TLRs recognize microbial molecules including lipoprotein (TLR2), LPS (TLR4), flagellin (TLR5), dsRNA (TLR3), ssRNA (TLR7/8), and CpG DNA motif (TLR9), and subsequently activate NF-κB and MAPK to trigger inflammatory process [1].
PRRs are able to sense the presence of conserved microbial molecules, including proteins, lipopolysaccharides, lipoproteins, and nucleic acids [ 1, 2].
Besides TLRs, there are also other pattern recognition receptors (PRRs) present in PBMCs that detect a variety of microbial molecules, including NLRs, that contain a NOD- and ligand-recognizing leucine-rich repeat [ 35].
The defense against microbes and microbial molecules includes two general types of responses: innate immunity and adaptive immunity.
MSCs secrete multiple molecules, including anti-inflammatory cytokines, growth factors, and anti-microbial peptides, and appear as fine-tuners of host inflammation [6].
We recently demonstrated that roots can incorporate large organic molecules including proteins and DNA [20], [21], and this implies that plants may be less dependent on microbial activity for break-down of organic matter than currently assumed.
The ligands are called pathogen associated molecular patterns (PAMPs) and include a broad array of microbial molecules build-up of proteins, nucleic acids, lipids, or synthetic chemicals.
PAMPs include a growing list of essential conserved microbial molecules that are not normally present in the plants such as lipopolysaccharides (LPS), flagellin, glycoproteins, chitin etc [19].
PRRs include Nod-like receptors (NLRs), which mediate cytosolic recognition of microbial molecules and promote their clearance [ 7– 9], and Toll-like receptors (TLRs), which are located at the cell surface or within endosomal membranes and recognize a wide range of microbial molecules such as lipopolysaccharide (LPS), peptidoglycan, lipoteichoic acid, flagellin, and zymosan [ 8, 10, 11].
These include an avoidance behaviour, triggered by the detection of specific microbial molecules, such as the cyclic pentadepsipeptide biosurfactant serrawettin W2, produced by some strains of Serratia marcescens (Pradel et al., 2007).
The first signal (also called the priming step) consists of microbial molecules or endogenous cytokines and is required for the upregulation of NLRP3 itself and the production of targeting cytokines by the inflammasome, including the pro-form of IL-1 β (pro-IL-1 β).
