Abstract
Balancing growth and defense under fluctuating environments is critical for plant resilience. This study uncovers how microRNA-mediated phosphorus (P) allocation regulates this equilibrium in soybean under variable light. We demonstrate that miR397a acts as a susceptibility factor by repressing laccase genes GmLAC7 and GmLAC12, compromising lignin-based structural defense and enhancing Soybean mosaic virus (SMV) accumulation under normal light. Conversely, miR399j enhances resistance through targeted suppression of the phosphate transporter GmPHT1-4, reprogramming systemic P distribution to favor leaf allocation and support defense capacity. Under shade, however, the relationship between light, P, and immunity becomes context-dependent: although early SMV accumulation is reduced-likely reflecting slowed viral replication or growth prioritization-shade simultaneously induces systemic P reallocation to roots, depleting shoot P pools. This creates a metabolic bottleneck that uncouples GmLAC7/12 up-regulation from functional lignin deposition in miR397a-silenced plants, ultimately impairing structural barriers and permitting viral spread during sustained infection. Critically, exogenous P application restores lignin synthesis and suppresses SMV under shade, confirming P availability as the limiting factor. Our findings establish a light-gated hierarchy of resource allocation: under optimal light, miR399j directs P toward aerial defense; under shade, P conservation in roots comes at the cost of inducible structural immunity. This mechanistic framework offers new strategies for optimizing crop resilience in heterogeneous light environments.