Physiological and Transcriptomic Response of Exogenous Abscisic Acid and Brassinosteroid on Citrus Under Heat Stress

Heat stress severely hinders citrus yield and fruit quality. This study employed integrated physiological and transcriptomic analyses to investigate the effects of the exogenous application of abscisic acid (ABA) and brassinosteroid (BR) on heat stress responses in citrus. The results showed that the exogenous application of ABA and BR increased the contents of soluble sugar, proline, and ABA, and enhanced the activities of peroxidase and catalase under heat stress. Transcriptome trend analysis identified profiles 1, 6, and 7 as significantly enriched across exogenous ABA, BR, and control conditions. Profile 6 exhibited rapid upregulation followed by stabilization and showed a significantly higher gene count under both ABA and BR treatments than under the control. KEGG enrichment analysis revealed that genes in profile 6 were primarily enriched in amino sugar, nucleotide sugar, galactose, amino acids, 2-oxocarboxylic acid, glycerophospholipid, glucosinolate metabolism, MAPK signaling pathway, plant hormone signal transduction, protein processing in the endoplasmic reticulum, plant–pathogen interaction, and endocytosis. Furthermore, four genes encoding heat shock proteins (HSP), including HSP21A, HSP21B, HSP70-17, and HSP70A, were induced under heat stress and showed significant upregulation in response to exogenous ABA and BR treatments. In conclusion, these findings indicated that exogenous ABA and BR regulated ABA and osmoprotectant accumulation and antioxidant defense activation in response to heat stress.