Poster: Hormones

Abs # 616: His-Asp phosphorelay signaling factors in maize leaves: Implications of the signal divergence by cytokinin-inducible response regulators in the cytosol and the nuclei

Presenter:	Sakakibara, Hitoshi , sakaki@postman.riken.go.jp
Authors	Sakakibara, Hitoshi  (A)   Asakura, Yasuharu  (A)   Hagino, Takeshi  (A)   Aoki, Koh  (A)   Yonekura-Sakakibara, Keiko  (A)   Yamaya, Tomoyuki  (A)
Affiliations:	(A): Plant Science Center, RIKEN (The Institute of Physical and Chemical Research)

We isolated genes for histidyl-aspartyl (His-Asp) phosphorelay components (His-containing phosphotransfer proteins, HP, and response regulators, RR) from Zea mays L. to characterize their function in cytokinin signaling. Six type-A RRs (ZmRR1, ZmRR2, ZmRR4 to ZmRR7), 3 type-B RRs (ZmRR8 to ZmRR10), and 3 HPs (ZmHP1 to ZmHP3) were found in leaves. ZmRR3 specifically expressed in the ear and silk. All type-A RR genes expressed in leaves were upregulated by cytokinin. Transient expression of fusion products of the signaling modules with GFP suggested cytosolic and nuclear localizations of ZmHPs, whereas type-B ZmRR8 was restricted to the nucleus. Type-A RRs were localized partly in the cytosol (ZmRR1, ZmRR2, and ZmRR3) and partly in the nucleus (ZmRR4, ZmRR5, and ZmRR6). In the yeast two-hybrid assay, ZmHP1 and ZmHP3 interacted with both cytosolic ZmRR1 and nuclear type-B ZmRRs. In vitro experiments demonstrated that ZmHPs function as a phospho-donor for ZmRRs; turnover rates of the phosphorylated state were several folds lower in ZmRR8 and ZmRR9 than in ZmRR1 and ZmRR4. These results suggest that the His-Asp phosphorelay signaling pathway might diverge into a cytosolic and a nuclear branch in leaves of maize, and that the biochemical nature of ZmRRs is different in terms of stability of the phosphorylated status.