Brassinosteroid-Mediated Improvement of Rice Yield and Phosphorus (P) use Efficiency Under P Deficiency: A Root–Rhizosphere Perspective

Brassinosteroids (BRs) play important roles in regulating nutrient uptake, and phosphorus (P) deficiency severely limits rice productivity. However, whether and how BRs mediate P use efficiency (PUE), particularly via root-rhizosphere processes, remains unclear. Over three years, we ran two pot experiments in a low-P soil (Olsen-P 6.8 mg kg⁻¹). Experiment 1 (Genotype × P): YG2 (strong low-P tolerant variety) and ZD88 (weak low-P tolerant variety) were grown under no P (0P) and normal P (NP) conditions. Experiment 2 (Chemical application× P): plant roots were irrigated with 2,4-epibrassinolide (2,4-EBL) or a BRs biosynthesis inhibitor under both 0P and NP rates, with distilled water as the control. Results showed that, relative to NP, 0P significantly decreased root BR (2,4-epibrassinolide and 2,8-homobrassinolide) content in both genotypes, with a smaller reduction in YG2 than in ZD88 under 0P. YG2 outperformed ZD88 in grain yield and PUE at both P rates, especially at 0P, mainly due to the enhancement of early-stage (before panicle initiation) P accumulation driven by its elevated BR content. Under 0P, YG2 also exhibited superior root morph-physiological traits, viz. root length, root activity, malate secretion, along with higher pyrroloquinoline quinone biosynthesis protein C (pqqC) gene copies and greater resin-P content in the rhizosphere. At 0P, applying 2,4-EBL increased root BR content, activated BR-signaling gene expression, improved root and rhizosphere traits, and enhanced early-stage P accumulation, whereas applying BRs biosynthesis inhibitor had opposite effects. Applying 2,4-EBL additionally favored recruitment of the phosphate-solubilizing bacterium Massilia. Correlation and structural equation model analyses supported a pathway whereby elevated BR content activated BR signaling and downstream cascades that strengthened root performance and enriched Massilia, thereby increasing absorptive capacity and rhizosphere P supply. Overall, BRs mediate grain yield and PUE by optimizing root-rhizosphere cooperation under P-deficiency conditions.