Spatiotemporal expression patterns of wheat amino acid transporters reveal their putative roles in nitrogen transport and responses to abiotic stress

Abstract

Amino acid transporters have roles in amino acid uptake from soil, long-distance transport, remobilization from vegetative tissues and accumulation in grain. Critically, the majority of wheat grain nitrogen is derived from amino acids remobilized from vegetative organs. However, no systematic analysis of wheat AAT genes has been reported to date. Here, 283 full length wheat AAT genes representing 100 distinct groups of homeologs were identified and curated by selectively consolidating IWGSC CSSv2 and TGACv1 Triticum aestivum genome assemblies and reassembling or mapping of IWGSC CSS chromosome sorted reads to fill any gaps. Gene expression profiling was performed using public RNA-seq data from root, leaf, stem, spike, grain and grain cells (transfer cell (TC), aleurone cell (AL), and starchy endosperm (SE)). AATs highly expressed in roots are good candidates for amino acid uptake from soil whilst AATs highly expressed in senescing leaves and stems may be involved in translocation to grain. AATs in TC (TaAAP2 and TaAAP19) and SE (TaAAP13) may play important roles in determining grain protein content and grain yield. The expression levels of AAT homeologs showed unequal contributions in response to abiotic stresses and development, which may aid wheat adaptation to a wide range of environments.Grain yield and protein content are dependent upon nitrogen accumulation in the grain. Greater than 70% of wheat grain nitrogen is remobilised and translocated from senescing leaves and stems 1, 2 , and amino acids represent the major transport form of organic nitrogen delivered to the endosperm cavity via the vascular strand 3, 4 . Large quantities of amino acids are imported into grain to meet the high nitrogen demand for synthesis of endosperm storage proteins, and for embryo development.Plasma membrane transporters for amino acids are required for root uptake, xylem loading in the roots, phloem loading in leaves, and for nitrogen import into seeds. Amino acid transporters (AATs) , and other crops [9][10][11] . Based on sequence similarities and uptake properties, amino acid transporter families in plants comprise of two subfamilies: the amino acid/auxin permeases (AAAP) and the amino acid polyamine and choline transporters (APC) [12][13][14][15] . The AAAP subfamily can be further divided into general amino acid permeases (AAPs), lysine-histidine transporters (LHTs), proline transporters (ProTs), gamma-aminobutyric acid transporters (GATs), aromatic and neutral amino acid transporters (ANTs) and indole-3-acetic acid transporters (AUXs). A new group, amino acid transporter-like (ATL), was recently identified in rice 6 . The APC subfamily is grouped into three further subfamilies: cationic amino acid transporter (CATs), bidirectional acid transporters (BATs) and L-type amino acid transporters (LATs).Within the amino acid transporter family, amino acid permeases (AAPs) have been functionally characterized in Arabidopsis. Eight members (AtAAP1-8) have been identified as amino acid permeases (AAP) that

Tissue specific expression of UMAMIT amino acid transporters in wheat

“…Likely, the tissues examined consist of more than one cell type with distinct functions, as revealed by recent single-cell RNAseq studies showing that the cells in the vascular bundle expressing mainly sugar and amino acid importers are distinct from those expressing the exporters 13 , 14 . In addition, polar localization of transporters within the same cell type might allow for the directional transport, as previously found for both plant 15 , 16 , 18 and animal 19 , 20 systems. Some of the UMAMITs might also be involved in amino acid transport across the intracellular membranes, analogous to AtUMAMIT24 7 .…”

Phylogeny and gene expression of the complete NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY in Triticum aestivum

“…This is also the case for members of another large N-related gene family in wheat, the amino acid transporter (AAT) family ( Wan et al , 2017 ). Whereas the AAT genes are mostly located close to the centromere of each chromosome ( Wan et al , 2017 ), the majority of the NPF genes are located distant from the centromere, with a favoured distribution on the long arm of the chromosomes ( Fig. 2 ).…”

Wheat amino acid transporters highly expressed in grain cells regulate amino acid accumulation in grain

“…Previously three TaAAP genes ( TaAAP2 , TaAAP13 , and TaAAP21 ) out of 100 homoeologous groups of amino acid transporters (283 genes) were shown to be highly expressed in the endosperm transfer cells, starchy endosperm cells and aleurone cells of wheat grains, respectively [ 10 ]. These genes were therefore selected to determine their gene expression patterns, spatial tissue localization and potential functions in grain nitrogen metabolism.…”

“…The expression patterns determined from RNA-seq data of endosperm transfer cells, starchy endosperm cells, and aleurone cells at 20 DPA (days post anthesis) [ 10 , 50 ] are shown in Fig 1A–1C . TaAAP2 (three homoeologs with the IWGSC RefSeq v1.1 IDs: TraesCS2A02G348600 ( TaAAP2A ), TraesCS2B02G367000 ( TaAAP2B ), TraesCS2D02G347000 ( TaAAP2D )) was highly expressed in endosperm transfer cells at 20 DPA (corresponding to the middle of grain filling), but had very low expression in starchy endosperm and aleurone cells at 20 DPA ( Fig 1A ).…”

“… (a-c), Expression of TaAAP2 (a), TaAAP13 (b), and TaAAP21 (c) in grain cells: endosperm transfer cells (ETC), starchy endosperm (SE), aleurone cells (AL) of Chinese Spring at 20DPA (days post anthesis) by RNA-seq data [ 10 , 32 ]. (d-f), Expression of TaAAP2B (d), TaAAP13D (e), and TaAAP21A (f) in different organs of wheat cultivar Hereward from whole caryopses at 5, 10, 14, 17, 21, and 28 DPA (days post anthesis), roots, leaves and stems at Zadoks 23 (2–3 tillers stage) and Zadoks 45 (booting stage) at two nitrogen levels, 0kg N/ha and 200kg N/ha, by RT-qPCR.…”