Abstract
Brassinosteroids (BRs) are essential phytohormones mainly perceived by a single-pass transmembrane receptor-like protein kinase (RLK), BRASSINOSTEROID INSENSITIVE 1 (BRI1). bri1-5 and bri1-9, two distinct mutants with point mutations in the extracellular domain of BRI1, show weak defective phenotypes. Previous studies indicated that bri1-5 and bri1-9 mutated proteins can be recognized and eliminated via an endoplasmic reticulum quality control (ERQC) mechanism. Most of these two proteins, therefore, cannot reach their destination, plasma membrane. Here, we report our functional characterization of bri1-301, another BRI1 mutant protein with an amino acid substitution in the cytoplasmic kinase domain. bri1-301 is a partially functional BR receptor with significantly decreased protein abundance. Interestingly, protein stability and subcellular localization of bri1-301 are temperature-sensitive. At 22°C, an optimal temperature for indoor Arabidopsis growth, bri1-301 shows a weak defective phenotype. At a lower temperature condition such as 18°C, bri1-301 exhibits subtle morphological defects. At a higher temperature condition such as 28°C, on the other hand, bri1-301 displays an extremely severe phenotype reminiscent to that of a null bri1 mutant due to greatly increased bri1-301 internalization and degradation. Our detailed analyses suggest that bri1-301 stability is controlled by ERQC and plasma membrane quality control (PMQC) systems. Since PMQC has not been well studied in plants, bri1-301 can be used as a model mutant for future genetic dissection of this critical process.
Highlights
Brassinosteroids (BRs) are a group of polyhydroxylated phytohormones widely identified in plant kingdom
BRASSINOSTEROID INSENSITIVE1 (BRI1) kinase activity is essential to the initiation of successful BR signaling (Friedrichsen et al, 2000; Wang et al, 2008; Oh et al, 2009; Jaillais et al, 2011)
The crucial role of kinase activity for BRI1 function has been demonstrated by severe morphological defects of null bri1 mutant alleles, such as bri1-1, bri1-101, bri1-117, and bri1-703 (Clouse et al, 1996; Li and Chory, 1997; Friedrichsen et al, 2000; Sun et al, 2017)
Summary
Brassinosteroids (BRs) are a group of polyhydroxylated phytohormones widely identified in plant kingdom. Binding of BRs to the extracellular domain of BRI1 relieves its kinase domain from an inhibitory state caused by both the interference of its C-terminal tail as well as an inhibitory binding protein, BRI1 KINASE INHIBITOR 1 (BKI1) (Wang et al, 2005b; Wang and Chory, 2006; Jaillais et al, 2011). Activated BRI1 triggers downstream signaling cascade predominantly via protein phosphorylation and dephosphorylation (Li and Nam, 2002; Mora-Garcia et al, 2004; Tang et al, 2008; Kim et al, 2009, 2011; Tang et al, 2011). Transcription factors, including well-characterized BZR1 and BES1, are subsequently activated and thousands of their target genes are transcriptionally regulated (Wang et al, 2002; Yin et al, 2002; He et al, 2005; Sun et al, 2010; Yu et al, 2011)
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