Poster: Hormones

Abs # 569: Cytokinin-mediated His-to-Asp phosphorelay signal transduction in Arabidopsis thaliana, (I) Functional interaction between the AHK family of cytokinin receptors and type-A ARRs

Presenter:	Kiba, Takatoshi , i002006d@mbox.nagoya-u.ac.jp
Authors	Kiba, Takatoshi  (A)   Yamada, Hisami  (A)   Mizuno, Takeshi  (A)
Affiliations:	(A): Grad. Sch. Bioagric. Sci., Nagoya Univ.

Cytokinins are a class of plant hormones that influence numerous aspects of plant development. Recently, it was demonstrated that the AHK2/3/4 His-kinases function as cytokinin-binding signal receptors. These cytokinin sensors presumably act in concert with downstream components, such as histidine-containing phosphotransfer factors(HPt) and response regulators(RR), through a His-Asp phosphorelay signal transduction pathway. Arabidopsis have 5 genes each coding HPt factor(AHP) and 22 genes each coding RR(ARR). The ARR family members are basically classified into two distinct subtypes. The type-A ARR members are rapidly induced by cytokinin at the level of transcription, whereas the type-B ARR members most likely act as DNA-binding transcriptional regulator. Thus, a certain multistep[AHK2/3/4→AHP→ARR] phosphorelay network was considered to be involved in a signaling network in response to cytokinin. In this paper, we attempted to gain further insight into the above scenario. It was found that, among 10 type-A ARR family members, the cytokinin-inducible expression of ARR15 and ARR16 genes in roots are markedly impaired in the cre1-1 mutant, which carries a loss-of-function mutation in the AHK4 gene, suggesting a link between these type-A ARRs and the AHK4-mediated cytokinin signal transduction. To provide further evince for this idea, we characterized not only T-DNA insertion mutants of ARR15, but also transgenic lines that ectopically overexpress ARR15 and ARR16 genes in a manner independent of cytokinin. The results supported the idea that ARR15 and ARR16 serve as negative regulators in a cytokinin signal transduction pathway. Physiological role of type-A ARRs will be discussed with special reference to the AHK4-mediated cytokinin signal transduction pathway.