Progress on the Signal Transduction and Crosstalk Mechanism of Humic Substances in Promoting Plant Root and Shoot Growth

Humic substances (HS), a natural polymer isomeric mixture, is an important component of terrestrial ecosystem. Its promotion effect on plant growth has been widely studied in recent years. Multiple signaling pathways play an important role in the process of HS promoting plant growth and development. The article summarizes the signaling pathway transduction and crosstalk mechanisms of HS in promoting the growth and development of plant roots and shoots. During the promotion of root growth, HS mainly involves the transduction of Indole-3-acetic acid (IAA), nitric oxide (NO), abscisic acid (ABA), reactive oxygen species (ROS) and Ca2+ signal, and the complex crosstalk between them; During the promotion of shoot growth, HS mainly involves the transmission of cytokinin (CTK) and ABA signaling pathways, and the complex crosstalk occurs between nitrate and CTK signaling pathways. HS regulates plant physiological metabolism through a complex metabolic network, involving multiple complicated signaling pathways and crosstalk processes. Among them, (PM) H+- ATPase is the key node of HS promoting root and shoot growth, and multiple signaling pathways mediate the signal transduction of cascade amplification via (PM) H+- ATPase. NO signal has a complex crosstalk process with multiple signaling pathways that regulate root and shoot growth, and is an important crosstalk signaling molecule involved in the promotion of plant growth by HS. There are other signaling pathways involved in the growth and development of roots and shoots triggered by HS, among which the defense signaling pathways salicylicacid (SA) and jasmonic acid (JA) are regulated by signaling pathways IAA, NO, ABA and CTK. Due to the dispersion, heterogeneity and complexity of biochemical research of HS, the relevant mechanism of the short-term promoting effect of HS on plant growth and the overall action mechanism on plant growth and development during the whole growth stage have not yet been clarified. Therefore, further in-depth analysis is needed to reveal the signal transduction, crossover, crosstalk and integration pathways involved in the growth promotion process, and the complex nutritional and metabolic pathways. The result of our research could provide theoretical basis for studying the action mechanism of HS physiological activity and its application in agricultural production.