Brassinosteroids (BRs) are a group of steroid hormones, essentially important for plant development and growth. BR signaling functions to promote cell expansion and cell division, and plays a role in etiolation and reproduction. As the phytohormone originally identified in the pollen grains of Brassica napus, BR promotes the elongation of stigma. Recent studies have revealed that BR is also critical for floral transition, inflorescence stem architecture formation and other aspects of plant reproductive processes. In this review, we focus on the current understanding of BRs in plant reproduction, the spatial and temporal control of BR signaling, and the downstream molecular mechanisms in both the model plant Arabidopsis and crops. The crosstalk of BR with environmental factors and other hormones in reproduction will also be discussed.Int. J. Mol. Sci. 2020, 21, 872 2 of 16 BRI1 KINASE INHIBITOR 1 (BKI1) [14][15][16][17]. Then the kinase domains of BRI1 and BAK1 are transphosphorylated and the phosphorylated BKI1 can associate with the 14-3-3 family proteins to further amplify BR signaling [16,18]. Another two plasma membrane-anchored cytoplasmic kinases, BRASSINOSTEROID-SIGNALLING KINASE 1 (BSK1) and CONSTITUTIVE DIFFERENTIAL GROWTH 1 (CDG1) are also phosphorylated by activated BRI1 to inactivate the phosphatase BRI1-SUPPESSOR 1 (BSU1) [19][20][21]. BSU1 in turn dephosphorylates a conserved tyrosine residue of BRASSINOSTEROID INSENSITIVE 2 (BIN2), leading to the inactivation of this GSK3-like kinase [22]. The function of BIN2 is to phosphorylate and inactivate two homologous transcription factors, BRASSINAZOLE RESISTANT 1 (BZR1) and BR1-EMS-SUPPRESSSOR 1 (BES1) in the absence of BR [23][24][25]. The phosphorylation leads to the deactivation of these two transcription factors [26]. In high BR level, BSU1 inactivates BIN2 and unphosphorylated BZR1 and BES1 can initiate BR regulated gene activation and repression [23,27]. BZR1 and BES1 initiate BR responsive gene expression by recognizing and binding to the BR response DNA cis-element BR-Response Element (BRRE, CGTGC/TG) and E-box (CANNTG) [23,28]. Gain-of-function bzr1-1D and bes1-D contain a proline to leucine mutation in the protein degradation domain and therefore exhibit BR constitutive phenotypes [24,25,29]. However, the bzr1-1D and bes1-D are morphologically different, indicating the two proteins are involved in distinct BR functions. A number of transcription factors and histone modifiers are identified to interact with BZR1/BES1 for the control of various BR responses [28,[30][31][32][33]. BZR1 and BES1 belong to a six-member small family clade, consisting another four homologs, BES1/BZR1 homolog 1 to 4 (BEH1-4), which also act as downstream transcription factors in BR signaling pathway [28].BR regulates a broad range of plant growth and development, including hypocotyl elongation, root development, skotomorphogenesis, vascular differentiation, floral transition, anther development, and pollen grain maturation. In this review, we will focus on the functions ...