Inhibition of legume nodulation by Pi deficiency is dependent on the autoregulation of nodulation (AON) pathway

Abstract

SUMMARY Inhibition of nodule development is one of the main adverse effects of phosphate (Pi) deficiency in legumes. Despite all of the efforts made over the last decades to understand how root nodules cope with Pi deficiency, the molecular mechanisms leading to the reduction in nodule number under Pi deficiency remain elusive. In the present study, we provide experimental evidence indicating that Pi deficiency activates the autoregulation of nodulation (AON) pathway, leading to a reduction in nodule numbers in both common bean and soybean. A transcriptional profile analysis revealed that the expression of the AON‐related genes PvNIN, PvRIC1, PvRIC2, and PvTML is upregulated under Pi deficiency conditions. The downregulation of the MYB transcription factor PvPHR1 in common bean roots significantly reduced the expression of these four AON‐related genes. Physiological analyses indicated that Pi deficiency does not affect the establishment of the root nodule symbiosis in the supernodulation mutant lines Pvnark and Gmnark. Reciprocal grafting and split‐roots analyses determined that the activation of the AON pathway was required for the inhibitory effect of Pi deficiency. Altogether, these data improve our understanding of the genetic mechanisms controlling the establishment of the root nodule symbiosis under Pi deficiency.

Systemic Optimization of Legume Nodulation: A Shoot-Derived Regulator, miR2111

“…The low Pi conditions induce RIC1, RIC2, and TML expression, suggesting the involvement of these genes in the low-nodulation phenotype of P. vulgaris under Pi-deficient conditions. RIC1 and RIC2 induction in response to Pi deficiency is likely dependent on PHOSPHATE STARVATION RESPONSE 1 (PHR1), a master transcription factor for Pi deficiency adaptation (Rubio et al, 2001;López-Arredondo et al, 2014;Isidra-Arellano et al, 2020). The putative cis-regulatory element P1BS, a PHR1-binding sequence (Bustos et al, 2010), was identified in RIC1 and RIC2 promoters, and this putative cis-element is conserved in promoter regions of RIC1 and RIC2 orthologs of other legumes, implying the involvement of PHR1 in this negative regulation of nodulation.…”

“…Indeed, RIC1 and RIC2 expression induction by Pi deficiency is not observed in PHR1-silenced P. vulgaris hairy roots. Moreover, grafting experiments using P. vulgaris and soybean nark mutants demonstrated that this nodulation restriction under Pi deficiency is also a shoot-controlled negative regulatory system that depends on NARK (Isidra-Arellano et al, 2020). Since elevated TML mRNA levels in roots have been observed in response to low Pi levels, it is not surprising that miR2111 is also involved in this regulation.…”

Nitrogen and Phosphorus Signaling and Transport During Legume–Rhizobium Symbiosis

“…These defects may be associated with an auxin-cytokinin imbalance, upregulation of ethylene, abscisic acid (ABA), and jasmonate-related signaling pathways under Pi deficiency in common bean ( Isidra-Arellano et al, 2018 ). Recent studies have shown that the AON-related genes PvNIN , PvRIC1 , PvRIC2 , and PvTML are induced under conditions of Pi deficiency and their transcriptional activation is likely to be dependent on PHR1 ( Isidra-Arellano et al, 2020 ; Figure 2 ). These studies suggest that Pi deficiency inhibits the root nodule symbiosis in common bean and soybean through constitutive activation of the AON pathway under Pi deficiency ( Isidra-Arellano et al, 2020 ).…”

“…Recent studies have shown that the AON-related genes PvNIN , PvRIC1 , PvRIC2 , and PvTML are induced under conditions of Pi deficiency and their transcriptional activation is likely to be dependent on PHR1 ( Isidra-Arellano et al, 2020 ; Figure 2 ). These studies suggest that Pi deficiency inhibits the root nodule symbiosis in common bean and soybean through constitutive activation of the AON pathway under Pi deficiency ( Isidra-Arellano et al, 2020 ).…”

The soybean β‐expansin gene GmINS1 contributes to nodule development in response to phosphate starvation

“…Low P availability severely suppresses nodule formation and development, two of the major processes that largely determine BNF capacity. It has been reported that phosphate (Pi) starvation negatively impacts critical early steps of rhizobial symbiosis through (1) limiting infection thread (IT) formation, (2) reducing the expression of genes participating in early nodulation processes, and (3) disrupting autoregulation of nodule numbers in Pi‐deficient seedlings (Isidra‐Arellano et al, 2018, 2020; Nasr‐Esfahani et al, 2017). After IT formation, another crucial step for effective N 2 fixation in legumes is nodule organogenesis, when rhizobia differentiate into intracellular N 2 ‐fixing bacteroids that subsequently expand into nodules (Oke & Long, 1999).…”