Phloem-Specific Translational Regulation of Soybean Nodulation: Insights from a Phloem-Targeted TRAP-Seq Approach

Soybean (Glycine max) root nodulation is a symbiotic process that requires complex molecular and cellular coordination. The phloem plays a crucial role not only in nutrient transport but also in long-distance signaling that regulates nodulation. However, the molecular mechanisms underlying phloem-specific regulation during nodulation remain poorly characterized. Here, we developed a phloem-specific Translating Ribosome Affinity Purification sequencing (TRAP-seq) system to investigate the translational dynamics of phloem-associated genes during nodulation. Using a phloem-specific promoter (Glyma.01G040700) combined with the GAL4-UAS amplification system, we successfully captured the translatome of soybean root phloem at early (72 hours post-inoculation, hpi) and late (21 days post-inoculation, dpi) nodulation stages. Differential expression analysis revealed dynamic translational reprogramming, with 2,636 differentially expressed genes (DEGs) at 72 hpi and 8,422 DEGs at 21 dpi. Gene ontology and pathway enrichment analyses showed stage-specific regulatory shifts, including early activation of ethylene and defense pathways and late-stage enhancement of nutrient transport and vascular development. Transcription factor analysis identified GmbHLH121 as a key phloem-specific regulator of nodulation. Functional validation using RNAi knockdown and overexpression experiments demonstrated that GmbHLH121 negatively regulates nodule formation, likely acting downstream of or independently from early nodulation signaling pathways. Additionally, we uncovered dynamic regulation of cell wall-modifying enzymes (PME and PMEI) in the phloem, implicating their role in modulating plasmodesmata permeability and facilitating symplastic connectivity during nodulation. Our findings highlight the critical role of phloem-mediated translational regulation in coordinating root nodulation, emphasizing the phloem as an active regulatory hub for long-distance signaling and symbiotic efficiency.