Systemic acquired resistance (SAR) is an induced immune mechanism in plants. Unlike vertebrate adaptive immunity, SAR is broad spectrum, with no specificity to the initial infection. An avirulent pathogen causing local programmed cell death can induce SAR through generation of mobile signals, accumulation of the defense hormone salicylic acid, and secretion of the antimicrobial PR (pathogenesis-related) proteins. Consequently, the rest of the plant is protected from secondary infection for a period of weeks to months. SAR can even be passed on to progeny through epigenetic regulation. The Arabidopsis NPR1 (nonexpresser of PR genes 1) protein is a master regulator of SAR. Recent study has shown that salicylic acid directly binds to the NPR1 adaptor proteins NPR3 and NPR4, regulates their interactions with NPR1, and controls NPR1 protein stability. However, how NPR1 interacts with TGA transcription factors to activate defense gene expression is still not well understood. In addition, redox regulators, the m...

Systemic Acquired Resistance: Turning Local Infection into Global Defense

Zhonglin Mou (Corresponding author)



The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant immune response Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network Full understanding of the mechanism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques However, elucidating genetic components involved in these pathways by conventional genetics, biochemistry, and molecular biology approaches will continue to be a major task of the community High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1744-7909.2011.01043.x

Salicylic Acid and its Function in Plant Immunity

Previous studies have described the development of control methods against bacterial wilt diseases caused by Ralstonia solanacearum. This review focused on recent advances in control measures, such as biological, physical, chemical, cultural, and integral measures, as well as biocontrol efficacy and suppression mechanisms. Biological control agents (BCAs) have been dominated by bacteria (90%) and fungi (10%). Avirulent strains of R. solanacearum, Pseudomonas spp., Bacillus spp., and Streptomyces spp. are well-known BCAs. New or uncommon BCAs have also been identified such as Acinetobacter sp., Burkholderia sp., and Paenibacillus sp. Inoculation methods for BCAs affect biocontrol efficacy, such as pouring or drenching soil, dipping of roots, and seed coatings. The amendment of different organic matter, such as plant residue, animal waste, and simple organic compounds, have frequently been reported to suppress bacterial wilt diseases. The combined application of BCAs and their substrates was shown to more effectively suppress bacterial wilt in the tomato. Suppression mechanisms are typically attributed to the antibacterial metabolites produced by BCAs or those present in natural products; however, the number of studies related to host resistance to the pathogen is increasing. Enhanced/modified soil microbial communities are also indirectly involved in disease suppression. New promising types of control measures include biological soil disinfection using substrates that release volatile compounds. This review described recent advances in different control measures. We focused on the importance of integrated pest management (IPM) for bacterial wilt diseases.

https://www.jstage.jst.go.jp/article/jsme2/advpub/0/advpub_ME14144/_pdf

Recent trends in control methods for bacterial wilt diseases caused by Ralstonia solanacearum

The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant im- mune response. Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network. Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network. Full understanding of the mech- anism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques. However, elucidating genetic components involved in these pathways by conventional ge- netics, biochemistry, and molecular biology approaches will continue to be a major task of the community. High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling.

Invited Expert Review Salicylic Acid and its Function in Plant Immunity

Plants have evolved sophisticated mechanisms to sense and respond to pathogen attacks. Resistance against necrotrophic pathogens generally requires the activation of the jasmonic acid (JA) signaling pathway, whereas the salicylic acid (SA) signaling pathway is mainly activated against biotrophic pathogens. SA can antagonize JA signaling and vice versa. Here, we report that the necrotrophic pathogen Botrytis cinerea exploits this antagonism as a strategy to cause disease development. We show that B. cinerea produces an exopolysaccharide, which acts as an elicitor of the SA pathway. In turn, the SA pathway antagonizes the JA signaling pathway, thereby allowing the fungus to develop its disease in tomato (Solanum lycopersicum). SA-promoted disease development occurs through Nonexpressed Pathogen Related1. We also show that the JA signaling pathway required for tomato resistance against B. cinerea is mediated by the systemin elicitor. These data highlight a new strategy used by B. cinerea to overcome the plant’s defense system and to spread within the host.

/pdf/botrytis-cinerea-manipulates-the-antagonistic-effects-4rfaol53dn.pdf

Botrytis cinerea Manipulates the Antagonistic Effects between Immune Pathways to Promote Disease Development in Tomato

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.

Ching-Fang Lu

Papers

Transgenic tomato plants expressing the Arabidopsis NPR1 gene display enhanced resistance to a spectrum of fungal and bacterial diseases