Synthetic elicitors are drug-like compounds that induce plant immune responses but are structurally distinct from natural defense elicitors. Using high-throughput screening, we previously identified 114 synthetic elicitors that activate the expression of a pathogen-responsive reporter gene in Arabidopsis (Arabidopsis thaliana). Here, we report on the characterization of one of these compounds, 2-(5-bromo-2-hydroxy-phenyl)-thiazolidine-4-carboxylic acid (BHTC). BHTC induces disease resistance of plants against bacterial, oomycete, and fungal pathogens and has a unique mode of action and structure. Surprisingly, we found that low doses of BHTC enhanced root growth in Arabidopsis, while high doses of this compound inhibited root growth, besides inducing defense. These effects are reminiscent of the hormetic response, which is characterized by low-dose stimulatory effects of a wide range of agents that are toxic or inhibitory at higher doses. Like its effects on defense, BHTC-induced hormesis in Arabidopsis roots is partially dependent on the WRKY70 transcription factor. Interestingly, BHTC-induced root hormesis is also affected in the auxin-response mutants axr1-3 and slr-1. By messenger RNA sequencing, we uncovered a dramatic difference between transcriptional profiles triggered by low and high doses of BHTC. Only high levels of BHTC induce typical defense-related transcriptional changes. Instead, low BHTC levels trigger a coordinated intercompartmental transcriptional response manifested in the suppression of photosynthesis- and respiration-related genes in the nucleus, chloroplasts, and mitochondria as well as the induction of development-related nuclear genes. Taken together, our functional characterization of BHTC links defense regulation to hormesis and provides a hypothetical transcriptional scenario for the induction of hormetic root growth.

Synthetic elicitors are drug-like compounds that are structurally distinct from natural defense elicitors. They can protect plants from diseases by activating host immune responses and can serve as tools for the dissection of the plant immune system as well as leads for the development of environmentally-safe pesticide alternatives. By high-throughput screening, we previously identified 114 synthetic elicitors that activate expression of the pathogen-responsive CaBP22⁻³³³::GUS reporter gene in Arabidopsis thaliana (Arabidopsis), 33 of which are [(phenylimino)methyl]phenol (PMP) derivatives or PMP-related compounds. Here we report on the characterization of one of these compounds, 2,4-dichloro-6-{(E)-[(3-methoxyphenyl)imino]methyl}phenol (DPMP). DPMP strongly triggers disease resistance of Arabidopsis against bacterial and oomycete pathogens. By mRNA-seq analysis we found transcriptional profiles triggered by DPMP to resemble typical defense-related responses.

Early blight (EB), caused by the necrotrophic pathogen Alternaria solani, is one of the most common and destructive diseases in the tomato (Solanum lycopersicum L.). The use of fungicides is a prominent tactic used to control EB; however, their undesirable effects on the environment and human health, as well as involvement in the development of resistant strains, have driven researchers to search for new alternatives. Plant defense elicitors are exogenous defense-triggering molecules that induce a plant’s defense system associated with extensive transcriptional- and metabolic reprogramming of the genome and do not cause direct toxicity to phytopathogens. Moreover, 2,6-dichloroisonicotinic acid (INA) was an early-identified and strong plant defense elicitor to various phytopathogens. Recently, the combination of chitosan oligomers and pectin-derived oligogalacturonides that can mimic the induction of plants by a pathogen or damaged-derived molecules (PAMP and DAMP) were characterized as defense elicitors, named FytoSol. In this study, the preventive roles of these two defense elicitors—FytoSol and INA—against EB disease and its molecular basis, were explored. According to the results, FytoSol significantly reduced disease severity by an average of 30% for almost one month with an AUDPC value of 399 compared to the control, which had an AUDPC value of 546. On the contrary, INA did not provide any protection against EB. Gene expression analyses of these two distinct plant defense elicitors indicated that the expression patterns of several SA-, JA-, or ET-pathway-related genes (Pti4, TPK1b, Pto kinase, TomloxD, PRB1-2, SABP2, WRKY33b, WRKY70, PR-5, and PR3) were induced by defense elicitors differently. FytoSol extensively upregulated gene expressions of PR3, downregulated the SA-related defense pathway, and provided remarkable protection against the necrotrophic pathogen Alternaria solani. On the contrary, INA mostly induced genes related to biotrophic and/or hemibiotrophic pathogen protection. Our results indicate that FytoSol is a promising plant defense elicitor against EB and the modes of action of the elicitors are important to characterize their effects against pathogens. Further research may extend the use of defense elicitors as alternatives to pesticides in agriculture.

Environmental and climatic fluctuations as well as abiotic stress factors affect agricultural production and cause a loss in quality and yield. It is important to find alternative solutions for the sustainability of agricultural production to feed the increasing population. Salt stress is one of the most devastating abiotic stress factors and tomato (Solanum lycopersicum L.) production is also affected by salt stress since it needs extensive irrigation for high yield. The exogenous application of some plant inducers showed promising results in the induction and improvement of plant tolerance to stress factors. Chitosan (2-amino-2-deoxy-b-D-glucosamine), one of the organic compounds, is getting significant attention in agriculture with its potential. Here, we evaluated the potential of chitosan application for salt stress tolerance on tomato. 0.03% and 0.05% chitosan solutions were applied as a foliar spray to the plant and salt tolerance improvement were investigated under untreated (0 mM NaCl) and 100 mM NaCl conditions. The growth-related (root and shoot diameters, above and below-ground biomass, number of leaves and branches, and plant height), photosynthetic parameters (chlorophyll a, b, total carotenoid content), and ion leakage were investigated. According to the results, chitosan application improves plant development in both untreated and salt-stress conditions and improved plant growth. Also, photosynthetic parameters showed that the application of chitosan increased chlorophyll contents under untreated conditions. Our result suggests that the application of chitosan may have a promising effect on salt stress tolerance and further research may shed light on its molecular mechanisms.

To achieve efficiency in seedling development, the seed must germinate quickly and homogeneously. Pre-sowing applications such as priming are practiced to eliminate or reduce the negative effects of environmental factors through germination and seedling development. This allows to reach a higher germination rate, yield, and desired plant density, especially by protecting against biotic and abiotic stresses. This study was carried out to examine the effects of pre-sowing seed applications on seedling growth and vigor of bread wheat (Triticum aestivum L.). Three different doses of β-amino-n-butyric acid (BABA) were used as seed priming agents and Dimethyl sulfoxide (DMSO) as control groups. The study was carried out according to the completely randomized factorial design with three replications and ten plants per replication. As a result of priming applications, significantly higher values in the number of roots, longest root length, total root length, average root length and shoot fresh weight were obtained compared to control. Our results, for the first time, showed that seed priming with BABA had a promoting effect on many aspects of early seedling growth in bread wheat. This is a preliminary study to understand the mechanism of seed priming and its effects on germination and root growth in wheat. Further studies may shed light on the molecular mechanisms of BABA or other compounds as seed priming agents, benefits for abiotic and biotic stress tolerance, and good stand establishment at the seedling or later stages.