The signaling pathways of brassinosteroids (BRs), a unique plant steroid hormone, are critically involved in a diverse range of plant growth and developmental processes as well as many important agronomic traits. Recent advances in the understanding of BR biosynthetic and signaling pathways in model organisms and crops have increased the feasibility of modulating BR responses in crop plants to enhance adaptation to various vulnerable environmental changes. In particular, the identification and functional analysis of BR signaling components in rice (Oryza sativa) present the possibility of their utilization to improve many agricultural traits involved in crop yields. In this review, we summarize recent advances and progress in the understanding of the BR signaling pathway and its interactions with diverse internal and external signaling cues. We also discuss how these physiological modulations of BR and the abundant signaling crosstalk can be applied to enhance rice productivity through the manipulation of plant architecture and fine-tuning of stress responses. Finally, we discuss how the complex regulation of BR signaling pathways could favor application in the molecular design of plant growth and development, precise breeding strategies, and cultivation methods for rice crop improvement.

Brassinosteroids (BRs) play crucial roles in the physiology and development of plants. In the model plant Arabidopsis, BR signaling is initiated at the level of membrane receptors, BRASSINOSTEROIDS INSENSITIVE 1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) complex, thus activating the transcription factors (TFs) BRASSINAZOLE RESISTANT 1/BRI1-EMS-SUPPRESSOR 1 (BZR1/BES1) to coordinate BR responsive genes. BRASSINOSTEROIDS INSENSITIVE 2 (BIN2), glycogen synthase kinase 3 (GSK3) like-kinase, negatively regulates BZR1/BES1 transcriptional activity through phosphorylation-dependent cytosolic retention and shuttling. However, it is still unknown whether this mechanism is conserved in Panax ginseng C. A. Mayer, a member of the Araliaceae family, which is a shade-tolerant perennial root crop. Despite its pharmacological and agricultural importance, the role of BR signaling in the development of P. ginseng and characterization of BR signaling components are still elusive. In this study, by utilizing the Arabidopsisbri1 mutant, we found that ectopic expression of the gain of function form of PgBZR1 (Pgbzr1-1D) restores BR deficiency. In detail, ectopic expression of Pgbzr1-1D rescues dwarfism, defects of floral organ development, and hypocotyl elongation of bri1-5, implying the functional conservation of PgBZR1 in P. ginseng. Interestingly, brassinolide (BL) and BRs biosynthesis inhibitor treatment in two-year-old P. ginseng storage root interferes with and promotes, respectively, secondary growth in terms of xylem formation. Altogether, our results provide new insight into the functional conservation and potential diversification of BR signaling and response in P. ginseng.