The MAP4 Kinase SIK1 Ensures Robust Extracellular ROS Burst and Antibacterial Immunity in Plants

Abstract

Microbial patterns are recognized by cell-surface receptors to initiate pattern-triggered immunity (PTI) in plants. Receptor-like cytoplasmic kinases (RLCKs), such as BIK1, and calcium-dependent protein kinases (CPKs) are engaged during PTI to activate the NADPH oxidase RBOHD for reactive oxygen species (ROS) production. It is unknown whether protein kinases besides CPKs and RLCKs participate in RBOHD regulation. We screened mutants in all ten Arabidopsis MAP4 kinases (MAP4Ks) and identified the conserved MAP4K SIK1 as a positive regulator of PTI. sik1 mutants were compromised in their ability to elicit the ROS burst in response to microbial features and exhibited compromised PTI to bacterial infection. SIK1 directly interacts with, phosphorylates, and stabilizes BIK1 in a kinase activity-dependent manner. Furthermore, SIK1 directly interacts with and phosphorylates RBOHD upon flagellin perception. Thus, SIK1 positively regulates immunity by stabilizing BIK1 and activating RBOHD to promote the extracellular ROS burst.

Quantitative phosphoproteomic analysis reveals common regulatory mechanisms between effector‐ and PAMP‐triggered immunity in plants

“…In particular, BIK1 phosphorylates RBOHD residues S343 and S347, and CPKs phosphorylate RBOHD residues S163 and, possibly, S347 during PTI. Recently, it was found that the MAP4K SIK1 is also involved in the full phosphorylation on S339 and S347 during PTI (M. Zhang et al, 2018).…”

“…It would be interesting to investigate the involvement of these RLCKs in RBOHD phosphorylation using multiple knockout mutants. Interestingly, the MAP4 kinase SIK1 has been found recently to be able to phosphorylate RBOHD during PTI (M. Zhang et al, 2018). However, it remains to be elucidated whether SIK1 contributes to RBOHD phosphorylation during ETI.…”

MAP 4K4 associates with BIK 1 to regulate plant innate immunity

“…The authors also found that map4k3/sik1 mutants showed an autoimmune phenotype only under specific conditions, indicating that growth conditions affect MAP4K3 function. Furthermore, flg22-induced BIK1 activation is decreased in map4k4, but not map4k3 [40], indicating that BIK1 hyper-phosphorylation was dependent on MAP4K4 but not MAP4K3. Comparing map4k3 and map4k4 mutants, they show some common phenotypes.…”

“…Similar to MAP4K4, MAP4K3/SIK1 was recently reported to positively regulate flg22-induced immunity responses by stabilizing BIK1 [40]. To determine the genetic redundancy between MAP4K3 and MAP4K4, we generated map4k3 map4k4 double mutants ( Fig 6E).…”

Convergence of cell‐surface and intracellular immune receptor signalling

“…Specifically, RBOHD S347 site is in CPK phosphorylation motif (φ‐X‐X‐X‐X‐S/T‐X‐B; Kadota et al ., ), and CPK5 was involved in the phosphorylation (Adachi & Yoshioka, ) while RBOHD S343 is not in the typical CPK phosphorylation motif. A more recent study suggested that Arabidopsis MAP4 kinase SIK1 positively regulated ROS production by phosphorylating RBOHD S347 during PTI (Zhang et al ., ). Thus, these mentioned kinases may phosphorylate RBOHD during ETI, perhaps in a redundant manner.…”