Leaf ethylene and auxin act as core factors in the regulation of temperature on thidiazuron-induced cotton defoliation

The defoliant thidiazuron is applied worldwide to enhance the fiber quality of machine-harvested cotton, and its efficiency is profoundly regulated by temperature. However, little is known about how the temperature after defoliant application affects the physiological and phytohormone mechanisms regulating thidiazuron-induced cotton defoliation. In this study, we conducted mixed field and greenhouse experiments in Xinjiang, China, involving four treatments with two temperature conditions (25 and 17°C) supported by artificial climate rooms and two defoliant rates (0 and 400 mg L−1). High temperatures after thidiazuron application promoted the formation of cotton abscission zones and increased the defoliation rate by 58.9 % compared with that induced by thidiazuron at low temperatures; however, they did not significantly affect ethylene content in abscission zones. The excellent defoliation performance noted at high temperatures was attributable to a significant decrease in break strength, leaf water content, and auxin content, together with an increase in leaf ethylene evolution. More thidiazuron-induced differentially expressed genes were identified at high temperatures, and signal transduction of plant hormones, mainly auxin and ethylene, was significantly enriched. The high temperature resulted in auxin degradation in both leaves and abscission zones but only caused an earlier and higher peak in leaf ethylene content. Five hub transcription factors and some potential target genes associated with auxin and ethylene were identified using co-expression network analysis, and they may regulate chemical defoliation in cotton. Our results enrich the understanding of how temperature regulates the physiological and phytohormonal responses during thidiazuron-induced cotton abscission, and they are important for enhancing the efficiency of cotton harvest aids and breeding cotton varieties sensitive to defoliants.