An Overdose of the Arabidopsis Coreceptor BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 or Its Ectodomain Causes Autoimmunity in a SUPPRESSOR OF BIR1-1-Dependent Manner

Abstract

The membrane-bound BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1) is a common coreceptor in plants and regulates distinct cellular programs ranging from growth and development to defense against pathogens. BAK1 functions through binding to ligand-stimulated transmembrane receptors and activating their kinase domains via transphosphorylation. In the absence of microbes, BAK1 activity may be suppressed by different mechanisms, like interaction with the regulatory BIR (for BAK1-INTERACTING RECEPTOR-LIKE KINASE) proteins. Here, we demonstrated that BAK1 overexpression in Arabidopsis (Arabidopsis thaliana) could cause detrimental effects on plant development, including growth arrest, leaf necrosis, and reduced seed production. Further analysis using an inducible expression system showed that BAK1 accumulation quickly stimulated immune responses, even under axenic conditions, and led to increased resistance to pathogenic Pseudomonas syringae pv tomato DC3000. Intriguingly, our study also revealed that the plasma membrane-associated BAK1 ectodomain was sufficient to induce autoimmunity, indicating a novel mode of action for BAK1 in immunity control. We postulate that an excess of BAK1 or its ectodomain could trigger immune receptor activation in the absence of microbes through unbalancing regulatory interactions, including those with BIRs. Consistently, mutation of SUPPRESSOR OF BIR1-1, which encodes an emerging positive regulator of transmembrane receptors in plants, suppressed the effects of BAK1 overexpression. In conclusion, our findings unravel a new role for the BAK1 ectodomain in the tight regulation of Arabidopsis immune receptors necessary to avoid inappropriate activation of immunity.

Oomycete small RNAs invade the plant RNA-induced silencing complex for virulence

“…To further explore the role of target genes in plant immunity, we attempted to generate overexpression lines of resistant target gene versions and achieved an overexpressor of the AtWNK2r version ( AtWNK2 r-OE) (Fig.S5a) in the atwnk2-2 background. AtWNK2r -OE plants showed ectopic cell death in distance from infection sites (Fig.S14a), as previously described for overexpression lines of other immunity factors, such as At BAK1 (Domínguez-Ferreras et al, 2015). Moreover, infection structures frequently displayed aberrant swelling-like structures and extensive branching of hyphae instead of the regular pyriform haustoria formed in atwnk-2-2 (Fig.S14b), further supporting a role for AtWNK2 in immune reaction.…”

Loss‐of‐function of Arabidopsis receptor‐like kinase BIR1 activates cell death and defense responses mediated by BAK1 and SOBIR1

“…From a suppressor screen of bir1‐1 , we found that BAK1 is also required for activation of cell death and defense responses in the bir1‐1 mutant, suggesting that BAK1 also plays a positive role in promoting cell death and plant defense. This is consistent with the notion that over‐expression of BAK1 leads to spontaneous cell death (Belkhadir et al ., ; Dominguez‐Ferreras et al ., ). As BAK1 functions as a general co‐receptor for multiple RLKs involved in different biological processes, the opposite functions of BAK1 in plant immunity are most likely facilitated by the receptor‐like kinases (RLKs) it interacts with.…”

The immune repressor BIR1 contributes to antiviral defense and undergoes transcriptional and post‐transcriptional regulation during viral infections

“…BIR1 is a negative regulator of several resistance pathways in which BAK1 and SOBIR1 have concerted roles (Gao et al ., ; Dominguez‐Ferreras et al ., ; Liu et al ., ). Here we provide compelling evidence that BIR1 transcription is positively regulated by SA and propose that TRV triggers NPR1‐dependent expression of BIR1 during the infection by increasing SA concentrations in the infected tissue.…”