Reduced root lignification resulting from enhanced brassinosteroid signaling compromises waterlogging tolerance in Nardostachys jatamansi seedlings.
The alpine plant Nardostachys jatamansi is highly sensitive to waterlogging stress, and phytohormone signaling plays a critical role in its adaptation to this abiotic stress. Nevertheless, the involvement of brassinosteroid signaling in the response of N. jatamansi to waterlogging stress remains poorly characterized. In this study, 5 μM 24-epibrassinolide was applied to N. jatamansi seedlings via foliar spraying, which promotes primary root elongation and lateral root initiation. However, in contrast to previous findings, this pretreatment reduced the tolerance of N. jatamansi seedlings to waterlogging stress, resulting in a survival rate decrease of approximately 12.5%. Transcriptome analysis revealed that the differentially expressed genes (DEGs) were primarily associated with cell wall synthesis and assembly, as well as lignin biosynthesis. Notably, in the 24-epibrassinolide-treated group, the expression levels of lignin biosynthesis-related genes (including COMT, CDA, and LACs) were less than 40% of those in the control group. Non-invasive micro-test technology measurements of radial oxygen dynamics and determination of lignin content in roots revealed that 24-epibrassinolide-induced root longitudinal elongation and radial expansion combined with reduced root lignification synergistically increased the radial oxygen loss by 24 times. This outcome impaired the plant's ability to maintain internal oxygen under waterlogged conditions, thereby compromising its adaptive response to hypoxia. Notably, 10 μM brassinazole treatment elicited effects antagonistic to those observed with BR treatment. Overall, brassinosteroid signaling allocates limited carbon to root elongation instead of lignin synthesis for physical barrier formation, resulting in severe radial oxygen loss and energy crisis. This represents the intrinsic mechanism underlying the negative regulation of waterlogging tolerance by BR in N. jatamansi seedlings.