Abstract: Two-component systems, consisting of a histidine (His) protein kinase that senses a signal input and a response regulator that mediates the output, are involved in various signal transduction pathways in many prokaryotes, fungi, slime molds, and plants. While in plants, there also exist more complex signaling systems that include a hybrid His kinase, a phosphorelay intermediate and a response regulator, which is referred to as multiple step two component systems. Recent studies indicated that two-component systems play important roles in signal transduction in response to environmental stimuli and growth regulators (such as ethylene, cytokinin), light and osmotic stress.