Torgny Näsholm - Soil Nitrogen Sources and Plant Nitrogen Acquisition

Traditionally, research on plant nitrogen nutrition has had a strong focus on the inorganic nitrogen sources ammonium and nitrate. Their abundance in soils, their rapid uptake and the ease by which they can be used to cultivate plants have constituted a strong motivation for this focus. However, recent studies have emphasised the ubiquitous competence of plants, both mycorrhizal and non-mycorrhizal, to uptake various organic nitrogen sources, in particular amino acids. This raises the fundamental question; what nitrogen sources do plants acquire from soils? We argue that the answer to this question depends on the two fundamental steps in plant nitrogen nutrition: (i) what nitrogen sources are plant roots encountering in the soil and (ii) to what extent are these nitrogen sources absorbed? (i): We have adopted a method, microdialysis, originally developed for monitoring neurotransmitters in rodent brains, to study soil nitrogen dynamics. Microdialysis probes share important characteristics with plant fine roots and allows for low-invasive monitoring of solutes in soil. We argue that soil microdialysis may give important insights regarding the nitrogen sources encountered by roots. Results from in situ microdialysis of soil nitrogen in boreal forests as well as in subtropical sugarcane fields will be discussed. (ii): Several transporters have been implicated as responsible for root uptake of amino acids. The Lysine Histidine Transporter 1 (LHT1) was shown to mediate root uptake of neutral and acidic amino acids by Hirner et al. (2006). Studies in our lab have confirmed the importance of LHT1 for plant amino acid nutrition and also shown that Amino Acid Permease 5 complements LHT1 by mediating uptake of basic amino acids. We have used Arabidopsis LHT1 mutants to examine the importance of amino acids in plant nitrogen nutrition. Results from these studies will be presented.