ARR1 Directly Activates Cytokinin Response Genes that Encode Proteins with Diverse Regulatory Functions

Plant cells respond to cytokinins by changing their gene expression patterns. The histidyl-aspartyl (His-Asp) phosphorelay mediates the signal from cytokinin receptors to type-B response regulators including ARR1, which transactivate cytokinin primary response genes. However, the overall architecture of the signal cascade leading to cytokinin-responsive phenomena is still unclear, mainly because it is not known how the His-Asp phosphorelay is connected to downstream phenomena. To reveal events immediately downstream from the phosphorelay-mediated transcriptional activation, we searched for direct-target genes of ARR1 by exploiting ARR1DeltaDDK-GR, a chimeric transcription factor that transactivates ARR1 direct-target genes in transgenic plants by glucocorticoid induction. We identified 23 direct-target genes, most of which were found to be cytokinin primary response genes. The arr1-1 mutation clearly affected the primary response in at least 17 genes, meaning that they respond primarily to cytokinins through the function of ARR1. The 17 genes encode proteins with diverse functions, including type-A response regulators, cytokinin metabolic enzymes and putative disease resistance response proteins. These results provide novel evidence indicating that the His-Asp phosphorelay is connected to diverse regulatory levels of cytokinin-responsive phenomena through ARR1 direct-target genes.