The Occurrence of Amino Acids in Agricultural Soil and their Uptake by Plants

The ability of plants to take up amino acids is widespread among plants, but the ecological and physiological implications of this ability are not fully understood. Therefore, in the investigations this thesis is based upon, key aspects of the uptake of amino acids by agricultural plants were explored in field studies (to ensure ecological relevance) and laboratory analyses (to ensure precision). Small tension lysimeters were used to collect soil solution from several agricultural soils with minimal disturbance. Concentrations of free amino acids were found to be low (012.7 μM). However, they may be continuously replenished from bound amino acid pools and were found to be sufficiently high (generally) for uptake by hydroponically grown barley, Hordeum vulgare L., and Arabidopsis, Arabidopsis thaliana L. Hence, the effective minimum concentrations for uptake by these species do not seem to exceed most of the field-measured concentrations. The uptake affinity in both barley and Arabidopsis was found to be comparable to reported values for nitrate at corresponding concentrations and for uptake of amino acids by soil micro-organisms. The amino acid transporters lysine histidine transporter 1 (LHT1) and amino acid permease 5 (AAP5) were found to be largely responsible for amino acid uptake in Arabidopsis at these concentrations. These transporters have complementary affinities for amino acids with differing properties; LHT1 transporting acidic and neutral amino acids, and AAP5 basic amino acids. Furthermore, the gene expression of LHT1 and AAP5 clearly increased after roots were exposed to amino acids, even in the presence of inorganic nitrogen, resulting in up to 15-fold increases in the rate of amino acid uptake. The induced amino acid uptake rates were up to 10-fold higher than nitrate uptake rates in Arabidopsis. According to standard textbooks, nitrate and ammonium are the major nitrogen sources for plants. However, the results of these studies indicate that plants have the capacity to take up amino acids at field concentrations in presence of nitrate and ammonium. This capacity requires gene expression, synthesis and regulation of amino acid transporters, and the ability of plants to sense and respond to amino acid concentrations at ambient concentrations. There is, therefore, little doubt that plants can take up amino acids in their natural environment. Thus, it is time to reconsider traditional views of the nitrogen compounds used by agricultural plants.