PI-55 (6-(2-hydroxy-3-methylbenzylamino)purine) is a cytokinin receptor inhibitor that exhibits structural similarity to 6-benzylaminopurine (BAP). It competitively inhibits BAP binding on the specific Arabidopsis receptors CRE1/AHK4 and AHK3. Furthermore, PI-55 effectively suppresses cytokinins-induced haustorium formation and enhances parasite aggressiveness [1].

High concentrations of PI-55 (10 μM and 100 μM) result in partial inhibition of early haustorial structure development, particularly in the presence of cytokinin. Further, PI-55 treatment, when combined with BAP, diminishes the virulence of P. ramosa compared to the application of BAP alone. This suggests the involvement of histidine kinase receptors analogous to CRE1/AHK4 and AHK3 in the signaling pathway that initiates early haustorial formation [1].