PTI summary

[ValWood]

[ValWood]

From https://www.sciencedirect.com/science/article/pii/S1674205215000878

• Flg22 perception by FLS2 PRR induces rapid (seconds) immune receptor complex formation at the plasma membrane and different auto- and trans-phosphorylations of the actors (1).

• BIK1 becomes quickly phosphorylated and released from the PRR complex (2).

• A Ca2+ burst occurs (∼30 s to 2 min) and reaches a peak at around 4–6 min (3). This Ca2+ influx induces opening of other membrane channels (influx of H+, efflux of K+, Cl– and nitrate), which lead to an extracellular alkalinization (∼1 min) and a depolarization of the plasma membrane (1–3 min) (4)

• A long-lasting increase (8 to more than 30 min) of free Ca2+ levels also occurs in the chloroplast stroma.

• An ROS burst then rapidly occurs (∼2–3 min) via RBOHD and peaks at around 10–14 min (5). Full activation of RBOHD requires phosphorylation by BIK1 and Ca2+-induced CDPKs (5). Ca2+, PA, and NO also regulate RBOHD through direct binding or modification of the protein (5).

• RBOHD produces O2.– in the apoplast, which is converted into H2O2 by superoxide dismutases (SOD) (6). H2O2 can enter the cytosol and the different organelles of the cell and is capable of inducing cytosolic Ca2+ elevations (7).

• Ca2+-induced CaM leads to synthesis of NO, which can regulate NPR1 and RBOHD via cysteine S-nitrosylation (8).

• PA production by PLD and PLC/DGK is rapidly induced (already at 2 min) and reaches a maximum at around 8 min (9). NO production is partly required for PA generation (10). PA can modulate the activity of CDPKs, PDK1, and RBOHD.

• PA activates OXI1 in a PDK1-dependent manner (11), and H2O2 can activate Pti1 kinases in an OXI1-dependent way (12). Pti1 kinases then may regulate MAPKs (13).

• AGB1, AGG1, and AGG2 G proteins are partly required for ROS burst and probably also MPK4 activation (14). 14-3-3 proteins modulate the activity of RBOHD, CDPKs, PDK1, and several ACS isoforms (15).

• MAPK modules are activated in a few minutes, probably in part in Ca2+-burst and BIK1/PBLs dependent ways, and, with Ca2+-induced CDPKs, phosphorylate a large spectrum of substrates such as ACS isoforms, leading to ET synthesis, and transcription factors (TFs) (16).

• TFs, Mediator subunits (Med.), and chromatin remodelers/modifiers (Chrom.) participate in the regulation of several thousand genes (17).

• First changes include notably genes involved in SA, JA, and ET signaling, synthesis of antimicrobial compounds and transcription regulatory factors (18). SA, JA, and ET signaling pathways then contribute to following gene regulation (19).

• Crosstalks also occur with other phytohormones (20).

• This complex signaling network finally leads to the implementation of plant-induced defenses, such as the production and secretion of antimicrobial compounds and the generation of toxic ROS (21).

Black arrows denote enzymatic pathways or transport, and red arrows denote regulation (direct or indirect activation/inhibition). Question marks indicate unidentified or unclear events.