The Arabidopsis L-Type Amino Acid Transporter 5 (LAT5/PUT5) Is Expressed in the Phloem and Alters Seed Nitrogen Content When Knocked Out

Abstract

The Arabidopsis L-type Amino Acid Transporter-5 (LAT5; At3g19553) was recently studied for its role in developmental responses such as flowering and senescence, under an assumption that it is a polyamine uptake transporter (PUT5). The LATs in Arabidopsis have a wide range of substrates, including amino acids and polyamines. This report extensively studied the organ and tissue-specific expression of the LAT5/PUT5 and investigated its role in mediating amino acid transport. Organ-specific quantitative RT-PCR detected LAT5/PUT5 transcripts in all organs with a relatively higher abundance in the leaves. Tissue-specific expression analysis identified GUS activity in the phloem under the LAT5/PUT5 promoter. In silico analysis identified both amino acid transporter and antiporter domains conserved in the LAT5/PUT5 protein. The physiological role of the LAT5/PUT5 was studied through analyzing a mutant line, lat5-1, under various growth conditions. The mutant lat5-1 seedlings showed increased sensitivity to exogenous leucine in Murashige and Skoog growth medium. In soil, the lat5-1 showed reduced leaf growth and altered nitrogen content in the seeds. In planta radio-labelled leucine uptake studies showed increased accumulation of leucine in the lat5-1 plants compared to the wild type when treated in the dark prior to the isotopic feeding. These studies suggest that LAT5/PUT5 plays a role in mediating amino acid transport.

Nitrogen assimilation and translocation in Arabidopsis seeds

“…The LAT5 is expressed in phloem and in siliques, and it possibly has a bi-directional amino acid transport capacity. However, knocking out this transporter caused increased nitrogen content in seeds [19]. It probably plays a role in amino acid homeostasis rather than importing amino acids into seeds.…”

“…In the apoplastic loading, amino acids are exported from mesophyll cells into the apoplasm, followed by active uptake into the sieve element-companion cell complex of the phloem [52,[57][58][59][60]. While amino acid exporters in leaf mesophyll cell plasma membrane are obscure, published reports suggest that Arabidopsis AAP2, 5, & 8, CAT6 & 9, ProT1, and LAT5 are either expressed in the phloem or demonstrated to have a role in phloem loading [16,19,51,[61][62][63][64][65][66]. The LAT4 is expressed in green carpel cells in the silique with a possible role in mobilizing amino acids from these tissues toward seeds [20].…”