Low R:FR Light Shifts the Growth-Immunity Balance in Rice via PIF-Mediated Suppression of SA and JA Pathways

High-density rice planting reduces light quality within the canopy, especially the red to far-red (R: FR) ratio, triggering a physiological shift that enhances elongation growth at the expense of weakened defence mechanisms. This is not a passive consequence but a coordinated regulation controlled by the Phytochrome B (PhyB)-Phytochrome Interacting Factor (PIF) signalling module. Under low R: FR, PhyB becomes inactive, stabilising key PIFs such as OsPIL13 and OsPIF4. These transcription factors promote shade-avoidance growth by enhancing auxin and gibberellin biosynthesis, which in turn suppresses salicylic acid (SA) and jasmonic acid (JA) signalling. They also directly repress the expression of core defence genes. Together, these changes lower immune readiness in shaded rice plants. Here, we propose a rice-specific model in which low R: FR light signals directly suppress immunity through PIF-mediated transcriptional repression, highlighting a monocot-specific mechanism that integrates light perception with immune downregulation.