Essential protein. Functions as a histidine kinase and transmits the stress signal to a downstream MAPK cascade. This protein undergoes an ATP-dependent autophosphorylation at a conserved histidine residue in the kinase core, and a phosphoryl group is then transferred to a conserved aspartate residue in the receiver domain. Required for the development of megagametophyte in female gametophyte (embryo sac) independently of cytokinin. Contributes to vascular bundle formation and secondary growth in a cytokinin-independent manner, probably by promoting the maintenance of mitotic activity and/or identity of procambial cells. Seems to influence and promote the cytokinin signaling pathway. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Homodimer. Interacts with AHP2 and AHP3. Expressed in vascular tissues of inflorescence stems and floral organs, especially in procambium cells, and in siliques. Present at low levels in developing ovules. Within mature female gametophytes, high levels in the central cell nucleus and weak levels in the egg cell nucleus. In fertilized ovules, expressed in the endosperm nuclei during 2 days after pollination. Cannot be transmitted through the female germ line. Impaired megagametogenesis visible from the four-nucleate stage, with two normal nuclei and two degenerated nuclei, leading to female sterility. Normal response to cytokinins.