A Mutant Allele Uncouples the Brassinosteroid-Dependent and Independent Functions of BRASSINOSTEROID INSENSITIVE 1

Abstract

Plants depend on various cell surface receptors to integrate extracellular signals with developmental programs. One of the beststudied receptors is BRASSINOSTEROID INSENSITIVE 1 (BRI1) in Arabidopsis (Arabidopsis thaliana). Upon binding of its hormone ligands, BRI1 forms a complex with a shape-complementary coreceptor and initiates a signal transduction cascade, which leads to a variety of responses. At the macroscopic level, brassinosteroid (BR) biosynthetic and receptor mutants have similar growth defects, which initially led to the assumption that the signaling pathways were largely linear. However, recent evidence suggests that BR signaling is interconnected with several other pathways through various mechanisms. We recently described that feedback from the cell wall is integrated at the level of the receptor complex through interaction with RECEPTOR-LIKE PROTEIN 44 (RLP44). Moreover, BRI1 is required for another function of RLP44: the control of procambial cell fate. Here, we report a BRI1 mutant, bri1 cnu4 , which differentially affects canonical BR signaling and RLP44 function in the vasculature. Although BR signaling is only mildly impaired, bri1 cnu4 mutants show ectopic xylem in place of procambium. Mechanistically, this is explained by an increased association between RLP44 and the mutated BRI1 protein, which prevents the former from acting in vascular cell fate maintenance. Consistent with this, the mild BR response phenotype of bri1 cnu4 is a recessive trait, whereas the RLP44-mediated xylem phenotype is semidominant. Our results highlight the complexity of plant plasma membrane receptor function and provide a tool to dissect BR signaling-related roles of BRI1 from its noncanonical functions. Plant cells perceive a multitude of extracellular signals through a battery of plasma membrane-bound receptors that are crucial for the integration of environmental and developmental signals. The response to the growth-regulatory brassinosteroid (BR) phytohormones is mediated by one of the best-characterized plant signaling pathways (Singh and Savaldi-Goldstein, 2015) initiated by a receptor complex containing the Leu-rich repeat receptor-like kinase BRASSINOSTE-ROID INSENSITIVE 1 (BRI1; Li and Chory, 1997) and its coreceptors of the SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family (Ma et al., 2016; Hohmann et al., 2017). Binding of the brassinosteroid ligand mediates hetero-dimerization of BRI1 and SERK family members such as BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1; Li et al., 2002; Nam and Li, 2002), which in turn triggers extensive autoand transphosphorylation of the intracellular BAK1 and BRI1 kinase domains (Hohmann et al., 2017). The activated kinases recruit and activate downstream BR signaling components, which eventually leads to vast changes in gene expression mediated by BR signalingregulated transcription factors such as BRASSINA-ZOLE-RESISTANT 1 (BZR1; Wang et al., 2002) and BRI1-EMS-SUPPRESSOR 1 (BES1; Yin et al., 2002). Among the transcriptional targets of these transcription factors, cell...

Optimal BR signalling is required for adequate cell wall orientation in the Arabidopsis root meristem

“…Quantification of aberrant cell walls in cortex cells revealed that the majority of bri1-116 meristems had a least one oblique cortex cross wall per median confocal section, a phenotype which was enhanced in bri-triple mutants (Figure 5B). We recently reported that some functions of BRI1 are independent of classical BR signalling outputs (Holzwart et al, 2018; Holzwart et al, 2019). However, cell shape defects in bri1 mutants seem to be caused by reduced BRI1/ligand dependent signalling, as the biosynthetic mutants cpd (Szekeres et al, 1996) and det2 (Chory et al, 1991), as well as plants in which endogenous BRs were depleted by the application of brassinazole (BRZ) (Asami et al, 2000), displayed phenotypes comparable to bri1 mutants (Figure 5B).…”

Moonlighting Proteins Shine New Light on Molecular Signaling Niches

“…An increasing body of evidence suggests BRI1 has functions that are independent of classical brassinosteroid signaling outputs mediated by the canonical brassionsteroid signaling pathway, that are mediated by receptor-like protein 44 (RLP44), which probably acts as a scaffold promoting association of BRI1 and BAK1/SERK3 [ 103 ]. RLP44 is under transcriptional control of BRI1 and is able to promote activity of a complex containing PSKR1, through an analogous scaffolding mechanism as observed for the activation of brassinosteroid signaling [ 104 ], with BRI1 and PSKR1 likely competing for RLP44 [ 105 ]. This is of interest as both BRI1 and PSKR1 contain moonlighting guanylate cyclase function that may result in cGMP-enriched scaffold complexes.…”

Phosphorylation-dependent routing of RLP44 towards brassinosteroid or phytosulfokine signalling

“…RLP44 directly interacts with both BRI1 and BAK1 and promotes their association (Holzwart et al, 2018). However, RLP44 also interacts with and promotes the activity of the receptor complex for the plant growth peptide phytosulfokine (PSK) (Holzwart et al, 2018, 2020; Sauter, 2015). The interaction between RLP44 and the PSK receptor PSKR1 is important for the maintenance of procambial cell fate, as both RLP44 and PSK-related mutants show ectopic xylem formation in the position of the procambium in seedling roots (Holzwart et al, 2018, 2020).…”