Response regulators are signal-transduction molecules present in bacteria, yeast and plants, acting as relays for environmental challenges. This paper reports the characterization of a Zea mays gene, ZmTCRR-1, that codes for a member of the type-A response regulator class of proteins. The gene was found to be expressed exclusively in the endosperm transfer-cell layer 8-14 days after pollination, when transfer-cell differentiation is most active. The promoter of ZmTCRR-1 was strongly transactivated in heterologous systems by the transfer cell-specific transcription factor ZmMRP-1. The ZmTCRR-1 protein was detected not only in the transfer-cell layer, but also in the conductive tissue deep inside the endosperm, where there is no transcription of the gene. This suggests that two-component systems might be involved in intercellular signal transmission, in contrast to the generally held belief that these systems are involved only in cell-autonomous pathways.