Cytokinin derivatives as tools to enhance plant stress resilience.

Cytokinins (CKs) are key regulators of plant growth and development. Recently, CKs have been increasingly recognized to have additional roles as modulators of plant adaptation to abiotic and biotic stresses. A large part of CK action in stress adaptation can be attributed to the promotion of antioxidative defense mechanisms. However, the contributions of CKs to stress tolerance can be complex and sometimes contradictory, as both increased and decreased CK signaling can enhance stress resilience depending on the context, a phenomenon referred to as the "cytokinin paradox". In addition, some effects of exogenously applied classical CKs, including inhibition of root elongation and stimulation of stomatal opening, may become detrimental under severe stress. Recent research has therefore focused on developing advanced CK-derived compounds that selectively enhance stress tolerance but possess reduced classical growth-regulatory activities of CKs. These advanced derivatives, including N9-substituted CKs, CKX inhibitors, and urea-based compounds, retain antioxidant and photoprotective functions while minimizing developmental penalties. Their potential as tools to improve the stress resilience of crop plants is discussed.