PHO1 family members transport phosphate from infected nodule cells to bacteroids inMedicago truncatula

Abstract

Legumes play an important role in the soil nitrogen availability via symbiotic nitrogen fixation (SNF). Phosphate (Pi) deficiency severely impacts SNF because of the high Pi requirement of symbiosis. Whereas PHT1 transporters are involved in Pi uptake into nodules, it is unknown how Pi is transferred from the plant infected cells to nitrogen-fixing bacteroids. We hypothesized that Medicago truncatula genes homologous to Arabidopsis PHO1, encoding a vascular apoplastic Pi exporter, are involved in Pi transfer to bacteroids. Among the seven MtPHO1 genes present in M. truncatula, we found that two genes, namely MtPHO1.1 and MtPHO1.2, were broadly expressed across the various nodule zones in addition to the root vascular system. Expressions of MtPHO1.1 and MtPHO1.2 in Nicotiana benthamiana mediated specific Pi export. Plants with nodule-specific downregulation of both MtPHO1.1 and MtPHO1.2 were generated by RNA interference (RNAi) to examine their roles in nodule Pi homeostasis. Nodules of RNAi plants had lower Pi content and a three-fold reduction in SNF, resulting in reduced shoot growth. Whereas the rate of 33Pi uptake into nodules of RNAi plants was similar to control, transfer of 33Pi from nodule cells into bacteroids was reduced and bacteroids activated their Pi-deficiency response. Our results implicate plant MtPHO1 genes in bacteroid Pi homeostasis and SNF via the transfer of Pi from nodule infected cells to bacteroids.

Argonaute5 and its associated small RNAs modulate the transcriptional response during the rhizobia-Phaseolus vulgaris symbiosis

“…We found that the expression of genes encoding Fe 2+ -, Mn 2+ , and K + transporters was significantly diminished in PvAGO5 -RNAi mature nodules ( Supplementary Figure 5 ). We also observed that the expression of Phvul.007g275300.1 that encodes the PHO1 transporter, which is required to transfer phosphate from infected nodule cells to bacteroids ( Nguyen et al., 2021 ), was significantly downregulated. In contrast, genes encoding different ABC and MATE transporter family members, as well as sugars- and amino acid- transporters, were upregulated.…”

“…Effective root nodule symbiosis depends on efficient nutrient exchange. Recent studies indicate that the Phosphate Transporter1.1 (PHO1.1) and PHO1.2 translocate phosphate from the infected cell to nitrogen-fixing bacteroids in M. truncatula ( Nguyen et al., 2021 ). Similarly, different types of iron transporters, among them Yellow Stripe-Like and Vacuolar Iron Transporter Like, aid the translocation of iron from infected nodule cells to nitrogen-fixing bacteria ( Liu et al., 2020 ; Castro-Rodríguez et al., 2021 ).…”

Mechanisms underlying legume–rhizobium symbioses

“…The expression of GmPAP12 is directly activated by GmPHR1 under Pi starvation conditions to maintain Pi homeostasis and nitrogen fixation (Wang et al, 2020). The vascular apoplastic Pi exporters MtPHO1.1 and MtPHO1.2, which are broadly expressed in nodules, function in Pi transport from infected nodule cells to bacteroids (Nguyen et al, 2021). SPX proteins (named after the Saccharomyces cerevisiae SYG1 and Pho81, and mammalian Xpr1) function as phosphate sensors and negatively regulate the binding affinity of PHRs.…”

Intracellular phosphate sensing and regulation of phosphate transport systems in plants

“…In rice, PHO1s are also expressed at the vasculature of the uppermost node connecting to the panicle and in developing seed tissues, where they are involved in the allocation of Pi during grain filling ( Che et al, 2020 ; Ma et al, 2021) . In Medicago ( Medicago truncatula ), PHO1 mediates the transfer of Pi from nodule infected cells to bacteroids ( Nguyen et al, 2021 ). Negative regulations of Arabidopsis PHO1 either by WRKY6 at the transcriptional level ( Chen et al, 2009 ) or by its upstream open-reading frame at the translational level ( Reis et al, 2020 ) were reported.…”