Overexpression of the Selaginella lepidophylla bHLH transcription factor enhances water-use efficiency, growth, and development in Arabidopsis

•The SlbHLHopt protein appears to be a stress-responsive bHLH protein.•SlbHLHopt overexpression results in enhanced plant growth, development, and water-use efficiency in Arabidopsis.•SlbHLHopt functions as an important regulator of water-deficit and NaCl stresses in transgenic Arabidopsis at germination.•SlbHLHopt-overexpressing lines have significantly higher flavonoid content.