N fertilization not only leads to overall increased growth and biomass production, but also results in alterations in the allocation of resources and in plant morphology. Recent experiments using transgenic plants with low NR activity have revealed that nitratemediated signaling triggers at least some of these changes in resource allocation and development.
Plants growing on low nitrate supply typically display a higher root-to-shoot ratio than plants adequately fed with this nutrient (Brouwer, 1962; Van de Werf and Nagel, 1996). This functional adjustment in the allocation of dry matter tends to reduce the demand for nitrate at the root surface when the external concentration of this anion is low, helping the plant to adapt to the decrease in N availability. Nitrate availability affects both root and shoot morphogenesis, shifting biomass allocation by concomitant decreases in shoot growth and increases in root growth. Leaf area development is poor when the nitrate supply is low while the root system becomes more finely branched (Grime et al., 1991; Fichtner and Schulze, 1992). Globally, these morphological changes are not restricted to nitrate but they resemble those observed by limitations in other nutrients, e.g. phosphate or sulfate. This suggests that they are general mechanisms of adaptation to low nutrient availability rather than specific responses to nitrate or nitrate-derived signals (Clarkson and Touraine, 1994). On the other hand, changing the availability of another inorganic N source, ammonium, does not trigger similar phenotypic responses. On the contrary, the growth of various plant species is inhibited when ammonium is supplied instead of nitrate as an exclusive N source (Chaillou et al., 1986; Raab and Terry, 1994). Therefore, while plants supplied with low nitrate are able to adapt their morphology in such a way as to enable better management of low N resources, they cannot achieve this display of phenotypic flexibility in response to limiting ammonium. In a recent study, Walch-Liu et al. (2000) showed that supplying tobacco with ammonium resulted in decreased rates of cell division and cell elongation in comparison to nitrate-fed plants. These authors concluded that the effects triggered by ammonium were not due to the ammonium ion per se (that is they ruled out the 'ammonium toxicity' hypothesis), but rather to lack of nitrate. Nitrate is hence required to maintain a sufficient flux of root-to-shoot cytokinin transport, as cytokinin mediates leaf morphogenesis. Lowered concentrations of cytokinins have been observed in the xylem sap of N-deprived potato plants compared to those supplied with nitrate (Sattelmacher and Marschner, 1978). Furthermore, abscisic acid increases in the xylem sap ofnitrate-deficient plants, suggesting that changes in the hormonal balance in the xylem sap control leaf morphogenesis in response to low nitrate (Clarkson and Touraine, 1994).
The hypothesis that nitrate ions, rather than a metabolite more downstream in the N assimilatory pathway, are involved in phenotypic adaptations to changes in external nitrate concentration is consistent with the results obtained using plants affected in NR activity. The higher accumulation of nitrate observed in low-NR tobacco transformants than in N-replete wild-type plants, was accompanied by higher shoot-to-root ratios (Scheible et al., 1997a), even though the plants with low NR activity were severely N-limited with respect to organic N. More precisely, split-root experiments have shown that the inhibition of root growth was triggered by the accumulation of nitrate in the shoot, but not in the root. This is indicative of systemic regulation involving interorgan signaling. However, considering that nitrate is quasi-excluded from the sieve sap, this ion is unlikely to be the signal translocated in the phloem from shoot to root. The nature of the nitrate-related signal that is translocated to roots and the mechanisms involved in the transduction of such an inter-organ signal, remain to be elucidated. The inhibition of root growth triggered by nitrate accumulation correlates with decreased allocation of carbon to the root (Scheible et al., 1997a) and with a decrease in the number of lateral roots (Stitt and Scheible, 1998; Stitt and Feil, 2000). Interestingly, Lexa and Cheeseman (1997) did not find any difference in the shoot-to-root ratio in a NR-deficient pea mutant. This result may, however, be connected with the ability of pea roots to form nodules, even though nodule formation is inhibited in the presence of nitrate.
The responses of root growth to nitrate availability are complex. In addition to the feedback inhibition of root growth at high nitrate, low nitrate has a positive effect on root development. Indeed, localized application of low nitrate leads to a localized stimulation of lateral root proliferation (Drew and Saker, 1976; Granato and Raper, 1989; Robinson, 1994). Localized application of low nitrate leads to the proliferation of lateral roots in tobacco (Scheible et al., 1997a) and Arabidopsis thaliana (Zhang and Forde, 1998), even in genotypes with very low NR activity. This response is not accompanied by increases in the local concentrations of either amino acids or proteins (Scheible et al., 1997a). This suggests that the effect involves nitrate-mediated signaling rather than a mechanism driven by the nutrient-mediated growth stimulation alone. Recently, a nitrate-inducible MADS-box transcription factor gene (ANR1) has been identified as a component of the signaling pathway. This is involved in the stimulation of lateral root growth by localized nitrate supply in A. thaliana (Zhang and Forde, 1998). The role of ANR1 in eliciting the developmental response to nitrate has been demonstrated using reverse genetics. ANR1 -repressed lines (antisense or co-suppressed sense) lack the capacity to respond via lateral root proliferation to localized nitrate supply. In the A. thaliana wild type, nitrate -stimulated lateral root development is due to increased root elongation. This was attributed to an increase in the rate of cell production in the lateral root meristem (Zhang et al., 1999). The stimulatory effect of nitrate was blocked in the axr4 auxin-resistant mutant, indicating that nitrate and auxin share common signaling pathways or components (Zhang et al., 1999). The sensitivity of lateral root development to inhibition by high nitrate concentrations was also higher in the ANR1 antisense lines. Nitrate sensitivity increased with the degree of ANR1 repression in the transgenic lines (Zhang and Forde, 1998). This result is consistent with the existence of dual mechanisms of nitrate regulation ofroot branching. The inhibitory effect of nitrate is pronounced in ANR1 -deficient plants because the ANR1-dependent localized stimulatory effect is blocked. The development of lateral roots in the nia1nia2 NR-deficient mutants was more sensitive to inhibition by high nitrate than it was in the wildtype (Zhang et al., 1999). These observations support the hypothesis that tissue nitrate plays a role in the production of an inhibitory signal (Scheible et al., 1997a). An overall model for the regulation of root branching by two opposite signals (external nitrate and internal plant N status) has been proposed by Zhang et al. (1999; Fig. 1).
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