BAK1 and Other Associated Proteins in MAMP Signalling
Following MAMP perception, PRRs trigger downstream events involving protein association/dissociation. BAK1 was initially identified as a co-receptor in BRI1-mediated brassinosteroid (BR) signalling, which modulates plant growth and development
[58][59]. Studies have shown that BAK1 and related somatic embryogenesis receptor kinases (SERK) proteins associate with other LRR-RLKs or LRR-RLPs, and regulate plant growth and immunity
[4][60][61].
Table 2 outlines different BAK1 and other associated proteins, and the implication of their KOs in plant MTI. In
A. thaliana, both FLS2 and EFR form a complex with the co-receptor BAK1 to elicit immune responses immediately upon flg22 or elf18 perception, respectively
[4][5][62][63]. Plants carrying
BAK1 mutants (
bak1-3 and
bak1-4), generated by T-DNA insertion, displayed abnormal early and late flagellin-triggered responses
[4][5]. In this regard, there was a significant reduction in the oxidative burst triggered by elf26 in
BAK1 mutants, indicating that EF-Tu is also affected by the mutation in BAK1
[4]. Interestingly,
BAK1 mutants were not completely impaired to flg22 or elf18 perception, indicating that BAK1 was not the only rate-limiting component and therefore suggests additional regulatory protein(s), such as BKK1, that are part of the FLS2 and EFR receptor complexes
[4][5][60]. BAK1-disrupted
N. benthamiana plants displayed decreased induction of MTI responses by the csp22 peptide (part of bacterial cold-shock protein) and INF1 (an oomycete elicitor)
[5]. Furthermore, Arabidopsis
BAK1 KO mutants exhibited increased susceptibility to necrotrophic fungal pathogens, such as
Botrytis cinerea and
Alternaria brassicicola [64]. These results suggest a central role for BAK1 in modulating other PRRs besides FLS2 and EFR in plant defence signalling. The exact mechanism by which BAK1 mediates defence signalling is, however, not resolved. A recent study also showed BAK1 involvement in the tomato FLS3 recognition of flgII-28 (another flagellin epitope) and resulting immune response signalling
[10].
BAK1 and BAK1-LIKE1 (BKK1) have dual physiological roles by positively regulating a BR-dependent plant growth pathway, and negatively regulating a BR-independent cell-death
[65]. Here, cell death-control mediated by BAK1 and BKK1 is SA-dependent
[66]. Upon flagellin perception, BIK1 as a RLCK, associates with the FLS2-BAK1 receptor complex to initiate plant innate immunity and cell death
[67]. There was a significant loss of flg22-induced resistance to
Pst DC3000 infection in
BIK1 mutant seedlings, however, the mutation did not affect flg22-induced FLS2 and BAK1 association. On the other hand,
BIK1 mutants were susceptible to necrotrophic pathogens but were resistant to a virulent bacterial pathogen
Pst DC3000
[68]. Chen et al.
[69] demonstrated that the
bik1 mutant displayed a strong SA-dependent resistance to
Plasmodiophora brassicae, an obligate biotroph protist that induces gall formation in cruciferous plants.
Bak1-4 bik1 double mutants exhibited increased expression of plant defence genes and cell death phenotypes compared to
BIK1 single mutant
[70], highlighting the cooperativity of BIK1 and BAK1 influence in plant immunity.
BIR2, a novel LRR-RLK, interacts with BAK1 in a kinase-dependent manner, and negatively regulates BAK1-dependent MAMP-triggered immune signalling
[71]. Upon ligand binding to FLS2, BAK1 is released from BIR2 and recruited to the FLS2 complex. Therefore, BIR2 inhibits autoimmune cell-death responses by keeping BAK1 under control. Gao et al.
[72] showed that BIR1, a BAK1-interacting RLK, negatively regulates multiple plant resistance signalling responses, and suppresses cell death in Arabidopsis.
BIR1 KO mutants (
bir1-1) showed activation of constitutive defence responses and extensive cell death. However, the LRR-RLK SUPPRESSOR OF BIR1-1 (SOBIR1) and BAK1 function as co-receptors for LRR-RLPs, BAK1, and not SOBIR1, acts a co-receptor for LRR-RLKs
[63]. SOBIR1, a co-receptor/adaptor for LRR-RLPs recruits BAK1 to SOBIR1-RLP23 and SOBIR1-RLP30 complex upon nlp20 and Sclerotinia culture filtrate elicitor1 (SCFE1) perception, respectively, in Arabidopsis
[73][74]. Here,
SOBIR1 mutant (
sobir1-12) was more susceptible to fungal
Sclerotinia sclerotiorum and
B. cineria [75]. The dissociation of BIR1 upon MAMP recognition by PRRs allows BAK1 to form an active complex with SOBIR1, which triggers downstream cell death and defence signalling
[76].
The Arabidopsis malectin-like LRR-RLK, IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) associated with PRRs FLS2, EFR and CERK1 in BAK1-dependent and -independent MTI responses
[77]. Arabidopsis
IOS1 mutant (
ios1-2) showed perturbations in the latter, including defective chitin responses and delayed upregulation of the PTI marker gene
FLG22-INDUCED RECEPTOR-LIKE KINASE1 (
FRK1), as well as reduced downy mildew infection
[74]. The malectin-like RLK FERONIA (FER), facilitates the ligand-induced complex formation of PRRs in Arabidopsis
[78][79]. As such, the EFR/FLS2-BAK1 complex formation has been shown to be promoted by FER and inhibited by Rapid Alkalinization Factor 23 (RALF23)
[77]. Furthermore, a
FER mutant (
fer-4) showed diminished ligand-induced EFR/FLS2 complex formation, with the co-receptor BAK1. In addition, AtFER is involved in the negative regulation of jasmonic acid (JA) and coronatine (COR) signalling
[79]. In support, BAK1 and other defence-responsive proteins were identified in
A. thaliana plasma membranes after
B. cepacia and
E. coli LPS treatments
[31][32]. Here, proteins identified were similar to some previously implicated proteins upon flg22 elicitation, suggesting that LPS perception and signalling could likely resemble that of flg22.