Nano-improved plant salinity tolerance: The importance of K+/Na+ homeostasis and crosstalk between Ca2+ and hormones

Salinity stress is a major constraint on plant organ morphogenesis, and agricultural production, mostly by disrupting ion homeostasis and plant water status, leading to detrimental K + /Na + imbalance. Maintaining subcellular ionic balance is a critical defense mechanism against abiotic stresses, and plants employ diverse strategies to mitigate ion toxicity. Nanobiotechnology offers a promising approach to enhance plant ion homeostasis under stressed environments, leveraging nanoparticles' (NPs) capacity to modulate stress-responsive signaling pathways in crops. Crucially, NPs initiate crosstalk between Ca²⁺ signaling and hormonal networks, which cooperate with reactive oxygen species (ROS), K + , and nitric oxide (NO) signaling to regulate transcription factors (TFs) essential for ionic equilibrium. This review examines the role of NPs in promoting K⁺/Na⁺ homeostasis during salinity stress by regulating molecular, physiological, anatomical, and morphological mechanisms. These NP-induced Ca²⁺/hormonal networks directly or indirectly regulate NO signaling to bolster organ morphogenesis and stress tolerance. NPs enhance salinity tolerance by upregulating key genes (e.g., SOS1 , SOS2 , SOS3 , HKT1 , NHX ), improving ion homeostasis and organ development. Moreover, NP-triggered crosstalk between Ca²⁺ signaling and hormones plays a pivotal role in regulating TFs such as bHLH , R2R3-MYB , WRKY , NAC , ZIP , ERFs, and NFX1 . Collectively, these signaling and TF networks orchestrated by NPs sustain a high K⁺/Na⁺ ratio by regulating K⁺ and Ca²⁺ transport/distribution and reducing Na⁺ toxicity. Improved K⁺/Na⁺ regulation enhances nutrient uptake, activates ROS scavenging systems, modulates phytohormone levels, boosts photosynthetic efficiency, and optimizes stomatal motions. Understanding the mechanistic basis of NP-mediated stress regulation will elucidate their mode of action and the associated signaling cascades, clarifying their contribution to ion homeostasis under salinity stress.