The multiple plant response to high ammonium conditions: the Lotus japonicus AMT1; 3 protein acts as a putative transceptor

Ammonia transporter RhBG initiates downstream signaling and functional responses by activating NFκB

Transceptors, solute transporters that facilitate intracellular entry of molecules and also initiate intracellular signaling events, have been primarily studied in lower-order species. Ammonia, a cytotoxic endogenous metabolite, is converted to urea in hepatocytes for urinary excretion in mammals. During hyperammonemia, when hepatic metabolism is impaired, nonureagenic ammonia disposal occurs primarily in skeletal muscle. Increased ammonia uptake in skeletal muscle is mediated by a membrane-bound, 12 transmembrane domain solute transporter, Rhesus blood group-associated B glycoprotein (RhBG). We show that in addition to its transport function, RhBG interacts with myeloid differentiation primary response-88 (MyD88) to initiate an intracellular signaling cascade that culminates in activation of NFκB. We also show that ammonia-induced MyD88 signaling is independent of the canonical toll-like receptor-initiated mechanism of MyD88-dependent NFκB activation. In silico, in vitro, and in situ experiments show that the conserved cytosolic J-domain of the RhBG protein interacts with the Toll-interleukin-1 receptor (TIR) domain of MyD88. In skeletal muscle from human patients, human-induced pluripotent stem cell-derived myotubes, and myobundles show an interaction of RhBG-MyD88 during hyperammonemia. Using complementary experimental and multiomics analyses in murine myotubes and mice with muscle-specific RhBG or MyD88 deletion, we show that the RhBG-MyD88 interaction is essential for the activation of NFkB but not ammonia transport. Our studies show a paradigm of substrate-dependent regulation of transceptor function with the potential for modulation of cellular responses in mammalian systems by decoupling transport and signaling functions of transceptors.

Interplay between nitric oxide and inorganic nitrogen sources in root development and abiotic stress responses

Nitrogen (N) is an essential nutrient for plants, and the sources from which it is obtained can differently affect their entire development as well as stress responses. Distinct inorganic N sources (nitrate and ammonium) can lead to fluctuations in the nitric oxide (NO) levels and thus interfere with nitric oxide (NO)-mediated responses. These could lead to changes in reactive oxygen species (ROS) homeostasis, hormone synthesis and signaling, and post-translational modifications of key proteins. As the consensus suggests that NO is primarily synthesized in the reductive pathways involving nitrate and nitrite reduction, it is expected that plants grown in a nitrate-enriched environment will produce more NO than those exposed to ammonium. Although the interplay between NO and different N sources in plants has been investigated, there are still many unanswered questions that require further elucidation. By building on previous knowledge regarding NO and N nutrition, this review expands the field by examining in more detail how NO responses are influenced by different N sources, focusing mainly on root development and abiotic stress responses.

Nitrogen Journey in Plants: From Uptake to Metabolism, Stress Response, and Microbe Interaction

Plants uptake and assimilate nitrogen from the soil in the form of nitrate, ammonium ions, and available amino acids from organic sources. Plant nitrate and ammonium transporters are responsible for nitrate and ammonium translocation from the soil into the roots. The unique structure of these transporters determines the specificity of each transporter, and structural analyses reveal the mechanisms by which these transporters function. Following absorption, the nitrogen metabolism pathway incorporates the nitrogen into organic compounds via glutamine synthetase and glutamate synthase that convert ammonium ions into glutamine and glutamate. Different isoforms of glutamine synthetase and glutamate synthase exist, enabling plants to fine-tune nitrogen metabolism based on environmental cues. Under stressful conditions, nitric oxide has been found to enhance plant survival under drought stress. Furthermore, the interaction between salinity stress and nitrogen availability in plants has been studied, with nitric oxide identified as a potential mediator of responses to salt stress. Conversely, excessive use of nitrate fertilizers can lead to health and environmental issues. Therefore, alternative strategies, such as establishing nitrogen fixation in plants through diazotrophic microbiota, have been explored to reduce reliance on synthetic fertilizers. Ultimately, genomics can identify new genes related to nitrogen fixation, which could be harnessed to improve plant productivity.

Agro-industrial-residues as potting media: physicochemical and biological characters and their influence on plant growth

Nursery cultivation is recognized globally as an intensive production system to support quality seedlings as well as to manage resources efficiently. Apart from other factors, potting media (PM) play a crucial role in determining the success of nursery cultivation. Worldwide, peat is the most commonly used substrate in PM because of its favorable physicochemical properties. However, due to ascending environmental and ecological concerns regarding the use of peat, a variety of new substrates have been used/tested by researchers/practitioners/growers as PM. Bark, coir pith, wood fiber, compost derived from various agro-residues, and vermicompost either alone or in combination are some of the commonly explored substrates and found to have the potential to replace peat to a greater extent. In lieu of availability, abundance, low cost, and no/low processing requirement, the use of agro-industrial residue (AIR) in the PM is the current trend. However, challenges associated with their adoption cannot be ignored. The present review is focused on providing collective information, scientific knowledge and detailed analysis of various AIR used in PM. The critical evidence-based review would help in developing a consistent approach for the identification, selection and characterization of a new renewable substrate. In addition, it would help in developing a rationale understanding of the practical and economic realities involved in the adoption of the same in PM.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13399-021-01998-6.

A proposed role for endomembrane trafficking processes in regulating tonoplast content and vacuole dynamics under ammonium stress conditions in Arabidopsis root cells

Ammonium (NH₄⁺) stress has multiple effects on plant physiology, therefore, plant responses are complex, and multiple mechanisms are involved in NH₄⁺ sensitivity and tolerance in plants. Root growth inhibition is an important quantitative readout of the effects of NH₄⁺ stress on plant physiology, and cell elongation appear as the principal growth inhibition target. We recently proposed autophagy as a relevant physiological mechanisms underlying NH₄⁺ sensitivity response in Arabidopsis. In a brief overview, the impaired macro-autophagic flux observed under NH₄⁺ stress conditions has a detrimental impact on the cellular energetic balance, and therefore on the energy-demanding plant growth. In contrast to its inhibitory effect on the autophagosomes flux to vacuole, NH₄⁺ toxicity induced a micro-autophagy-like process. Consistent with the reduced membrane flux to the vacuole related to macro-autophagy inhibition and the increased tonoplast degradation due to enhanced micro-autophagy, the vacuoles of the root cells of the NH₄⁺-stressed plants showed lower tonoplast content and a decreased perimeter/area ratio. As the endosome-to-vacuole trafficking is another important process that contributes to membrane flux toward the vacuole, we evaluated the effects of NH₄⁺ stress on this process. This allows us to propose that autophagy could contribute to vacuole development as well as possible avenues to follow for future studies.

The Lotus japonicus NPF3.1 Is a Nodule-Induced Gene That Plays a Positive Role in Nodule Functioning

Nitrogen-fixing nodules are new organs formed on legume roots as a result of the beneficial interaction with the soil bacteria, rhizobia. Proteins of the nitrate transporter 1/peptide transporter family (NPF) are largely represented in the subcategory of nodule-induced transporters identified in mature nodules. The role of nitrate as a signal/nutrient regulating nodule functioning has been recently highlighted in the literature, and NPFs may play a central role in both the permissive and inhibitory pathways controlling N₂-fixation efficiency. In this study, we present the characterization of the Lotus japonicus LjNPF3.1 gene. LjNPF3.1 is upregulated in mature nodules. Promoter studies show transcriptional activation confined to the cortical region of both roots and nodules. Under symbiotic conditions, Ljnpf3.1-knockout mutant's display reduced shoot development and anthocyanin accumulation as a result of nutrient deprivation. Altogether, LjNPF3.1 plays a role in maximizing the beneficial outcome of the root nodule symbiosis.

Post-reclamation microbial diversity and functions in hexachlorocyclohexane (HCH) contaminated soil in relation to spontaneous HCH tolerant vegetation

The toxicity, volatility and persistence of the obsolete organochlorine pesticide hexachlorocyclohexane (HCH), makes reclamation of contaminated areas a priority for the health and welfare of neighboring human communities. Microbial diversity and functions and their relation to spontaneous vegetation in post-excavation situations, are essential indicators to consider in bioaugmentation or microbe-assisted phytoremediation strategies at field scale. Our study aimed to evaluate the effects of long-term HCH contamination on soil and plant-associated microbial communities, and whether contaminated soil has the potential to act as a bacterial inoculum in post-excavation bioremediation strategies. To scrutinize the role of vegetation, the potential nitrogen fixation of free-living and symbiotic diazotrophs of the legume Lotus tenuis was assessed as a measure of nutrient cycling functions in soil under HCH contamination. Potential nitrogen fixation was generally not affected by HCH, with the exception of lower nifH gene counts in excavated contaminated rhizospheres, most probably a short-term HCH effect on early bacterial succession in this compartment. HCH shaped microbial communities in long-term contaminated bulk soil, where we identified possible HCH tolerants such as Sphingomonas and Altererythrobacter. In L. tenuis rhizosphere, microbial community composition was additionally influenced by plant growth stage. Sphingobium and Massilia were the bacterial genera characteristic for HCH contaminated rhizospheres. Long-term HCH contamination negatively affected L. tenuis growth and development. However, root-associated bacterial community composition was driven solely by plant age, with negligible HCH effect. Results showed that L. tenuis acquired possible HCH tolerant bacteria such as the Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium clade, Sphingomonas, Massilia or Pantoea which could simultaneously offer plant growth promoting (PGP) benefits for the host. Finally, we identified an inoculum with possibly HCH tolerant, PGP bacteria transferred from the contaminated bulk soil to L. tenuis roots through the rhizosphere compartment, consisting of Mesorhizobium loti, Neorhizobium galegae, Novosphingobium lindaniclasticum, Pantoea agglomerans and Lysobacter bugurensis.

Concentration-dependent physiological and transcriptional adaptations of wheat seedlings to ammonium

Conventional wheat production utilizes fertilizers of various nitrogen forms. Sole ammonium nutrition has been shown to improve grain quality, despite the potential toxic effects of ammonium at elevated concentrations. We therefore investigated the responses of young seedlings of winter wheat to different nitrogen sources (NH₄ NO₃ = NN, NH₄ Cl = NNH₄ ⁺ and KNO₃ = NNO₃ ^- ). Growth with ammonium-nitrate was superior. However, an elevated concentration of sole ammonium caused severe toxicity symptoms and significant decreases in biomass accumulation. We addressed the molecular background of the ammonium uptake by gathering an overview of the ammonium transporter (AMT) of wheat (Triticum aestivum) and characterized the putative high-affinity TaAMT1 transporters. TaAMT1;1 and TaAMT1;2 were both active in yeast and Xenopus laevis oocytes and showed saturating high-affinity ammonium transport characteristics. Interestingly, nitrogen starvation, as well as ammonium resupply to starved seedlings triggered an increase in the expression of the TaAMT1s. The presence of nitrate seamlessly repressed their expression. We conclude that wheat showed the ability to respond robustly to sole ammonium supply by adopting distinct physiological and transcriptional responses.

Evidence that exogenous urea acts as a potent cue to alleviate ammonium-inhibition of root system growth of cotton plant (Gossypium hirsutum)

Many plants grown with low-millimolar concentration of NH₄ ⁺ as a sole nitrogen source develop NH₄ ⁺ -toxicity symptoms. To date, crucial molecular identities and a practical approach involved in the improvement of plant NH₄ ⁺ -tolerance remain largely unknown. By phenotyping of upland cotton grown on varied nitrogen forms, we came across a phenomenon that caused sub-millimolar concentrations of urea (e.g., up 50 μM) to repress the growth inhibition of roots and whole plant cultivated in a NH₄ ⁺ -containing nutrient solution. A growth-recovery assay revealed that the relief in NH₄ ⁺ -inhibited growth required only a short-term exposure (≧12 h) of the roots to urea, implying that urea could elicit an internal signaling and be involved in antagonizing NH₄ ⁺ -sensitivity. Intriguingly, split-root experiments demonstrated that low urea occurrence in one root-half could efficaciously stimulate not only supplied root but also the root-half grown in NH₄ ⁺ -solution without urea, indicating the existence of urea-triggered local and systemic long-distance signaling. In the split-root experiment we also observed high arginase activity, strong arginine reduction and remarkable upregulation of polyamine biosynthesis-related genes (ADC1/2, SPDS and SPMS). Therefore, we suggest that external urea might serve as an effective cue (signal molecule) in an arginine-/polyamine-related process for ameliorating NH₄ ⁺ -suppressed root growth, providing a novel aspect for deeper exploring and understanding plant NH₄ ⁺ -tolerance.

The functional characterization of LjNRT2.4 indicates a novel, positive role of nitrate for an efficient nodule N2 -fixation activity

Atmospheric nitrogen (N₂₎ -fixing nodules are formed on the roots of legume plants as result of the symbiotic interaction with rhizobia. Nodule functioning requires high amounts of carbon and energy, and therefore legumes have developed finely tuned mechanisms to cope with changing external environmental conditions, including nutrient availability and flooding. The investigation of the role of nitrate as regulator of the symbiotic N₂ fixation has been limited to the inhibitory effects exerted by high external concentrations on nodule formation, development and functioning. We describe a nitrate-dependent route acting at low external concentrations that become crucial in hydroponic conditions to ensure an efficient nodule functionality. Combined genetic, biochemical and molecular studies are used to unravel the novel function of the LjNRT2.4 gene. Two independent null mutants are affected by the nitrate content of nodules, consistent with LjNRT2.4 temporal and spatial profiles of expression. The reduced nodular nitrate content is associated to a strong reduction of nitrogenase activity and a severe N-starvation phenotype observed under hydroponic conditions. We also report the effects of the mutations on the nodular nitric oxide (NO) production and content. We discuss the involvement of LjNRT2.4 in a nitrate-NO respiratory chain taking place in the N₂ -fixing nodules.

Function and Regulation of Ammonium Transporters in Plants

Ammonium transporter (AMT)-mediated acquisition of ammonium nitrogen from soils is essential for the nitrogen demand of plants, especially for those plants growing in flooded or acidic soils where ammonium is dominant. Recent advances show that AMTs additionally participate in many other physiological processes such as transporting ammonium from symbiotic fungi to plants, transporting ammonium from roots to shoots, transferring ammonium in leaves and reproductive organs, or facilitating resistance to plant diseases via ammonium transport. Besides being a transporter, several AMTs are required for the root development upon ammonium exposure. To avoid the adverse effects of inadequate or excessive intake of ammonium nitrogen on plant growth and development, activities of AMTs are fine-tuned not only at the transcriptional level by the participation of at least four transcription factors, but also at protein level by phosphorylation, pH, endocytosis, and heterotrimerization. Despite these progresses, it is worth noting that stronger growth inhibition, not facilitation, unfortunately occurs when AMT overexpression lines are exposed to optimal or slightly excessive ammonium. This implies that a long road remains towards overcoming potential limiting factors and achieving AMT-facilitated yield increase to accomplish the goal of persistent yield increase under the present high nitrogen input mode in agriculture.

Water impacts nutrient dose responses genome-wide to affect crop production

Changes in nutrient dose have dramatic effects on gene expression and development. One outstanding question is whether organisms respond to changes in absolute nutrient amount (moles) vs. its concentration in water (molarity). This question is particularly relevant to plants, as soil drying can alter nutrient concentration, without changing its absolute amount. To compare the effects of amount vs. concentration, we expose rice to a factorial matrix varying the dose of nitrogen (N) and water (W) over a range of combinations, and quantify transcriptome and phenotype responses. Using linear models, we identify distinct dose responses to either N-moles, W-volume, N-molarity (N/W), or their synergistic interaction (N×W). Importantly, genes whose expression patterns are best explained by N-dose and W interactions (N/W or N×W) in seedlings are associated with crop outcomes in replicated field trials. Such N-by-W responsive genes may assist future efforts to develop crops resilient to increasingly arid, low nutrient soils.

Multiple Transceptors for Macro- and Micro-Nutrients Control Diverse Cellular Properties Through the PKA Pathway in Yeast: A Paradigm for the Rapidly Expanding World of Eukaryotic Nutrient Transceptors Up to Those in Human Cells

The nutrient composition of the medium has dramatic effects on many cellular properties in the yeast Saccharomyces cerevisiae. In addition to the well-known specific responses to starvation for an essential nutrient, like nitrogen or phosphate, the presence of fermentable sugar or a respirative carbon source leads to predominance of fermentation or respiration, respectively. Fermenting and respiring cells also show strong differences in other properties, like storage carbohydrate levels, general stress tolerance and cellular growth rate. However, the main glucose repression pathway, which controls the switch between respiration and fermentation, is not involved in control of these properties. They are controlled by the protein kinase A (PKA) pathway. Addition of glucose to respiring yeast cells triggers cAMP synthesis, activation of PKA and rapid modification of its targets, like storage carbohydrate levels, general stress tolerance and growth rate. However, starvation of fermenting cells in a glucose medium for any essential macro- or micro-nutrient counteracts this effect, leading to downregulation of PKA and its targets concomitant with growth arrest and entrance into G0. Re-addition of the lacking nutrient triggers rapid activation of the PKA pathway, without involvement of cAMP as second messenger. Investigation of the sensing mechanism has revealed that the specific high-affinity nutrient transporter(s) induced during starvation function as transporter-receptors or transceptors for rapid activation of PKA upon re-addition of the missing substrate. In this way, transceptors have been identified for amino acids, ammonium, phosphate, sulfate, iron, and zinc. We propose a hypothesis for regulation of PKA activity by nutrient transceptors to serve as a conceptual framework for future experimentation. Many properties of transceptors appear to be similar to those of classical receptors and nutrient transceptors may constitute intermediate forms in the development of receptors from nutrient transporters during evolution. The nutrient-sensing transceptor system in yeast for activation of the PKA pathway has served as a paradigm for similar studies on candidate nutrient transceptors in other eukaryotes and we succinctly discuss the many examples of transceptors that have already been documented in other yeast species, filamentous fungi, plants, and animals, including the examples in human cells.

Ammonium as a signal for physiological and morphological responses in plants

Ammonium is a major inorganic nitrogen source for plants. At low external supplies, ammonium promotes plant growth, while at high external supplies it causes toxicity. Ammonium triggers rapid changes in cytosolic pH, in gene expression, and in post-translational modifications of proteins, leading to apoplastic acidification, co-ordinated ammonium uptake, enhanced ammonium assimilation, altered oxidative and phytohormonal status, and reshaped root system architecture. Some of these responses are dependent on AMT-type ammonium transporters and are not linked to a nutritional effect, indicating that ammonium is perceived as a signaling molecule by plant cells. This review summarizes current knowledge of ammonium-triggered physiological and morphological responses and highlights existing and putative mechanisms mediating ammonium signaling and sensing events in plants. We put forward the hypothesis that sensing of ammonium takes place at multiple steps along its transport, storage, and assimilation pathways.

Identification of Ftr1 and Zrt1 as iron and zinc micronutrient transceptors for activation of the PKA pathway in Saccharomyces cerevisiae

Multiple types of nutrient transceptors, membrane proteins that combine a transporter and receptor function, have now been established in a variety of organisms. However, so far all established transceptors utilize one of the macronutrients, glucose, amino acids, ammonium, nitrate, phosphate or sulfate, as substrate. This is also true for the Saccharomyces cerevisiae transceptors mediating activation of the PKA pathway upon re-addition of a macronutrient to glucose-repressed cells starved for that nutrient, re-establishing a fermentable growth medium. We now show that the yeast high-affinity iron transporter Ftr1 and high-affinity zinc transporter Zrt1 function as transceptors for the micronutrients iron and zinc. We show that replenishment of iron to iron-starved cells or zinc to zinc-starved cells triggers within 1-2 minutes a rapid surge in trehalase activity, a well-established PKA target. The activation with iron is dependent on Ftr1 and with zinc on Zrt1, and we show that it is independent of intracellular iron and zinc levels. Similar to the transceptors for macronutrients, Ftr1 and Zrt1 are strongly induced upon iron and zinc starvation, respectively, and they are rapidly downregulated by substrate-induced endocytosis. Our results suggest that transceptor-mediated signaling to the PKA pathway may occur in all cases where glucose-repressed yeast cells have been starved first for an essential nutrient, causing arrest of growth and low activity of the PKA pathway, and subsequently replenished with the lacking nutrient to re-establish a fermentable growth medium. The broadness of the phenomenon also makes it likely that nutrient transceptors use a common mechanism for signaling to the PKA pathway.

A novel Zn2 Cys6 transcription factor BcGaaR regulates D-galacturonic acid utilization in Botrytis cinerea

D-galacturonic acid (GalA) is the most abundant monosaccharide component of pectin. Previous transcriptome analysis in the plant pathogenic fungus Botrytis cinerea identified eight GalA-inducible genes involved in pectin decomposition, GalA transport and utilization. Co-expression of these genes indicates that a specific regulatory mechanism occurs in B. cinerea. In this study, promoter regions of these genes were analysed and eight conserved sequence motifs identified. The Bclga1 promoter, containing all these motifs, was functionally analysed and the motif designated GalA Responsive Element (GARE) was identified as the crucial cis-regulatory element in regulation of GalA utilization in B. cinerea. Yeast one-hybrid screening with the GARE motif led to identification of a novel Zn2 Cys6 transcription factor (TF), designated BcGaaR. Targeted knockout analysis revealed that BcGaaR is required for induction of GalA-inducible genes and growth of B. cinerea on GalA. A BcGaaR-GFP fusion protein was predominantly localized in nuclei in mycelium grown in GalA. Fluorescence in nuclei was much stronger in mycelium grown in GalA, as compared to fructose and glucose. This study provides the first report of a GalA-specific TF in filamentous fungi. Orthologs of BcGaaR are present in other ascomycete fungi that are able to utilize GalA, including Aspergillus spp., Trichoderma reesei and Neurospora crassa.

Yeast nutrient transceptors provide novel insight in the functionality of membrane transporters

In the yeast Saccharomyces cerevisiae several nutrient transporters have been identified that possess an additional function as nutrient receptor. These transporters are induced when yeast cells are starved for their substrate, which triggers entry into stationary phase and acquirement of a low protein kinase A (PKA) phenotype. Re-addition of the lacking nutrient triggers exit from stationary phase and sudden activation of the PKA pathway, the latter being mediated by the nutrient transceptors. At the same time, the transceptors are ubiquitinated, endocytosed and sorted to the vacuole for breakdown. Investigation of the signaling function of the transceptors has provided a new read-out and new tools for gaining insight into the functionality of transporters. Identification of amino acid residues that bind co-transported ions in symporters has been challenging because the inactivation of transport by site-directed mutagenesis is not conclusive with respect to the cause of the inactivation. The discovery of nontransported agonists of the signaling function in transceptors has shown that transport is not required for signaling. Inactivation of transport with maintenance of signaling in transceptors supports that a true proton-binding residue was mutagenised. Determining the relationship between transport and induction of endocytosis has also been challenging, since inactivation of transport by mutagenesis easily causes loss of all affinity for the substrate. The use of analogues with different combinations of transport and signaling capacities has revealed that transport, ubiquitination and endocytosis can be uncoupled in several unexpected ways. The results obtained are consistent with transporters undergoing multiple substrate-induced conformational changes, which allow interaction with different accessory proteins to trigger specific downstream events.

Nitrogen modulation of legume root architecture signaling pathways involves phytohormones and small regulatory molecules

Nitrogen, particularly nitrate is an important yield determinant for crops. However, current agricultural practice with excessive fertilizer usage has detrimental effects on the environment. Therefore, legumes have been suggested as a sustainable alternative for replenishing soil nitrogen. Legumes can uniquely form nitrogen-fixing nodules through symbiotic interaction with specialized soil bacteria. Legumes possess a highly plastic root system which modulates its architecture according to the nitrogen availability in the soil. Understanding how legumes regulate root development in response to nitrogen availability is an important step to improving root architecture. The nitrogen-mediated root development pathway starts with sensing soil nitrogen level followed by subsequent signal transduction pathways involving phytohormones, microRNAs and regulatory peptides that collectively modulate the growth and shape of the root system. This review focuses on the current understanding of nitrogen-mediated legume root architecture including local and systemic regulations by different N-sources and the modulations by phytohormones and small regulatory molecules.

From transporter to transceptor: signaling from transporters provokes re-evaluation of complex trafficking and regulatory controls: endocytic internalization and intracellular trafficking of nutrient transceptors may, at least in part, be governed by their signaling function

When cells are starved of their substrate, many nutrient transporters are induced. These undergo rapid endocytosis and redirection of their intracellular trafficking when their substrate becomes available again. The discovery that some of these transporters also act as receptors, or transceptors, suggests that at least part of the sophisticated controls governing the trafficking of these proteins has to do with their signaling function rather than with control of transport. In yeast, the general amino acid permease Gap1 mediates signaling to the protein kinase A pathway. Its endocytic internalization and intracellular trafficking are subject to amino acid control. Other nutrient transceptors controlling this signal transduction pathway appear to be subject to similar trafficking regulation. Transporters with complex regulatory control have also been suggested to function as transceptors in other organisms. Hence, precise regulation of intracellular trafficking in nutrient transporters may be related to the need for tight control of nutrient-induced signaling.