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Abstract
The conquest of the land by plants required dramatic morphological and metabolic adaptations. Complex developmental programs under tight regulation evolved during this process. Key regulators of plant development are phytohormones, such as cytokinins. Cytokinins are adenine derivatives that affect various processes in plants. The cytokinin signal transduction system, which is mediated via a multistep variant of the bacterial two-component signaling system, is well characterized in the model plant Arabidopsis (Arabidopsis thaliana). To understand the origin and evolutionary pattern of this signaling pathway, we surveyed the genomes of several sequenced key plant species ranging from unicellular algae, moss, and lycophytes, to higher land plants, including Arabidopsis and rice (Oryza sativa), for proteins involved in cytokinin signal transduction. Phylogenetic analysis revealed that the hormone-binding receptor and a class of negative regulators first appeared in land plants. Other components of the signaling pathway were present in all species investigated. Furthermore, we found that the receptors evolved under different evolutionary constraints from the other components of the pathway: The number of receptors remained fairly constant, while the other protein families expanded.
Highlights
The conquest of the land by plants required dramatic morphological and metabolic adaptations
The signal is transferred by phosphorylation of His phosphotransmitter proteins (HPts), which translocated to the nucleus
For a complete analysis involving all members of a particular pathway of a given species it is essential to have access to the full genome sequence. With this caveat in mind, we focused our analysis on the nuclear genomes of Ostreococcus tauri (Derelle et al, 2006), Chlamydomonas reinhardtii, as model species for unicellular green algae (Merchant et al, 2007), and Volvox carteri, the multicellular green algae, which serves as a model for the evolution of multicellularity and differentiation (Schmitt, 2001)
Summary
The conquest of the land by plants required dramatic morphological and metabolic adaptations. The signaling pathway of this hormone is mediated by a multistep His-to-Asp phospho-relay system, a variant of the bacterial twocomponent system (TCS; Supplemental Fig. S1) While this type of signaling system is widespread in prokaryotes, it is unique to plants among higher eukaryotes (West and Stock, 2001; Heyl and Schmulling, 2003). Unraveling the Evolution of Cytokinin Signaling transcription factors initiate the transcription of their target genes, one group of which are the type-A RRs. Type-A RR proteins have been shown to be involved in a negative feedback mechanism regulating the activity of the cytokinin signaling pathway (Hwang and Sheen, 2001; To et al, 2004). Several pseudo RRs have been identified in Arabidopsis, but were shown to play a role in the regulation of the circadian rhythm and not in cytokinin signaling (Mizuno and Nakamichi, 2005)
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