Brassinosteroids: Multidimensional Regulators of Plant Growth, Development, and Stress Responses

Abstract

Brassinosteroids (BRs) are a group of polyhydroxylated plant steroid hormones that are crucial for many aspects of a plant's life. BRs were originally characterized for their function in cell elongation, but it is becoming clear that they play major roles in plant growth, development, and responses to several stresses such as extreme temperatures and drought. A BR signaling pathway from cell surface receptors to central transcription factors has been well characterized. Here, we summarize recent progress toward understanding the BR pathway, including BR perception and the molecular mechanisms of BR signaling. Next, we discuss the roles of BRs in development and stress responses. Finally, we show how knowledge of the BR pathway is being applied to manipulate the growth and stress responses of crops. These studies highlight the complex regulation of BR signaling, multiple points of crosstalk between BRs and other hormones or stress responses, and the finely tuned spatiotemporal regulation of BR signaling.

Figure 1.

Overview of the BR Signaling Pathway. When BRs are absent (left), PM-localized receptors BRI1 and BAK1 are inhibited by several factors, including BKI1 and BIR3. Additionally, BIN2 kinase functions as a negative regulator and phosphorylates BES1 and BZR1 TFs to inhibit their activity through multiple mechanisms. BSS1 forms a complex with BES1 and BZR1 in the cytoplasm, and THXh5 reduces BZR1 in the nucleus, further inactivating these TFs. This leads to relatively low expression of BR-induced genes and higher expression of BR-repressed genes. When BRs such as BL are present, they bind to the receptor BRI1 and coreceptor BAK1 to initiate BR signaling (right). BKI1 and BIR3 dissociate from the receptor complex, allowing BRI1 and BAK1 to become phosphorylated and activated. BSKs/CDGs are phosphorylated and activate BSU1 phosphatase to inhibit BIN2. Dephosphorylation by PP2A allows BES1 and BZR1 to function with other TFs and cofactors to promote BR-induced gene expression and inhibit BR-repressed gene expression. Figure was created with the software BioRender (BioRender.com). BRRE, BR Response Element; BSU1, BRI1 SUPPRESSOR1; P, phosphorylation; PUB12/13, PLANT U-BOX12/13; SDG8, SET DOMAIN GROUP8; SH, reduced Cys residue; SOH, oxidized Cys residue; TPL, TOPLESS; TRXh5, THIOREDOXIN H-TYPE5; Ub, ubiquitination.

Figure 2.

Mechanisms Regulating BIN2 Activity. In addition to canonical dephosphorylation and inactivation of BIN2 by BSU1 in the presence of BRs, several other mechanisms also regulate BIN2 activity. BIN2 is ubiquitinated by the E3 ubiquitin ligase KIB1 and degraded by the proteasome in the presence of BRs. Deacetylation by HDA6 inhibits BIN2 activity, whereas oxidation by ROS promotes BIN2 activity. ABA also activates BIN2 through the inhibition of ABI1/2 phosphatases that dephosphorylate BIN2. Finally, BIN2 is sequestered in a cell-type–specific manner by OPS in the phloem or POLAR and BASL in the stomatal cell lineage. Figure was created with the software BioRender (BioRender.com). Ac, acetylation; BSU1, BRI1 SUPPRESSOR1; OPS, OCTOPUS; P, phosphorylation; SOH, oxidized Cys residue; Ub, ubiquitination.

Figure 3.

Diverse Regulatory Mechanisms Controlling BES1 and BZR1 Activity. BES1 and BZR1 activity is modulated by multiple modes of regulation. BES1 transcripts are subject to alternative splicing, with a longer BES1-L isoform displaying increased nuclear localization. Phosphorylation by BIN2 inactivates BES1 and BZR1, whereas MPK6 phosphorylation of BES1 in response to bacterial pathogens or pathogen-associated molecular patterns leads to its activation. PP2A dephosphorylates and activates BES1 and BZR1 in the presence of BRs. The production of H₂O₂ is promoted by BRs and activates BES1 and BZR1 via oxidation, whereas TRXh5 reduces BZR1. BES1 and BZR1 can be inactivated by cytoplasmic sequestration, photoreceptors that respond to UV, red and blue light, or ubiquitination. Several families of E3 ubiquitin ligases target BZR1 or BES1 in different tissues or in response to environmental cues, leading to their degradation by the proteasome or autophagy. DSK2 mediates selective autophagy for BES1 degradation during stress. Figure was created with the software BioRender (BioRender.com). ATG8, AUTOPHAGY-RELATED PROTEIN8; COP1, CONSTITUTIVE PHOTOMORPHOGENIC1; CRY1, CRYPTOCHROME1; P, phosphorylation; PhyB, PHYTOCHROME B; PUB40, PLANT U-BOX40; SH, reduced Cys residue; SOH, oxidized Cys residue; TRXh5, THIOREDOXIN H-TYPE5; Ub, ubiquitination; UVR8, UV-B-RESISTANCE8.

Figure 4.

Summary of BR Regulated Developmental Processes in Arabidopsis. Temperature and light modulate PHYB activity, regulate the concentration of PIF4, and indirectly determine the levels of PIF4–BES1 heterodimerization. The interaction of these TFs determines their gene targets and leads to different cellular responses. Xylem differentiation is governed by the TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR signaling pathway. GSK3s are crucial components in this pathway, which act as negative regulators of xylem differentiation and enable crosstalk with the BR signaling pathway. Stomatal development is fine-tuned by the dual role of BIN2 and is dependent on its subcellular localization. When located in the nucleus, BIN2 mainly acts as a negative regulator of SPCH activity, whereas in complex with BASL and POLAR, it relocalizes to the PM polarized region of MMC and acts as a negative regulator of YDA and MKKs, leading to SPCH activation. BRs inhibit flowering by promoting the expression of flowering inhibitor FLC. Additionally, the expression BR biosynthetic genes exhibits diurnal changes. During the root epidermal cell fate determination process, BIN2 phosphorylates EGL3, leading to its trafficking from the nucleus to cytosol in trichoblast cells, which facilitates its movement from trichoblast to atrichoblast cells. BIN2 can also phosphorylate TTG1 to inhibit the activity of the WER–GL3/EGL3–TTG1 transcriptional complex. In the root apical meristem, BRs control the size of the stem cell niche by balancing the expression of BRAVO, which negatively regulates cell divisions in the quiescent center. BR signaling levels increase along the longitudinal axis, with higher levels present in cells closer to the differentiation/elongation zone. Arrows indicate activation and blunt-ended lines indicate inhibition. BRAVO, BRASSINOSTEROIDS AT VASCULAR AND ORGANIZING CENTER; BSU1, BRI1 SUPPRESSOR1; EGL3, ENHANCER OF GLABRA3; EPF1/2, EPIDERMAL PATTERNING FACTOR 1/2; FLC, FLOWERING LOCUS C; GL2, GLABRA2; MKK4/5/7/9, MITOGEN-ACTIVATED PROTEIN KINASE KINASE4/5/7/9; MMC, Meristemoid mother cell; P, phosphorylation; PHYB, PHYTOCHROME B; QC, Quiescent center; TDIF, TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR; TDR, TDIF RECEPTOR; TTG1, TRANSPARENT TESTA GLABRA1; WER, WEREWOLF; WOX4, WUSCHEL RELATED HOMEOBOX4; YDA, YODA.