AHK4 Antibody

This antibody targets AHK4, a cytokinin receptor related to bacterial two-component regulators. It binds to both the naturally occurring cytokinin and the synthetic urea-type cytokinin thidiazuron, a potent defoliant and herbicide. AHK4 functions as a histidine kinase, transmitting stress signals to a downstream MAPK cascade. The protein undergoes ATP-dependent autophosphorylation at a conserved histidine residue within the kinase core, subsequently transferring a phosphoryl group to a conserved aspartate residue in the receiver domain. In the presence of cytokinin, AHK4 feeds phosphate to phosphorelay-integrating histidine phosphotransfer protein (HPt), activating the subsequent cascade. Conversely, in the absence of cytokinin, it removes phosphate from HPt proteins, decreasing the system phosphoload. AHK4 plays a crucial role in meristems establishment in seedlings and acts as a redundant negative regulator of drought, salt stress responses, and abscisic acid (ABA) signaling, in a cytokinin-dependent manner. It is required for establishing vascular asymmetric cell divisions that define phloem and procambium cell lines. AHK4 acts as a redundant positive regulator of cytokinin signaling, influencing numerous developmental processes, including seed germination, cell division, seed size, chlorophyll retention during leaf senescence, root repression, and shoot promotion. AHK4 interacts with various isoprenoid-type cytokinins, including trans-zeatin (tZ and tZR), isopentenyladenine (iP), and isopentenyladenosine (iPR), as well as the meta hydroxylated derivative of benzyladenine m-topolin, buta-2,3-dienyladenine (HA-8), penta-2,3-dienyladenine (HA-1), 4-methyl-penta-2,3-dienyladenine (HA-10), 4-hydroxy-2-butynyladenine (RM1), 2-butynyladenine (RM6), and to a lesser extent, with cis-zeatin (cZ), zeatin riboside, and dihydrozeatin (DZ). Alongside AHK3, it participates in the cytokinin-dependent responses to Pi starvation and sucrose stresses. AHK4 is required for the formation of auxin-transporting vascular tissues in the hypocotyl, and primary and lateral roots, but not in adventitious roots, thus leading to auxin basipetal transport that regulates root development and branching. AHK4 also plays a role in alkamides (e.g. N-isobutyl decanamide) and N-acylethanolamides (NAE) signaling, controlling meristematic activity and differentiation processes during plant development. Moreover, it prevents sulfate uptake by mediating cytokinin-dependent down-regulation of high-affinity sulfate transporters (e.g. SULTR1;1 and SULTR1;2) expression in roots. Together with AHK2, it is required for growth and reproduction promotion stimulated by the endophytic fungus Piriformospora indica in a trans-zeatin-dependent manner. AHK4 is necessary to trigger the phytotoxic effect of the snapdragon (Antirrhinum majus) flowers volatile organic compound (VOC) methyl benzoate (MB). It also plays a role in the cytokinin-mediated repression of the iron uptake pathway.