ABSTRACT
We examined the expression of a family of Arabidopsis response regulators (ARR) and found that the steady-state levels of RNA for most are elevated very rapidly by cytokinin. Using nuclear run-on assays we demonstrated that this increase in ARR transcript levels in response to cytokinin is due, at least in part, to increased transcription. The start site of transcription for the ARR5 gene was identified using primer extension analysis. A DNA fragment comprised of 1.6 kb upstream of the ARR5 transcript start site conferred cytokinin-inducible gene expression when fused to a beta-glucuronidase reporter, confirming that the transcription rate of ARR5 is elevated by cytokinin. This reporter construct was also used to examine the spatial pattern of ARR5 expression. The highest levels of expression were observed in the root and shoot apical meristems, at the junction of the pedicle and the silique, and in the central portion of mature roots. The expression of ARR5 in the apical meristems was confirmed by whole mount in situ analysis of seedlings and is consistent with a role for cytokinin in regulating cell division in vivo.
Subject(s)
Arabidopsis/drug effects , Cytokinins/pharmacology , Plant Growth Regulators/pharmacology , Plant Proteins/genetics , Amino Acid Sequence , Arabidopsis/genetics , Arabidopsis/growth & development , Base Sequence , Blotting, Northern , Cycloheximide/pharmacology , Gene Expression Regulation, Developmental/drug effects , Gene Expression Regulation, Plant/drug effects , Glucuronidase/genetics , Glucuronidase/metabolism , Molecular Sequence Data , Multigene Family , Phylogeny , Plants, Genetically Modified , Promoter Regions, Genetic/genetics , RNA, Plant/drug effects , RNA, Plant/genetics , RNA, Plant/metabolism , Recombinant Fusion Proteins/drug effects , Recombinant Fusion Proteins/genetics , Recombinant Fusion Proteins/metabolism , Sequence Homology, Amino Acid , Time Factors , Transcription, Genetic/drug effectsABSTRACT
Homologs of bacterial two-component signal transduction elements are emerging as key players in eukaryotic signaling systems. The recent identification of a large gene family in Arabidopsis that is similar to two-component response regulators emphasizes the importance of this signaling mechanism in plants. The understanding of the function of these response regulator genes is only rudimentary but the transcriptional induction of a subset by cytokinin suggests a role for some of these regulators in the response to this important plant hormone.
Subject(s)
Arabidopsis/genetics , Arabidopsis/physiology , Plant Growth Regulators/physiology , Signal Transduction/physiology , Arabidopsis/enzymology , Bacterial Physiological Phenomena , Genes, Plant/genetics , Genes, Plant/physiology , Phylogeny , Plant Growth Regulators/classification , Sequence Homology, Amino AcidABSTRACT
Cytokinins have been implicated in many aspects of plant development, including a crucial role in regulating cell proliferation. Recent studies indicate that cytokinins may elevate cell division rates by induction of expression of CycD3, which encodes a D-type cyclin thought to play a role in the G1-->M transition of the cell cycle. Progress has also been made in our understanding of cytokinin perception as homologs of two-component phosphorelay systems have emerged as likely signaling elements.
