The aim of our research is to understand the molecular processes of plant nitrogen nutrition, with a focus on amino acid transport. We seek to understand how transport of amino acids contributes to nitrogen use efficiency and to optimization of plant growth. To accomplish this, we use a methodologically integrative approach, ranging from cell and molecular biology to ecophysiology.

Ulrika Ganetag 1150Nitrogen availability is considered a bottleneck for plant biomass production in terrestrial ecosystems. In addition, the demand for nitrogen in different parts of the plant varies during different stages of plant growth and development. Therefore, the ability to appropriately allocate and subsequently re-use nitrogen, together with nitrogen uptake, are the major determinants of nitrogen use efficiency, and hence for optimized plant growth. Being the major form of trans- ported nitrogen, amino acids are the currency of nitrogen within plants. There is therefore a complex network of transport of amino acids during plant growth and development, implying a demand for an intricate and well-orchestrated transport system. How this network is regulated to optimize nitrogen use efficiency in response to developmental and environmental cues during plant growth is at present not very well known. Besides being a key determinant for plant growth, nitrogen is also regarded as a major pollutant, resulting for example in changes in biodiversity. In addition, human perturbation of the global nitrogen cycle is the second largest driver of global climate change. Plant uptake of nitrogen from the soil and internal nitrogen fluxes are key processes in the global nitrogen cycle. Therefore, the molecular dissection of amino acid transport in plants has broad significance for our understanding of how nitrogen fluxes contribute to the overall plant nitrogen budget. This knowledge is also important from a more applied perspective, opening up new avenues to optimize nitrogen fertilization in both agriculture and forestry.

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Arabidopsis plants deficient in the amino acid transporter LHT1 Arabidopsis plants deficient in the amino acid transporter AAP5 (upper left corner) are insensitive to toxic concentrations of arginine.

We have identified two candidate Arabidopsis amino acid transporters, LHT1 and AAP5, for involvement in root uptake of amino acids from the soil solution. The process of amino acid uptake in plants has been demonstrated both in laboratory and field settings, and has thus been well established. However, the ecological significance of organic Nitrogen uptake for plant nitrogen nutrition is still a matter of intense debate. One of our projects aims to resolve the molecular mechanisms and ecological significance of root amino acid uptake. To accomplish this we use a collection of transgenic Arabidopsis and Populus with different amino acid uptake profiles in our studies.
We also want to understand how the processes of nitrogen allocation and remobilization contribute to nitrogen use efficiency in plants. This work focuses on how amino acid transporters are orchestrated to allocate and redistribute nitrogen in response to developmental and environmental cues. Being mediated by a gene family comprised of 50 genes or more, the investigation of amino acid transport is a complex task (at least 53 and 90 genes have been annotated as putative amino acid/auxin permeases in the Arabidopsis and Populus genomes, respectively). It is believed that amino acid transporters are functionally separated by their substrate specificity and their temporal and spatial expression patterns. However, many amino acid transporters have been shown to have multiple functions in plants. Similarly, LHT1 has been identified as being involved in redistribution of amino acids in leaf mesophyll cells, besides its function in root amino acid uptake. We are investigating the role of LHT1 and a number of other key amino acid transporters in the allocation and remobilization of amino acids. Wild type and transgenic Populus are being analyzed with respect to, for example, gene expression, nutrient uptake, growth and nitrogen allocation, in response to developmental and environmental cues under controlled conditions and in the field.
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