Since salicylic acid (SA) was discovered as an elicitor of tobacco plants inducing the resistance against Tobacco mosaic virus (TMV) in 1979, increasing reports suggest that SA indeed is a key plant hormone regulating plant immunity. In addition, recent studies indicate that SA can regulate many different responses, such as tolerance to abiotic stress, plant growth and development, and soil microbiome. In this review, we focused on the recent findings on SA's effects on resistance to biotic stresses in different plant-pathogen systems, tolerance to different abiotic stresses in different plants, plant growth and development, and soil microbiome. This allows us to discuss about the safe and practical use of SA as a plant defense activator and growth regulator. Crosstalk of SA with different plant hormones, such as abscisic acid, ethylene, jasmonic acid, and auxin in different stress and developmental conditions were also discussed.

Plant growth promoting rhizobacteria (PGPR) is not only enhancing plant growth, but also inducing resistance against a broad range of pathogens, thus providing effective strategies to substitute chemical products. In this study, Burkholderia contaminans AY001 (AY001) is isolated based on its broad-spectrum antifungal activity. AY001 not only inhibited fungal pathogen growth in dual culture and culture filtrate assays, but also showed various PGPR traits, such as nitrogen fixation, phosphate solubilization, extracellular protease production, zinc solubilization and indole-3-acetic acid (IAA) biosynthesis activities. Indeed, AY001 treatment significantly enhanced growth of tomato plants and enhanced resistance against two distinct pathogens, F. oxysporum f.sp. lycopersici and Pseudomonas syringae pv. tomato. Real-time qPCR analyses revealed that AY001 treatment induced jasmonic acid/ethylene-dependent defense-related gene expression, suggesting its Induced Systemic Resistance (ISR)-eliciting activity. Gas chromatography–mass spectrometry (GC-MS) analysis of culture filtrate of AY001 revealed production of antimicrobial compounds, including di(2-ethylhexyl) phthalate and pyrrolo [1,2-a]pyrazine-1,4-dione, hexahydro-3-(phenylmethyl). Taken together, our newly isolated AY001 showed promising PGPR and ISR activities in tomato plants, suggesting its potential use as a biofertilizer and biocontrol agent.

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In this study, we aimed to screen antagonistic microorganisms against Acidovorax citrulli, the causal agent of bacterial fruit blotch, which is known to induce sever diseases in cucurbit crops. From 240 bacterial strains isolated, only one unknown bacterial isolate, named YM002, showed significant antagonistic activity against A. citrulli KACC17909. Further experiments revealed that YM002 shows antagonistic activity against all tested A. citrulli strains, including KACC17000, KACC17001 and KACC17005, to different degrees. The phylogenetic analysis of 16S rRNA sequences identified YM002 as Paenibacillus tianmuensis. Importantly, pretreatment of cucumber (Cucumis sativus) leaves with YM002 enhanced disease resistance as observed by significantly reduced necrotic symptom development and bacterial growth. YM002-induced resistance accompanied by enhanced expression of defense-related genes, such as PAL1, PR1-1a and CTR1. Importantly, culture filtrate of YM002 significantly suppressed biofilm formation and swimming motility of A. citrulli, which is indispensable for its full virulence. In addition to its antagonistic activity, YM002 showed a various plant growth promotion (PGP)-related traits, such as production of ammonia production, amylase production, ACC deaminase production, inodole-3-acetic acid production, extracellular protease production, siderophore production, and zinc solubilization activities. Indeed, treatment of cucumber roots with YM002 significantly enhanced plant growth parameters, such as fresh and dry weight of leaves or roots. This study suggests the potential of YM002 as an effective PGPR with biological control activity against Acidovorax citrulli in cucumber plants.

Chemical and biological agents were evaluated to inhibit Colletotrichum fructicola, Phytophthora cactorum, and Lasiodiplodia theobromae causing strawberry diseases. Mycelial growths of C. fructicola were gradually arrested by increasing concentrations of fungicides pyraclostrobin and iminoctadine tris (albesilate). P. cactorum and L. theobromae were more sensitive to pyraclostrobin compared to C. fructicola, but iminoctadine tris (albesilate) was not or less effective to limit P. cactorum or L. theobromae, respectively. Bacillus siamensis H30-3 was antagonistic against the three pathogens by diffusible as well as volatile molecules, and evidently reduced aerial mycelial formation of P. cactorum. B. siamensis H30-3 growth was declined by at least 0.025 mg/ml of pyraclostrobin. The two fungicides additively inhibited mycelial growths of C. fructicola, but not of P. cactorum and L. theobromae. B. siamensis H30-3 volatiles led to less growth of C. fructicola than one reduced by the fungicides. Taken together, in vitro antimicrobial activities of the two fungicides together with or without B. siamensis H30-3 volatiles may be cautiously incorporated into integrated management of strawberry diseases dependent on causal pathogens.