Since Blackman and Davies found root-sourced chemical signals formed when soil was drying, many subsequent experiments have elucidated how root-shoot communication might operate. Chemical root-sourced signals are transmitted long-distance to aboveground parts via the transpiration stream, lowering the stomatal conductance,and inhibiting the transpiration rate so as to improve the water use efficiency of plants. It has been shown that the chemical signals act as an "early-warning" response to soil drying in plants. These root-sourced chemical signals include abscisic acid (ABA), cytokinins (CTK), auxin, xylem pH, Ca²⁺, of which the ABA-based root signal pathway has been the most extensively and thoroughly studied in plants. This paper summarizes the basic properties, main functions and regulatory mechanisms of several major root-sourced chemical agents. Most content is focused on current research progress on stomatal behavior regulation, differential gene expression, and plant growth and development processes under the operation of these signals. Over the last two decades, most research regarding the eco-physiology of chemical root-sourced signals has been accumulated on the crops such as wheat, maize and legumes. According to Xiong and Fan, the soil water content interval (SWCI) from "switching on" to "turning off" root signals was wider in modern wheat varieties than in in older varieties. There is a significant correlation between SWCI and other parameters such as lethal leaf water potential, maintenance rate of grain yield and water use efficiency. The results support the influence of root signals on regulatory mechanisms of reactive oxygen species, osmotica and antioxidant enzyme system in wheat and legume crops. On the other hand, the molecular mechanism of ABA regulating plant drought adaptation as a major chemical root-sourced signal is one of the hottest issues in this field. Important progress was achieved by Miyazonol et al. who showed that the PYR/PYL/RCAR family of START proteins received ABA to inhibit the phosphatase activity of the group-A protein phosphatases 2C (PP2Cs), which acted as major negative regulators in ABA signalling. The crystal structures of the ABA receptor PYL1 were bound with (+)-ABA, and produced a critical complex by the further binding of (+)-ABA-bound PYL1 with the PP2C protein ABI1. This result provided a breakthrough understanding on the structural basis of the mechanism of (+)-ABA-dependent inhibition of ABI1 by PYL1 in ABA signalling. In addition, other types of chemical root-sourced signals have been found to act in a different regulatory role. As a whole, the physiological effects of the various signals mutually interact and are involved with a range of complex processes from the molecular to the population level. Future research will investigate the key substances produced from the network of crosstalk and explore the regulatory mechanism of these substances at the molecular and physiological level. The eco-physiology for root-sourced chemical signals is following a trend of "revealing the coupling structural and functional relationship at different scales under the basis of molecular and physiological mechanisms".