Rhizobium symbiotic efficiency meets CEP signaling peptides

Apoplastic barriers are essential for nodule formation and nitrogen fixation in Lotus japonicus

Establishment of the apoplastic root barrier known as the Casparian strip occurs early in root development. In legumes, this area overlaps with nitrogen-fixing nodule formation, which raises the possibility that nodulation and barrier formation are connected. Nodules also contain Casparian strips, yet, in this case, their role is unknown. We established mutants with defective barriers in Lotus japonicus. This revealed that effective apoplastic blockage in the endodermis is important for root-to-shoot signals underlying nodulation. Our findings further revealed that in nodules, the genetic machinery for Casparian strip formation is shared with roots. Apoplastic blockage controls the metabolic source-sink status required for nitrogen fixation. This identifies Casparian strips as a model system to study spatially constrained symbiotic plant-microbe relationships.

The Small Key to the Treasure Chest: Endogenous Plant Peptides Involved in Symbiotic Interactions

Plant growth and development are inextricably connected with rhizosphere organisms. Plants have to balance between strong defenses against pathogens while modulating their immune responses to recruit beneficial organisms such as bacteria and fungi. In recent years, there has been increasing evidence that regulatory peptides are essential in establishing these symbiotic relationships, orchestrating processes that include nutrient acquisition, root architecture modification, and immune modulation. In this review, we provide a comprehensive summary of the peptide families that facilitate beneficial relationships between plants and rhizosphere organisms.

TML1 and TML2 synergistically regulate nodulation and affect arbuscular mycorrhiza in Medicago truncatula

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant's need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of nodulation (AON) and autoregulation of mycorrhizal symbiosis (AOM) both negatively regulate their respective processes and share multiple components-plants that make too many nodules usually have higher arbuscular mycorrhiza (AM) fungal root colonization. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON in Lotus japonicus. Medicago truncatula has two sequence homologs: MtTML1 and MtTML2. We report the generation of stable single and double mutants harboring multiple allelic variations in MtTML1 and MtTML2 using CRISPR-Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations in MtTML1 or MtTML2 produced two to three times the nodules of wild-type plants, whereas plants containing mutations in both genes displayed a synergistic effect, forming 20× more nodules compared to wild-type plants. Examination of expression and heterozygote effects suggests that genetic compensation may play a role in the observed synergy. Plants with mutations in both TMLs only showed mild increases in AM fungal root colonization at later timepoints in our experiments, suggesting that these genes may also play a minor role in AM symbiosis regulation. The mutants created will be useful tools to dissect the mechanism of synergistic action of MtTML1 and MtTML2 in M. truncatula symbiosis with beneficial microbes.

Small peptides: novel targets for modulating plant-rhizosphere microbe interactions

The crucial role of rhizosphere microbes in plant growth and their resilience to environmental stresses underscores the intricate communication between microbes and plants. Plants are equipped with a diverse set of signaling molecules that facilitate communication across different biological kingdoms, although our comprehension of these mechanisms is still evolving. Small peptides produced by plants (SPPs) and microbes (SPMs) play a pivotal role in intracellular signaling and are essential in orchestrating various plant development stages. In this review, we posit that SPPs and SPMs serve as crucial signaling agents for the bidirectional cross-kingdom communication between plants and rhizosphere microbes. We explore several potential mechanistic pathways through which this communication occurs. Additionally, we propose that leveraging small peptides, inspired by plant-rhizosphere microbe interactions, represents an innovative approach in the field of holobiont engineering.

TML1 AND TML2 SYNERGISTICALLY REGULATE NODULATION AND AFFECT ARBUSCULAR MYCORRHIZA IN MEDICAGO TRUNCATULA

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant's need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of Nodulation (AON) and Autoregulation of Mycorrhization (AOM) both negatively regulate their respective processes and share multiple components - plants that make too many nodules usually have higher AM fungal root colonization. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON in Lotus japonicus . M. truncatula has two sequence homologs: Mt TML1 and Mt TML2. We report the generation of stable single and double mutants harboring multiple allelic variations in MtTML1 and MtTML2 using CRISPR-Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations in Mt TML1 or Mt TML2 produced 2-3 times the nodules of wild-type plants whereas plants containing mutations in both genes displayed a synergistic effect, forming 20x more nodules compared to wild type plants. Examination of expression and heterozygote effects suggest genetic compensation may play a role in the observed synergy. Plants with mutations in both TMLs only showed mild increases in AM fungal root colonization at later timepoints in our experiments, suggesting these genes may also play a minor role in AM symbiosis regulation. The mutants created will be useful tools to dissect the mechanism of synergistic action of Mt TML1 and Mt TML2 in M. truncatula symbiosis with beneficial microbes.