The Raf-like kinase ILK1 and the high affinity K+ transporter HAK5 are required for Innate Immunity and Abiotic Stress Response

Abstract

Plant perception of pathogen-associated molecular patterns (PAMPs) and other environmental stresses trigger transient ion fluxes at the plasma membrane. Apart from the role of Ca 2+ uptake in signaling, the regulation and significance of PAMPinduced ion fluxes in immunity remain unknown. We characterized the functions of INTEGRIN-LINKED KINASE1 (ILK1) that encodes a Raf-like MAP2K kinase with functions insufficiently understood in plants. Analysis of ILK1 mutants impaired in the expression or kinase activity revealed that ILK1 contributes to plant defense to bacterial pathogens, osmotic stress sensitivity, and cellular responses and total ion accumulation in the plant upon treatment with a bacterial-derived PAMP, flg22. The calmodulin-like protein CML9, a negative modulator of flg22-triggered immunity, interacted with, and suppressed ILK1 kinase activity. ILK1 interacted with and promoted the accumulation of HAK5, a putative (H + )/K + symporter that mediates a high-affinity uptake during K + deficiency. ILK1 or HAK5 expression was required for several flg22 responses including gene induction, growth arrest, and plasma membrane depolarization. Furthermore, flg22 treatment induced a rapid K + efflux at both the plant and cellular levels in wild type, while mutants with impaired ILK1 or HAK5 expression exhibited a comparatively increased K + loss. Taken together, our results position ILK1 as a link between plant defense pathways and K + homeostasis.

From Chaos to Harmony: Responses and Signaling upon Microbial Pattern Recognition

“…Interestingly, flg22 treatment triggers a considerably higher peak value of Cl − efflux than of H + influx, suggesting the involvement of anion channels in establishing PM depolarization (66). Integrin-linked kinase 1 (ILK1), which interacts with the high-affinity K + transporter HAK5, positively regulates flg22-induced PM depolarization, implicating the involvement of K + efflux in PM depolarization (16). Ca 2+ signaling likely functions upstream of PM depolarization and extracellular alkalinization, as lanthanum, a PM Ca 2+ channel blocker, inhibits flg22-induced PM depolarization (66).…”

Insights into the Structure, Function, and Ion-Mediated Signaling Pathways Transduced by Plant Integrin-Linked Kinases

“…Animal ILKs contain abundant substitutions in conserved residues of the KD and, although their kinase activity may be possible, extensive analyses substantiate the view that they are true pseudokinases (Wickström et al, 2010). By comparison, plant ILKs contain fewer substitutions and some, at least, can catalyze a phosphotransfer reaction, in both autophosphorylation and substrate phosphorylation assays (Chinchilla et al, 2008; Brauer et al, 2016). …”

“…Notably, despite lacking transmembrane regions, several ILKs were found associated with the PM or intracellular membranous organelles. ILK1 co-localized with the PM and ER markers (Brauer et al, 2016), and ILK4 and ILK6 were identified in the PM, Golgi and tonoplast (Benschop et al, 2007; Whiteman et al, 2008; Elmore et al, 2012; Heard et al, 2015). ILK4 and ILK5 may be nuclear-localized as predicted by the bipartite or monopartite nuclear localization signals within their C-termini ( Figure 1 ); an ILK5-GFP fusion was detected both in the cytosol and nucleus (Koroleva et al, 2005; Ito et al, 2011).…”

“…The absence of ILK in epidermal cells led to defective cell differentiation and aberrant translocation of the CaSR calcium-sensing receptor (Sayedyahossein et al, 2016). In plants, CML9, a Ca 2+ sensor, and negative PTI modulator interacted with ILK1 in vivo and inhibited ILK1 kinase activity (Brauer et al, 2016), suggesting that ILK1-CML9 are part of a putative pathway mediated by pathogen-triggered Ca 2+ waves. Although limited, these findings support the existence of reciprocal regulatory relationships between ILK signaling and Ca 2+ , which may thus influence multiple cellular functions.…”

“…Plant ILKs were first described in Medicago and Arabidopsis as ankyrin kinases proposed to act in nodulation and secondary root growth (Chinchilla et al, 2003, 2008). Recently, ILK1 was demonstrated to regulate plants’ sensitivity to osmotic and salt stress, response to nutrient availability, and susceptibility to bacterial pathogens (Brauer et al, 2016). Most of the roughly 50 plant RAFs have unknown functions and poorly understood signaling mechanisms.…”