GmVTL1a is an iron transporter on the symbiosome membrane of soybean with an important role in nitrogen fixation

Abstract

Legumes establish symbiotic relationships with soil bacteria (rhizobia), housed in nodules on roots. The plant supplies carbon substrates and other nutrients to the bacteria in exchange for fixed nitrogen. The exchange occurs across a plant-derived symbiosome membrane (SM), which encloses rhizobia to form a symbiosome. Iron supplied by the plant is crucial for rhizobial enzyme nitrogenase that catalyses nitrogen fixation, but the SM iron transporter has not been identified. We use yeast complementation, real-time PCR and proteomics to study putative soybean (Glycine max) iron transporters GmVTL1a and GmVTL1b and have characterized the role of GmVTL1a using complementation in plant mutants, hairy root transformation and microscopy. GmVTL1a and GmVTL1b are members of the vacuolar iron transporter family and homologous to Lotus japonicus SEN1 (LjSEN1), which is essential for nitrogen fixation. GmVTL1a expression is enhanced in nodule infected cells and both proteins are localized to the SM. GmVTL1a transports iron in yeast and restores nitrogen fixation when expressed in the Ljsen1 mutant. Three GmVTL1a amino acid substitutions that block nitrogen fixation in Ljsen1 plants reduce iron transport in yeast. We conclude GmVTL1a is responsible for transport of iron across the SM to bacteroids and plays a crucial role in the nitrogen-fixing symbiosis.

The Medicago truncatula Vacuolar iron Transporter‐Like proteins VTL4 and VTL8 deliver iron to symbiotic bacteria at different stages of the infection process

“…MtVTL4 , MtVTL5 , MtVTL6 and MtVTL7 are closely related paralogues on chromosome 4 which are more similar to Arabidopsis thaliana VTL5 than to other AtVTL genes. In contrast, MtVTL8 has no direct homologue in Arabidopsis thaliana , but is closely related to SEN1 in L. japonicus (Hakoyama et al ., 2011) and NODULIN‐21 of soybean ( GmNod21 , Delauney et al ., 1990; renamed GmVTL1 in Brear et al ., 2020 and Liu et al ., 2020). The amino acid sequences of MtVTL8 and LjSEN1 share 75.7% similarity (63.0% identity).…”

“…In particular VTL8 is critical for delivering iron to the bacteroids and for establishment of the nitrogen‐fixing bacteria. While this article was undergoing review, two studies describing a specific VTL in soybean nodule development were also accepted for publication (Brear et al ., 2020; Liu et al ., 2020).…”

Structure and function of the vacuolar Ccc1/VIT1 family of iron transporters and its regulation in fungi

“…In A. thaliana , most VTL transporters localize to the tonoplast and participate in iron homeostasis [25] . Interestingly, recent studies have shown that VTL1a from Glycine max , VTL8 from Medicago truncatula, and probably SEN1 from Lotus japonicum localize to the symbiosome membrane and deliver iron from the plant to the bacteroid in order to supply iron-demands derived from the symbiosis interaction [41] , [42] , [43] , [44] .…”

The iron will of the research community: advances in iron nutrition and interactions in lockdown times

“…Functional studies in several legume species have shown that MATE proteins homologous to FRD3 in Arabidopsis facilitate iron transport from the nodule xylem into the nodule ( Wang et al , 2017 ; Kryvoruchko et al , 2018 ). An NRAMP homologue then transports iron into the infected cell, and VIT-like transporters export the iron into the peribacteroid space ( Brear et al , 2020 ; Liu et al , 2020 ; Walton et al , 2020 ), from where the nitrogen-fixing bacteria can take it up with as yet unknown transporters.…”