**4. Concluding remarks on crossroads between abiotic stresses and micro/nanoplastics and nanoparticles**

Adsorption or uptake of micro/nanoplastics through root tips leads to toxicity and activates mechanoreceptors similar to other osmotic stresses. Apparently, plants share common stress perception and signal transduction crossroads between familiar abiotic factors and novel micro/nanoplastics and nanoparticles, even though evolutionary adaptations have not introduced these pollutants to plants before. Micro/nanoplastics may cause mechanical stress by physical blockage, disconnecting cells, and consequently reducing signal transmission. Obstructing properties prevent plants to uptake water and nutrients, hence reducing germination. Due to the extremely small size of nanoparticles, rapid and relatively uncontrolled penetration and translocation to various cell compartments occur. Transport proteins or ion channels mentioned earlier utilize the proportional entry of nanoparticles as well as endocytosis. Especially metallic forms of nanoparticles, such as Cu, Ni, Zn, TiO2, and CeO2, may lead to excessive ROS production through the Fenton reactions by altering oxidative states. Occasionally, nanoparticles can decrease intracellular H2O2 concentrations and lipid peroxidation by increasing the efficiency of redox reactions by playing a central role in electron retransmission. On the other hand, nanoparticle exposure may increase the production of <sup>1</sup> O2; hence, creating unique ROS signatures to be decoded to appropriate abiotic stress response. Ca2+ ions play a vital role in increasing plant tolerance during abiotic stresses by modulating stress signaling and responses. Ca nanoparticle applications lead to better utilization of mineral elements. CaO nanoparticles undertake a key role in stress signaling processes to maintain ion homeostasis in plants. ABA is a notorious phytohormone for stress signaling and abiotic stress responses. Nanoparticle exposure rapidly impacts the ABA signaling pathway. Ag nanoparticles induced ABA signaling by enhancing ROS and altering root growth. La2O3 nanoparticles induce rapidly detectable ABA fluctuations through the ABA receptors. Similarly, ZnO nanoparticles exposure mediates the transcript level of ABA synthesis and catabolism-related genes. Further proteomic, transcriptomic, epigenetic, and other omic-based examinations will provide insight into the regulatory role of nano-sized pollutants in the stress resistance of future plant cultivars [109–112].
