Soybean Yellow Stripe-like7 is a symbiosome membrane peptide transporter essential for nitrogen fixation

Abstract

Legumes form a symbiosis with rhizobia that convert atmospheric nitrogen (N2) to ammonia which they provide to the plant in return for a carbon and nutrient supply. Nodules, developed as part of the symbiosis, harbor rhizobia which are enclosed in the plant-derived symbiosome membrane (SM), to form a symbiosome. In the mature nodule all exchanges between the symbionts occur across the SM. Here we characterize GmYSL7, a member of Yellow stripe-like family which is localized to the SM in soybean nodules. It is expressed specifically in nodule infected cells with expression peaking soon after nitrogenase becomes active. Although most members of the family transport metal complexed with phytosiderophores, GmYSL7 does not. It transports oligopeptides of between four and 12 amino acids. Silencing of GmYSL7 reduces nitrogenase activity and blocks development when symbiosomes contain a single bacteroid. RNAseq of nodules in which GmYSL7 is silenced suggests that the plant initiates a defense response against the rhizobia. There is some evidence that metal transport in the nodules is dysregulated, with upregulation of genes encoding ferritin and vacuolar iron transporter family and downregulation of a gene encoding nicotianamine synthase. However, it is not clear whether the changes are a result of the reduction of nitrogen fixation and the requirement to store excess iron or an indication of a role of GmYSL7 in regulation of metal transport in the nodules. Further work to identify the physiological substrate for GmYSL7 will allow clarification of this role.One sentence summaryGmYSL7 is a symbiosome membrane peptide transporter that is essential for symbiotic nitrogen fixation that when silenced blocks symbiosome development.

Medicago truncatula Yellow Stripe‐Like7 encodes a peptide transporter participating in symbiotic nitrogen fixation

“…The fact that it is upregulated exclusively in roots may indicate that a signal controlling whole plant metal iron allocation has been disrupted rather than one related only to nitrogen‐fixing cells. Similarly, GmYSL7 , a close orthologue of MtYSL7 in soybean, can functionally complement the ysl7 phenotype when driven under the MtYSL7 promoter (Gavrin et al, 2021). GmYSL7 is a nodule‐specific protein, located on the symbiosome membrane, with similar peptide transport characteristics, that might synchronize the metabolism/nutrition between the macro‐ and microsymbionts, including metal homeostasis genes.…”

Iron Transport across Symbiotic Membranes of Nitrogen-Fixing Legumes

“…The most widely studied ferric iron transporters in plants are the YSL (Yellow Stripe Like) proteins [ 52 , 53 ], part of the larger Oligopeptide Transport (OPT) family. Nodule-enhanced YSL proteins have been identified in both soybean [ 51 , 54 ] and M. truncatula [ 55 ]. GmYSL7 is localised to the SM, while MtYSL7 is located in the nodule cortex, and interference with their expression compromises nodule development and nitrogen fixation and, in the case of MtYSL7, iron distribution in the nodule.…”