Towards engineering of hormonal crosstalk in plant immunity

Exploration of a multifunctional biocontrol agent Streptomyces sp. JCK-8055 for the management of apple fire blight

Apple fire blight, caused by the bacterium Erwinia amylovora, is a devastating disease of apple and pear trees. Biological control methods have attracted much attention from researchers to manage plant diseases as they are eco-friendly and viable alternatives to synthetic pesticides. Herein, we isolated Streptomyces sp. JCK-8055 from the root of pepper and investigated its mechanisms of action against E. amylovora. Streptomyces sp. JCK-8055 produced aureothricin and thiolutin, which antagonistically affect E. amylovora. JCK-8055 and its two active metabolites have a broad-spectrum in vitro activity against various phytopathogenic bacteria and fungi. They also effectively suppressed tomato bacterial wilt and apple fire blight in in vivo experiments. Interestingly, JCK-8055 colonizes roots as a tomato seed coating and induces apple leaf shedding at the abscission zone, ultimately halting the growth of pathogenic bacteria. Additionally, JCK-8055 can produce the plant growth regulation hormone indole-3-acetic acid (IAA) and hydrolytic enzymes, including protease, gelatinase, and cellulase. JCK-8055 treatment also triggered the expression of salicylate (SA) and jasmonate (JA) signaling pathway marker genes, such as PR1, PR2, and PR3. Overall, our findings demonstrate that Streptomyces sp. JCK-8055 can control a wide range of plant diseases, particularly apple fire blight, through a combination of mechanisms such as antibiosis and induced resistance, highlighting its excellent potential as a biocontrol agent. KEY POINTS: • JCK-8055 produces the systemic antimicrobial metabolites, aureothricin, and thiolutin. • JCK-8055 treatment upregulates PR gene expression in apple plants against E. amylovora. • JCK-8055 controls plant diseases with antibiotics and induced resistance.

CaIAA2-CaARF9 module mediates the trade-off between pepper growth and immunity

To challenge the invasion of various pathogens, plants re-direct their resources from plant growth to an innate immune defence system. However, the underlying mechanism that coordinates the induction of the host immune response and the suppression of plant growth remains unclear. Here we demonstrate that an auxin response factor, CaARF9, has dual roles in enhancing the immune resistance to Ralstonia solanacearum infection and in retarding plant growth by repressing the expression of its target genes as exemplified by Casmc4, CaLBD37, CaAPK1b and CaRROP1. The expression of these target genes not only stimulates plant growth but also negatively impacts pepper resistance to R. solanacearum. Under normal conditions, the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 is active when promoter-bound CaARF9 is complexed with CaIAA2. Under R. solanacearum infection, however, degradation of CaIAA2 is triggered by SA and JA-mediated signalling defence by the ubiquitin-proteasome system, which enables CaARF9 in the absence of CaIAA2 to repress the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 and, in turn, impeding plant growth while facilitating plant defence to R. solanacearum infection. Our findings uncover an exquisite mechanism underlying the trade-off between plant growth and immunity mediated by the transcriptional repressor CaARF9 and its deactivation when complexed with CaIAA2.

Evaluation of Tunisian wheat endophytes as plant growth promoting bacteria and biological control agents against Fusarium culmorum

Plant growth-promoting rhizobacteria (PGPR) applications have emerged as an ideal substitute for synthetic chemicals by their ability to improve plant nutrition and resistance against pathogens. In this study, we isolated fourteen root endophytes from healthy wheat roots cultivated in Tunisia. The isolates were identified based from their 16S rRNA gene sequences. They belonged to Bacillota and Pseudomonadota taxa. Fourteen strains were tested for their growth-promoting and defense-eliciting potentials on durum wheat under greenhouse conditions, and for their in vitro biocontrol power against Fusarium culmorum, an ascomycete responsible for seedling blight, foot and root rot, and head blight diseases of wheat. We found that all the strains improved shoot and/or root biomass accumulation, with Bacillus mojavensis, Paenibacillus peoriae and Variovorax paradoxus showing the strongest promoting effects. These physiological effects were correlated with the plant growth-promoting traits of the bacterial endophytes, which produced indole-related compounds, ammonia, and hydrogen cyanide (HCN), and solubilized phosphate and zinc. Likewise, plant defense accumulations were modulated lastingly and systematically in roots and leaves by all the strains. Testing in vitro antagonism against F. culmorum revealed an inhibition activity exceeding 40% for five strains: Bacillus cereus, Paenibacillus peoriae, Paenibacillus polymyxa, Pantoae agglomerans, and Pseudomonas aeruginosa. These strains exhibited significant inhibitory effects on F. culmorum mycelia growth, sporulation, and/or macroconidia germination. P. peoriae performed best, with total inhibition of sporulation and macroconidia germination. These finding highlight the effectiveness of root bacterial endophytes in promoting plant growth and resistance, and in controlling phytopathogens such as F. culmorum. This is the first report identifying 14 bacterial candidates as potential agents for the control of F. culmorum, of which Paenibacillus peoriae and/or its intracellular metabolites have potential for development as biopesticides.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Harnessing Phytohormones: Advancing Plant Growth and Defence Strategies for Sustainable Agriculture

Phytohormones, pivotal regulators of plant growth and development, are increasingly recognized for their multifaceted roles in enhancing crop resilience against environmental stresses. In this review, we provide a comprehensive synthesis of current research on utilizing phytohormones to enhance crop productivity and fortify their defence mechanisms. Initially, we introduce the significance of phytohormones in orchestrating plant growth, followed by their potential utilization in bolstering crop defences against diverse environmental stressors. Our focus then shifts to an in-depth exploration of phytohormones and their pivotal roles in mediating plant defence responses against biotic stressors, particularly insect pests. Furthermore, we highlight the potential impact of phytohormones on agricultural production while underscoring the existing research gaps and limitations hindering their widespread implementation in agricultural practices. Despite the accumulating body of research in this field, the integration of phytohormones into agriculture remains limited. To address this discrepancy, we propose a comprehensive framework for investigating the intricate interplay between phytohormones and sustainable agriculture. This framework advocates for the adoption of novel technologies and methodologies to facilitate the effective deployment of phytohormones in agricultural settings and also emphasizes the need to address existing research limitations through rigorous field studies. By outlining a roadmap for advancing the utilization of phytohormones in agriculture, this review aims to catalyse transformative changes in agricultural practices, fostering sustainability and resilience in agricultural settings.

The Role of Cytokinins and Abscisic Acid in the Growth, Development and Virulence of the Pathogenic Fungus Stagonospora nodorum (Berk.)

Cytokinins (CKs) and abscisic acid (ABA) play an important role in the life of both plants and pathogenic fungi. However, the role of CKs and ABA in the regulation of fungal growth, development and virulence has not been sufficiently studied. We compared the ability of two virulent isolates (SnB and Sn9MN-3A) and one avirulent isolate (Sn4VD) of the pathogenic fungus Stagonospora nodorum Berk. to synthesize three groups of hormones (CKs, ABA and auxins) and studied the effect of exogenous ABA and zeatin on the growth, sporulation and gene expression of necrotrophic effectors (NEs) and transcription factors (TFs) in them. Various isolates of S. nodorum synthesized different amounts of CKs, ABA and indoleacetic acid. Using exogenous ABA and zeatin, we proved that the effect of these hormones on the growth and sporulation of S. nodorum isolates can be opposite, depends on both the genotype of the isolate and on the concentration of the hormone and is carried out through the regulation of carbohydrate metabolism. ABA and zeatin regulated the expression of fungal TF and NE genes, but correlation analysis of these parameters showed that this effect depended on the genotype of the isolate. This study will contribute to our understanding of the role of the hormones ABA and CKs in the biology of the fungal pathogen S. nodorum.

ABI5 binding proteins: key players in coordinating plant growth and development

During the course of terrestrial evolution, plants have developed complex networks that involve the coordination of phytohormone signalling pathways in order to adapt to an ever-changing environment. Transcription factors coordinate these responses by engaging in different protein complexes and exerting both positive and negative effects. ABA INSENSITIVE 5 (ABI5) binding proteins (AFPs), which are closely related to NOVEL INTERACTOR OF JAZ (NINJA)-like proteins, are known for their fundamental role in plants' morphological and physiological growth. Recent studies have shown that AFPs regulate several hormone-signalling pathways, including abscisic acid (ABA) and gibberellic acid (GA). Here, we review the genetic control of AFPs and their crosstalk with plant hormone signalling, and discuss the contributions of AFPs to plants' growth and development.

Alternaria TeA toxin activates a chloroplast retrograde signaling pathway to facilitate JA-dependent pathogenicity

The chloroplast is a critical battleground in the arms race between plants and pathogens. Among microbe-secreted mycotoxins, tenuazonic acid (TeA), produced by the genus Alternaria and other phytopathogenic fungi, inhibits photosynthesis, leading to a burst of photosynthetic singlet oxygen (1O2) that is implicated in damage and chloroplast-to-nucleus retrograde signaling. Despite the significant crop damage caused by Alternaria pathogens, our understanding of the molecular mechanism by which TeA promotes pathogenicity and cognate plant defense responses remains fragmentary. We now reveal that A. alternata induces necrotrophic foliar lesions by harnessing EXECUTER1 (EX1)/EX2-mediated chloroplast-to-nucleus retrograde signaling activated by TeA toxin-derived photosynthetic 1O2 in Arabidopsis thaliana. Mutation of the 1O2-sensitive EX1-W643 residue or complete deletion of the EX1 singlet oxygen sensor domain compromises expression of 1O2-responsive nuclear genes and foliar lesions. We also found that TeA toxin rapidly induces nuclear genes implicated in jasmonic acid (JA) synthesis and signaling, and EX1-mediated retrograde signaling appears to be critical for establishing a signaling cascade from 1O2 to JA. The present study sheds new light on the foliar pathogenicity of A. alternata, during which EX1-dependent 1O2 signaling induces JA-dependent foliar cell death.

Bacillus velezensis WB induces systemic resistance in watermelon against Fusarium wilt

BACKGROUND

Our previous findings indicated that Bacillus velezensis WB could control Fusarium wilt by changing the structure of the microbial community in the watermelon rhizosphere. However, there are few studies on its mechanism in the pathogen resistance of watermelon. Therefore, in this study, we determined the mechanism of B. velezensis WB-induced systemic resistance in watermelon against Fusarium wilt through glasshouse pot experiments.

RESULTS

The results showed that B. velezensis WB significantly reduced the incidence and disease index of Fusarium wilt in watermelon. B. velezensis WB can enhance the basal immunity of watermelon plants by: increasing the activity of phenylalanine ammonia-lyase (PAL), peroxidase (POD), superoxide dismutase (SOD) and β-1,3-glucanase; accumulating lignin, salicylic acid (SA) and jasmonic acid (JA); reducing malondialdehyde (MDA) concentrations; and inducing callus deposition in watermelon plant cells. RNA-seq analysis showed that 846 watermelon genes were upregulated and 612 watermelon genes were downregulated in the WF treatment. This process led to the activation of watermelon genes associated with auxin, gibberellin, SA, ethylene and JA, and the expression of genes in the phenylalanine biosynthetic pathway was upregulated. In addition, transcription factors involved in plant resistance to pathogens, such as MYB, NAC and WRKY, were induced. Gene correlation analysis showed that Cla97C10G195840 and Cla97C02G049930 in the phenylalanine biosynthetic pathway, and Cla97C02G041360 and Cla97C10G197290 in the plant hormone signal transduction pathway showed strong correlations with other genes.

CONCLUSION

Our results indicated that B. velezensis WB is capable of inducing systemic resistance in watermelon against Fusarium wilt. © 2023 Society of Chemical Industry.

Crosstalk and trade-offs: Plant responses to climate change-associated abiotic and biotic stresses

As sessile organisms, plants are constantly challenged by a dynamic growing environment. This includes fluctuations in temperature, water availability, light levels, and changes in atmospheric constituents such as carbon dioxide (CO2 ) and ozone (O3 ). In concert with changes in abiotic conditions, plants experience changes in biotic stress pressures, including plant pathogens and herbivores. Human-induced increases in atmospheric CO2 levels have led to alterations in plant growth environments that impact their productivity and nutritional quality. Additionally, it is predicted that climate change will alter the prevalence and virulence of plant pathogens, further challenging plant growth. A knowledge gap exists in the complex interplay between plant responses to biotic and abiotic stress conditions. Closing this gap is crucial for developing climate resilient crops in the future. Here, we briefly review the physiological responses of plants to elevated CO2 , temperature, tropospheric O3 , and drought conditions, as well as the interaction of these abiotic stress factors with plant pathogen pressure. Additionally, we describe the crosstalk and trade-offs involved in plant responses to both abiotic and biotic stress, and outline targets for future work to develop a more sustainable future food supply considering future climate change.

Yet uninfected? Resolving cell states of plants under pathogen attack

Although we have made significant strides in unraveling plant responses to pathogen attacks at the tissue or major cell type scale, a comprehensive understanding of individual cell responses still needs to be achieved. Addressing this gap, Zhu et al. employed single-cell transcriptome analysis to unveil the heterogeneous responses of plant cells when confronted with bacterial pathogens.

Tissue-Specific Hormone Signalling and Defence Gene Induction in an In Vitro Assembly of the Rapeseed Verticillium Pathosystem

Priming plants with beneficial microbes can establish rapid and robust resistance against numerous pathogens. Here, compelling evidence is provided that the treatment of rapeseed plants with Trichoderma harzianum OMG16 and Bacillus velezensis FZB42 induces defence activation against Verticillium longisporum infection. The relative expressions of the JA biosynthesis genes LOX2 and OPR3, the ET biosynthesis genes ACS2 and ACO4 and the SA biosynthesis and signalling genes ICS1 and PR1 were analysed separately in leaf, stem and root tissues using qRT-PCR. To successfully colonize rapeseed roots, the V. longisporum strain 43 pathogen suppressed the biosynthesis of JA, ET and SA hormones in non-primed plants. Priming led to fast and strong systemic responses of JA, ET and SA biosynthesis and signalling gene expression in each leaf, stem and root tissue. Moreover, the quantification of plant hormones via UHPLC-MS analysis revealed a 1.7- and 2.6-fold increase in endogenous JA and SA in shoots of primed plants, respectively. In roots, endogenous JA and SA levels increased up to 3.9- and 2.3-fold in Vl43-infected primed plants compared to non-primed plants, respectively. Taken together, these data indicate that microbial priming stimulates rapeseed defence responses against Verticillium infection and presumably transduces defence signals from the root to the upper parts of the plant via phytohormone signalling.

Differences in the transcriptional immune response to Albugo candida between white rust resistant and susceptible cultivars in Brassica rapa L

Albugo candida causing white rust disease decreases the yield of Brassica rapa vegetables greatly. Resistant and susceptible cultivars in B. rapa vegetables have different immune responses against A. candida inoculation, however, the mechanism of how host plants respond to A. candida is still unknown. Using RNA-sequencing, we identified differentially expressed genes (DEGs) between A. candida inoculated [48 and 72 h after inoculation (HAI)] and non-inoculated samples in resistant and susceptible cultivars of komatsuna (B. rapa var. perviridis). Functional DEGs differed between the resistant and susceptible cultivars in A. candida inoculated samples. Salicylic acid (SA) responsive genes tended to be changed in their expression levels by A. candida inoculation in both resistant and susceptible cultivars, but different genes were identified in the two cultivars. SA-dependent systemic acquired resistance (SAR) involving genes were upregulated following A. candida inoculation in the resistant cultivar. Particular genes categorized as SAR that changed expression levels overlapped between A. candida and Fusarium oxysporum f. sp. conglutinans inoculated samples in resistant cultivar, suggesting a role for SAR in defense response to both pathogens particularly in the effector-triggered immunity downstream pathway. These findings will be useful for understanding white rust resistance mechanisms in B. rapa.

Synthetic dual hormone-responsive promoters enable engineering of plants with broad-spectrum resistance

In plant immunity, the mutually antagonistic hormones salicylic acid (SA) and jasmonic acid (JA) are implicated in resistance to biotrophic and necrotrophic pathogens, respectively. Promoters that can respond to both SA and JA signals are urgently needed to engineer plants with enhanced resistance to a broad spectrum of pathogens. However, few natural pathogen-inducible promoters are available for this purpose. To address this problem, we have developed a strategy to synthesize dual SA- and JA-responsive promoters by combining SA- and JA-responsive cis elements based on the interaction between their cognate trans-acting factors. The resulting promoters respond rapidly and strongly to both SA and Methyl Jasmonate (MeJA), as well as different types of phytopathogens. When such a synthetic promoter was used to control expression of an antimicrobial peptide, transgenic plants displayed enhanced resistance to a diverse range of biotrophic, necrotrophic, and hemi-biotrophic pathogens. A dual-inducible promoter responsive to the antagonistic signals auxin and cytokinin was generated in a similar manner, confirming that our strategy can be used for the design of other biotically or abiotically inducible systems.

Detection of type III effector-induced transcription factors that regulate phytohormone content during symbiosis establishment in soybean

Soybean is a pivotal protein and oil crop that utilizes atmospheric nitrogen via symbiosis with rhizobium soil bacteria. Rhizobial type III effectors (T3Es) are essential regulators during symbiosis establishment. However, how the transcription factors involved in the interaction between phytohormone synthesis and type III effectors are connected is unclear. To detect the responses of phytohormone and transcription factor genes to rhizobial type III effector NopAA and type III secretion system, the candidate genes underlying soybean symbiosis were identified using RNA sequencing (RNA-seq) and phytohormone content analysis of soybean roots infected with wild-type Rhizobium and its derived T3E mutant. Via RNA-seq analysis the WRKY and ERF transcription factor families were identified as the most differentially expressed factors in the T3E mutant compared with the wild-type. Next, qRT-PCR was used to confirm the candidate genes Glyma.09g282900, Glyma.08g018300, Glyma.18g238200, Glyma.03g116300, Glyma.07g246600, Glyma.16g172400 induced by S. fredii HH103, S. fredii HH103ΩNopAA, and S. fredii HH103ΩRhcN. Since the WRKY and ERF families may regulate abscisic acid (ABA) content and underlying nodule formation, we performed phytohormone content analysis at 0.5 and 24 h post-inoculation (hpi). A significant change in ABA content was found between wild Rhizobium and type III effector mutant. Our results support that NopAA can promote the establishment of symbiosis by affecting the ABA signaling pathways by regulating WRKY and ERF which regulate the phytohormone signaling pathway. Specifically, our work provides insights into a signaling interaction of prokaryotic effector-induced phytohormone response involved in host signaling that regulates the establishment of symbiosis and increases nitrogen utilization efficiency in soybean plants.

Plant Hormonomics: A Key Tool for Deep Physiological Phenotyping to Improve Crop Productivity

Agriculture is particularly vulnerable to climate change. To cope with the risks posed by climate-related stressors to agricultural production, global population growth, and changes in food preferences, it is imperative to develop new climate-smart crop varieties with increased yield and environmental resilience. Molecular genetics and genomic analyses have revealed that allelic variations in genes involved in phytohormone-mediated growth regulation have greatly improved productivity in major crops. Plant science has remarkably advanced our understanding of the molecular basis of various phytohormone-mediated events in plant life. These findings provide essential information for improving the productivity of crops growing in changing climates. In this review, we highlight the recent advances in plant hormonomics (multiple phytohormone profiling) and discuss its application to crop improvement. We present plant hormonomics as a key tool for deep physiological phenotyping, focusing on representative plant growth regulators associated with the improvement of crop productivity. Specifically, we review advanced methodologies in plant hormonomics, highlighting mass spectrometry- and nanosensor-based plant hormone profiling techniques. We also discuss the applications of plant hormonomics in crop improvement through breeding and agricultural management practices.

Iron Availability and Homeostasis in Plants: A Review of Responses, Adaptive Mechanisms, and Signaling

Iron is an essential element for all living organisms, playing a major role in plant biochemistry as a redox catalyst based on iron redox properties. Iron is the fourth most abundant element of the Earth's crust, but its uptake by plants is complex because it is often in insoluble forms that are not easily accessible for plants to use. The physical and chemical speciation of iron, as well as rhizosphere activity, are key factors controlling the bioavailability of Fe. Iron can be under reduced (Fe²⁺) or oxidized (Fe³⁺) ionic forms, adsorbed onto mineral surfaces, forming complexes with organic molecules, precipitated to form poorly crystalline hydroxides to highly crystalline iron oxides, or included in crystalline Fe-rich mineral phases. Plants must thus adapt to a complex and changing iron environment, and their response is finely regulated by multiple signaling pathways initiated by a diversity of stimulus perceptions. Higher plants possess two separate strategies to uptake iron from rhizosphere soil: the chelation strategy and the reduction strategy in grass and non-grass plants, respectively. Molecular actors involved in iron uptake and mobilization through the plant have been characterized for both strategies. All these processes that contribute to iron homeostasis in plants are highly regulated in response to iron availability by downstream signaling responses, some of which are characteristic signaling signatures of iron dynamics, while others are shared with other environmental stimuli. Recent research has thus revealed key transcription factors, cis-acting elements, post-translational regulators, and other molecular mechanisms controlling these genes or their encoded proteins in response to iron availability. In addition, the most recent research is increasingly highlighting the crosstalk between iron homeostasis and nutrient response regulation. These regulatory processes help to avoid plant iron concentrations building up to potential cell functioning disruptions that could adversely affect plant fitness. Indeed, when iron is in excess in the plant, it can lead to the production and accumulation of dangerous reactive oxygen species and free radicals (H₂O₂, HO^•, O₂^•-, HO^•₂) that can cause considerable damages to most cellular components. To cope with iron oxidative stress, plants have developed defense systems involving the complementary action of antioxidant enzymes and molecular antioxidants, safe iron-storage mechanisms, and appropriate morphological adaptations.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

A R2R3 MYB Transcription Factor, TaMYB391, Is Positively Involved in Wheat Resistance to Puccinia striiformis f. sp. tritici

A biotrophic fungus, Puccinia striiformis f.sp. tritici (Pst), which causes stripe rust disease in wheat is the most yield-limiting factor in wheat production. Plants have complex defense mechanisms against invading pathogens. Hypersensitive response (HR), a kind of programmed cell death (PCD) at the infection site, is among these defense mechanisms. Transcription factors (TFs) play a crucial role in plant defense response against invading pathogens. Myeloblastosis (MYB) TFs are among the largest TFs families that are involved in response to both biotic and abiotic stresses. However, little is known about the mechanisms of MYB TFs during the interaction between wheat and the stripe rust fungus. Here, we identified an R2R3 MYB TF from wheat, designated as TaMYB391, and characterized its functional role during wheat-Pst interaction. Our data indicated that TaMYB391 is induced by Pst infection and exogenous application of salicylic acid (SA) and abscisic acid (ABA). TaMYB391 is localized in the nucleus of both wheat and Nicotiana benthamiana. Transient overexpression of TaMYB391 in N. benthamiana triggered HR-related PCD accompanied by increased electrolyte leakage, high accumulation of reactive oxygen species (ROS), and transcriptional accumulation of SA defense-related genes and HR-specific marker genes. Overexpression of TaMYB391 in wheat significantly enhanced wheat resistance to stripe rust fungus through the induction of pathogenesis-related (PR) genes, ROS accumulation and hypersensitive cell death. On the other hand, RNAi-mediated silencing of TaMYB391 decreased the resistance of wheat to Pst accompanied by enhanced growth of the pathogen. Together our findings demonstrate that TaMYB391 acts as a positive regulator of HR-associated cell death and positively contributes to the resistance of wheat to the stripe rust fungus by regulating certain PR genes, possibly through SA signaling pathways.

Ambivalent response in pathogen defense: A double-edged sword?

Plants possess effective immune systems that defend against most microbial attackers. Recent plant immunity research has focused on the classic binary defense model involving the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our current understanding comes from studies that relied on information derived from a limited number of pathosystems, newer studies concerning the incredibly diverse interactions between plants and microbes are providing additional insights into other novel mechanisms. Here, we review the roles of both classical and more recently identified components of defense signaling pathways and stress hormones in regulating the ambivalence effect during responses to diverse pathogens. Because of their different lifestyles, effective defense against biotrophic pathogens normally leads to increased susceptibility to necrotrophs, and vice versa. Given these opposing forces, the plant potentially faces a trade-off when it mounts resistance to a specific pathogen, a phenomenon referred to here as the ambivalence effect. We also highlight a novel mechanism by which translational control of the proteins involved in the ambivalence effect can be used to engineer durable and broad-spectrum disease resistance, regardless of the lifestyle of the invading pathogen.

A plant growth-promoting bacteria Priestia megaterium JR48 induces plant resistance to the crucifer black rot via a salicylic acid-dependent signaling pathway

Xanthomonas campestris pv. campestris (Xcc)-induced black rot is one of the most serious diseases in cruciferous plants. Using beneficial microbes to control this disease is promising. In our preliminary work, we isolated a bacterial strain (JR48) from a vegetable field. Here, we confirmed the plant-growth-promoting (PGP) effects of JR48 in planta, and identified JR48 as a Priestia megaterium strain. We found that JR48 was able to induce plant resistance to Xcc and prime plant defense responses including hydrogen peroxide (H₂O₂) accumulation and callose deposition with elevated expression of defense-related genes. Further, JR48 promoted lignin biosynthesis and raised accumulation of frees salicylic acid (SA) as well as expression of pathogenesis-related (PR) genes. Finally, we confirmed that JR48-induced plant resistance and defense responses requires SA signaling pathway. Together, our results revealed that JR48 promotes plant growth and induces plant resistance to the crucifer black rot probably through reinforcing SA accumulation and response, highlighting its potential as a novel biocontrol agent in the future.

Class I TCP transcription factor AtTCP8 modulates key brassinosteroid-responsive genes

The plant-specific TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor family is most closely associated with regulating plant developmental programs. Recently, TCPs were also shown to mediate host immune signaling, both as targets of pathogen virulence factors and as regulators of plant defense genes. However, comprehensive characterization of TCP gene targets is still lacking. Loss of function of the class I TCP gene AtTCP8 attenuates early immune signaling and, when combined with mutations in AtTCP14 and AtTCP15, additional layers of defense signaling in Arabidopsis (Arabidopsis thaliana). Here, we focus on TCP8, the most poorly characterized of the three to date. We used chromatin immunoprecipitation and RNA sequencing to identify TCP8-bound gene promoters and differentially regulated genes in the tcp8 mutant; these datasets were heavily enriched in signaling components for multiple phytohormone pathways, including brassinosteroids (BRs), auxin, and jasmonic acid. Using BR signaling as a representative example, we showed that TCP8 directly binds and activates the promoters of the key BR transcriptional regulatory genes BRASSINAZOLE-RESISTANT1 (BZR1) and BRASSINAZOLE-RESISTANT2 (BZR2/BES1). Furthermore, tcp8 mutant seedlings exhibited altered BR-responsive growth patterns and complementary reductions in BZR2 transcript levels, while TCP8 protein demonstrated BR-responsive changes in subnuclear localization and transcriptional activity. We conclude that one explanation for the substantial targeting of TCP8 alongside other TCP family members by pathogen effectors may lie in its role as a modulator of BR and other plant hormone signaling pathways.

Molecular basis for host responses to Xanthomonas infection

This review highlights the most relevant and recent updated information available on the defense responses of selected hosts against Xanthomonas spp. Xanthomonas is one of the most important genera of Gram-negative phytopathogenic bacteria, severely affecting the productivity of economically important crops worldwide, colonizing either the vascular system or the mesophyll tissue of the host. Due to its rapid propagation, Xanthomonas poses an enormous challenge to farmers, because it is usually controlled using huge quantities of copper-based chemicals, adversely impacting the environment. Thus, developing new ways of preventing colonization by these bacteria has become essential. Advances in genomic and transcriptomic technologies have significantly elucidated at molecular level interactions between various crops and Xanthomonas species. Understanding how these hosts respond to the infection is crucial if we are to exploit potential approaches for improving crop breeding and cutting productivity losses. This review focuses on our current knowledge of the defense response mechanisms in agricultural crops after Xanthomonas infection. We describe the molecular basis of host-bacterium interactions over a broad spectrum with the aim of improving our fundamental understanding of which genes are involved and how they work in this interaction, providing information that can help to speed up plant breeding programs, namely using gene editing approaches.

Bacterial cyclodipeptides elicit Arabidopsis thaliana immune responses reducing the pathogenic effects of Pseudomonas aeruginosa PAO1 strains on plant development

Plants being sessile organisms are exposed to various biotic and abiotic factors, thus causing stress. The Pseudomonas aeruginosa bacterium is an opportunistic pathogen for animals, insects, and plants. Direct exposure of Arabidopsis thaliana to the P. aeruginosa PAO1 strain induces plant death by producing a wide variety of virulence factors, which are regulated mainly by quorum sensing systems. Besides virulence factors, P. aeruginosa PAO1 also produces cyclodipeptides (CDPs), which possess auxin-like activity and promote plant growth through activation of the target of the rapamycin (AtTOR) pathway. On the other hand, plant defense mechanisms are regulated through the production of phytohormones, such as salicylic acid (SA) and jasmonic acid (JA), which are induced in response to pathogen-associated molecular patterns (PAMPs), activating defense genes associated with SA and JA such as PATHOGENESIS-RELATED-1 (PR-1) and LIPOXYGENASE2 (LOX2), respectively. PR proteins are suggested to play critical roles in coordinating the Systemic Acquired Resistance (SAR). In contrast, LOX proteins (LOX2, LOX3, and LOX4) have been associated with the production of JA by producing its precursors, oxylipins. The activation of defense mechanisms involves signaling cascades such as Mitogen-Activated Protein Kinases (MAPKs) or the TOR pathway as a switch for re-directing energy towards defense or growth. In this work, we challenged A. thaliana (wild type, mpk6 or mpk3 mutants, and overexpressing TOR) seedlings with P. aeruginosa PAO1 strains to identify the role of bacterial CDPs in the plant immune response. Results showed that the pre-exposure of these Arabidopsis seedlings to CDPs significantly reduced plant infection of the pathogenic P. aeruginosa PAO1 strains, indicating that plants that over-express AtTOR or lack MPK3/MPK6 protein-kinases are more susceptible to the pathogenic effects. In addition, CDPs induced the GUS activity only in the LOX2::GUS plants, indicative of JA-signaling activation. Our findings indicate that the CDPs are molecules that trigger SA-independent and JA-dependent defense responses in A. thaliana; hence, bacterial CDPs may be considered elicitors of the Arabidopsis immune response to pathogens.

Transcriptome Analysis Reveals the Response Mechanism of Frl-Mediated Resistance to Fusarium oxysporum f. sp. radicis-lycopersici (FORL) Infection in Tomato

Tomato Fusarium crown and root rot (FCRR) is an extremely destructive soil-borne disease. To date, studies have shown that only plants with tomato mosaic virus (TMV) resistance exhibit similar resistance to tomato Fusarium oxysporum f. sp. radicis-lycopersici (FORL) and have identified a single relevant gene, Frl, in Peruvian tomato. Due to the relative lack of research on FCRR disease-resistance genes in China and elsewhere, transcriptome data for FORL-resistant (cv. ‘19912’) and FORL-susceptible (cv. ‘Moneymaker’) tomato cultivars were analysed for the first time in this study. The number of differentially expressed genes (DEGs) was higher in Moneymaker than in 19912, and 189 DEGs in the ‘plant–pathogen interaction’ pathway were subjected to GO and KEGG enrichment analyses. MAPK and WRKY genes were enriched in major metabolic pathways related to plant disease resistance; thus, we focused on these two gene families. In the early stage of tomato infection, the content of JA and SA increased, but the change in JA was more obvious. Fourteen genes were selected for confirmation of their differential expression levels by qRT-PCR. This study provides a series of novel disease resistance resources for tomato breeding and genetic resources for screening and cloning FORL resistance genes.

Salicylic acid and jasmonic acid crosstalk in plant immunity

The phytohormones salicylic acid (SA) and jasmonic acid (JA) are major players in plant immunity. Numerous studies have provided evidence that SA- and JA-mediated signaling interact with each other (SA-JA crosstalk) to orchestrate plant immune responses against pathogens. At the same time, SA-JA crosstalk is often exploited by pathogens to promote their virulence. In this review, we summarize our current knowledge of molecular mechanisms for and modulations of SA-JA crosstalk during pathogen infection.

Strigolactones Modulate Salicylic Acid-Mediated Disease Resistance in Arabidopsis thaliana

Strigolactones are low-molecular-weight phytohormones that play several roles in plants, such as regulation of shoot branching and interactions with arbuscular mycorrhizal fungi and parasitic weeds. Recently, strigolactones have been shown to be involved in plant responses to abiotic and biotic stress conditions. Herein, we analyzed the effects of strigolactones on systemic acquired resistance induced through salicylic acid-mediated signaling. We observed that the systemic acquired resistance inducer enhanced disease resistance in strigolactone-signaling and biosynthesis-deficient mutants. However, the amount of endogenous salicylic acid and the expression levels of salicylic acid-responsive genes were lower in strigolactone signaling-deficient max2 mutants than in wildtype plants. In both the wildtype and strigolactone biosynthesis-deficient mutants, the strigolactone analog GR24 enhanced disease resistance, whereas treatment with a strigolactone biosynthesis inhibitor suppressed disease resistance in the wildtype. Before inoculation of wildtype plants with pathogenic bacteria, treatment with GR24 did not induce defense-related genes; however, salicylic acid-responsive defense genes were rapidly induced after pathogenic infection. These findings suggest that strigolactones have a priming effect on Arabidopsis thaliana by inducing salicylic acid-mediated disease resistance.

Geminivirus C4 proteins inhibit GA signaling via prevention of NbGAI degradation, to promote viral infection and symptom development in N. benthamiana

The phytohormone gibberellin (GA) is a vital plant signaling molecule that regulates plant growth and defense against abiotic and biotic stresses. To date, the molecular mechanism of the plant responses to viral infection mediated by GA is still undetermined. DELLA is a repressor of GA signaling and is recognized by the F-box protein, a component of the SCF^SLY1/GID2 complex. The recognized DELLA is degraded by the ubiquitin-26S proteasome, leading to the activation of GA signaling. Here, we report that ageratum leaf curl Sichuan virus (ALCScV)-infected N. benthamiana plants showed dwarfing symptoms and abnormal flower development. The infection by ALCScV significantly altered the expression of GA pathway-related genes and decreased the content of endogenous GA in N. benthamiana. Furthermore, ALCScV-encoded C4 protein interacts with the DELLA protein NbGAI and interferes with the interaction between NbGAI and NbGID2 to prevent the degradation of NbGAI, leading to inhibition of the GA signaling pathway. Silencing of NbGAI or exogenous GA₃ treatment significantly reduces viral accumulation and disease symptoms in N. benthamiana plants. The same results were obtained from experiments with the C4 protein encoded by tobacco curly shoot virus (TbCSV). Therefore, we propose a novel mechanism by which geminivirus C4 proteins control viral infection and disease symptom development by interfering with the GA signaling pathway.

Gibberellins (GAs) are plant hormones essential for many developmental processes in plants. Plant virus infection can induce abnormal flower development and influence the GA pathway, resulting in plant dwarfing symptoms, but the underlying mechanisms are still not well described. Here, we demonstrate that the geminivirus-encoded C4 protein regulates the GA signaling pathway to promote viral accumulation and disease symptom development. By directly interacting with NbGAI, the C4 protein interferes with the interaction between NbGAI and NbGID2, which inhibits the degradation of NbGAI. As a result, the GA signaling pathway is blocked, and the infected plants display symptoms of typical dwarfing and delayed flowering. Our results reveal a novel mechanism by which geminivirus C4 proteins influence viral pathogenicity by interfering with the GA signaling pathway and provide new insights into the interaction between the virus and host.

The Crosstalk of the Salicylic Acid and Jasmonic Acid Signaling Pathways Contributed to Different Resistance to Phytoplasma Infection Between the Two Genotypes in Chinese Jujube

Jujube witches’ broom disease (JWB), one of the most serious phytoplasma diseases, usually results in the destruction of Chinese jujube (Ziziphus jujuba Mill.). Although most jujube cultivars are sensitive to JWB, we found a few genotypes that are highly resistant to JWB. However, the molecular mechanism of phytoplasma resistance has seldom been studied. Here, we used Chinese jujube “T13,” which has strong resistance to JWB, and a typical susceptible cultivar, “Pozao” (“PZ”), as materials to perform comparative transcriptome, hormone, and regulation analyses. After phytoplasma infection, the differential expression genes (DEGs) were detected at all three growth phases (S1, S2, and S3) in “PZ,” but DEGs were detected only at the first growth phase in “T13.” Meanwhile, no phytoplasma was detected, and the symptoms especially witches’ broom caused by JWB were not observed at the last two growth phases (S2 and S3) in “T13.” Protein–protein interaction analysis also showed that the key genes were mainly involved in hormone and reactive oxygen species (ROS) signaling. In addition, during the recovered growth phase in “T13” from S1 to S2, the level of hydrogen peroxide (H₂O₂) was significantly increased and then decreased from S2 to S3. Moreover, jasmonic acid (JA) was significantly accumulated in “PZ” diseased plants, especially at the S2 phase and at the S2 phase in “T13,” while the content of salicylic acid (SA) decreased significantly at the S2 phase of “T13” compared to that in “PZ.” The changes in H₂O₂ and JA or SA were consistent with the changes in their key synthesis genes in the transcriptome data. Finally, exogenous application of an SA inhibitor [1-aminobenzotriazole (ABT)] rescued witches’ broom symptoms, while the contents of both JA and MeJA increased after ABT treatment compared to the control, demonstrating that exogenous application of an SA inhibitor rescued the symptoms of jujube after phytoplasma infection by decreasing the contents of SA and increasing the contents of JA and MeJA. Collectively, our study provides a new perspective on the transcriptional changes of Chinese jujube in response to JWB and novel insights that the crosstalk of JA and SA signaling communicated together to contribute to “T13” JWB resistance.

Sweet Immunity Aspects during Levan Oligosaccharide-Mediated Priming in Rocket against Botrytis cinerea

New strategies are required for crop protection against biotic stress. Naturally derived molecules, including carbohydrates such as fructans, can be used in priming or defense stimulation. Rocket (Eruca sativa) is an important leafy vegetable and a good source of antioxidants. Here, we tested the efficacy of fructan-induced immunity in the Botrytis cinerea pathosystem. Different fructan types of plant and microbial origin were considered and changes in sugar dynamics were analyzed. Immune resistance increased significantly after priming with natural and sulfated levan oligosaccharides (LOS). No clear positive effects were observed for fructo-oligosaccharides (FOS), inulin or branched-type fructans. Only sulfated LOS induced a direct ROS burst, typical for elicitors, while LOS behaved as a genuine priming compound. Total leaf sugar levels increased significantly both after LOS priming and subsequent infection. Intriguingly, apoplastic sugar levels temporarily increased after LOS priming but not after infection. We followed LOS and small soluble sugar dynamics in the apoplast as a function of time and found a temporal peak in small soluble sugar levels. Although similar dynamics were also found with inulin-type FOS, increased Glc and FOS levels may benefit B. cinerea. During LOS priming, LOS- and/or Glc-dependent signaling may induce downstream sweet immunity responses.

Silencing GhIAA43, a member of cotton AUX/IAA genes, enhances wilt resistance via activation of salicylic acid-mediated defenses

Auxin-mediated degradation of Aux/IAA proteins is a crucial step in auxin signaling. Recent researches indicate that Aux/IAA members also play a role in biotic and abiotic stresses. For example, Pseudomonas syringae infection causes Arabidopsis Aux/IAA protein (AXR2, AXR3) turnover. Here, by analyzing RNA-seq data we found that several cotton Aux/IAA genes are responsive to Verticillium dahliae infection, one of these named GhIAA43, was investigated for its role in cotton defense against V. dahliae infection. We demonstrate that the transcript levels of GhIAA43 were responsive to both V. dahliae infection and exogenous IAA application. By producing transgenic Arabidopsis plants overexpressing GhIAA43-GUS fusion, we show that IAA treatment and V. dahliae infection promoted GhIAA43 protein turnover. Silencing GhIAA43 in cotton enhanced wilt resistance, suggesting that GhIAA43 is a negative regulator in cotton defense against V. dahliae attack. By monitoring SA marker gene expression and measurement of SA content in GhIAA43-silenced cotton plants, we found that the enhanced resistance in GhIAA43-silenced cotton plants is due to the activation of SA-related defenses, and the activated defenses specifically occurred in the presence of V. dahliae. Furthermore, exogenous IAA application improve wilt resistance in cotton plants tested. Our results provide novel connection between auxin signaling and SA-related defenses in cotton upon V. dahliae attack.

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

Cell wall integrity is tightly regulated and maintained given that non-physiological modification of cell walls could render plants vulnerable to biotic and/or abiotic stresses. Expansins are plant cell wall-modifying proteins active during many developmental and physiological processes, but they can also be produced by bacteria and fungi during interaction with plant hosts. Cell wall alteration brought about by ectopic expression, overexpression, or exogenous addition of expansins from either eukaryote or prokaryote origin can in some instances provide resistance to pathogens, while in other cases plants become more susceptible to infection. In these circumstances altered cell wall mechanical properties might be directly responsible for pathogen resistance or susceptibility outcomes. Simultaneously, through membrane receptors for enzymatically released cell wall fragments or by sensing modified cell wall barrier properties, plants trigger intracellular signaling cascades inducing defense responses and reinforcement of the cell wall, contributing to various infection phenotypes, in which expansins might also be involved. Here, we review the plant immune response activated by cell wall surveillance mechanisms, cell wall fragments identified as responsible for immune responses, and expansin's roles in resistance and susceptibility of plants to pathogen attack.

Network organization of the plant immune system: from pathogen perception to robust defense induction

The plant immune system has been explored essentially through the study of qualitative resistance, a simple form of immunity, and from a reductionist point of view. The recent identification of genes conferring quantitative disease resistance revealed a large array of functions, suggesting more complex mechanisms. In addition, thanks to the advent of high-throughput analyses and system approaches, our view of the immune system has become more integrative, revealing that plant immunity should rather be seen as a distributed and highly connected molecular network including diverse functions to optimize expression of plant defenses to pathogens. Here, we review the recent progress made to understand the network complexity of regulatory pathways leading to plant immunity, from pathogen perception, through signaling pathways and finally to immune responses. We also analyze the topological organization of these networks and their emergent properties, crucial to predict novel immune functions and test them experimentally. Finally, we report how these networks might be regulated by environmental clues. Although system approaches remain extremely scarce in this area of research, a growing body of evidence indicates that the plant response to combined biotic and abiotic stresses cannot be inferred from responses to individual stresses. A view of possible research avenues in this nascent biology domain is finally proposed.

Early Defense Mechanisms of Brassica oleracea in Response to Attack by Xanthomonas campestris pv. campestris

Black rot disease, caused by Xanthomonas campestris pv. campestris (Xcc), results in significant yield losses in Brassica oleracea crops worldwide. To find black rot disease-resistant cabbage lines, we carried out pathogenicity assays using the scissor-clipping method in 94 different B. oleracea lines. By comparing the lesion areas, we selected a relatively resistant line, Black rot Resistance 155 (BR155), and a highly susceptible line, SC31. We compared the two cabbage lines for the Xcc-induced expression pattern of 13 defense-related genes. Among them, the Xcc-induced expression level of PR1 and antioxidant-related genes (SOD, POD, APX, Trx H, and CHI) were more than two times higher in BR155 than SC31. Nitroblue tetrazolium (NBT) and diaminobenzidine tetrahydrochloride (DAB) staining analysis showed that BR155 accumulated less Xcc-induced reactive oxygen species (ROS) than did SC31. In addition, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assays showed that BR155 had higher antioxidant activity than SC31. This study, focused on the defense responses of cabbage during the early biotrophic stage of infection, indicated that Xcc-induced ROS might play a role in black rot disease development. We suggest that non-enzymatic antioxidants are important, particularly in the early defense mechanisms of cabbage against Xcc.

CaWRKY30 Positively Regulates Pepper Immunity by Targeting CaWRKY40 against Ralstonia solanacearum Inoculation through Modulating Defense-Related Genes

The WRKY transcription factors (TFs) network is composed of WRKY TFs’ subset, which performs a critical role in immunity regulation of plants. However, functions of WRKY TFs’ network remain unclear, particularly in non-model plants such as pepper (Capsicum annuum L.). This study functionally characterized CaWRKY30—a member of group III Pepper WRKY protein—for immunity of pepper against Ralstonia solanacearum infection. The CaWRKY30 was detected in nucleus, and its transcriptional expression levels were significantly upregulated by R. solanacearum inoculation (RSI), and foliar application ethylene (ET), abscisic acid (ABA), and salicylic acid (SA). Virus induced gene silencing (VIGS) of CaWRKY30 amplified pepper’s vulnerability to RSI. Additionally, the silencing of CaWRKY30 by VIGS compromised HR-like cell death triggered by RSI and downregulated defense-associated marker genes, like CaPR1, CaNPR1, CaDEF1, CaABR1, CaHIR1, and CaWRKY40. Conversely, transient over-expression of CaWRKY30 in pepper leaves instigated HR-like cell death and upregulated defense-related maker genes. Furthermore, transient over-expression of CaWRKY30 upregulated transcriptional levels of CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40. On the other hand, transient over-expression of CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40 upregulated transcriptional expression levels of CaWRKY30. The results recommend that newly characterized CaWRKY30 positively regulates pepper’s immunity against Ralstonia attack, which is governed by synergistically mediated signaling by phytohormones like ET, ABA, and SA, and transcriptionally assimilating into WRKY TFs networks, consisting of CaWRKY6, CaWRKY22, CaWRKY27, and CaWRKY40. Collectively, our data will facilitate to explicate the underlying mechanism of crosstalk between pepper’s immunity and response to RSI.

Transcriptomic Analysis Reveals Candidate Genes Responding Maize Gray Leaf Spot Caused by Cercospora zeina

Gray leaf spot (GLS), caused by the fungal pathogen Cercospora zeina (C. zeina), is one of the most destructive soil-borne diseases in maize (Zea mays L.), and severely reduces maize production in Southwest China. However, the mechanism of resistance to GLS is not clear and few resistant alleles have been identified. Two maize inbred lines, which were shown to be resistant (R6) and susceptible (S8) to GLS, were injected by C. zeina spore suspensions. Transcriptome analysis was carried out with leaf tissue at 0, 6, 24, 144, and 240 h after inoculation. Compared with 0 h of inoculation, a total of 667 and 419 stable common differentially expressed genes (DEGs) were found in the resistant and susceptible lines across the four timepoints, respectively. The DEGs were usually enriched in 'response to stimulus' and 'response to stress' in GO term analysis, and 'plant-pathogen interaction', 'MAPK signaling pathways', and 'plant hormone signal transduction' pathways, which were related to maize's response to GLS, were enriched in KEGG analysis. Weighted-Genes Co-expression Network Analysis (WGCNA) identified two modules, while twenty hub genes identified from these indicated that plant hormone signaling, calcium signaling pathways, and transcription factors played a central role in GLS sensing and response. Combing DEGs and QTL mapping, five genes were identified as the consensus genes for the resistance of GLS. Two genes, were both putative Leucine-rich repeat protein kinase family proteins, specifically expressed in R6. In summary, our results can provide resources for gene mining and exploring the mechanism of resistance to GLS in maize.

Multiparameter in vivo imaging in plants using genetically encoded fluorescent indicator multiplexing

Biological processes are highly dynamic, and during plant growth, development, and environmental interactions, they occur and influence each other on diverse spatiotemporal scales. Understanding plant physiology on an organismic scale requires analyzing biological processes from various perspectives, down to the cellular and molecular levels. Ideally, such analyses should be conducted on intact and living plant tissues. Fluorescent protein (FP)-based in vivo biosensing using genetically encoded fluorescent indicators (GEFIs) is a state-of-the-art methodology for directly monitoring cellular ion, redox, sugar, hormone, ATP and phosphatidic acid dynamics, and protein kinase activities in plants. The steadily growing number of diverse but technically compatible genetically encoded biosensors, the development of dual-reporting indicators, and recent achievements in plate-reader-based analyses now allow for GEFI multiplexing: the simultaneous recording of multiple GEFIs in a single experiment. This in turn enables in vivo multiparameter analyses: the simultaneous recording of various biological processes in living organisms. Here, we provide an update on currently established direct FP-based biosensors in plants, discuss their functional principles, and highlight important biological findings accomplished by employing various approaches of GEFI-based multiplexing. We also discuss challenges and provide advice for FP-based biosensor analyses in plants.

Pathogen and drought stress affect cell wall and phytohormone signaling to shape host responses in a sorghum COMT bmr12 mutant

BACKGROUND

As effects of global climate change intensify, the interaction of biotic and abiotic stresses increasingly threatens current agricultural practices. The secondary cell wall is a vanguard of resistance to these stresses. Fusarium thapsinum (Fusarium stalk rot) and Macrophomina phaseolina (charcoal rot) cause internal damage to the stalks of the drought tolerant C4 grass, sorghum (Sorghum bicolor (L.) Moench), resulting in reduced transpiration, reduced photosynthesis, and increased lodging, severely reducing yields. Drought can magnify these losses. Two null alleles in monolignol biosynthesis of sorghum (brown midrib 6-ref, bmr6-ref; cinnamyl alcohol dehydrogenase, CAD; and bmr12-ref; caffeic acid O-methyltransferase, COMT) were used to investigate the interaction of water limitation with F. thapsinum or M. phaseolina infection.

RESULTS

The bmr12 plants inoculated with either of these pathogens had increased levels of salicylic acid (SA) and jasmonic acid (JA) across both watering conditions and significantly reduced lesion sizes under water limitation compared to adequate watering, which suggested that drought may prime induction of pathogen resistance. RNA-Seq analysis revealed coexpressed genes associated with pathogen infection. The defense response included phytohormone signal transduction pathways, primary and secondary cell wall biosynthetic genes, and genes encoding components of the spliceosome and proteasome.

CONCLUSION

Alterations in the composition of the secondary cell wall affect immunity by influencing phenolic composition and phytohormone signaling, leading to the action of defense pathways. Some of these pathways appear to be activated or enhanced by drought. Secondary metabolite biosynthesis and modification in SA and JA signal transduction may be involved in priming a stronger defense response in water-limited bmr12 plants.

A 13-Lipoxygenase, GhLOX2, positively regulates cotton tolerance against Verticillium dahliae through JA-mediated pathway

Verticillium wilt is a major limiting factor for sustainable production of cotton but the mechanism of controlling this disease is still poorly understood. Lipoxygenase (LOX)-derived oxylipins have been implicated in defense responses against diverse pathogens; however there is limited information about the functional characterization of LOXs in response to Verticillium dahliae infection. In this study, we report the characterization of a cotton LOX gene, GhLOX2, which phylogenetically clustered into 13-LOX subfamily and is closely related to Arabidopsis LOX2 gene. GhLOX2 was predominantly expressed in leaves and strongly induced following V. dahliae inoculation and treatment of methyl jasmonate (MeJA). RNAi-mediated knock-down of GhLOX2 enhanced cotton susceptibility to V. dahliae and was coupled with suppression of jasmonic acid (JA)-related genes both after inoculation with the cotton defoliating strain V991 or MeJA treatment. Interestingly, lignin contents, transcripts of lignin synthesis genes and H₂O₂ contents were also decreased in GhLOX2-silenced plants. This study suggests that GhLOX2 is involved in defense responses against infection of V. dahliae in cotton and supports that JA is one of the major defense hormones against this pathogen.

Arabidopsis cell wall composition determines disease resistance specificity and fitness

Plant cells are surrounded by an extracellular matrix known as the cell wall. We have analyzed the contribution of the Arabidopsis cell wall to disease resistance to pathogens with different parasitic styles. Here, we demonstrate that plant cell walls are determinants of immune responses since modification of their composition in a set of Arabidopsis cell wall mutants has an impact on their disease resistance and fitness phenotypes. In these genotypes, we identified specific correlations between the amounts of specific wall carbohydrate epitopes and disease resistance/fitness phenotypes through mathematical analyses. These data support the relevant and specific function of plant cell wall composition in plant immune responses and provide the basis for using wall traits in crop breeding programs.

Plant cell walls are complex structures subject to dynamic remodeling in response to developmental and environmental cues and play essential functions in disease resistance responses. We tested the specific contribution of plant cell walls to immunity by determining the susceptibility of a set of Arabidopsis cell wall mutants (cwm) to pathogens with different parasitic styles: a vascular bacterium, a necrotrophic fungus, and a biotrophic oomycete. Remarkably, most cwm mutants tested (29/34; 85.3%) showed alterations in their resistance responses to at least one of these pathogens in comparison to wild-type plants, illustrating the relevance of wall composition in determining disease-resistance phenotypes. We found that the enhanced resistance of cwm plants to the necrotrophic and vascular pathogens negatively impacted cwm fitness traits, such as biomass and seed yield. Enhanced resistance of cwm plants is not only mediated by canonical immune pathways, like those modulated by phytohormones or microbe-associated molecular patterns, which are not deregulated in the cwm tested. Pectin-enriched wall fractions isolated from cwm plants triggered immune responses in wild-type plants, suggesting that wall-mediated defensive pathways might contribute to cwm resistance. Cell walls of cwm plants show a high diversity of composition alterations as revealed by glycome profiling that detect specific wall carbohydrate moieties. Mathematical analysis of glycome profiling data identified correlations between the amounts of specific wall carbohydrate moieties and disease resistance phenotypes of cwm plants. These data support the relevant and specific function of plant wall composition in plant immune response modulation and in balancing disease resistance/development trade-offs.

A Nod factor- and type III secretion system-dependent manner for Robinia pseudoacacia to establish symbiosis with Mesorhizobium amorphae CCNWGS0123

Under nitrogen-limiting conditions, symbiotic nodulation promotes the growth of legume plants via the fixation of atmospheric nitrogen to ammonia by rhizobia in root nodules. The rhizobial Nod factor (NF) and type III secretion system (T3SS) are two key signaling pathways for establishing the legume-rhizobium symbiosis. However, whether NF signaling is involved in the nodulation of Robinia pseudoacacia and Mesorhizobium amorphae CCNWGS0123, and its symbiotic differences compared with T3SS signaling remain unclear. Therefore, to elucidate the function of NF signaling in nodulation, we mutated nodC in M. amorphae CCNWGS0123, which aborted NF synthesis. Compared with the plants inoculated with the wild type strain, the plants inoculated with the NF-deficient strain exhibited shorter shoots with etiolated leaves. These phenotypic characteristics were similar to those of the plants inoculated with the T3SS-deficient strain, which served as a Nod- (non-effective nodulation) control. The plants inoculated with both the NF- and T3SS-deficient strains formed massive root hair swellings, but no normal infection threads were detected. Sections of the nodules showed that inoculation with the NF- and T3SS-deficient strains induced small, white bumps without any rhizobia inside. Analyzing the accumulation of 6 plant hormones and the expression of 10 plant genes indicated that the NF- and T3SS-deficient strains activated plant defense reactions while suppressing plant symbiotic signaling during the perception and nodulation processes. The requirement for NF signaling appeared to be conserved in two other leguminous trees that can establish symbiosis with M. amorphae CCNWGS0123. In contrast, the function of the T3SS might differ among species, even within the same subfamily (Faboideae). Overall, this work demonstrated that nodulation of R. pseudoacacia and M. amorphae CCNWGS0123 was both NF and T3SS dependent.

Inoculation with arbuscular mycorrhizal fungus modulates defense-related genes expression in banana seedlings susceptible to wilt disease

Banana as an important economic crop worldwide, often suffers from serious damage caused by Fusarium oxysporum f. sp. Cubense. Arbuscular mycorrhizal (AM) fungi have been considered as one of the promising plant biocontrol agents in preventing from root pathogens. This study examined the effect of AM fungal inoculation on plant growth and differential expressions of growth- and defense-related genes in banana seedlings. Tissue-cultured seedlings of Brazilian banana (Musa acuminate Cavendish cv. Brail) were inoculated with AM fungus (Rhizophagus irregularis, Ri), and developed good mycorrhizal symbiosis from 4 to 11 weeks after inoculation with an infection rate up to 71.7% of the roots system. Microbial abundance revealed that Ri abundance in banana roots was 1.85×10⁶ copies/ml at 11 weeks after inoculaiton. Inoculation improved plant dry weights by 47.5, 124, and 129% for stem, leaf, and the whole plant, respectively, during phosphate depletion. Among a total of 1411 differentially expressed genes (DEGs) obtained from the transcriptome data analysis, genes related to plant resistance (e.g. POD, PAL, PYR, and HBP-1b) and those related to plant growth (e.g. IAA, GH3, SAUR, and ARR8) were up-regulated in AM plants. This study demonstrates that AM fungus effectively promoted the growth of banana plants and induced defense-related genes which could help suppress wilt disease. The outcomes of this study form a basis for further study on the mechanism of banana disease resistance induced by AM fungi.

The transcriptional response to salicylic acid plays a role in Fusarium yellows resistance in Brassica rapa L

Fusarium yellows resistant and susceptible lines in Brassica rapa showed different salicylic acid responses; the resistant line showed a similar response to previous reports, but the susceptible line differed. Fusarium yellows caused by Fusarium oxysporum f. sp. conglutinans (Foc) is an important disease. Previous studies showed that genes related to salicylic acid (SA) response were more highly induced following Foc infection in Brassica rapa Fusarium yellows resistant lines than susceptible lines. However, SA-induced genes have not been identified at the whole genome level and it was unclear whether they were up-regulated by Foc inoculation. Transcriptome analysis with and without SA treatment in the B. rapa Fusarium yellows susceptible line 'Misugi' and the resistant line 'Nanane' was performed to obtain insights into the relationship between SA sensitivity/response and Fusarium yellows resistance. 'Nanane's up-regulated genes were related to SA response and down-regulated genes were related to jasmonic acid (JA) or ethylene (ET) response, but differentially expressed genes in 'Misugi' were not. This result suggests that Fusarium yellows resistant and susceptible lines have a different SA response and that an antagonistic transcription between SA and JA/ET responses was found only in a Fusarium yellows resistant line. SA-responsive genes were induced by Foc inoculation in Fusarium yellows resistant (RJKB-T23) and susceptible lines (RJKB-T24). By contrast, 39 SA-induced genes specific to RJKB-T23 might function in the defense response to Foc. In this study, SA-induced genes were identified at the whole genome level, and the possibility, the defense response to Foc observed in a resistant line could be mediated by SA-induced genes, is suggested. These results will be useful for future research concerning the SA importance in Foc or other diseases resistance in B. rapa.

Jasmonic Acid- and Ethylene-Induced Mitochondrial Alternative Oxidase Stimulates Marssonina brunnea Defense in Poplar

Mitochondrial processes are implicated in plant response to biotic stress caused by viruses, actinomyces, bacteria and pests, but their function in defense against fungal invasion remains unclear. Here, we investigated the role and regulation of mitochondrial alternative oxidase (AOX) in response to black spot disease caused by the hemibiotrophic fungus Marssonina brunnea in poplar. M. brunnea inoculation induced the transcription of the AOX1a gene in the mitochondrial electron transport chain and of jasmonic acid (JA) and ethylene (ET) biosynthetic genes, with the accumulation of these phytohormones in poplar leaf, while inhibiting the transcript amount of the mitochondrial cytochrome c oxidase gene (COX6b) and genes related to salicylic acid (SA). Enhanced AOX reduced poplar susceptibility to M. brunnea with a higher ATP/ADP ratio while the repressed AOX caused the reverse effect. Exogenous JA and 1-aminocyclopropane-1-carboxylic acid (ACC, a biosynthetic precursor of ET) inhibited the transcript amount of COX6b and consequently increased the ratio of AOX pathway to total respiration. Furthermore, the transcription of CYS C1 and CYS D1 genes catalyzing cyanide metabolism was induced, while the cysteine (CYS) substrate levels reduced upon M. brunnea inoculation; exogenous JA and ACC mimicked the effect of M. brunnea infection on cysteine. Exogenous SA enhanced, while JA and ACC reduced, poplar susceptibility to M. brunnea. Moreover, inhibiting AOX completely prohibited JA- and ET-increased tolerance to M. brunnea in poplar. These observations indicate that the JA- and ET-induced mitochondrial AOX pathway triggers defense against M. brunnea in poplar. This effect probably involves cyanide. These findings deepen our understanding of plant-pathogenic fungi interactions.

Arabidopsis cell wall composition determines disease resistance specificity and fitness

Plant cell walls are complex structures subject to dynamic remodeling in response to developmental and environmental cues and play essential functions in disease resistance responses. We tested the specific contribution of plant cell walls to immunity by determining the susceptibility of a set of Arabidopsis cell wall mutants (cwm) to pathogens with different parasitic styles: a vascular bacterium, a necrotrophic fungus, and a biotrophic oomycete. Remarkably, most cwm mutants tested (29/34; 85.3%) showed alterations in their resistance responses to at least one of these pathogens in comparison to wild-type plants, illustrating the relevance of wall composition in determining disease-resistance phenotypes. We found that the enhanced resistance of cwm plants to the necrotrophic and vascular pathogens negatively impacted cwm fitness traits, such as biomass and seed yield. Enhanced resistance of cwm plants is not only mediated by canonical immune pathways, like those modulated by phytohormones or microbe-associated molecular patterns, which are not deregulated in the cwm tested. Pectin-enriched wall fractions isolated from cwm plants triggered immune responses in wild-type plants, suggesting that wall-mediated defensive pathways might contribute to cwm resistance. Cell walls of cwm plants show a high diversity of composition alterations as revealed by glycome profiling that detect specific wall carbohydrate moieties. Mathematical analysis of glycome profiling data identified correlations between the amounts of specific wall carbohydrate moieties and disease resistance phenotypes of cwm plants. These data support the relevant and specific function of plant wall composition in plant immune response modulation and in balancing disease resistance/development trade-offs.

Ethylene-Cytokinin Interaction Determines Early Defense Response of Wheat against Stagonospora nodorum Berk

Ethylene, salicylic acid (SA), and jasmonic acid are the key phytohormones involved in plant immunity, and other plant hormones have been demonstrated to interact with them. The classic phytohormone cytokinins are important participants of plant defense signaling. Crosstalk between ethylene and cytokinins has not been sufficiently studied as an aspect of plant immunity and is addressed in the present research. We compared expression of the genes responsible for hormonal metabolism and signaling in wheat cultivars differing in resistance to Stagonospora nodorum in response to their infection with fungal isolates, whose virulence depends on the presence of the necrotrophic effector SnTox3. Furthermore, we studied the action of the exogenous cytokinins, ethephon (2-chloroethylphosphonic acid, ethylene-releasing agent) and 1-methylcyclopropene (1-MCP, inhibitor of ethylene action) on infected plants. Wheat susceptibility was shown to develop due to suppression of reactive oxygen species production and decreased content of active cytokinins brought about by SnTox3-mediated activation of the ethylene signaling pathway. SnTox3 decreased cytokinin content most quickly by its activated glucosylation in an ethylene-dependent manner and, furthermore, by oxidative degradation and inhibition of biosynthesis in ethylene-dependent and ethylene-independent manners. Exogenous zeatin application enhanced wheat resistance against S. nodorum through inhibition of the ethylene signaling pathway and upregulation of SA-dependent genes. Thus, ethylene inhibited triggering of SA-dependent resistance mechanism, at least in part, by suppression of the cytokinin signaling pathway.

Enhanced Biosynthesis of Chlorogenic Acid and Its Derivatives in Methyl-Jasmonate-Treated Gardenia jasminoides Cells: A Study on Metabolic and Transcriptional Responses of Cells

Chlorogenic acid and its derivatives (CQAs) are considered as important bioactive secondary metabolites in Gardenia jasminoides Ellis (G. jasminoides). However, few studies have investigated the biosynthesis and regulation of CQAs in G. jasminoides. In this study, methyl jasmonate (MeJA) was used to enhance CQAs accumulation in cultured G. jasminoides cells. Moreover, the possible molecular mechanism of MeJA-mediated accumulation of CQAs is also explored. To this end, time-course transcriptional profiles of G. jasminoides cells responding to MeJA were used to investigate the mechanism from different aspects, including jasmonate (JAs) biosynthesis, signal transduction, biosynthesis of precursor, CQAs biosynthesis, transporters, and transcription factors (TFs). A total of 57,069 unigenes were assembled from the clean reads, in which 80.7% unigenes were successfully annotated. Furthermore, comparative transcriptomic results indicated that differentially expressed genes (DEGs) were mainly involved in JAs biosynthesis and signal transduction (25 DEGs), biosynthesis of precursor for CQAs (18 DEGs), CQAs biosynthesis (19 DEGs), and transporters (29 DEGs). Most of these DEGs showed continuously upregulated expressions over time, which might activate the jasmonic acid (JA) signal transduction network, boost precursor supply, and ultimately stimulate CQAs biosynthesis. Additionally, various TFs from different TF families also responded to MeJA elicitation. Interestingly, 38 DEGs from different subgroups of the MYB family might display positive or negative regulations on phenylpropanoids, especially on CQAs biosynthesis. Conclusively, our results provide insight into the possible molecular mechanism of regulation on CQAs biosynthesis, which led to a high CQAs yield in the G. jasminoides cells under MeJA treatment.

Deciphering Trichoderma-Plant-Pathogen Interactions for Better Development of Biocontrol Applications

Members of the fungal genus Trichoderma (Ascomycota, Hypocreales, Hypocreaceae) are ubiquitous and commonly encountered as soil inhabitants, plant symbionts, saprotrophs, and mycoparasites. Certain species have been used to control diverse plant diseases and mitigate negative growth conditions. The versatility of Trichoderma’s interactions mainly relies on their ability to engage in inter- and cross-kingdom interactions. Although Trichoderma is by far the most extensively studied fungal biocontrol agent (BCA), with a few species already having been commercialized as bio-pesticides or bio-fertilizers, their wide application has been hampered by an unpredictable efficacy under field conditions. Deciphering the dialogues within and across Trichoderma ecological interactions by identification of involved effectors and their underlying effect is of great value in order to be able to eventually harness Trichoderma’s full potential for plant growth promotion and protection. In this review, we focus on the nature of Trichoderma interactions with plants and pathogens. Better understanding how Trichoderma interacts with plants, other microorganisms, and the environment is essential for developing and deploying Trichoderma-based strategies that increase crop production and protection.

Protein Phosphorylation in Plant Cell Signaling

Owing to their sessile nature, plants have evolved sophisticated sensory mechanisms to respond quickly and precisely to the changing environment. The extracellular stimuli are perceived and integrated by diverse receptors, such as receptor-like protein kinases (RLKs) and receptor-like proteins (RLPs), and then transmitted to the nucleus by complex cellular signaling networks, which play vital roles in biological processes including plant growth, development, reproduction, and stress responses. The posttranslational modifications (PTMs) are important regulators for the diversification of protein functions in plant cell signaling. Protein phosphorylation is an important and well-characterized form of the PTMs, which influences the functions of many receptors and key components in cellular signaling. Protein phosphorylation in plants predominantly occurs on serine (Ser) and threonine (Thr) residues, which is dynamically and reversibly catalyzed by protein kinases and protein phosphatases, respectively. In this review, we focus on the function of protein phosphorylation in plant cell signaling, especially plant hormone signaling, and highlight the roles of protein phosphorylation in plant abiotic stress responses.

NPR1 is required for root colonization and the establishment of a mutualistic symbiosis between the beneficial bacterium Rhizobium radiobacter and barley

Non-expressor of pathogenesis-related genes 1 (NPR1) is a key regulator of plant innate immunity and systemic disease resistance. The model for NPR1 function is based on experimental evidence obtained largely from dicots; however, this model does not fit all aspects of Poaceae family, which includes major crops such as wheat, rice and barley. In addition, there is little scientific data on NPR1's role in mutualistic symbioses. We assessed barley (Hordeum vulgare) HvNPR1 requirement during the establishment of mutualistic symbiosis between barley and beneficial Alphaproteobacterium Rhizobium radiobacter F4 (RrF4). Upon RrF4 root-inoculation, barley NPR1-knockdown (KD-hvnpr1) plants lost the typical spatiotemporal colonization pattern and supported less bacterial multiplication. Following RrF4 colonization, expression of salicylic acid marker genes were strongly enhanced in wild-type roots; whereas in comparison, KD-hvnpr1 roots exhibited little to no induction. Both basal and RrF4-induced root-initiated systemic resistance against virulent Blumeria graminis were impaired in leaves of KD-hvnpr1. Besides these immune-related differences, KD-hvnpr1 plants displayed higher root and shoot biomass than WT. However, RrF4-mediated growth promotion was largely compromised in KD-hvnpr1. Our results demonstrate a critical role for HvNPR1 in establishing a mutualistic symbiosis between a beneficial bacterium and a cereal crop.