Cytokinin Senescence Delay Is Shaped by Receptor Specificity and Metabolic Stability

Each of the four different cytokinin (CK) base forms, trans Zeatin (tZ), isopentenyladenine (iP), dihydrozeatin (DHZ), and cis Zeatin (cZ) have distinct chemical metabolism and affinity to the CK Histidine Kinase (CHK) receptors. However, it remains unclear how specific biochemical features of each form such receptor specificity or metabolic differences drives distinct tissue-specific physiological hormone output in response to application of these CK bases. Here, we show that CK receptor preference and metabolic persistence together shape isoform-specific CK signaling strength, including tissue-dependent hormone responses in Arabidopsis leaf versus root assays. Physiological, genetic, and multi-omics integration was used to show that tZ and iP anti senescence activity is matched by DHZ through a distinct receptor metabolic mechanism. DHZ requires Arabidopsis Histidine Kinase 3 (AHK3) signaling to be fully effective in a leaf Dark Induced Senescence (DIS) assay and where it overcomes its lower receptor affinity through higher metabolic persistence, accumulating at levels [~]2.5-fold above tZ and iP early in a senescence time course. Together, these findings provide a framework for integration of receptor preference and metabolic stability to determine CK isoform activity.