A signal peptide peptidase is required for ER-symbiosome proximal association and protein secretion

Essential roles of nodule cysteine-rich peptides in maintaining the viability of terminally differentiated bacteroids in legume-rhizobia symbiosis

Investigations into the nitrogen-fixing symbiosis between legumes and rhizobia can yield innovative strategies for sustainable agriculture. Legume species of the Inverted Repeat-Lacking Clade (IRLC) and the Dalbergioids, can utilize nodule cysteine-rich (NCR) peptides, a diverse family of peptides characterized by four or six highly conserved cysteine residues, to communicate with their microbial symbionts. These peptides, many of which exhibit antimicrobial properties, induce profound differentiation of bacteroids (semi-autonomous forms of bacteria) within nodule cells. This terminal differentiation endows the bacteroids with the ability to fix nitrogen, at the expense of their reproductive capacity. Notably, a significant number of NCR peptides is expressed in the nodule fixation zone, where the bacteroids have already reached terminal differentiation. Recent discoveries, through forward genetics approaches, have revealed that the functions of NCR peptides extend beyond antimicrobial effects and the promotion of differentiation. They also play a critical role in sustaining the viability of terminally differentiated bacteroids within nodule cells. These findings underscore the multifaceted functions of NCR peptides and highlight the importance of these peptides in mediating communications between host cells and the terminally differentiated bacteroids.

The symbiosome-a transient organelle in evolution

This article comments on: Casaes PA, Ferreira dos Santos JM, Silva VC, Rhem MFK, Teixeira Cota MM, de Faria SM, Rando JG, James EK, Gross E. 2024. The radiation of nodulated Chamaecrista species from the rainforest into more diverse habitats has been accompanied by a reduction in growth form and a shift from fixation threads to symbiosomes. Journal of Experimental Botany 75, 3643-3662.

Signal Peptide Peptidase and PI4Kβ1/2 play opposite roles in plant ER stress response and immunity

The Arabidopsis pi4kβ1,2 mutant is mutated in the phosphatidylinositol 4-kinase (PI4K) β1 and PI4Kβ2 enzymes which are involved in the biosynthesis of phosphatidylinositol 4-phosphate (PI4P), a minor membrane lipid with important signaling roles. pi4kβ1,2 plants display autoimmunity and shorter roots. Though the pi4kβ1,2 mutant has been extensively characterized, the source of its autoimmunity remains largely unknown. In this study, through a genetic suppressor screen, we identified multiple partial loss-of-function alleles of signal peptide peptidase (spp) that can suppress all the defects of pi4kβ1,2. SPP is an intramembrane cleaving aspartic protease. Interestingly, pi4kβ1,2 plants display enhanced ER stress response and mutations in SPP can suppress such phenotype. Furthermore, reduced ER stress responses were observed in the spp single mutants. Overall, our study reveals a previously unknown function of PI4Kβ and SPP in ER stress and plant immunity.

Targeted mutagenesis of Medicago truncatula Nodule-specific Cysteine-Rich (NCR) genes using the Agrobacterium rhizogenes-mediated CRISPR/Cas9 system

The host-produced nodule specific cysteine-rich (NCR) peptides control the terminal differentiation of endosymbiotic rhizobia in the nodules of IRLC legumes. Although the Medicago truncatula genome encodes about 700 NCR peptides, only few of them have been proven to be crucial for nitrogen-fixing symbiosis. In this study, we applied the CRISPR/Cas9 gene editing technology to generate knockout mutants of NCR genes for which no genetic or functional data were previously available. We have developed a workflow to analyse the mutation and the symbiotic phenotype of individual nodules formed on Agrobacterium rhizogenes-mediated transgenic hairy roots. The selected NCR genes were successfully edited by the CRISPR/Cas9 system and nodules formed on knockout hairy roots showed wild type phenotype indicating that peptides NCR068, NCR089, NCR128 and NCR161 are not essential for symbiosis between M. truncatula Jemalong and Sinorhizobium medicae WSM419. We regenerated stable mutants edited for the NCR068 from hairy roots obtained by A. rhizogenes-mediated transformation. The analysis of the symbiotic phenotype of stable ncr068 mutants showed that peptide NCR068 is not required for symbiosis with S. meliloti strains 2011 and FSM-MA either. Our study reports that gene editing can help to elicit the role of certain NCRs in symbiotic nitrogen fixation.