SPÍCHAL, Lukáš, WERNER, T., POPA, Igor, RIEFLER, M., SCHMÜLLING, T., STRNAD, Miroslav. The purine derivative PI-55 blocks cytokinin action via receptor inhibition. FEBS Journal. 2009, 276(1), 244-253. ISSN 1742-464X. E-ISSN 1742-4658

We report the identification of the first known molecule antagonizing the activity of the plant hormone cytokinin at the receptor level. This compound, 6-(2-hydroxy-3-methylbenzylamino)purine, designated PI-55, is structurally related to cytokinin 6-benzylaminopurine, but substitutions at specific positions of the aromatic side chain strongly diminished its cytokinin activity and conferred antagonistic properties. PI-55 competitively inhibited the binding of the natural ligand trans-zeatin to the Arabidopsis cytokinin receptors CRE1/AHK4 and AHK3 and repressed induction of the cytokinin response gene ARR5:GUS. Genetic analysis revealed that CRE1/AHK4 is the primary target of PI-55. Cytokinin bioassays also demonstrated the anticytokinin effect of PI-55 in several other species. PI-55 accelerated the germination of Arabidopsis seeds and promoted the root growth and formation of lateral roots, thus phenocopying the known consequences of a lowered cytokinin status and demonstrating its potential to inhibit cytokinin perception in planta.