants respond to changes in the environment. C4 plants are considered to utilize light, water and N more efficiently by virtue of the evolution of an ATP-driven C02 pump that maximizes assimilation. The pump machinery expressed inC4 leaf mesophyll cells consists primarily of two key enzymes, C4-type phosphoenolpyruvate (PEP) carboxylase (C4Ppc1) and pyruvate, orthophosphate dikinase (C4Ppdk). These enzymes function in collaboration with the reductive pentose phosphate (RPP) pathway that operates in the chloroplasts of the leaf bundle sheath cells. Leaf levels of the C assimilatory enzymes including Rubisco in leaves are considered to represent an enzymatic limitation on the rate of photosynthesis. They are therefore key to the biomass accumulation and productivity of plants (Sugiyama et al., 1998). N partitioning into C assimilatory enzymes is different
in C,and C. plants in terms of responses to N. The allocation ofN into Rubisco is selectively increased by N availability in some C3 plants, whereas the allocation is decreased in maize (Brown, 1978; Sugiyama et al., 1984; Yamazaki et al., 1986; Sugiharto et al., 1990). Instead, the allocation of N into C4Ppc1 and C4Ppdk is up regulated by N availability, similar to the situation with Rubisco in C3 plants (Sugiyama et al., 1984). This may reflect the essential requirement of these enzymes for the C4 pathway of photosynthesis in maize, which is an important appendage to the RPP pathway. The N-responsive accumulation of the two enzymes is regulated at the level of protein synthesis and is accompanied by mRNA accumulation (Sugiharto et al., 1990). A similar mode ofregulation has also been demonstrated in the genes encoding carbonic anhydrase (C4Ca) in maize (Sugiharto et al., 1992a,b), alanine aminotransferase (AlaAT) (Son et al., 1992) and aspartate aminotransferase (AspAT) (Taniguchi et al., 1995) in Panicum miliaceum, which also function in the C4 pathway. Further investigations into the nature of N partitioning in C4 plants will help us to understand how plants regulate N partitioning into the C assimilating machinery.
Inorganic N sources act as signals for the regulation of expression of genes encoding proteins involved in the assimilation of C and N. Signals from N sources may be derived from the source itself, from intermediates formed during assimilation, or from other cellular constituents derived from cross-talk between metabolic pathways and even between organs and tissues. In addition, signals derived directly or indirectly from N sources may interact with other cellular signals including other nutrients and environmental stimuli. Regardless of the complex nature ofsignal formation, signaling ofN availability can lead to the modification of gene expression. Transcriptional and post-transcriptional controls have been found. Modes of regulation are based on gene expression and involve controls on transcript abundance, as well as translational controls. These form the basis for regulation of N partitioning into protein.
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