Potassium Stress Induces Compensatory Root Adaptive Responses in Trifoliate Orange Through Reconfigured Auxin Signaling

Potassium (K+) is essential for plant growth and development, influencing numerous physiological processes and stress responses. While the importance of K+ in overall plant performance is well-established, its specific effects on root system architecture and the underlying molecular mechanisms in woody perennials remain poorly understood. This knowledge gap is particularly significant for citrus rootstocks like trifoliate orange (Poncirus trifoliata L.), where root system optimization directly impacts drought resistance, nutrient acquisition, and overall orchard productivity. Here, we investigated how varying K+ concentrations (K0, K2, K6, and K12) affect trifoliate orange seedling development by comprehensively analyzing root architecture parameters, root hair morphology, endogenous hormone levels, and expression patterns of cell-wall-modifying and auxin-related genes. We found that moderate K+ levels (K6) optimized root architectural development while both deficiency (K0, K2) and excess (K12) inhibited overall growth and root architecture but enhanced root hair development. This morphological dichotomy corresponded to distinct hormonal profiles, showing reduced auxin (IAA), gibberellins (GAs), and zeatin riboside (ZR) levels under K+ stress conditions. Gene expression analysis revealed significant upregulation of expansins (PtEXPA4, PtEXPA5, PtEXPA7) and reconfiguration of auxin biosynthesis (TAA/TAR/YUC) and transport (AUX/LAX/ABCB/PIN) machinery under non-optimal K+ conditions. Our findings suggest that K+ availability modulates trifoliate orange root development through coordinated regulation of hormone homeostasis and gene expression, particularly within the auxin signaling network. These findings elucidate K+-responsive root developmental plasticity as a potential adaptive strategy, providing valuable insights for optimizing fertilization strategies in citrus cultivation and identifying potential molecular targets for enhancing potassium use efficiency in woody perennials.