Distinct chemical resistance-inducing stimuli result in common transcriptional, metabolic, and nematode community signatures in rice root and rhizosphere

Induced resistance (IR), a phenotypic state induced by an exogenous stimulus and characterized by enhanced resistance to future (a)biotic challenge, is an important component of plant immunity. Numerous IR-inducing stimuli have been described in various plant species, but relatively little is known about 'core' systemic responses shared by these distinct IR stimuli and the effects of IR on plant-associated microbiota. In this study, rice (Oryza sativa) leaves were treated with four distinct IR stimuli (β-aminobutyric acid, acibenzolar-S-methyl, dehydroascorbic acid, and piperonylic acid) capable of inducing systemic IR against the root-knot nematode Meloidogyne graminicola and evaluated their effect on the root transcriptome and exudome, and root-associated nematode communities. Our results reveal shared transcriptional responses-notably induction of jasmonic acid and phenylpropanoid metabolism-and shared alterations to the exudome that include increased amino acid, benzoate, and fatty acid exudation. In rice plants grown in soil from a rice field, IR stimuli significantly affected the composition of rhizosphere nematode communities 3 d after treatment, but by 14 d after treatment these changes had largely reverted. Notably, IR stimuli did not reduce nematode diversity, which suggests that IR might offer a sustainable option for managing plant-parasitic nematodes.

Ellman's method-based determination of acibenzolar-S-methyl in tobacco by transesterification with methanol and SABP2-catalyzed hydrolysis

Acibenzolar-S-methyl (ASM) is the most commercially successful biological antibacterial agent used for crop through systemic acquired resistance (SAR). In this study, a reproducible and accurate procedure, based on the spectrophotometric/microplate reader analysis, has been developed to detect ASM in tobacco leaves. This method involves oxidation of methyl mercaptan by the Ellman's reagent 5,5'-dithio-bis (2-nitrobenzoic acid) (DTNB) to form the yellow derivative 5'-thio-2-nitrobenzoic acid (TNB2-), measurable at 412 nm. Methyl mercaptan can be produced by either the ASM transesterification with methanol or the SA-binding protein 2 (SABP2)-catalyzed ASM hydrolysis. The proportions of methanol, reaction time, temperature, the concentrations of EDTA and DTNB were optimized in a 96-well plate. The calibration curve of ASM was linear over the range of 25.2-315 μg g-1. The results of the intra- and inter-day accuracy and precision data were within the FDA acceptance criteria. With ASM as substrate, the turnover number of SABP2 was determined, with the kcat value of 31.1 min-1 using the Michaelis-Menten equation. In tobacco plants treated with 100 μM ASM, it was decreased as time elapsed in treated tobacco, reaching negligible values 72 h after treatment. The optimized method was applied for the determination of ASM transesterification with methanol and the kinetic data determination of SABP2-catalyzed ASM hydrolysis.

Roles of calcium-dependent protein kinases mediated reactive oxygen species homeostasis in inducing resistance of apples by acibenzolar-S-methyl

This study was carried out to investigate the effect of acibenzolar-S-methyl (ASM) and ethylenebis (oxyethylenenitrilo) tetraacetic acid (EGTA) treatments on calcium-dependent protein kinases (CDPKs) and reactive oxygen species (ROS) metabolism in apples. Postharvest ASM treatment increased H₂O₂ content, reduced glutathione and ascorbic acid contents, and NADPH oxidase, peroxidase, ascorbate peroxidase, superoxide dismutase and glutathione reductase activities and retarded catalase activity and MdCAT expression in apples. ASM treatment enhanced MdSOD, MdPOD, MdAPX, MdGR, MdCDPK1, MdCDPK4, MdCDPK5, MdCDPK7, and MdCDPK21 expressions in apples. However, EGTA + ASM treatments suppressed H₂O₂, glutathione and ascorbic acid contents, NADPH oxidase, peroxidase, superoxide dismutase, ascorbate peroxidase and glutathione reductase activities. EGTA + ASM treatments suppressed the selected genes expressions in ROS metabolism and CDPKs, but up-regulated MdCAT expression in apples. These findings suggest that CDPKs play a vital role in regulating ROS metabolism and involve in inducing resistance in apples by ASM.

Acibenzolar-S-Methyl Activates Stomatal-Based Defense Systemically in Japanese Radish

Acibenzolar-S-methyl (ASM) is a well-known plant activator, which is a synthetic analog of salicylic acid (SA). Recently, copper fungicides and antibiotics are major strategies for controlling bacterial diseases. However, resistant strains have already been found. Therefore, there is an increasing demand for sustainable new disease control strategies. We investigated the ASM disease control effect against Pseudomonas cannabina pv. alisalensis (Pcal), which causes bacterial blight on Japanese radish. In this study, we demonstrated that ASM effectively suppressed Pcal disease symptom development associated with reduced bacterial populations on Japanese radish leaves. Interestingly, we also demonstrated that ASM activated systemic acquired resistance (SAR), including stomatal-based defense on ASM-untreated upper and lower leaves. Reactive oxygen species (ROS) are essential second messengers in stomatal-based defense. We found that ASM induced stomatal closure by inducing ROS production through peroxidase. These results indicate that stomatal closure induced by ASM treatment is effective for preventing Pcal pathogen invasion into plants, and in turn reduction of disease development.

actinidiae in absence and presence of acibenzolar-S-methyl

BACKGROUND

Since 2007, bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) has become a pandemic disease leading to important economic losses in every country where kiwifruit is widely cultivated. Options for controlling this disease are very limited and rely primarily on the use of bactericidal compounds, such as copper, and resistance inducers. Among the latter, the most widely studied is acibenzolar-S-methyl. To elucidate the early molecular reaction of kiwifruit plants (Actinidia chinensis var. chinensis) to Psa infection and acibenzolar-S-methyl treatment, a RNA seq analysis was performed at different phases of the infection process, from the epiphytic phase to the endophytic invasion on acibenzolar-S-methyl treated and on non-treated plants. The infection process was monitored in vivo by confocal laser scanning microscopy.

RESULTS

De novo assembly of kiwifruit transcriptome revealed a total of 39,607 transcripts, of which 3360 were differentially expressed during the infection process, primarily 3 h post inoculation. The study revealed the coordinated changes of important gene functional categories such as signaling, hormonal balance and transcriptional regulation. Among the transcription factor families, AP2/ERF, MYB, Myc, bHLH, GATA, NAC, WRKY and GRAS were found differentially expressed in response to Psa infection and acibenzolar-S-methyl treatment. Finally, in plants treated with acibenzolar-S-methyl, a number of gene functions related to plant resistance, such as PR proteins, were modulated, suggesting the set-up of a more effective defense response against the pathogen. Weighted-gene coexpression network analysis confirmed these results.

CONCLUSIONS

Our work provides an in-depth description of the plant molecular reactions to Psa, it highlights the metabolic pathway related to acibenzolar-S-methyl-induced resistance and it contributes to the development of effective control strategies in open field.

Direct nematicidal effects of methyl jasmonate and acibenzolar-S-methyl against Meloidogyne incognita

The aim of this study was to examine the nematicidal properties of two defence inducers against the root-knot nematode Meloidogyne incognita. A direct-contact bioassay was applied to evaluate the nematicidal effects of acibenzolar-S-methyl (ASM) and methyl jasmonate (MEJA) on second-stage juveniles (J2). Nematodes were incubated in different concentrations of these compounds, and the numbers of immobile nematodes were counted after 24 and 48 h post incubation. Tap water was then added to verify whether the nematodes recovered or remained dead at 72 h. The percentage of dead nematodes was used as indicator for the toxicity of the different solutions. Our results show that ASM, in the formulation of Bion®, and MEJA have nematicidal properties.