Brassinosteroids, the Sixth Class of Phytohormones: A Molecular View from the Discovery to Hormonal Interactions in Plant Development and Stress Adaptation

Abstract

Phytohormones are natural chemical messengers that play critical roles in the regulation of plant growth and development as well as responses to biotic and abiotic stress factors, maintaining plant homeostasis, and allowing adaptation to environmental changes. The discovery of a new class of phytohormones, the brassinosteroids (BRs), almost 40 years ago opened a new era for the studies of plant growth and development and introduced new perspectives in the regulation of agronomic traits through their use in agriculture. BRs are a group of hormones with significant growth regulatory activity that act independently and in conjunction with other phytohormones to control different BR-regulated activities. Genetic and molecular research has increased our understanding of how BRs and their cross-talk with other phytohormones control several physiological and developmental processes. The present article provides an overview of BRs' discovery as well as recent findings on their interactions with other phytohormones at the transcriptional and post-transcriptional levels, in addition to clarifying how their network works to modulate plant growth, development, and responses to biotic and abiotic stresses.

Keywords: brassinosteroids; hormonal cross-talk; plant hormones.

Figure 1

Natural brassinosteroids isolated from or detected in plant sources.

Figure 1

Natural brassinosteroids isolated from or detected in plant sources.

Figure 2

Current model of the signaling pathway in the presence or absence of brassinosteroids (BRs) in Arabidopsis. In the absence of BR, the receptor kinase BRI1 (BRASSINOSTEROID INSENSITIVE 1) does not heterodimerize with its coreceptor BAK1 (BRI1-ASSOCIATED RECEPTOR KINASE 1), maintaining their inactive forms. Consequently, BIN2 (BRASSINOSTEROID-INSENSITIVE 2), a negative regulator of BR signaling pathway, is free to constitutively phosphorylate BZR1 (BRASSINAZOLE RESISTANT 1) and BES1 (BRI1-EMS SUPPRESSOR 1), the two master transcription factors of BR-induced responses, inducing their interactions with 14-3-3 proteins that, in turn, promotes the cytoplasmic retention of BZR1/BES1, suppressing their DNA-binding activity. On the other hand, in the presence of BR, the activation of BRI1 triggers its autophosphorylation and partial kinase activity and dissociation from its inhibitor BKI1, which is attached at the BRI1 kinase domain. This leads to its heterodimerization with BAK1, and transphosphorylation to complete BRI1 kinase activity. Activated BRI1 then phosphorylates BSKs (BR-SIGNALING KINASES) and CDG1 (CONSTITUTIVE DIFFERENTIAL GROWTH 1) which both phosphorylate BSU1 (BRI1 SUPPRESSOR 1), leading to BIN2 dephosphorylation. BIN2 is subsequently restrained by KIB1 (KINK SUPPRESSED IN BZR1-1D), which prevents the association of BIN2 with BZR1/BES1 and facilitates its ubiquitination and degradation. The inactivated form of BIN2 allows BZR1 and BES1 to enter into the nucleus and regulate the expression of BR target genes. Additionally, PP2A (PHOSPHATASE 2A) also positive regulates BR signaling by dephosphorylating BZR1 and BES1, whereas SBI1 (SUPPRESSOR OF BRI1) deactivates BRI1 through the methylation of PP2A.

Figure 3

Schematic working model of regulatory interactions between BR and auxin in root and hypocotyl growth. The green arrows represent the post-transcriptional activation of AUXIN RESPONSE FACTOR (ARF) by BRASSINOSTEROID-INSENSITIVE 2 (BIN2). The blue arrows represent the transcriptional activation of ARF and auxin-responsive genes in the root transition elongation zone by BRASSINAZOLE RESISTANT 1 (BZR1). The red arrows represent the transcriptional repression of ARF and auxin-responsive genes in the root quiescence center (QC) by BZR1. The negative feedback of biosynthetic genes coordinated by BZR1 in both root and hypocotyl elongation is also represented by red arrows.

Figure 4

The integrated model for BR–Gibberellin (GA) cross-talk. BR activates BRASSINAZOLE RESISTANT 1/BRI1-EMS SUPPRESSOR 1 (BZR1/BES1) to promote GA biosynthesis and production. As a consequence, GAs degrade DELLA proteins, releasing their repressive action on BZR1/BES1 activity. Some critical factors, emphasized above, may influence and alter this interaction over time and should be considered when discussing BR–GA coordination in plants (e.g.; BR-induced accumulation of DELLA at dawn in the early stages of Arabidopsis development).

Figure 5

A putative interplay between brassinosteroid (BR), cytokinin (CK), and abscisic acid (ABA). ABA is responsible for inhibiting BR effects during stress conditions by upregulating BIN2 (BRASSINOSTEROID-INSENSITIVE 2), a major negative regulator of BR signaling, whereas BR is responsible for inhibiting ABA effects during growth processes through PP2C (PROTEIN PHOSPHATASE 2C), a major negative regulator of Snark proteins (positive regulators of ABA signaling). ABA is also responsible for inhibiting CK signaling by upregulating CKX (CK oxidases/dehydrogenases), which play a major role in inactivating bioactive CKs. Despite ABA’s role, BR and CK present positive interactions. While CK upregulates BR biosynthetic (DFW4) and signaling (BRI1, BAK1) genes, BR upregulates IPT (isopentenyltransferases), which are major enzymes responsible for the biosynthesis of bioactive CKs.

Figure 6

A general simplified model of BR and ethylene cross-talk. The perception of BR begins in its receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) which activates BR signaling, which, in turn, controls ethylene biosynthesis in a dose-dependent manner. (A) High levels of BR decrease the activity of BRASSINAZOLE RESISTANT 1/BRI1-EMS SUPPRESSOR 1 (BZR1/BES1), the major transcription factors of the BR signaling pathway while enhancing the stability of the 1-aminocyclopropane-1-carboxylic acid (ACC)-synthase enzyme (ACS) proteins by preventing its degradation by the 26S proteasome and consequently, activating ethylene biosynthesis. (B) Low levels of BR increase the activity of BZR1/BES1, which, in turn, bind to the promoter of the ACC-synthase (ACS) and ACC-oxidase (ACO) genes, inhibiting their transcription and consequently, repressing ethylene biosynthesis. PM represents the plasma membrane.

Figure 7

BR and ABA cross-talk relies on protein activity modulation and gene expression regulation. (A) In the presence of BR, the complex formed by BRI1-EMS SUPPRESSOR 1 (BES1), TOPLESS/(TPL/TPR) and HISTONE DEACETYLASE 19 (HDAC19) inhibits ABA Insensitive 3 (ABI3) expression by interacting with E-box promoter sequences. The transcription factor BRASSINAZOLE RESISTANT 1 (BZR1) interacts with the G-box sequences of the ABI5 promoter, leading to gene repression. Repression of the ABI3 and ABI5 genes results in lower expression of ABA-regulated genes and decreased stress responses. At low levels of BR, stress responses are stimulated by SnRK2.3 activation by BRASSINOSTEROID-INSENSITIVE 2 (BIN2). Additionally, the BR-repressor BIN2 phosphorylates the transcription factor ABI5, resulting in the expression of ABA-related genes. (B) In guard cells, BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) phosphorylates the kinase SnRK2.6 at low levels of BR, driving stomatal closure responses. At low levels of ABA, the PHOSPHATASE 2C (PP2C) phosphatase ABI1 and ABI2 repress SnRK2.6 phosphorylation by BAK1 and also the phosphorylation of SnRK2.3 by BIN2, decreasing stress responses related to ABA–BR cross-talk.

Figure 8

A suggested model of brassinosteroid (BR) regulation of immunity at multiple levels. BAK1 (BRI1-ASSOCIATED RECEPTOR KINASE 1) is considered to mediate the growth and immunity tradeoff because it serves as a coreceptor for both BR-mediated responses via BRI1 and innate immunity mediated responses via flg-sensing 2 (FLS2). The scheme suggests an inhibition of FLS2-mediated immune signaling by BR, independent of complex formation with coreceptor BAK1, when the inhibition occurs downstream of BAK1. BRI1 (BRASSINOSTEROID-INSENSITIVE 1) represents a BR receptor; flg22 (flagellin 22) is a type of pathogen- and microbe-associated molecule pattern (MAMP/PAMP); FLS2 (flg-sensing2) is a flg22 receptor; BIK1 (BOTRYTIS-INDUCED KINASE 1) is a coreceptor of FLS2; BZR1/BES1 (BRASSINAZOLE RESISTANT 1/BRI1-EMS SUPPRESSOR 1, respectively) are the major transcriptional factors of the BR signaling pathway; MAPKs (mitogen activated protein kinases) are a class of marker proteins which indicate various stress conditions.