A Co-opted Regulator of Lateral Root Development Controls Nodule Organogenesis in Lotus

An isoflavone reductase-like protein MtIFL negatively regulates nodule symbiosis in Medicago truncatula

Flavonoids are valuable metabolites produced by legumes, including Medicago truncatula, and play crucial roles in signaling communication during legume-rhizobium symbiosis. Isoflavone reductase proteins (IFRs) are involved in the biosynthesis of isoflavones and plant defense regulation. However, their role in symbiotic nitrogen fixation remains mostly elusive. Here, 13 putative IFR gene family members were identified, and an IFR-like gene named MtIFL was functionally characterized through expression patterns, phenotypic characterizations, flavonoid metabolome, and transcriptome analyses. Furthermore, the binding partner of MtIFL was investigated using Y1H, EMSA, and Dual-LUC assays. MtIFL was strongly induced in the process of nodule development and expressed in the meristem and infection zone of mature nodules. Knockout of MtIFL promoted nodulation, while overexpression of MtIFL induced premature senescence of nodules. Further investigations revealed that MtIFL negatively regulates nitrogen-fixing symbiosis by mediating isoflavone metabolism, and MtNIN regulates MtIFL expression by binding to the hNRE motif. Overall, our findings suggest that MtIFL negatively regulates nodule formation, thereby influencing the survival of rhizobia in nodule cells. MtNIN regulates MtIFL expression and may play a role in isoflavone metabolism. These results provide novel insights into the function of IFRs in symbiotic nodulation and the role of flavonoids in nodule development.

NIN-at the heart of NItrogen-fixing Nodule symbiosis

Legumes and actinorhizal plants establish symbiotic relationships with nitrogen-fixing bacteria, resulting in the formation of nodules. Nodules create an ideal environment for nitrogenase to convert atmospheric nitrogen into biological available ammonia. NODULE INCEPTION (NIN) is an indispensable transcription factor for all aspects of nodule symbiosis. Moreover, NIN is consistently lost in non-nodulating species over evolutions. Here we focus on recent advances in the signaling mechanisms of NIN during nodulation and discuss the role of NIN in the evolution of nitrogen-fixing nodule symbiosis.

Processing of NODULE INCEPTION controls the transition to nitrogen fixation in root nodules

[Figure: see text].

Control of the Rhizobia Nitrogen-Fixing Symbiosis by Common Bean MADS-Domain/AGL Transcription Factors

Plants MADS-domain/AGL proteins constitute a large transcription factor (TF) family that controls the development of almost every plant organ. We performed a phylogeny of (ca. 500) MADS-domain proteins from Arabidopsis and four legume species. We identified clades with Arabidopsis MADS-domain proteins known to participate in root development that grouped legume MADS-proteins with similar high expression in roots and nodules. In this work, we analyzed the role of AGL transcription factors in the common bean (Phaseolus vulgaris) - Rhizobium etli N-fixing symbiosis. Sixteen P. vulgaris AGL genes (PvAGL), out of 93 family members, are expressed - at different levels - in roots and nodules. From there, we selected the PvAGL gene denominated PvFUL-like for overexpression or silencing in composite plants, with transgenic roots and nodules, that were used for phenotypic analysis upon inoculation with Rhizobium etli. Because of sequence identity in the DNA sequence used for RNAi-FUL-like construct, roots, and nodules expressing this construct -referred to as RNAi_AGL- showed lower expression of other five PvAGL genes highly expressed in roots/nodules. Contrasting with PvFUL-like overexpressing plants, rhizobia-inoculated plants expressing the RNAi_AGL silencing construct presented affection in the generation and growth of transgenic roots from composite plants, both under non-inoculated or rhizobia-inoculated condition. Furthermore, the rhizobia-inoculated plants showed decreased rhizobial infection concomitant with the lower expression level of early symbiotic genes and increased number of small, ineffective nodules that indicate an alteration in the autoregulation of the nodulation symbiotic process. We propose that the positive effects of PvAGL TF in the rhizobia symbiotic processes result from its potential interplay with NIN, the master symbiotic TF regulator, that showed a CArG-box consensus DNA sequence recognized for DNA binding of AGL TF and presented an increased or decreased expression level in roots from non-inoculated plants transformed with OE_FUL or RNAi_AGL construct, respectively. Our work contributes to defining novel transcriptional regulators for the common bean - rhizobia N-fixing symbiosis, a relevant process for sustainable agriculture.

Home sweet home: how mutualistic microbes modify root development to promote symbiosis

Post-embryonic organogenesis has uniquely equipped plants to become developmentally responsive to their environment, affording opportunities to remodel organism growth and architecture to an extent not possible in other higher order eukaryotes. It is this developmental plasticity that makes the field of plant-microbe interactions an exceptionally fascinating venue in which to study symbiosis. This review article describes the various ways in which mutualistic microbes alter the growth, development, and architecture of the roots of their plant hosts. We first summarize general knowledge of root development, and then examine how association of plants with beneficial microbes affects these processes. Working our way inwards from the epidermis to the pericycle, this review dissects the cell biology and molecular mechanisms underlying plant-microbe interactions in a tissue-specific manner. We examine the ways in which microbes gain entry into the root, and modify this specialized organ for symbiont accommodation, with a particular emphasis on the colonization of root cortical cells. We present significant advances in our understanding of root-microbe interactions, and conclude our discussion by identifying questions pertinent to root endosymbiosis that at present remain unresolved.