Biodiversity is adversely affected by the growing levels of synthetic chemicals released into the environment due to agricultural activities. This has been the driving force for embracing sustainable agriculture. Plant secondary metabolites offer promising alternatives for protecting plants against microbes, feeding herbivores, and weeds. Terpenes are the largest among PSMs and have been extensively studied for their potential as antimicrobial, insecticidal, and weed control agents. They also attract natural enemies of pests and beneficial insects, such as pollinators and dispersers. However, most of these research findings are shelved and fail to pass beyond the laboratory and greenhouse stages. This review provides an overview of terpenes, types, biosynthesis, and their roles in protecting plants against microbial pathogens, insect pests, and weeds to rekindle the debate on using terpenes for the development of environmentally friendly biopesticides and herbicides.

Fungi infections cause approximately 60–70% yield loss through diseases such as rice blast, powdery mildew, Fusarium rot, downy mildew, etc. Plants naturally respond to these infections by eliciting an array of protective metabolites to confer physical or chemical protection. Among plant metabolites, lignin, a phenolic compound, thickens the middle lamella and the secondary cell walls of plants to curtail fungi infection. The biosynthesis of monolignols (lignin monomers) is regulated by genes whose transcript abundance significantly improves plant defense against fungi. The catalytic activities of lignin biosynthetic enzymes also contribute to the accumulation of other defense compounds. Recent advances focus on modifying the lignin pathway to enhance plant growth and defense against pathogens. This review presents an overview of monolignol regulatory genes and their contributions to fungi immunity, as reported over the last five years. This review expands the frontiers in lignin pathway engineering to enhance plant defense.

RNA interference (RNAi) caused by exogenous double-stranded RNA (dsRNA) has developed into a powerful technique in functional genomics, and to date it is widely used to down-regulate crucial physiology-related genes to control pest insects. A molt-regulating transcription factor gene, HaHR3, of cotton bollworm (Helicoverpa armigera) was selected as the target gene. Four different fragments covering the coding sequence (CDS) of HaHR3 were cloned into vector L4440 to express dsRNAs in Escherichia coli. The most effective silencing fragment was then cloned into a plant over-expression vector to express a hairpin RNA (hpRNA) in transgenic tobacco (Nicotiana tabacum). When H. armigera larvae were fed the E. coli or transgenic plants, the HaHR3 mRNA and protein levels dramatically decreased, resulting developmental deformity and larval lethality. The results demonstrate that both recombinant bacteria and transgenic plants could induce HaHR3 silence to disrupt H. armigera development, transgenic plant-mediated RNAi is emerging as a powerful approach for controlling insect pests.

Fusarium oxysporum is an important soilborne fungal pathogen with many different formae speciales that can colonize the plant vascular system and cause serious crop wilt disease worldwide. We found a glycoside hydrolase family 12 protein FoEG1, secreted by F. oxysporum, that acted as a pathogen‐associated molecular pattern (PAMP) targeting the apoplast of plants to induce cell death. Purified FoEG1 protein triggered cell death in different plants and induced the plant defence response to enhance the disease resistance of plants. The ability of FoEG1 to induce cell death was mediated by leucine‐rich repeat (LRR) receptor‐like kinases BAK1 and SOBIR1, and this ability was independent of its hydrolase activity. The mutants of cysteine residues did not affect the ability of FoEG1 to induce cell death, and an 86 amino acid fragment from amino acid positions 144 to 229 of FoEG1 was sufficient to induce cell death in Nicotiana benthamiana. In addition, the expression of FoEG1 was strongly induced in the early stage of F. oxysporum infection of host plants, and FoEG1 deletion or loss of enzyme activity reduced the virulence of F. oxysporum. Therefore, our results suggest that FoEG1 can contribute to the virulence of F. oxysporum depending on its enzyme activity and can also act as a PAMP to induce plant defence responses.

APETALA2/ethylene‐responsive factor (AP2/ERF) family transcription factors are well‐documented in plant responses to a wide range of biotic and abiotic stresses, but their roles in mediating elicitor‐induced disease resistance remains largely unexplored. PevD1 is a Verticillium dahliae secretory effector that can induce disease resistance in cotton and tobacco plants. In our previous work, Nicotiana benthamiana ERF114 ( NbERF114 ) was identified in a screen of genes differentially expressed in response to PevD1 infiltration. Here, we found that the ortholog of NbERF114 in Arabidopsis thaliana ( ERF114 ) also strongly responded to PevD1 treatment and transcripts were induced by Pseudomonas syringae pv. tomato (Pst) DC3000 infection. Loss of ERF114 function caused impaired disease resistance, while overexpressing ERF114 ( OE‐ERF114 ) enhanced resistance to Pst DC3000. Moreover, ERF114 mediated PevD1‐induced disease resistance. RNA‐sequencing analysis revealed that the transcript level of phenylalanine ammonia‐lyase1 ( PAL1 ) and its downstream genes were significantly suppressed in erf114 mutants compared with A. thaliana Col‐0. Reverse transcription‐quantitative PCR (RT‐qPCR) analysis further confirmed that the PAL1 mRNA level was significantly elevated in overexpressing OE‐ERF114 plants but reduced in erf114 mutants compared with Col‐0. Chromatin immunoprecipitation‐qPCR (ChIP‐qPCR) and electrophoretic mobility shift assay verified that ERF114 directly bound to the promoter of PAL1 . The gene expression profiles of ERF114 and PAL1 in oestradiol‐inducible transgenic plants confirmed ERF114 could activate PAL1 transcriptional expression. Further investigation revealed that ERF114 positively modulated PevD1‐induced lignin and salicylic acid accumulation, probably by activating PAL1 transcription.

BcGs1, a cell wall-degrading enzyme (CWDE), was originally derived from Botrytis cinerea. Our previous study revealed that BcGs1 could trigger defense responses and protect plants against various pathogens. We researched the defense response mechanism underlying this BcGs1 elicitation in tomato. We revealed that the two domains were required for BcGs1’s full necrosis activity. According to analysis and quantitative real-time PCR of the up-regulated proteins and genes filtered by iTRAQ-based quantitative proteome approach, oxidative metabolism and phenylpropanoid metabolism were speculated to be involved in BcGs1-triggered defense response in tomato. Furthermore, experimental evidence showed that BcGs1 triggered reactive oxygen species (ROS) burst and increased the level of phenylalanine-ammonia lyase (PAL) and peroxidase (POD) enzyme activity, as well as lignin accumulation. Moreover, histochemical analysis revealed that infiltration of BcGs1 in tomato leaves exhibited cell wall thickening compared with untreated plants. The results suggested that BcGs1 activated the basal defense response included lignin metabolism contributed to BcGs1-induced resistance to Botrytis. cinerea infection in tomato.