Atypical response regulators expressed in the maize endosperm transfer cells link canonical two component systems and seed biology.

Two component systems (TCS) are phosphotransfer-based signal transduction pathways first discovered in bacteria, where they perform most of the sensing tasks. They present a highly modular structure, comprising a receptor with histidine kinase activity and a response regulator which regulates gene expression or interacts with other cell components. A more complex framework is usually found in plants and fungi, in which a third component transfers the phosphate group from the receptor to the response regulator. They play a central role in cytokinin mediated functions in plants, affecting processes such as meristem growth, phyllotaxy, seed development, leaf senescence or tissue differentiation. We have previously reported the expression and cellular localization of a type A response regulator, ZmTCRR-1, in the transfer cells of the maize seed, a tissue critical for seed filling and development, and described its regulation by a tissue specific transcription factor. In this work we investigate the expression and localization of other components of the TCS signalling routes in the maize seed and initiate the characterization of their interactions. The discovery of a new type A response regulator, ZmTCRR-2, specifically expressed in the transfer cells and controlled by a tissue specific transcription factor suggests a previously unknown role for TCS in the biology of transfer cells. We have characterized other canonical TCS molecules, including 6 histidine kinases and 3 phosphotransfer proteins, potentially involved in the atypical transduction pathway defined by ZmTCRR-1 and 2. We have identified potential upstream interactors for both proteins and shown that they both move into the developing endosperm. Furthermore, ZmTCRR-1 expression in an heterologous system (Arabidopsis thaliana) is directed to xylem parenchyma cells, probably involved in transport processes, one of the major roles attributed to the transfer cell layer. Our data prove the expression of the effector elements of a TCS route operating in the transfer cells under developmental control. Its possible role in integrating external signals with seed developmental processes is discussed.