SUMMARY The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type‐B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type‐A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the ricePHPgenes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three ricePHPsfail to complement an Arabidopsisphpmutant (aphp1/ahp6). Disruption of the three ricePHPsresults in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type‐A RR and cytokinin oxidase genes. The triplephpmutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the ricePHPsdoes not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.
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Role of the Cytokinin-Activated Type-B Response Regulators in Hormone Crosstalk
Cytokinin is an important phytohormone that employs a multistep phosphorelay to transduce the signal from receptors to the nucleus, culminating in activation of type-B response regulators which function as transcription factors. Recent chromatin immunoprecipitation-sequencing (ChIP-seq) studies have identified targets of type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) and integrated these into the cytokinin-activated transcriptional network. Primary targets of the type-B ARRs are enriched for genes involved in hormonal regulation, emphasizing the extensive crosstalk that can occur between cytokinin, auxin, abscisic acid, brassinosteroids, gibberellic acid, ethylene, jasmonic acid, and salicylic acid. Examination of hormone-related targets reveals multiple regulatory points including biosynthesis, degradation/inactivation, transport, and signal transduction. Here, we consider this early response to cytokinin in terms of the hormones involved, points of regulatory crosstalk, and physiological significance.
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- PAR ID:
- 10151630
- Date Published:
- Journal Name:
- Plants
- Volume:
- 9
- Issue:
- 2
- ISSN:
- 2223-7747
- Page Range / eLocation ID:
- 166
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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