PI-55 at high concentrations (10 μM and 100 μM), which causes an incomplete blocking of early haustorial structure development, especially when cytokinin activity promotes it. PI-55 treatment also reduces the overall aggressiveness of P. ramosa when applied with BAP in comparison with BAP alone, suggesting that the signaling pathway leading to early haustorial structure formation involves histidine kinase receptors homologous to CRE1/AHK4 and AHK3.