Editorial: Induced resistance and priming against pests and pathogens

Due to the rapidly changing climate and increasingly restrictive regulations on the use of pesticides, there is an urgent need to discover and develop new and more sustainable strategies of crop protection that meet the present and future needs of a growing world population. Fundamental research on plant-microbe and plant-insect interactions – both pathogenic and beneficial – is of key importance to gain a better molecular, physiological and ecological understanding of these complex interactions and so generate the tools necessary to develop new crop protection strategies. Induced resistance (IR) develops after treatment of plants with pathogens, pests, beneficial microorganisms, chemical agents, physical wounding, or herbivory. Plants exposed to such stimuli increase their level of basal resistance against future attacks compared to non-stimulated plants. IR is often based on a priming of basal defense mechanisms, which enables a faster and/or stronger defense response upon secondary challenge. Given its long-lasting nature and broad-spectrum effectiveness, IR has long been recognized for its value in integrated pest and disease management approaches. This Research Topic highlights the latest advances in research on IR and priming presented at the IOBC-PR-IR2022 conference in Sheffield, UK, from 4th to 7th April 2022, which is organized by the working group of the International Organization for Biological Control. In addition to reviewing the scientific significance of this work, we discuss future challenges in IR research and the potential application of IR in future crop protection strategies

A quest for long-distance signals: The epidermis as central regulator of pipecolic acid-associated systemic acquired resistance.

AGD2-like DEFENSE RESPONSE PROTEIN 1 (ALD1) is an aminotransferase that is necessary for the biosynthesis of the immune-active non-protein amino acid pipecolic acid (Pip). Pip and its N-hydroxylated derivative, N-hydroxy-Pip (NHP), have been suggested as possible long-distance signals moving in plants from infected to systemic, uninfected sites to enhance immunity. Jiang et al. (2021) show that accumulation of ALD1 in epidermal chloroplasts at local, infected sites promotes systemic immunity. Their results highlight the epidermis as a site of active immune signaling and ALD1 as an important upstream regulator of long-distance signal transmission in systemic acquired resistance (SAR)

Ethylene responsive factors in the orchestration of stress responses in monocotyledonous plants.

The APETALA2/Ethylene-Responsive Factor (AP2/ERF) superfamily of transcription factors (TFs) regulates physiological, developmental and stress responses. Most of the AP2/ERF TFs belong to the ERF family in both dicotyledonous and monocotyledonous plants. ERFs are implicated in the responses to both biotic and abiotic stress and occasionally impart multiple stress tolerance. Studies have revealed that ERF gene function is conserved in dicots and monocots. Moreover, successful stress tolerance phenotypes are observed on expression in heterologous systems, making ERFs promising candidates for engineering stress tolerance in plants. In this review, we summarize the role of ERFs in general stress tolerance, including responses to biotic and abiotic stress factors, and endeavor to understand the cascade of ERF regulation resulting in successful signal-to-response translation in monocotyledonous plants

Plant defense and long-distance signaling in the phloem.

Systemic signals are perceived in distant plant tissues and initiate systemic stress resistance through priming or induction of defense responses. This chapter provides an overview of how distant tissues and organs are alerted to possible attacks by signals that move through vascular bundles. It discusses systemic wound response (SWR), systemic acquired resistance (SAR), and systemic acquired acclimation (SAA). It also reviews the systemic signals associated with SWR, SAR, and SAA that are thought to move, at least partially, through the phloem. Various signaling compounds that are involved in inducible stress resistance responses are transported systemically through the vasculature. Not only Jasmonic acid (JA) and systemin, but also electrical signals and possibly hydrogen peroxide (H2O2), play major parts in SWR. Salicylic acid (SA), lipid transfer protein (LTPs), and associated lipid derived signals, peptides, nitric oxide (NO), and H2O2 are mainly associated with SAR

New molecules in plant defence against pathogens.

Plants host a multipart immune signalling network to ward off pathogens. Pathogen attack upon plant tissues can often lead to an amplified state of (induced) defence against subsequent infections in distal tissues; this is known as systemic acquired resistance (SAR). The interaction of plants with beneficial microbes of the rhizosphere microbiome can also lead to an induced resistance in above-ground plant tissues, known as induced systemic resistance. Second messengers such as calcium (Ca2+), reactive oxygen species (ROS), and nitric oxide (NO) are necessary for cell-to-cell signal propagation during SAR and show emergent roles in the mediation of other SAR metabolites. These include the lysine-derived signals pipecolic acid (Pip) and N-hydroxypipecolic acid (NHP), which are key signalling metabolites in SAR. Emerging evidence additionally pinpoints plant volatiles as modulators of defence signalling within and between plants. Plant volatile organic compounds (VOCs) such as monoterpenes can promote SAR by functioning through ROS. Furthermore, plant-derived and additionally also microbial VOCs can target both salicylic acid and jasmonic acid signalling pathways in plants and modulate defence against pathogens. In this review, an overview of recent findings in induced defence signalling, with a particular focus on newer signalling molecules and how they integrate into these networks is discussed

Salicylic acid, a multifaceted hormone to combat disease.

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed

Analysis of innate immune responses against pathogenic bacteria in <em>Arabidopsis</em>, tomato, and barley.

The immune status of plants can be evaluated by monitoring the propagation of pathogens. Plants defend themselves against pathogen attack through an intricate network of phytohormone-driven innate immune responses. Of these, salicylic acid (SA)-dependent defense responses can be assessed in planta by monitoring the propagation of biotrophic and hemi-biotrophic pathogens. Here, we describe methods to monitor the propagation of the hemi-biotrophic bacterial pathogen Pseudomonas syringae in Arabidopsis thaliana leaves. We describe protocols to (i) propagate the plants to the appropriate growth stage for infection, (ii) prepare the bacterial inoculum, (iii) inoculate plants using spray and infiltration techniques, and (iv) analyze the resulting in planta bacterial titers. The latter bacterial titers serve as a measure of plant susceptibility and negatively correlate with immunity. Based on the methods used with the A. thaliana-P. syringae model pathosystem, we include complementary methods allowing the analysis of innate immunity in the crop plants Solanum lycopersicum (tomato) in interaction with P. syringae and Hordeum vulgare (barley) in interaction with Xanthomonas translucens

Volatile terpenes - mediators of plant-to-plant communication.

Plants interact with other organisms employing volatile organic compounds (VOCs). The largest group of plant-released VOCs are terpenes, comprised of isoprene, monoterpenes and sesquiterpenes. Mono- and sesquiterpenes are well known communication compounds in plant-insect interactions whereas the smallest, most commonly emitted terpene, isoprene, is rather assigned a function in combating abiotic stresses. Recently, it has become evident that different volatile terpenes also act as plant-to-plant signaling cues. Upon being perceived, specific volatile terpenes can sensitize distinct signaling pathways in receiver plant cells, which in turn trigger plant innate immune responses. This vastly extends the range of action of volatile terpenes that not only protect plants from various biotic and abiotic stresses, but also convey information about environmental constraints within and between plants. As a result, plant-insect and plant-pathogen interactions, which are believed to influence each other through phytohormone cross talk, are likely equally sensitive to reciprocal regulation via volatile terpene cues. Here, we review the current knowledge of terpenes as volatile semiochemicals and discuss why and how volatile terpenes make good signaling cues. We discuss how volatile terpenes may be perceived by plants, what are possible downstream signaling events in receiver plants, and how responses to different terpene cues might interact to orchestrate the net plant response to multiple stresses. Finally, we discuss how the signal can be further transmitted to the community level leading to a mutually beneficial community scale response or distinct signaling with near kin

Barley susceptibility factor RACB modulates transcript levels of signalling protein genes in compatible interaction with Blumeria graminis f.sp hordei.

RHO (rat sarcoma homologue) GTPases (guanosine triphosphatases) are regulators of downstream transcriptional responses of eukaryotes to intracellular and extracellular stimuli. For plants, little is known about the function of Rho-like GTPases [called RACs (rat sarcoma-related C botulinum substrate) or ROPs (RHO of plants)] in transcriptional reprogramming of cells. However, in plant hormone response and innate immunity, RAC/ROP proteins influence gene expression patterns. The barley RAC/ROP RACB is required for full susceptibility of barley to the powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). We compared the transcriptomes of barley plants either silenced for RACB or over-expressing constitutively activated RACB with and without inoculation with Bgh. This revealed a large overlap of the barley transcriptome during the early response to Bgh and during the over-expression of constitutively activated RACB. Global pathway analyses and stringent analyses of differentially expressed genes suggested that RACB influences, amongst others, the expression of signalling receptor kinases. Transient induced gene silencing of RACB-regulated signalling genes (a leucine-rich repeat protein, a leucine-rich repeat receptor-like kinase and an S-domain SD1-receptor-like kinase) suggested that they might be involved in RACB-modulated susceptibility to powdery mildew. We discuss the function of RACB in regulating the transcriptional responses of susceptible barley to Bgh