As a class of plant polyhydroxy steroids, Brassinosteroids (BR) regulate several plant growth and development processes like vascular differentiation, reproduction, senescence, cell elongation, stress response and photomorphogenesis.  A family of plant specific transcription factors, named BRASSINAZOLE-RESISTANT (BZR) act as positive regulators of the BR signaling pathway in different crops (Clouse et al., 1996, Li and Chory et al., 1997., Clouse., 2011).

BZR1 and BRASSINOSTEROID-INSENSITIVE 1-EMS-SUPPRESSOR 1 (BES1)/BZR2 are well characterized BR signaling regulators and Arabidopsis, BEH1-4 (BES1/BZR1 homolog 1-4) show high sequence identity with BZR1 and BES1 and most likely play redundant role in BR signaling. From in vivo and in vitro studies, members of this family share a most conserved N-terminal DNA binding domain called the BZR domain. BZR1 and BES1 through its atypical basic N-terminal Helix-Loop-Helix motif binds to E-box (CANNTG) designated as MBS (MYB binding site) and BRRE (CGTGT/CG) respectively. The central region of BZR1 and BES1 consist of GSK3 kinase family phosphorylation site P and PEST motif involved in protein degradation followed by carboxyl terminal domain (Yin et al., 2005). Based on sequence analysis and expression studies in response to several abiotic stress and organ specific development of 15 BrBZR identified from genome wide analysis of Brassica rapa, 6 BrBZR were constitutively expressed during flower development implying their flower specific role. Most of the BrBZRs were upregulated in cold resistant lines as compared to cold susceptible lines suggesting their role in cold stress resistance. Differential transcript abundance expression levels were observed in response to salt and drought stress by 6 and 11 BrBZR respectively. Also, in response to exogenous ABA treatment, several fold up-regulation in BrBZRs were observed Thus in addition to BR signaling, BrBZR plays a role in stress resistance (Saha et al., 2015).

Eleven BZR members are present in Zea mays. Organ specific expression of 11 ZmBZRs revealed no transcript specific accumulation for 5 of them whereas the remaining 6 were ubiquitously expressed in whole plant implying their role as regulators in maize development. In response to abiotic stress like N starvation, hypoxia, and salt stress, all 6 ZmBZR were upregulated specifically ZmBZR4 and ZmBZR11 in response to hypoxia and ZmBZR9 and ZmBZR11 in response to salt stress, however in response to heat stress all ZmBZR were down regulated. Thus, ZmBZR are involved in the maize stress signaling pathway (Manoli et al., 2017). ZmBES1/BZR1-5 positively regulates kernel size, since overexpression of ZmBES1/BZR1-5 increased seed size and weight in Arabidopsis and maize Mu transposon insertion and EMS mutants resulted in smaller kernels. This transcription factor localizes to the nucleus, because of regulation by bHLH domain and phosphorylation of BAM domain by casein kinase II subunit b (ZmCKIIb4) and functions as a homodimer through its BAM domain. This TF also targets the promoter of AP2/EREBP genes and inhibits their transcription based on ChIP-seq and dual luciferase assay (Sun et al., 2021). Based on the qRT-PCR results, ZmBES1/BZR1-3 and ZmBES1/BZR1-9 are predominantly expressed in roots and localizes to the nucleus where homodimerization is required for exhibiting transcriptional activity. They act as a negative regulator of drought tolerance, since the transgenic lines expressing ZmBES1/BZR1-3 or ZmBES1/BZR1-9 showed increased drought sensitivity caused due to suppressed root development (Feng et al., 2022). Through BR signaling, ZmBZR1 regulates organ size in Arabidopsis by regulating the expression of organ size related genes like Germination Repression and Cell Expansion Receptor like kinase (GRACE) and KIP-RELATED PROTEIN6 (KRP6) thus implying its role in progressive increase in cell during plant development (Zhang et al., 2020). TaBZR2 was identified from wheat drought transcriptome data analysis and its overexpression resulted in increased drought tolerance in wheat transgenic lines whereas TaBZR2 RNAi wheat lines showed increased drought sensitivity. Based on EMSA and luciferase reporter assay, TaBZR2 transcriptionally upregulating the expression of TaGST1 which plays a positive role in scavenging drought induced O2-, therefore mediating cross talk between BR and drought signaling pathway (Cui et al., 2019). In Arabidopsis, BZRs play crucial role in both BR signaling and anther development. Knockout of all BZRs (bzr-h mutant) resulted in phenotype identical to BR receptor null mutant (bri-t). Based on real-time PCR and in situ hybridization expression of SPOROCYTELESS (SPL) encoding vital transcription factor in anther locule development was barely detectable in bzr-h mutant thus resulting in loculeless anther in this mutant unlike bri-t mutant therefore suggesting BZR role in anther locule development irrespective of BR-receptor mechanism (Chen et al., 2019). BZR1 regulates outer integument growth by regulating the expression of INNER NO OUTER (INO) gene encoding essential regulator of outer integument growth through both BRI1 dependent and independent pathway since Arabidopsis thaliana BR receptor null mutant bri1-116 showed impaired outer integument growth and embryo sac development and the defects were elevated by overexpression of increased transcriptional activity of BZR1 (Jia et al., 2020).

Last updated June 2023 by Ankita Abnave

References:

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