Pia Walch-Liu

Nitrate signalling mediated by the NRT1.1 nitrate transporter antagonises l‐glutamate‐induced changes in root architecture

Arabidopsis root architecture is highly responsive to changes in the nitrogen supply. External NO(3)(-) stimulates lateral root growth via a signalling pathway involving the ANR1 MADS box transcription factor, while the presence of exogenous l-glutamate (Glu) at the primary root tip slows primary root growth and stimulates root branching. We have found that NO(3)(-), in conjunction with Glu, has a hitherto unrecognized role in regulating the growth of primary roots. Nitrate was able to stimulate primary root growth, both directly and by antagonising the inhibitory effect of Glu. Each response depended on direct contact between the primary root tip and the NO(3)(-), and was not elicited by an alternative N source (NH(4)(+)). The chl1-5 mutant, which is defective in the NRT1.1 (CHL1) NO(3)(-) transporter, was insensitive to NO(3)(-) antagonism of Glu signalling, while an anr1 mutant retained its sensitivity. Sensitivity to NO(3)(-) was restored in a chl1-5 mutant constitutively expressing NRT1.1. However, expression in chl1-5 of a transport-competent but non-phosphorylatable form of NRT1.1 not only failed to restore NO(3)(-) sensitivity but also had a dominant-negative effect on Glu sensitivity. Our results indicate the existence of a NO(3)(-) signalling pathway at the primary root tip that can antagonise the roots response to Glu, and they further suggest that NRT1.1 has a direct NO(3)(-) sensing role in this pathway. We discuss how the observed signalling interactions between NO(3)(-) and Glu could provide a mechanism for modulating root architecture in response to changes in the relative abundance of organic and inorganic N.

Signaling mechanisms integrating root and shoot responses to changes in the nitrogen supply

During their life cycle, plants must be able to adapt to wide variations in the supply of soil nitrogen (N). Changes in N availability, and in the relative concentrations of NO3−and NH4+, are known to have profound regulatory effects on the N uptake systems in the root, on C and N metabolism throughout the plant, and on root and shoot morphology. Optimising the plant’s responses to fluctuations in the N supply requires co-ordination of the pathways of C and N assimilation, as well as establishment of the appropriate allocation of resources between root and shoot growth. Achieving this integration of responses at the whole plant level implies long-distance signaling mechanisms that can communicate information about the current availability of N from root-to-shoot, and information about the C/N status of the shoot in the reverse direction. In this review we will discuss recent advances which have contributed to our understanding of these long-range signaling pathways.

Nitrate and glutamate sensing by plant roots

The architecture of a root system plays a major role in determining how efficiently a plant can capture water and nutrients from the soil. Growth occurs at the root tips and the process of exploring the soil volume depends on the behaviour of large numbers of individual root tips at different orders of branching. Each root tip is equipped with a battery of sensory mechanisms that enable it to respond to a range of environmental signals, including nutrients, water potential, light, gravity and touch. We have previously identified a MADS (MCM1, agamous, deficiens and SRF) box gene (ANR1) in Arabidopsis thaliana that is involved in modulating the rate of lateral root growth in response to changes in the external NO(3)(-) supply. Transgenic plants have been generated in which a constitutively expressed ANR1 protein can be post-translationally activated by treatment with dexamethasone (DEX). When roots of these lines are treated with DEX, lateral root growth is markedly stimulated but there is no effect on primary root growth, suggesting that one or more components of the regulatory pathway that operate in conjunction with ANR1 in lateral roots may be absent in the primary root tip. We have recently observed some very specific effects of low concentrations of glutamate on root growth, resulting in significant changes in root architecture. Experimental evidence suggests that this response involves the sensing of extracellular glutamate by root tip cells. We are currently investigating the possible role of plant ionotropic glutamate receptors in this sensory mechanism.

Response of shoot and root growth to supply of different nitrogen forms is not related to carbohydrate and nitrogen status of tobacco plants

In the present study, we investigated effects of homogeneous or localized supply of different nitrogen (N) forms on shoot and root growth of tobacco. While homogeneous supply of NH4+ and N deprivation inhibited shoot growth compared with application of NO3—, the N form had no significant effect on root growth. In contrast, in a split-root experiment, application of NH4+ or N deprivation in one half of the root system repressed root growth compared with the other part of the root, which was supplied with NO3—. However, shoot growth was not affected by localized NH4+ application or local N deprivation. Inhibitory effects on shoot and root growth by variations of N supply could not be related to limitations in N or C status of the plants or to NH4+ toxicity. A possible involvement of NO3— as a signal compound including of phytohormones is discussed. Reaktionen des Spross- und Wurzel-wachstums auf verschiedene N-Formen bei Tabak sind unabhangig vom C- und N-Status der Pflanzen In der vorliegenden Arbeit wurde der Einfluss von homogenem und lokalem Angebot verschiedener N-Formen auf das Spross- und Wurzelwachstum bei Tabak untersucht. Wahrend homogenes NH4+-Angebot oder N-Mangel im Vergleich zu NO3—-Ernahrung das Sprosswachstum verminderte, wurde die Wurzelbiomasse von der Form des N-Angebotes nicht beeinflusst. Im Gegensatz dazu wurde in einem Split-Root Experiment das Wurzelwachstum durch halbseitiges NH4+-Angebot oder einseitigen N-Entzug im Vergleich zur Wurzelhalfte mit NO3—-Angebot gehemmt. Das Sprosswachs-tum dagegen wurde bei lokalem Angebot von NH4+- oder einseitigem N-Entzug kaum beeintrachtigt. Die beschriebenen Hemmwirkungen auf das Spross- und Wurzelwachstum bei unterschiedlichem N-Angebot konnten nicht auf einen limitierten C- oder N-Status der Pflanzen oder auf Ammoniumtoxizitat zuruckgefuhrt werden. Eine mogliche Beteiligung von NO3— als Signalsubstanz und von Phytohormonen wird diskutiert.

L-Glutamate as a Novel Modifier of Root Growth and Branching: What's the Sensor?

Exogenous L-glutamate (L-Glu) has been shown to be able to elicit major changes in Arabidopsis root architecture at micromolar concentrations. The root response, which is strongly genotype-dependent, is specific to L-Glu and involves both inhibition of primary root growth and stimulation of root branching behind the primary root tip. The L-Glu appears to be sensed directly at the root tip, where it inhibits meristematic activity. An intriguing and still unanswered question is whether members of the family of Glu receptor-like genes (GLRs) have a role in mediating this response. A pharmacological approach described here, using agonists and antagonists of mammalian ionotropic Glu receptors, has failed to resolve the issue. Progress towards identifying the genes involved in the root response to L-Glu is likely to come through the application of forward and reverse genetics, in combination with quantitative trait loci (QTL) mapping.

Elevated atmospheric CO2 concentration favors nitrogen partitioning into roots of tobacco plants under nitrogen deficiency by decreasing nitrogen demand of the shoot

Anthropogenic increase of atmospheric CO2 concentration is likely to affect plant growth in natural and agricultural ecosystems. Since nitrogen (N) is one of the major factors limiting agricultural plant production, we investigated the effect of elevated atmospheric CO2 concentration on N partitioning at the whole-plant level and the cellular level at limited N supply. Tobacco was grown at ambient (400 ppm) and elevated (800 ppm) concentrations of atmospheric CO2 under conditions of defined N supply with the same amount of N supplied to all plants, independent of CO2-induced changes of the actual growth rate. Under conditions of N deficiency, high CO2 concentration promoted root growth whereas shoot growth was only slightly increased, which resulted in an increased root/shoot ratio. At low N supply, elevated atmospheric CO2 concentration decreased N concentrations in the shoot tissue, but not in roots. Obviously elevated CO2 supply stimulated N partitioning into roots relative to the shoots, which coincided with relatively stronger root growth. At the cellular level, Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) protein decreased under N deficiency and elevated CO2. This was associated with increasing starch concentrations, while sugar concentrations were not affected. We suggest that root growth under N limitation is restricted by the internal N availability rather than by other factors such as carbon supply. The present results suggest that N partitioning into roots is favored by elevated CO2 supply due to a decreased N demand of the shoot.

QTL analysis of the developmental response to L-glutamate in Arabidopsis roots and its genotype-by-environment interactions

A major QTL controlling glutamate sensitivity of Arabidopsis root development has been mapped and additional loci identified that epistatically regulate its response to environmental changes.

Pflanzenartenunterschiede im Wurzelwachstum bei Verschiedener N-Ernährung: N-Form-Effekt und/oder pH-Effekt?

The form of N supply may influence root growth not only by physiological effects within the plants such as carbohydrate demand for N assimilation, but also indirectly by changes in rhizosphere pH. The aim of the present study was to distinguish between these mechanisms in various plant species (tobacco, potato, rice) by growing the plants in nutrient solution culture at pH 4.5 or 7 with either NO3 − or NH4 + supply. In tobacco and potato, the impairment of root extension growth in NO3 − fed plants was caused by the increase in the rhizosphere pH, and accordingly was disposed by growing the plants at pH 4.5. Reversely, roots of NH4 +-fed plants which showed good extension growth at low pH became stunted when the pH was raised to 7. In contrast to tobacco and potato, root growth of rice was neither influenced by the form of N supply nor by pH. The ability of rice for good root extrusion at pH 7 even when supplied with NO3 − was attributed to the fact that the rizosphere of nodal roots was acidified under these conditions.