Specification of cellular fate in the Arabidopsis root depends on signaling by the hormone cytokinin. Mähönen et al. have now analyzed how cytokinin regulates and stabilizes choices in vascular cell fate. Protoxylem differentiation is a default choice, a direction that is blocked by cytokinin. The AHP6 locus promoted protoxylem differentiation and encodes a protein with resemblances to phosphotransfer proteins except for an amino acid residue critical for phosphotransfer. Nonetheless, it inhibits a cytokinin-directed phosphorelay system. AHP6 expression is spatially localized such that it can block cytokinin function in specific regions, thus allowing protoxylem specification in those locations. Cytokinin and AHP6 interact together in a feedback loop to create specific cellular domains that remain less responsive to cytokinins.
A. P. Mähönen, A. Bishopp, M. Higuchi, K. M. Nieminen, K. Kinoshita, K. Törmäkangas, Y. Ikeda, A. Oka, T. Kakimoto, Y. Helariutta, Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development. Science 311, 94-98 (2006). [Abstract] [Full Text]