Arabidopsis UMAMIT24 and 25 are amino acid exporters involved in seed loading

Julien Besnard,Sakiko Okumoto,Jean-Christophe Avice,Stanislav Vitha,Guillaume Pilot,Chengsong Zhao,Ayumi Hyodo

doi:10.1093/jxb/ery302

Abstract

Phloem-derived amino acids are the major source of nitrogen supplied to developing seeds. Amino acid transfer from the maternal to the filial tissue requires at least one cellular export step from the maternal tissue prior to the import into the symplasmically isolated embryo. Some members of UMAMIT (usually multiple acids move in an out transporter) family (UMAMIT11, 14, 18, 28, and 29) have previously been implicated in this process. Here we show that additional members of the UMAMIT family, UMAMIT24 and UMAMIT25, also function in amino acid transfer in developing seeds. Using a recently published yeast-based assay allowing detection of amino acid secretion, we showed that UMAMIT24 and UMAMIT25 promote export of a broad range of amino acids in yeast. In plants, UMAMIT24 and UMAMIT25 are expressed in distinct tissues within developing seeds; UMAMIT24 is mainly expressed in the chalazal seed coat and localized on the tonoplast, whereas the plasma membrane-localized UMAMIT25 is expressed in endosperm cells. Seed amino acid contents of umamit24 and umamit25 knockout lines were both decreased during embryogenesis compared with the wild type, but recovered in the mature seeds without any deleterious effect on yield. The results suggest that UMAMIT24 and 25 play different roles in amino acid translocation from the maternal to filial tissue; UMAMIT24 could have a role in temporary storage of amino acids in the chalaza, while UMAMIT25 would mediate amino acid export from the endosperm, the last step before amino acids are taken up by the developing embryo.

Highlights

Transfer of nutrients from the mother plant to the reproductive tissues is critical to ensure proper seed development
Available microarray data showed that UMAMIT23, UMAMIT24, and UMAMIT25 are almost exclusively expressed in the developing siliques and seeds (Schmid et al, 2005), in which amino acid exporters play an important role in nitrogen transfer to the filial tissue
We have shown here that UMAMIT24 and UMAMIT25, previously uncharacterized members of the UMAMIT family, function as amino acid exporters when expressed in yeast (Fig. 1).These results are in good agreement with previous studies reporting amino acid export activities for other UMAMIT genes (Ladwig et al, 2012; Müller et al, 2015; Besnard et al, 2016)

Summary

Introduction

Transfer of nutrients from the mother plant to the reproductive tissues is critical to ensure proper seed development. Phloem-derived amino acids are the main form of nitrogenous compounds transported to the seeds and are stored as proteins, necessary for seed metabolism during seed maturation and germination (Pate et al, 1975;Atkins et al, 1979).The quantity of amino acids delivered through the phloem to the developing seed correlates with seed protein and lipid content, suggesting. For each of the UMAMIT proteins found so far to be expressed within the seed, the phenotype caused by a single loss-of-function mutation is relatively benign (i.e. no loss of seed viability), suggesting the involvement of multiple amino acid exporters, including previously uncharacterized UMAMITs, in amino acid transport to the embryo

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Conclusion