The Beneficial Effect of 24-Epibrassinolide Against High-Temperature Stress in Gracilariopsis lemaneiformis Revealed by Physiological Response and Transcriptomic Profiling

The commercial seaweed Gracilariopsis lemaneiformis is mainly used as raw material for agar production and feed for abalone. The heat-resistant strains G. lemaneiformis are extensively cultivated in the northern and southern coasts of China, yet high temperature in the summertime in southern coasts has hindered the growth and limited the cultivated periods of this seaweed. A vast majority of reports have manifested that exogenous phytohormone brassinosteroids (BRs) can improve the plant heat-tolerance. However, little is known about the effect and its underlying mechanism of BRs in algae. In this study, the effect of 24-epibrassinolide (EBR) on the physiological and transcriptional levels was investigated in the high-temperature stressed G. lemaneiformis. Physiological data indicated that EBR application could improve the growth with 1.43-fold on day 5 and non-photochemical quenching parameter, reduce the productions of reactive oxygen species (ROS) and malondialdehyde (MDA), enhance the accumulations of proline, trehalose, and the levels of endogenous 2-methylthio-N6-isopentenyladenine (2MeSiP), 2-methylthio-cis-zeatin riboside (2MeScZR), jasmonic acid (JA), N-[(-)-jasmonoyl]-(l)-phenylalanine (JA-Phe) and salicylic acid 2-O-β-glucoside (SAG) under high-temperature condition. In addition, transcriptomic analysis identified 656 upregulated- and 680 downregulated-genes following the EBR treatment compared to the control group, revealed that EBR activated the metabolic pathways of the glycolysis, tricarboxylic acid (TCA) cycle, pentose phosphate pathway, and synthesis of threonine, methionine, and serine. Taken together, the aforementioned results highlighted the beneficial effect of EBR via alleviating the oxidative stress, promoting the accumulations of osmolytes and stress-related phytohormones, and activating the metabolisms of carbohydrate and amino acid, thereby resulting in the mitigating of growth inhibition by high-temperature stress in G. lemaneiformis.