Cytokinins are plant hormones with profound roles in growth and development, including control of leaf longevity. Although the cytokinin signal is known to be perceived by histidine kinase receptors, the underlying molecular mechanism and specificity of the receptors leading to delayed leaf senescence have not yet been elucidated. Here, we found that AHK3, one of the three cytokinin receptors in Arabidopsis, plays a major role in controlling cytokinin-mediated leaf longevity through a specific phosphorylation of a response regulator, ARR2. This result was obtained through identification of a gain-of-function Arabidopsis mutant that shows delayed leaf senescence because of a missense mutation in the extracellular domain of AHK3. A loss-of-function mutation in AHK3, but not of the other cytokinin receptors, conferred a reduced sensitivity to cytokinin in cytokinin-dependent delay of leaf senescence and abolished cytokinin-dependent phosphorylation of ARR2. Consistently, transgenic overexpression of wild-type, but not an unphosphorylatable mutant ARR2, led to delayed senescence of leaves.histidine kinase 3 ͉ cytokinin ͉ leaf senescence L eaf senescence is typically observed in autumn leaves and during the death process of monocarpic plants (1). During senescence, leaf cells experience striking changes in cellular metabolism and structure (2). The first visible change is yellowing of the leaves caused by chlorophyll breakdown during chloroplast disassembly (3), accompanied by hydrolysis of macromolecules such as lipids, proteins, and nucleic acids. The products of this catabolic process are reallocated to developing organs, such as younger leaves and germinating seeds (4). Therefore, senescence is not a simple degenerative process but is considered as a genetically programmed, active process that contributes to the fitness of plants. Although occurring in an age-dependent manner, leaf senescence is greatly affected by various endogenous and environmental signals to attune the life span of leaves to optimized fitness of plants (5, 6). Among the endogenous developmental signals, the plant hormones cytokinins have a particularly profound effect on longevity of plant organs (7-9). For example, it has been shown that the transgenic modification of cytokinin biosynthesis during the senescence phase can significantly delay the senescence of plant organs, including leaves, and improve plant productivity by up to 50% (8, 10).Besides control of longevity, cytokinins have many critical functions in plants, such as the control of cell proliferation, shoot formation, nutrient relocation, and shoot branching (11,12). In Arabidopsis, cytokinins are known to be perceived by three cytokinin receptors, AHK2, AHK3, and AHK4͞CRE1͞WOL, which possess the structural features of hybrid histidine kinases (13,14). In the model proposed for the cytokinin signaltransduction pathway (15-17), a histidine protein kinase initiates the cytokinin signal-transduction pathway through phosphorelay to histidine phosphotransfer (AHP) proteins. AHP protei...

Phytohormone brassinosteroids (BRs) play critical roles in plant growth and development. BR acts by modulating the phosphorylation status of two key transcriptional factors, BRI1 EMS SUPPRESSOR1 and BRASSINAZOLE RESISTANT1 (BZR1), through the action of BRASSINOSTEROID INSENSITIVE1/BRI1 ASSOCIATED RECEPTOR KINASE1 receptors and a GSK3 kinase, BRASSINOSTEROID INSENSITIVE2 (BIN2). It is still unknown how the perception of BR at the plasma membrane connects to the expression of BR target genes in the nucleus. We show here that BZR1 functions as a nucleocytoplasmic shuttling protein and GSK3-like kinases induce the nuclear export of BZR1 by modulating BZR1 interaction with the 14-3-3 proteins. BR-activated phosphatase mediates rapid nuclear localization of BZR1. Besides the phosphorylation domain for 14-3-3 binding, another phosphorylation domain in BZR1 is required for the BIN2-induced nuclear export of BZR1. Mutations of putative phosphorylation sites in two distinct domains enhance the nuclear retention of BZR1 and BR responses in transgenic plants. We propose that the spatial redistribution of BZR1 is critical for proper BR signaling in plant growth and development.