Dual-Transcriptome Dissection of the Mechanisms Underlying Alfalfa Phenotypic Differences Induced by Two Rhizobial...

Area of Science:

• Plant-microbe interactions
• Molecular biology
• Symbiotic nitrogen fixation

Background:

• Rhizobial symbiosis with legumes like alfalfa is crucial for nitrogen fixation.
• Distinct rhizobial strains can elicit varied symbiotic outcomes in the host plant.
• Molecular mechanisms underlying strain-specific symbiotic phenotypes at the mature nodule stage are not fully understood.

Purpose of the Study:

• To investigate the molecular differences in alfalfa nodules induced by two distinct rhizobial strains (WE2 and WWL2) at the mature stage.
• To compare gene expression profiles in both the alfalfa host and the rhizobia to understand strain-specific symbiotic phenotypes.
• To identify candidate genes and pathways involved in regulating symbiotic efficiency.

Main Methods:

• Analysis of 21-day post-inoculation alfalfa nodules induced by rhizobial strains WE2 and WWL2.
• Measurement of nitrogenase activity using the acetylene reduction assay (ARA).
• Dual RNA sequencing of both host and rhizobial transcripts to compare gene expression.

Main Results:

• Strain WE2-induced nodules exhibited higher expression of host nodule marker genes (ENOD93, leghemoglobin) and transporter genes (SWEET, amino acid/peptide).
• Host gene expression was enriched in pathways for transmembrane transport, redox functions, and microaerobic condition maintenance.
• Rhizobia in WE2 nodules showed higher expression of microaerobic respiration and nitrogen fixation (nif/fix) genes, while WWL2 rhizobia had higher expression of transport and motility genes.

Conclusions:

• Coordinated differences exist between host resource supply/microaerobic pathways and rhizobial respiration/nitrogen fixation programs.
• Strain WE2 appears to promote a more efficient symbiotic state characterized by enhanced host transport and rhizobial nitrogen fixation.
• Candidate genes and pathways were identified for future functional validation and development of improved alfalfa inoculants.