Kinetics of NO3(-) net fluxes in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association, as affected by the presence of NO3(-) and NH4+

Nitrogen and phosphate metabolism in ectomycorrhizas

1047 I. Introduction 1047 II. Mobilization of soil N/P by ECM fungi 1048 III. N/P uptake 1048 IV. N/P assimilation 1049 V. N/P storage and remobilization 1049 VI. Hyphal N/P efflux at the plant-fungus interface 1052 VII. Conclusion and research needs 1054 Acknowledgements 1055 References 1055 SUMMARY: Nutrient homeostasis is essential for fungal cells and thus tightly adapted to the local demand in a mycelium with hyphal specialization. Based on selected ectomycorrhizal (ECM) fungal models, we outlined current concepts of nitrogen and phosphate nutrition and their limitations, and included knowledge from Baker's yeast when major gaps had to be filled. We covered the entire pathway from nutrient mobilization, import and local storage, distribution within the mycelium and export at the plant-fungus interface. Even when nutrient import and assimilation were broad issues for ECM fungi, we focused mainly on nitrate and organic phosphorus uptake, as other nitrogen/phosphorus (N/P) sources have been covered by recent reviews. Vacuolar N/P storage and mobilization represented another focus point of this review. Vacuoles are integrated into cellular homeostasis and central for an ECM mycelium at two locations: soil-growing hyphae and hyphae of the plant-fungus interface. Vacuoles are also involved in long-distance transport. We further discussed potential mechanisms of bidirectional long-distance nutrient transport (distances from millimetres to metres). A final focus of the review was N/P export at the plant-fungus interface, where we compared potential efflux mechanisms and pathways, and discussed their prerequisites.

Uptake and metabolism of ammonium and nitrate in response to drought stress in Malus prunifolia

Using a hydroponics culture system, we monitored morphological, physiological, and molecular changes in Malus prunifolia seedlings when drought conditions induced by 5% polyethylene glycol (PEG) were combined with a low or normal supply of N (0.05 mM or 1 mM NH₄NO₃, respectively). Under either nutrient level, drought stress negatively inhibited seedling performance, as manifested by reduced photosynthesis and biomass production, decreased accumulations of total N, and inhibited root growth. Concentrations of NO₃^- and NH₄⁺ and the activities of enzymes involved in N metabolism (nitrate reductase, glutamine synthetase, and glutamate synthase) were also significantly decreased under drought stress. The net influx of NO₃^- at the surface of the fine roots declined while that of NH₄⁺ rose markedly, suggesting that the latter may play a more important role in improving drought tolerance in M. prunifolia. Consistently, two ammonium transporters (AMT1;2 and AMT4;2) were notably up-regulated in response to drought stress, whereas most genes related to nitrate uptake, reduction, and N metabolism were down-regulated. At the normal N level, PEG-treated plants showed higher values for biomass production, root growth, and N uptake/reduction when compared with plants exposed to the lower N supply. These results suggest that the negative effect of drought stress on M. prunifolia may be alleviated when more nitrogen is available.

Net ammonium and nitrate fluxes in wheat roots under different environmental conditions as assessed by scanning ion-selective electrode technique

Wheat is one of the most important food crops in the world, its availability affects global food security. In this study, we investigated variations in NH4(+) and NO3(-) fluxes in the fine roots of wheat using a scanning ion-selective electrode technique in the presence of different nitrogen (N) forms, N concentrations, and pH levels as well as under water stress. Our results show that the fine roots of wheat demonstrated maximum NH4(+) and NO3(-) influxes at 20 mm and 25 mm from the root tip, respectively. The maximal net NH4(+) and NO3(-) influxes were observed at pH 6.2 in the presence of a 1/4 N solution. We observed N efflux in two different cultivars following the exposure of roots to a 10% PEG-6000 solution. Furthermore, the drought-tolerant cultivar generally performed better than the drought-intolerant cultivar. Net NH4(+) and NO3(-) fluxes may be determined by plant growth status, but environmental conditions can also affect the magnitude and direction of N flux. Interestingly, we found that NO3(-) was more sensitive to environmental changes than NH4(+). Our results may be used to guide future hydroponic experiments in wheat as well as to aid in the development of effective fertilisation protocols for this crop.

Ectomycorrhizas with Paxillus involutus enhance cadmium uptake and tolerance in Populus × canescens

Ectomycorrhizas (EMs), which are symbiotic organs formed between tree roots and certain fungi, can mediate cadmium (Cd) tolerance of host plants, but the underlying physiological and molecular mechanisms are not fully understood. To investigate EMs mediated Cd tolerance in woody plants, Populus × canescens was inoculated with Paxillus involutus (strain MAJ) to establish mycorrhizal roots. Mycorrhizal poplars and non-mycorrhizal controls were exposed to 0 or 50 μM CdSO4 . EMs displayed higher net Cd(2+) influx than non-mycorrhizal roots. Net Cd(2+) influx was coupled with net H(+) efflux and inactivation of plasma membrane (PM) H(+) -ATPases reduced Cd(2+) uptake of EMs less than of non-mycorrhizal roots. Consistent with higher Cd(2+) uptake in EMs, in most cases, transcript levels of genes involved in Cd(2+) uptake, transport and detoxification processes were increased in EMs compared to non-mycorrhizal roots. Higher CO2 assimilation, improved nutrient and carbohydrate status, and alleviated oxidative stress were found in mycorrhizal compared to non-mycorrhizal poplars despite higher Cd(2+) accumulation. These results indicate that mycorrhizas increase Cd(2+) uptake, probably by an enlarged root volume and overexpression of genes involved in Cd(2+) uptake and transport, and concurrently enhance Po. × canescens Cd tolerance by increased detoxification, improved nutrient and carbohydrate status and defence preparedness.

Nitrogen metabolism of two contrasting poplar species during acclimation to limiting nitrogen availability

To investigate N metabolism of two contrasting Populus species in acclimation to low N availability, saplings of slow-growing species (Populus popularis, Pp) and a fast-growing species (Populus alba × Populus glandulosa, Pg) were exposed to 10, 100, or 1000 μM NH4NO3. Despite greater root biomass and fine root surface area in Pp, lower net influxes of NH4(+) and NO3(-) at the root surface were detected in Pp compared to those in Pg, corresponding well to lower NH4(+) and NO3(-) content and total N concentration in Pp roots. Meanwhile, higher stable N isotope composition (δ(15)N) in roots and stronger responsiveness of transcriptional regulation of 18 genes involved in N metabolism were found in roots and leaves of Pp compared to those of Pg. These results indicate that the N metabolism of Pp is more sensitive to decreasing N availability than that of Pg. In both species, low N treatments decreased net influxes of NH4(+) and NO3(-), root NH4(+) and foliar NO3(-) content, root NR activities, total N concentration in roots and leaves, and transcript levels of most ammonium (AMTs) and nitrate (NRTs) transporter genes in leaves and genes involved in N assimilation in roots and leaves. Low N availability increased fine root surface area, foliar starch concentration, δ(15)N in roots and leaves, and transcript abundance of several AMTs (e.g. AMT1;2) and NRTs (e.g. NRT1;2 and NRT2;4B) in roots of both species. These data indicate that poplar species slow down processes of N acquisition and assimilation in acclimation to limiting N supply.

Biotrophic transportome in mutualistic plant-fungal interactions

Understanding the mechanisms that underlie nutrient use efficiency and carbon allocation along with mycorrhizal interactions is critical for managing croplands and forests soundly. Indeed, nutrient availability, uptake and exchange in biotrophic interactions drive plant growth and modulate biomass allocation. These parameters are crucial for plant yield, a major issue in the context of high biomass production. Transport processes across the polarized membrane interfaces are of major importance in the functioning of the established mycorrhizal association as the symbiotic relationship is based on a 'fair trade' between the fungus and the host plant. Nutrient and/or metabolite uptake and exchanges, at biotrophic interfaces, are controlled by membrane transporters whose regulation patterns are essential for determining the outcome of plant-fungus interactions and adapting to changes in soil nutrient quantity and/or quality. In the present review, we summarize the current state of the art regarding transport systems in the two major forms of mycorrhiza, namely ecto- and arbuscular mycorrhiza.

LbNrt RNA silencing in the mycorrhizal symbiont Laccaria bicolor reveals a nitrate-independent regulatory role for a eukaryotic NRT2-type nitrate transporter

Fungal nitrogen metabolism plays a fundamental role in function of mycorrhizal symbiosis and consequently in nutrient cycling of terrestrial ecosystems. Despite its global ecological relevance the information on control and molecular regulation of nitrogen utilization in mycorrhizal fungi is very limited. We have extended the nitrate utilization RNA silencing studies of the model mycorrhizal basidiomycete, Laccaria bicolor, by altering the expression of LbNrt, the sole nitrate transporter-encoding gene of the fungus. Here we report the first nutrient transporter mutants for mycorrhizal fungi. Silencing of LbNrt results in fungal strains with minimal detectable LbNrt transcript levels, significantly reduced growth capacity on nitrate and altered symbiotic interaction with poplar. Transporter silencing also creates marked co-downregulation of whole Laccaria fHANT-AC (fungal high-affinity nitrate assimilation cluster). Most importantly, this effect on the nitrate utilization pathway appears independent of extracellular nitrate or nitrogen status of the fungus. Our results indicate a novel and central nitrate uptake-independent regulatory role for a eukaryotic nitrate transporter. The possible cellular mechanisms behind this regulation mode are discussed in the light of current knowledge on NRT2-type nitrate transporters in different eukaryotes.

Net fluxes of ammonium and nitrate in association with H+ fluxes in fine roots of Populus popularis

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH4(+)) and/or nitrate (NO3(-)) to improve yields. However, little is known about net NH4(+)/NO3(-) fluxes and their relation with H(+) fluxes in poplar roots. In this study, net NH4(+)/NO3(-) fluxes in association with H(+) fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH4(+) and NO3(-) fluxes was found along root tips of P. popularis. The maximal net uptake of NH4(+) and NO3(-) occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH4(+) uptake was induced by ca. 48 % with provision of NO3(-) together, but net NO3(-) uptake was inhibited by ca. 39 % with the presence of NH4(+) in poplar roots. Furthermore, inactivation of plasma membrane (PM) H(+)-ATPases by orthovanadate markedly inhibited net NH4(+)/NO3(-) uptake and even led to net NH4(+) release with NO3(-) co-provision. Linear correlations were observed between net NH4(+)/NO3(-) and H(+) fluxes in poplar roots except that no correlation was found between net NH4(+) and H(+) fluxes in roots exposed to NH4Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH4(+)/NO3(-) uptake and that net NH4(+)/NO3(-) fluxes and the interaction of net fluxes of both ions are tightly associated with H(+) fluxes in poplar roots.

fHANT-AC genes of the ectomycorrhizal fungus Laccaria bicolor are not repressed by l-glutamine allowing simultaneous utilization of nitrate and organic nitrogen sources

In boreal and temperate forest ectomycorrhizal fungi play a crucial role in nitrogen cycling by assimilating nitrogenous compounds from soil and transferring them to tree hosts. The expression profile of fHANT-AC genes, nitrate transporter (Lbnrt), nitrate reductase (Lbnr) and nitrite reductase (Lbnir), responsible for nitrate utilization in the ectomycorrhizal fungus Laccaria bicolor, was studied on variable N regimens. The three genes were shown to be under a common regulation: repressed in the presence of ammonium while growth on nitrate resulted in high transcripts accumulation. The presence of nitrate was shown not to be indispensable for activation of Laccaria fHANT-AC as also N starvation and growth on urea and l-asparagine resulted in high transcript levels. Equally high expression of Laccaria fHANT-AC genes was detected in mycelia grown on variable concentrations of l-glutamine. This finding shows that in L. bicolor N metabolite repression of fHANT-AC is not signalled via l-glutamine like described in ascomycetes. The expression patterns of Lbnrt and Lbnir were also studied in an Lbnr RNA-silenced Laccaria strain. No differences were observed on the N source regulation or the degree of transcript accumulation of these genes, indicating that the presence of high nitrate reductase activity is not a core regulator of L. bicolor fHANT-AC expression. The simultaneous utilization of nitrate and organic N sources, already suggested by high transcript levels of Laccaria fHANT-AC genes on organic N, was supported by the increase of culture medium pH as a result of nitrate transporter activity. The possible ecological and evolutionary significance of the herein reported high regulatory flexibility of Laccaria nitrate utilization pathway for ectomycorrizal fungi and the ectomycorrhizal symbiosis is discussed.

Functional properties and differential mode of regulation of the nitrate transporter from a plant symbiotic ascomycete

Nitrogen assimilation by plant symbiotic fungi plays a central role in the mutualistic interaction established by these organisms, as well as in nitrogen flux in a variety of soils. In the present study, we report on the functional properties, structural organization and distinctive mode of regulation of TbNrt2 (Tuber borchii NRT2 family transporter), the nitrate transporter of the mycorrhizal ascomycete T. borchii. As revealed by experiments conducted in a nitrate-uptake-defective mutant of the yeast Hansenula polymorpha, TbNrt2 is a high-affinity transporter (K(m)=4.7 microM nitrate) that is bispecific for nitrate and nitrite. It is expressed in free-living mycelia and in mycorrhizae, where it preferentially accumulates in the plasma membrane of root-contacting hyphae. The TbNrt2 mRNA, which is transcribed from a single-copy gene clustered with the nitrate reductase gene in the T. borchii genome, was specifically up-regulated following transfer of mycelia to nitrate- (or nitrite)-containing medium. However, at variance with the strict nitrate-dependent induction commonly observed in other organisms, TbNrt2 was also up-regulated (at both the mRNA and the protein level) following transfer to a nitrogen-free medium. This unusual mode of regulation differs from that of the adjacent nitrate reductase gene, which was expressed at basal levels under nitrogen deprivation conditions and required nitrate for induction. The functional and expression properties, described in the present study, delineate TbNrt2 as a versatile transporter that may be especially suited to cope with the fluctuating (and often low) mineral nitrogen concentrations found in most natural, especially forest, soils.