Fig. 1. Several typical plant secondary metabolites with special interests.

Fig. 2. Schematic illustration of the sequential defense reactions in plants induced by elicitors. In many particular cases, only some of these events were detected.

Fig. 3. Schematic illustration of biosynthetic pathway of JA related and other oxylipins, and their mediation of elicitor signal transduction leading to accumulation of plant secondary metabolites. Elicitors or avirulent determinants are perceived by specific receptors localized to the plasma membrane (or in the cytoplasm). The activated receptors may then activate ion channels and G-proteins and then activate phospholipases (such as PLA2), through Ca²⁺ signaling or by G-protein coupling. Phospholipases hydrolyze phospholipids, such as PC, into fatty acid and lysoPC; the former can be a precursor for biosynthesis of JA and related oxylipins via the octadecanoid pathway, or peroxidized by reactive oxygen species to produce another class of pentacyclic oxylipin phytoprostanes. All these pentacyclic oxylipins can activate biosynthesis of plant secondary metabolites. In addition, the oxidized fatty acids HPOD (13S-hydroperoxy-9(Z),11(E)-octadecadienoic acid.) and HPOT (13S-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid) by LOX can also be used by HPL-pathway for C6-aldehyde and C12 oxoacid generation. Hexenal (from HPOD) or cis-3-hexenal (from HPOT), as well as traumatin (from C12 oxoacid), can act as defense signals to induce accumulation of plant defensive secondary metabolites.

Fig. 4. Lipid messengers derived from hydrolysis of the plasma membrane. Proteins in the plasma membrane and the plasma membrane itself can perceive various stresses at first. Phospholipase A (PLA), phospholipase C (PLC), and phospholipase D (PLD) are most often activated by various environmental and hormonal stresses, and hydrolyze membrane phospholipids, such as phosphatidylcholine (PC) and PIP₂, to produce various lipid signal molecules. Free fatty acid products from PLA action can be used for oxylipin signaling pathway, and another hydrolysis product of PC by PLA, lysophosphatidylcholine (lyso PC), has multiple roles in cellular processes, including activation of proton pumping from endomembrane systems to the cytosol that will induce cytoplasmic acidification and activate biosynthesis of secondary metabolites. PLC hydrolyzes PIP₂ into DAG and IP₃, which can activate protein kinases and mobilize Ca²⁺ from intracellular Ca stores. As an emerging messenger, PA can be produced either by PLD hydrolyzing phospholipids or through DAG phosphorylation by DAG kinase (DAGK), while PA can also be converted into DAG by PA phosphatase (PAP). PA may directly or indirectly affect plant secondary metabolism.

Fig. 5. A comprehensive schematic illustration of elicitor signal transduction network. Proteinaceous or carbohydrate elicitor molecules are recognized by specific receptors on the plasma membrane. G-proteins may be coupled to receptors and mediate elicitor-induced ion channel activation. Ion fluxes, especially Ca²⁺ influx, cause cytosolic free Ca²⁺ spiking which causes activation of protein kinases, peroxidases, NADPH oxidases, and phospholipases, which further generate other signaling messengers, such as reactive oxygen species, DAG, IP₃, cAMP, lysoPC, JA, ethylene, NO, cADP ribose, and SA. All these messengers compose paralleling or cross-linking pathways to integrate these signals onto regulation of transcription factors (TFs). Various transcription factors integrate these signaling to activate gene expression by transcription machinery. Phenylpropanoid pathway is one of earlier induced pathways for cell wall reinforcement. Most genes for secondary metabolite synthesis are later response genes. The circled “P” shows protein phosphorylation and dephosphorylation-dependent regulation while the circled “Ca” shows Ca²⁺-dependent regulation. Lipid elicitor such as syringolide or cerebroside could be perceived by receptors localized to the cytoplasm. Lipid elicitors also induce Ca²⁺ signaling and oxidative burst, and eventually stimulate the accumulation of plant secondary metabolites. NOS: nitric oxide synthase; NR: nitrate reductase; AC: AMP cyclase; GC: GMP cyclase. It is must be noted that a plant may only use parts of this elicitor signaling network to fulfill their responses to elicitors.

Elicitors identified with their binding sites or receptors in plants

Elicitor signal transduction involved in production of plant secondary metabolites

Abbreviations for genes and enzymes: 4CL, 4-coumarate: CoA ligase; 5eAS, 5-epi-aristolochene synthase; ADH, aristolochene/deoxy-capsidiol hydroxylase; AS, anthranilate synthase; bAS, β-amyrin synthase; BBE, berberine bridge enzyme; BMT, SAM-bergaptol-O-methyltransferase; C4H, cinnamate 4-hydroxylase; CCoAMT, caffeoyl-CoA 3-O-methyltransferase; CFS, (S)-cheilanthifoline synthase; CHR, chalcone reductase; CHS, chalcone synthase; CYP79, cytochrome P450 CYP79; CYP83, cytochrome P450 CYP83;. D4H, desacetoxyvindoline 4-hydroxylase; DAT, deacetylvindoline 4-O-acetyltransferase; DHBO, dihydrobenzophenanthridine oxidase; DMID, 7,2′-dihydroxy-4′-methoxyisoflavanol dehydratase; DP6H, dihydroxypterocarpan 6a-hydroxylase; F3H, flavanone 3-hydroxylase; F6H, flavonoid 6-hydoxylase; FPPS, farnesyl pyrophosphate synthase; GGDPS, geranylgeranyl diphosphate synthase; GPP, geranyl pyrophosphate synthase; HBG, 4-hydroxybenzoate 3-geranyltransferase; HHT, hydroxyanthranilate hydroxycinnamoyltransferase; HMGR, 3-hydroxy-3-methylglutaryl coenzyme A reductase; IFR, isoflavone reductase; IPP isomerase, isoprepenyl pyrophosphate isomerase; InGPS, indole-3-glycerolphosphate synthase; MSH, (S)-cis-N-methylstylopine 14-hydroxylase; N7M, naringenin 7-o-methyltransferase; N8DT, naringenin 8-dimethylallyltransferase; PAI, phosphoribosyl-anthranilate isomerase; PAL, phenylalanine ammonia lyase; PAT, phosphoribosylanthranilate transferase; PKR, polyketide reductase; PKS, polyketide synthase; PMT, putrescine: SAM N-methyltransferase, PPH, protopine 6-hydroxylase; SGD, strictosidine β-d-glucosidase; SGT: scopoletin glucocyltransferase SPS, (S)-stylopine synthase; SQE, squalene epoxidase SQS, squalene synthase; STR, strictosidine synthase; STS, stilbene synthase; TDC, tryptophan decarboxylase; THB-NMT, tetrahydroberberine-N-methyltransferase THC-NMT, tetrahydrocortisine-N-methyltransferase TSA, tryptophan synthase UGSGT, UDP-glucuronic acid: soyasapogenol β-glucuronosyltransferase; VR, vestitone reductase.