Root-secreted chemicals mediate multi-partite interactions in the rhizosphere, where plant roots continually respond to and alter their immediate environment. Increasing evidence suggests that root exudates initiate and modulate dialogue between roots and soil microbes. For example, root exudates serve as signals that initiate symbiosis with rhizobia and mycorrhizal fungi. In addition, root exudates maintain and support a highly specific diversity of microbes in the rhizosphere of a given particular plant species, thus suggesting a close evolutionary link. In this review, we focus mainly on compiling the information available on the regulation and mechanisms of root exudation processes, and provide some ideas related to the evolutionary role of root exudates in shaping soil microbial communities.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2009.01926.x

Regulation and function of root exudates.

Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

†Background Productive agriculture needs a large amount of expensive nitrogenous fertilizers. Improving nitrogen use efficiency (NUE) of crop plants is thus of key importance. NUE definitions differ depending on whether plants are cultivated to produce biomass or grain yields. However, for most plant species, NUE mainly depends on how plants extract inorganic nitrogen from the soil, assimilate nitrate and ammonium, and recycle organic nitrogen. Efforts have been made to study the genetic basis as well as the biochemical and enzymatic mechanisms involved in nitrogen uptake, assimilation, and remobilization in crops and model plants. The detection of the limiting factors that could be manipulated to increase NUE is the major goal of such research. †Scope An overall examination of the physiological, metabolic, and genetic aspects of nitrogen uptake, assimilation and remobilization is presented in this review. The enzymes and regulatory processes manipulated to improve NUE components are presented. Results obtained from natural variation and quantitative trait loci studies are also discussed. †Conclusions This review presents the complexity of NUE and supports the idea that the integration of the numerous data coming from transcriptome studies, functional genomics, quantitative genetics, ecophysiology and soil science into explanatory models of whole-plant behaviour will be promising.

REVIEW: PART OF A SPECIAL ISSUE ON PLANT NUTRITION Nitrogen uptake, assimilation and remobilization in plants: challenges for sustainable and productive agriculture

https://www.zora.uzh.ch/id/eprint/148899/1/Sasse_TIPS_2017.pdf

Feed Your Friends: Do Plant Exudates Shape the Root Microbiome?

Languishing for many years in the shadow of plant inorganic nitrogen (N) nutrition research, studies of organic N uptake have attracted increased attention during the last decade. The capacity of plants to acquire organic N, demonstrated in laboratory and field settings, has thereby been well established. Even so, the ecological significance of organic N uptake for plant N nutrition is still a matter of discussion. Several lines of evidence suggest that plants growing in various ecosystems may access organic N species. Many soils display amino acid concentrations similar to, or higher than, those of inorganic N, mainly as a result of rapid hydrolysis of soil proteins. Transporters mediating amino acid uptake have been identified both in mycorrhizal fungi and in plant roots. Studies of endogenous metabolism of absorbed amino acids suggest that L- but not D-enantiomers are efficiently utilized. Dual labelled amino acids supplied to soil have provided strong evidence for plant uptake of organic N in the field but have failed to provide information on the quantitative importance of this process. Thus, direct evidence that organic N contributes significantly to plant N nutrition is still lacking. Recent progress in our understanding of the mechanisms underlying plant organic N uptake may open new avenues for the exploration of this subject.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02751.x

Uptake of organic nitrogen by plants.

Plant nitrogen (N) uptake is a key process in the global N cycle and is usually considered a “bottleneck” for biomass production in land ecosystems. Earlier, mineral N was considered the only form available to plants. Recent studies have questioned this dogma and shown that plants may access organic N sources such as amino acids. The actual mechanism enabling plants to access amino acid N is still unknown. However, a recent study suggested the Lysine Histidine Transporter 1 (LHT1) to be involved in root amino acid uptake. In this study, we isolated mutants defective in root amino acid uptake by screening Arabidopsis (Arabidopsis thaliana) seeds from ethyl methanesulfonate-treated plants and seeds from amino acid transporter T-DNA knockout mutants for resistance against the toxic d-enantiomer of alanine (Ala). Both ethyl methanesulfonate and T-DNA knockout plants identified as d-Ala resistant were found to be mutated in the LHT1 gene. LHT1 mutants displayed impaired capacity for uptake of a range of amino acids from solutions, displayed impaired growth when N was supplied in organic forms, and acquired substantially lower amounts of amino acids than wild-type plants from solid growth media. LHT1 mutants grown on mineral N did not display a phenotype until at the stage of flowering, when premature senescence of old leaf pairs occurred, suggesting that LHT1 may fulfill an important function at this developmental stage. Based on the broad and unbiased screening of mutants resistant to d-Ala, we suggest that LHT1 is an important mediator of root uptake of amino acids. This provides a molecular background for plant acquisition of organic N from the soil.

Comprehensive Screening of Arabidopsis Mutants Suggests the Lysine Histidine Transporter 1 to Be Involved in Plant Uptake of Amino Acids

Summary
• Specific transporters mediate uptake of amino acids by plant roots. Earlier studies have indicated that the lysine histidine transporter 1 and amino acid permease 1 participate in this process, but although plant roots have been shown to absorb cationic amino acids with high affinity, neither of these transporters seems to mediate transport of L-arginine (L-Arg) or L-lysine (L-Lys).
• Here, a collection of T-DNA knockout mutants were screened for alterations in Arabidopsis root uptake rates of L-Arg and it was found that only the AAP5 mutant displayed clear phenotypic divergence on high concentrations of L-Arg. A second screen using low concentrations of 15N-labelled L-Arg in the growth media also identified AAP5 as being involved in L-Arg acquisition.
• Momentaneous root uptake of basic amino acids was strongly affected in AAP5 mutant lines, but their uptake of other types of amino acids was only marginally affected. Comparisons of the root uptake characteristics of AAP5 and LHT1 mutants corroborated the hypothesis that the two transporters have distinct affinity spectra in planta.
• Root uptake of all tested amino acids, except L-aspartic acid (L-Asp), was significantly affected in double AAP5*LHT1 mutants, suggesting that these two transporters account for a major proportion of roots’ uptake of amino acids at low concentrations.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2008.02589.x

Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease 5

Summary
• Various amino acids, including both L- and D-enantiomers, may be present in soils, and recent studies have indicated that plants may access such nitrogen (N) forms. Here, the capacity of Arabidopsis to utilize different L- and D-amino acids is investigated and the constraints on this process are explored.
• Mutants defective in the lysine histidine transporter 1 (LHT1) and transgenic plants overexpressing LHT1 as well as plants expressing D-amino acid-metabolizing enzymes, were used in studies of uptake and growth on various N forms.
• Arabidopsis absorbed all tested N-forms, but D-enantiomers at lower rates than L-forms. Several L- but no D-forms were effective as N sources. Plants deficient in LHT1 displayed strong growth reductions and plants overexpressing LHT1 showed strong growth enhancement when N was supplied as amino acids, in particular when these were supplied at low concentrations. Several D- amino acids inhibited growth of wild-type plants, while transgenic Arabidopsis-expressing genes encoding D-amino acid-metabolizing enzymes could efficiently utilize such compounds for growth.
• These results suggest that several amino acids, and in particular L-Gln and L-Asn, promote growth of Arabidopsis, and increased expression of specific amino acid transporters enhances growth on amino acids. The efficiency by which transgenic plants exploit D-amino acids illustrates how plants can be engineered to utilize specific N sources otherwise inaccessible to them.

Capacities and constraints of amino acid utilization in Arabidopsis.

Recent studies of Arabidopsis have identified several transporters as being important for amino acid uptake. We used Arabidopsis plants with altered expression of lysine histidine transporter 1 (LHT1), amino acid permease 1 (AAP1) and amino acid permease 5 (AAP5) with the aim of disentangling the roles of each transporter in the uptake of different amino acids at naturally occurring concentrations (2-50 μM). LHT1 mutants displayed reduced uptake rates of L-Gln, L-Ala, L-Glu and L-Asp but not of L-Arg or L-Lys, while AAP5 mutants were affected in the uptake of L-Arg and L-Lys only. Double mutants (lht1aap5) exhibited reduced uptake of all tested amino acids. In the concentration range tested, AAP1 mutants did not display altered uptake rates for any of the studied amino acids. Expression analysis of amino acid transporter genes with important root functions revealed no major differences in the individual mutants other than for genes targeted for mutation. We conclude that LHT1 and AAP5, but not AAP1, are crucial for amino acid uptake at concentrations typically found in soils. LHT1 and AAP5 displayed complementary affinity spectra, and no redundancy with respect to gene expression was found between the two transporters, suggesting these two transporters have separate roles in amino acid uptake.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2011.03699.x

Ulrika Ganeteg

Papers

Uptake of organic nitrogen by plants.

Comprehensive Screening of Arabidopsis Mutants Suggests the Lysine Histidine Transporter 1 to Be Involved in Plant Uptake of Amino Acids

Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease 5

Capacities and constraints of amino acid utilization in Arabidopsis.

Transporters in Arabidopsis roots mediating uptake of amino acids at naturally occurring concentrations