Ethylene promotes lateral root formation following waterlogging in an epiphytic orchid, Cymbidium tracyanum

Waterlogging is a major abiotic stress that severely impairs plant growth, development and productivity. However, the mechanisms governing recovery from waterlogging stress in plants, particularly in epiphytes, remain poorly understood. Focused on lateral root (LR) formation, this study explored the physiological changes and key gene expression dynamics underlying root development in Cymbidium tracyanum during waterlogging and recovery to elucidate recovery mechanisms in epiphytic orchids. Significant morphological changes in C. tracyanum occurred during the recovery period rather than the waterlogging period. Transcriptome analysis revealed extensive transcriptional reprogramming during waterlogging and recovery, particularly in genes related to phytohormone biosynthesis and signaling pathways. Phytohormone profiling showed rapid ethylene accumulation during waterlogging, with earlier activation of ethylene biosynthesis and signaling genes compared to other hormones. Exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) under non-stressed condition promoted LR formation and triggered jasmonate accumulation, mimicking the natural recovery response. Weighted gene co-expression network analysis, along with gene function annotation and expression analyses, identified CtNAC68 and CtWRKY65 as key genes responsive to ethylene and jasmonate signaling pathways. Collectively, these results demonstrated that lateral root formation, a process regulated by ethylene and jasmonate signaling, was a strategy employed by C. tracyanum to adapt to waterlogging stress. The findings provide the first robust evidence for the role of lateral root formation in waterlogging adaptation of epiphytic orchids, offering novel insights into their ecological adaptation and evolution.