Leaf senescence is one of the final stages of plant development. It is a highly regulated degradation process that involves changes in cell structure, metabolism, gene expression, and also environmental signals. In particular, it is associated with substantial changes in the expression of genes involved in degrading proteins into Altiratinib
and membrane lipids into fatty acids and sugars (Sarwat et al., 2013). Plant senescence involves enhanced chlorophyll catabolism activity and the degradation of leaf proteins, membrane lipids and RNA; these processes are largely responsible for the color changes of autumn leaves. There is extensive crosstalk between plant hormones such as CKs, auxins, ethylene and abscisic acid, which establishes an endogenous regulatory pathway for age-controlled senescence (Lim et al., 2007). CKs play an essential role in this process by delaying the onset of senescence. In Arabidopsis, the AHK3 receptor seems to play a major role in regulating cytokinin-mediated leaf longevity because it promotes the phosphorylation
of the cytokinin response regulator ARR2 ( Kim et al., 2006). In this context it is interesting that bacterial receptor assays have shown that the AHK3 and AHK4 receptors have important differences in their ligand preferences ( Lomin et al., 2015 and Spichal et al.,
2004). While iP and tZ are preferred ligands for both receptors, only AHK3 is activated by CK ribosides and ribotides. In addition, AHK3 also has a higher affinity for BAP and other ArCKs, which generally exhibit low activity in bacterial receptor tests despite their significant biological activity in planta ( Spichal et al., 2004).