The potential applications of cytokinins and cytokinin oxidase/ dehydrogenase inhibitors for mitigating abiotic stresses in model and non-model plant species

Cytokinins (CKs) are important phytohormones which are used by plants to optimize responses against abiotic stresses such as drought, salinity, temperature and nutrient stresses known to repress germination, and influencing general plant growth and development. Such stresses often trigger phenotypic plasticity and lead to low yields. Yet, the beneficial effect of CKs is counteracted by cytokinin oxidase/dehydrogenase (CKO/CKX, EC 1.5.99.12) enzymes and by N- and/or O-glycosylation. Additionally, research on CKs and CKX is often limited to model plants studied in isolation, and sparsely covers non-model plants exposed to abiotic stresses. Thus, this review explored the role of CKs and CKX inhibitors in mitigating abiotic stresses in model and non-model plants. We also examined possible crosstalk mechanisms of CKs with auxins, polyamines, and other major phytohormones. A detailed literature search was conducted using several databases including Web of Science, Google Scholar, ScienceDirect, Scopus, and PubMed. Upon perception of environmental stimuli, CKs [e.g., N6-(Delta 2isopent-2-enyl)adenine (iP), trans-zeatin (tZ) and cis-zeatin (cZ)] induce abiotic stress tolerance in a CK - dependent manner or by forming intermolecular pathways with abscisic acid, ethylene, auxins and polyamines. Regulatory motifs of type-B ARRs code for transcriptional responses via DNA-binding. Inhibitors of CKX (e.g., 3TFM-2HE, INCYDE, F-INCYDE and anisiflupurin) act as promoters of growth and stress-tolerance through the inhibition of catabolic CKXs and regulate an increase in endogenous CKs (e.g., iP, tZ and cZ) in plants. The ability of CKX inhibitors to intercept CKX gene regulation is an indication of their potential applications in agriculture and other industries that rely on plant-based products.