Altered Xylem-Phloem Transfer of Amino Acids Affects Metabolism and Leads to Increased Seed Yield and Oil Content in Arabidopsis  

Abstract

Seed development and nitrogen (N) storage depend on delivery of amino acids to seed sinks. For efficient translocation to seeds, amino acids are loaded into the phloem in source leaves and along the long distance transport pathway through xylem-phloem transfer. We demonstrate that Arabidopsis thaliana AMINO ACID PERMEASE2 (AAP2) localizes to the phloem throughout the plant. AAP2 T-DNA insertion lines showed changes in source-sink translocation of amino acids and a decrease in the amount of seed total N and storage proteins, supporting AAP2 function in phloem loading and amino acid distribution to the embryo. Interestingly, in aap2 seeds, total carbon (C) levels were unchanged, while fatty acid levels were elevated. Moreover, branch and silique numbers per plant and seed yield were strongly increased. This suggests changes in N and C delivery to sinks and subsequent modulations of sink development and seed metabolism. This is supported by tracer experiments, expression studies of genes of N/C transport and metabolism in source and sink, and by phenotypic and metabolite analyses of aap2 plants. Thus, AAP2 is key for xylem to phloem transfer and sink N and C supply; moreover, modifications of N allocation can positively affect C assimilation and source-sink transport and benefit sink development and oil yield.

Cytosolic Glutamine Synthetase Gln1;2 Is the Main Isozyme Contributing to GS1 Activity and Can Be Up-Regulated to Relieve Ammonium Toxicity

“…This localization indicates that Gln-1;2 plays a role in export of N from developed source leaves to developing sink leaves. After leaf development, ammonium liberated from protein turnover and amino acid catabolism is assimilated into Gln, which is translocated in the vascular bundles to new sinks (Zhang et al, 2010;Pratelli et al, 2012). Among the new sinks are trichomes that are the first cells forming during new leaf development (Zhou et al, 2013).…”

Improving Plant Nitrogen Use Efficiency through Alteration of Amino Acid Transport Processes

“…Nitrogen utilization efficiency (NUtE) is defined as total seed yield relative to total shoot N content (Moll et al, 1982) and is affected by several physiological factors, including N uptake, metabolism, allocation, and remobilization (Habash et al, 2001;Tsay et al, 2011;Girondé et al, 2015). In addition, regulated transport of N within the plant strongly influences the amount of N allocated to seeds and final seed yields (Rolletschek et al, 2005;Schmidt et al, 2007;Weigelt et al, 2008;Tan et al, 2010;Zhang et al, 2010Zhang et al, , 2015Carter and Tegeder, 2016;Santiago and Tegeder, 2016 Araus et al, 2016;Chen et al, 2017), nitrate allocation (Tsay et al, 2011), N metabolism (Ameziane et al, 2000Chichkova et al, 2001;Habash et al, 2001;Yamaya et al, 2002;Seiffert et al, 2004;Good et al, 2007;Shrawat et al, 2008;Peña et al, 2017), and its regulation (Yanagisawa et al, 2004). While generally successful, these studies often did not address the different components of NUE (i.e.…”

“…In legumes like pea (Pisum sativum), amino acid synthesis occurs mainly in the roots followed by its transport via the xylem to mature source leaves (Atkins et al, 1983). Within leaves, the amino-N is utilized for a variety of metabolic processes, transiently stored as amino acids or protein, or loaded into the phloem in order to supply N to developing sinks, such as fruit and seeds (Urquhart and Joy, 1982;Tegeder and Rentsch, 2010;Zhang et al, 2010;Tegeder, 2014;Santiago and Tegeder, 2016). Some of the root-derived amino acids also might be transferred from the xylem to the phloem for direct N supply of sinks (Urquhart and Joy, 1982;Zhang et al, 2010;Tegeder, 2014) or exit the xylem along the transport pathway for transient storage in the stem (McNeil et al, 1979).…”

“…The uptake of N by the root is influenced by the availability of soil N and controlled by plant N assimilation processes and N metabolite levels as well as the N demand of the plant (Ruffel et al, 2011;Nacry et al, 2013;Stahl et al, 2016). In addition, recent work has resolved that amino acid transport processes in root and shoot strongly influence N uptake and metabolism, most probably through a feedback regulatory mechanism and shootto-root signaling (Miller et al, 2008;Sanders et al, 2009;Tan et al, 2010;Zhang et al, 2010Zhang et al, , 2015 Forde, 2014;Santiago and Tegeder, 2016). Nitrogen utilization efficiency (NUtE) is defined as total seed yield relative to total shoot N content (Moll et al, 1982) and is affected by several physiological factors, including N uptake, metabolism, allocation, and remobilization (Habash et al, 2001;Tsay et al, 2011;Girondé et al, 2015).…”

“…Nitrogen utilization efficiency (NUtE) is defined as total seed yield relative to total shoot N content (Moll et al, 1982) and is affected by several physiological factors, including N uptake, metabolism, allocation, and remobilization (Habash et al, 2001;Tsay et al, 2011;Girondé et al, 2015). In addition, regulated transport of N within the plant strongly influences the amount of N allocated to seeds and final seed yields (Rolletschek et al, 2005;Schmidt et al, 2007;Weigelt et al, 2008;Tan et al, 2010;Zhang et al, 2010Zhang et al, , 2015Carter and Tegeder, 2016;Santiago and Tegeder, 2016).…”