Title : Activation of Innate Plant Immunity as a Sustainable Strategy for Pest Management
Abstract:
Plants adopt sophisticated molecular mechanisms to activate immune response against pathogen and parasite attacks. Immune response in plants is regulated by several low molecular weight molecules known as phytohormones, i.e. salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) which also regulate many aspects of plant growth, photosynthesis, flowering, reproduction, seed production and response to environmental abiotic challenges. Defense following pest attacks and also colonization from beneficial microorganisms by plants goes through a relatively unspecific response, a basal defense (PTI, PAMPs Triggered Immunity), in which some molecular patterns (PAMPs, Pathogen Associated Molecular Patterns), such as chitin from fungi, peptidoglycans/flagellin from bacteria, and lipochitooligosaccharides from symbionts are recognized in the apoplastic spaces by cell-surface receptors known as NLR proteins (nucleotide binding domain (NDB), leucine-rich repeats (LRR)). However, adapted pathogens can circumvent PTI by delivering effector molecules directly into the cells. Specific effectors can be recognized by resistant plants carrying intracellular NLRs known encoded by resistance genes (R-genes) in the so-called Effector-Triggered Immunity (ETI). ETI is characterized by a Hypersensitive Reaction (HR) in which production of Reactive Oxygen Species (ROS), SA, and antimicrobial enzymes occur leading to tissue necrosis which stops the spread of the infection. Primary infections of necrotrophs in the green parts of the plant produce diffusible immune signals moving upwards to uninfected leaves, which prime plants against subsequent infections by a wide array of biotrophic pathogen/parasites; this defense mechanism has long since been recognized as Systemic Acquired Resistance (SAR). The executors of SAR are the so-called Pathogenesis Related Proteins (PR-proteins). Over-expression of PR-genes is strictly associated with SAR induction in leaves. Our study has proved that SAR can be induced also in roots against soil-borne parasites such as the biotrophic root-knot nematodes (RKNs). SAR can be induced by minimal amounts of exogenously-added SA and natural functional analogs like methyl-SA, or synthetically produced analogs such as benzothiadiazole (BTH) and 2,6-?dichloroisonicotinic acid (INA). Research is on-going to monitor some other classes of chemicals, such as strobilurins, used as fungicides at present, as potential inducers of resistance against nematodes and insects. Furthermore, enrichment of soil of potted tomato and egg plants by Arbuscular Mycorrhizal Fungi (AMF) or antagonistic symbiotic fungi such as some strains of Trichoderma spp., commercially available in mixtures, has been proved to induce resistance (MIR, Micorrhiza Induced Resistance; TIR, Trichoderma Induced Resistance) to RKNs; propagules of Glomus spp. (MIR) mixed with soil induced resistance against the miner insect Tuta absoluta in tomato. MIR has been recognized by over-expression of PR-1 either in roots or in leaves.
Application of this strategy in PMI programs will foster the development of all omics techniques, such as metabolomics for immune signal identification, epigenomics for assay of DNA methylation and acetylation in chromatin rearrangements following priming, genomics and proteomics for assays of PR-gene expression, PR-protein enzyme activities, protein identification by specific antibodies, etc. Activation of plant immunity will make plants more responsive to environmental challenges and open the way to a more sustainable pest control as indicated by scientific European communities.
