Brassinosteroids antagonize gibberellin- and salicylate-mediated root immunity in rice

A multifaceted crosstalk between brassinosteroid and gibberellin regulates the resistance of cucumber to Phytophthora melonis

Cucumber plants are highly susceptible to the hemibiotroph oomycete Phytophthora melonis. However, the mechanism of resistance to cucumber blight remains poorly understood. Here, we demonstrated that cucumber plants with impairment in the biosynthesis of brassinosteroids (BRs) or gibberellins (GAs) were more susceptible to P. melonis. By contrast, increasing levels of endogenous BRs or exogenously application of 24-epibrassinolide enhanced the resistance of cucumber plants against P. melonis. Furthermore, we found that both knockout and overexpression of the BR biosynthesis gene CYP85A1 reduced the endogenous GA3 content compared with that of wild-type plants under the condition of inoculation with P. melonis, and the enhancement of disease resistance conferred by BR was inhibited in plants with silencing of the GA biosynthetic gene GA20ox1 or KAO. Together, these findings suggest that GA homeostasis is an essential factor mediating BRs-induced disease resistance. Moreover, BZR6, a key regulator of BR signaling, was found to physically interact with GA20ox1, thereby suppressing its transcription. Silencing of BZR6 promoted endogenous GA biosynthesis and compromised GA-mediated resistance. These findings reveal multifaceted crosstalk between BR and GA in response to pathogen infection, which can provide a new approach for genetically controlling P. melonis damage in cucumber production.

Brassinosteroid decreases cadmium accumulation via regulating gibberellic acid accumulation and Cd fixation capacity of root cell wall in rice (Oryza sativa)

The precise mechanism behind the association between plants' reactions to cadmium (Cd) stress and brassinosteroid (BR) remains unclear. In the current investigation, Cd stress quickly increased the endogenous BR concentration in the rice roots. Exogenous BR also increased the hemicellulose level in the root cell wall, which in turn increased its capacity to bind Cd. Simultaneously, the transcription level of genes responsible for root Cd absorption was decreased, including Natural Resistance-Associated Macrophage Protein 1/5 (OsNRAMP1/5) and a major facilitator superfamily gene called OsCd1. Ultimately, the increased expression of Heavy Metal ATPase 3 (OsHMA3) and the decreased expression of OsHMA2, which was in charge of separating Cd into vacuoles and translocating Cd to the shoots, respectively, led to a decrease in the amount of Cd that accumulated in the rice shoots. In contrast, transgenic rice lines overexpressing OsGSK2 (a negative regulator in BR signaling) accumulated more Cd, while OsGSK2 RNA interference (RNAi) rice line accumulated less Cd. Furthermore, BR increased endogenous Gibberellic acid (GA) level, and applying GA could replicate its alleviative effect. Taken together, BR decreased Cd accumulation in rice by mediating the cell wall's fixation capacity to Cd, which might relied on the buildup of the GA.

Recent Advances in the Crosstalk between Brassinosteroids and Environmental Stimuli

Due to their sessile lifestyle, plants need to optimize their growth in order to adapt to ever-changing environments. Plants receive stimuli from the environment and convert them into cellular responses. Brassinosteroids (BRs), as growth-promoting steroid hormones, play a significant role in the tradeoff between growth and environmental responses. Here, we provide a comprehensive summary for understanding the crosstalk between BR and various environmental stresses, including water availability, temperature fluctuations, salinization, nutrient deficiencies and diseases. We also highlight the bottlenecks that need to be addressed in future studies. Ultimately, we suppose to improve plant environmental adaptability and crop yield by excavating natural BR mutants or modifying BR signaling and its targets.

Comparative transcriptome and metabolome analysis revealed diversity in the response of resistant and susceptible rose (Rosa hybrida) varieties to Marssonina rosae

Rose black spot disease caused by Marssonina rosae is among the most destructive diseases that affects the outdoor cultivation and production of roses; however, the molecular mechanisms underlying the defensive response of roses to M. rosae have not been clarified. To investigate the diversity of response to M. rosae in resistant and susceptible rose varieties, we performed transcriptome and metabolome analyses of resistant (KT) and susceptible (FG) rose varieties and identified differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) in response to M. rosae at different time points. In response to M. rosae, DEGs and DAMs were mainly upregulated compared to the control and transcription factors were concentrated in the WRKY and AP2/ERF families. Gene Ontology analysis showed that the DEGs of FG were mainly enriched in biological processes, such as the abscisic acid-activated signaling pathway, cell wall, and defense response, whereas the DEGs of KT were mainly enriched in Golgi-mediated vesicle transport processes. Kyoto Encyclopedia of Genes and Genomes analysis showed that the DEGs of both varieties were concentrated in plant-pathogen interactions, plant hormone signal transduction, and mitogen-activated protein kinase signaling pathways, with the greatest number of DEGs associated with brassinosteroid (BR) in the plant hormone signal transduction pathway. The reliability of the transcriptome results was verified by qRT-PCR. DAMs of KT were significantly enriched in the butanoate metabolism pathway, whereas DAMs of FG were significantly enriched in BR biosynthesis, glucosinolate biosynthesis, and tryptophan metabolism. Moreover, the DAMs in these pathways were significantly positively correlated with the DEGs. Disease symptoms were aggravated when FG leaves were inoculated with M. rosae after 24-epibrassinolide treatment, indicating that the response of FG to M. rosae involves the BR signaling pathway. Our results provide new insights into the molecular mechanisms underlying rose response to M. rosae and lay a theoretical foundation for formulating rose black spot prevention and control strategies and cultivating resistant varieties.

Leaf rust (Puccinia recondita f. sp. secalis) triggers substantial changes in rye (Secale cereale L.) at the transcriptome and metabolome levels

BACKGROUND

Rye (Secale cereale L.) is a cereal crop highly tolerant to environmental stresses, including abiotic and biotic stresses (e.g., fungal diseases). Among these fungal diseases, leaf rust (LR) is a major threat to rye production. Despite extensive research, the genetic basis of the rye immune response to LR remains unclear.

RESULTS

An RNA-seq analysis was conducted to examine the immune response of three unrelated rye inbred lines (D33, D39, and L318) infected with compatible and incompatible Puccinia recondita f. sp. secalis (Prs) isolates. In total, 877 unique differentially expressed genes (DEGs) were identified at 20 and 36 h post-treatment (hpt). Most of the DEGs were up-regulated. Two lines (D39 and L318) had more up-regulated genes than down-regulated genes, whereas the opposite trend was observed for line D33. The functional classification of the DEGs helped identify the largest gene groups regulated by LR. Notably, these groups included several DEGs encoding cytochrome P450, receptor-like kinases, methylesterases, pathogenesis-related protein-1, xyloglucan endotransglucosylases/hydrolases, and peroxidases. The metabolomic response was highly conserved among the genotypes, with line D33 displaying the most genotype-specific changes in secondary metabolites. The effect of pathogen compatibility on metabolomic changes was less than the effects of the time-points and genotypes. Accordingly, the secondary metabolome of rye is altered by the recognition of the pathogen rather than by a successful infection. The results of the enrichment analysis of the DEGs and differentially accumulated metabolites (DAMs) reflected the involvement of phenylpropanoid and diterpenoid biosynthesis as well as thiamine metabolism in the rye immune response.

CONCLUSION

Our work provides novel insights into the genetic and metabolic responses of rye to LR. Numerous immune response-related DEGs and DAMs were identified, thereby clarifying the mechanisms underlying the rye response to compatible and incompatible Prs isolates during the early stages of LR development. The integration of transcriptomic and metabolomic analyses elucidated the contributions of phenylpropanoid biosynthesis and flavonoid pathways to the rye immune response to Prs. This combined analysis of omics data provides valuable insights relevant for future research conducted to enhance rye resistance to LR.

TOR coordinates cytokinin and gibberellin signals mediating development and defense

Plants constantly perceive and process environmental signals and balance between the energetic demands of growth and defense. Growth arrest upon pathogen attack was previously suggested to result from a redirection of the plants' metabolic resources towards the activation of plant defense. The energy sensor Target of Rapamycin (TOR) kinase is a conserved master coordinator of growth and development in all eukaryotes. Although TOR is positioned at the interface between development and defense, little is known about the mechanisms by which TOR may potentially regulate the relationship between these two modalities. The plant hormones cytokinin (CK) and gibberellin (GA) execute various aspects of plant development and defense. The ratio between CK and GA was reported to determine the outcome of developmental programmes. Here, investigating the interplay between TOR-mediated development and TOR-mediated defense in tomato, we found that TOR silencing resulted in rescue of several different aberrant developmental phenotypes, demonstrating that TOR is required for the execution of developmental cues. In parallel, TOR inhibition enhanced immunity in genotypes with a low CK/GA ratio but not in genotypes with a high CK/GA ratio. TOR-inhibition mediated disease resistance was found to depend on developmental status, and was abolished in strongly morphogenetic leaves, while being strongest in mature, differentiated leaves. CK repressed TOR activity, suggesting that CK-mediated immunity may rely on TOR downregulation. At the same time, TOR activity was promoted by GA, and TOR silencing reduced GA sensitivity, indicating that GA signalling requires normal TOR activity. Our results demonstrate that TOR likely acts in concert with CK and GA signalling, executing signalling cues in both defense and development. Thus, differential regulation of TOR or TOR-mediated processes could regulate the required outcome of development-defense prioritisation.

Does Constitutive Expression of Defense-Related Genes and Salicylic Acid Concentrations Correlate with Field Resistance of Potato to Black Scurf Disease?

Black scurf disease on potato caused by Rhizoctonia solani AG3 occurs worldwide and is difficult to control. The use of potato cultivars resistant to black scurf disease could be part of an integrated control strategy. Currently, the degree of resistance is based on symptom assessment in the field, but molecular measures could provide a more efficient screening method. We hypothesized that the degree of field resistance to black scurf disease in potato cultivars is associated with defense-related gene expression levels and salicylic acid (SA) concentration. Cultivars with a moderate and severe appearance of disease symptoms on tubers were selected and cultivated in the same field. In addition, experiments were conducted under controlled conditions in an axenic in vitro culture and in a sand culture to analyze the constitutive expression of defense-related genes and SA concentration. The more resistant cultivars did not show significantly higher constitutive expression levels of defense-related genes. Moreover, the level of free SA was increased in the more resistant cultivars only in the roots of the plantlets grown in the sand culture. These results indicate that neither expression levels of defense-related genes nor the amount of SA in potato plants can be used as reliable predictors of the field resistance of potato genotypes to black scurf disease.

Sl-lncRNA47980, a positive regulator affects tomato resistance to Phytophthora infestans

Emerging evidence suggests that long non-coding RNAs (lncRNAs) involve in defense respond against pathogen attack and show great potentials to improve plant resistance. Tomato late blight, a destructive plant disease, is caused by the oomycete pathogen Phytophthora infestans, which seriously affects the yield and quality of tomato. Our previous research has shown that Sl-lncRNA47980 is involved in response to P. infestans infection, but its molecular mechanism is unknown. Gain- and loss-of-function experiments revealed that Sl-lncRNA47980 as a positive regulator, played a crucial role in enhancing tomato resistance to P. infestans. The Sl-lncRNA47980-overexpressing transgenic plants exhibited an improved ability to scavenge reactive oxygen species (ROS), decreased contents of endogenous gibberellin (GA) and salicylic acid (SA), and increased contents of jasmonic acid (JA), while silencing of Sl-lncRNA47980 showed an opposite trend in the levels of these hormones. Furthermore, it was found that Sl-lncRNA47980 could upregulate the expression of SlGA2ox4 gene through activation of the promoter of SlGA2ox4 to affect GA content. The increased expression of the tomato GA signaling repressor SlDELLA could activate JA-related genes and inhibit SA-related genes to varying degrees respectively. In addition, exogenous application of GA3 and GA synthesis inhibitor uniconazole could increase disease susceptibility of Sl-lncRNA47980-overexpressing plants and the resistance of Sl-lncRNA47980-silenced plants, respectively, to P. infestans. From thus, it was speculated that Sl-lncRNA47980 conferred tomato resistance to P. infestans, which was related to the decrease in endogenous GA content. Our study provided information to link Sl-lncRNA47980 with changes in ROS accumulation and phytohormone levels in plant immunity, thus providing a new candidate gene for tomato breeding.

Salicylic acid interacts with other plant growth regulators and signal molecules in response to stressful environments in plants

Salicylic acid (SA) is one of the potential plant growth regulators (PGRs) that regulate plant growth and development by triggering many physiological and metabolic processes. It is also known to be a crucial component of plant defense mechanisms against environmental stimuli. In stressed plants, it is documented that it can effectively modulate a myriad of metabolic processes including strengthening of oxidative defense system by directly or indirectly limiting the buildup of reactive nitrogen and oxygen radicals. Although it is well recognized that it performs a crucial role in plant tolerance to various stresses, it is not fully elucidated that whether low or high concentrations of this PGR is effective to achieve optimal growth of plants under stressful environments. It is also not fully understood that to what extent and in what manner it cross-talks with other potential growth regulators and signalling molecules within the plant body. Thus, this critical review discusses how far SA mediates crosstalk with other key PGRs and molecular components of signalling pathways mechanisms, particularly in plants exposed to environmental cues. Moreover, the function of SA exogenously applied in regulation of growth and development as well as reinforcement of oxidative defense system of plants under abiotic stresses is explicitly elucidated.

Brassinosteroid Promotes Grape Berry Quality-Focus on Physicochemical Qualities and Their Coordination with Enzymatic and Molecular Processes: A Review

Brassinosteroid (BR) is an important endogenous phytohormone that plays a significant role in fruit quality regulation. The regulation of BR biosynthesis and its physiological effects have been well-studied in various fruits. External quality (fruit longitudinal and transverse diameters, firmness, single berry weight, color) and internal quality (sugars, aroma, anthocyanin, stress-related metabolites) are important parameters that are modified during grape berry development and ripening. Grapevines are grown all over the world as a cash crop and utilized for fresh consumption, wine manufacture, and raisin production. In this paper, the biosynthesis and signaling transduction of BR in grapevine were summarized, as well as the recent developments in understanding the role of BR in regulating the external quality (fruit longitudinal and transverse diameters, firmness, single berry weight, and color) and internal quality (sugars, organic acids, aroma substances, anthocyanins, antioxidants) of grapes. Additionally, current advancements in exogenous BR strategies for improving grape berries quality were examined from the perspectives of enzymatic activity and transcriptional regulation. Furthermore, the interaction between BR and other phytohormones regulating the grape berry quality was also discussed, aiming to provide a reliable reference for better understanding the potential value of BR in the grape/wine industry.

Negative Interactions Balance Growth and Defense in Plants Confronted with Herbivores or Pathogens

Plants have evolved a series of defensive mechanisms against pathogens and herbivores, but the defense response always leads to decreases in growth or reproduction, which has serious implications for agricultural production. Growth and defense are negatively regulated not only through metabolic consumption but also through the antagonism of different phytohormones, such as jasmonic acid (JA) and salicylic acid (SA). Meanwhile, plants can limit the expression of defensive metabolites to reduce the costs of defense by producing constitutive defenses such as glandular trichomes or latex and accumulating specific metabolites, determining the activation of plant defense or the maintenance of plant growth. Interestingly, plant defense pathways might be prepared in advance which may be transmitted to descendants. Plants can also use external organisms to protect themselves, thus minimizing the costs of defense. In addition, plant relatives exhibit cooperation to deal with pathogens and herbivores, which is also a way to regulate growth and defense.

TMT-based quantitative membrane proteomics identified PRRs potentially involved in the perception of MSP1 in rice leaves

Pathogen-associated molecular patterns (PAMPs) play a key role in triggering PAMPs triggered immunity (PTI) in plants. In the case of the rice-Magnaporthe oryzae pathosystem, fewer PAMPs and their pattern recognition receptors (PRRs) have been characterized. Recently, a M. oryzae snodprot1 homolog protein (MSP1) has been identified that functions as PAMP and triggering the PTI responses in rice. However, the molecular mechanism underlying MSP1-induced PTI is currently elusive. Therefore, we generated MSP1 overexpressed transgenic lines of rice, and a tandem mass tag (TMT)-based quantitative membrane proteomic analysis was employed to decipher the potential MSP1-induced signaling in rice using total cytosolic as well as membrane protein fractions. This approach led to the identification of 8033 proteins of which 1826 were differentially modulated in response to overexpression of MSP1 and/or exogenous jasmonic acid treatment. Of these, 20 plasma membrane-localized receptor-like kinases (RLKs) showed increased abundance in MSP1 overexpression lines. Moreover, activation of proteins related to the protein degradation and modification, calcium signaling, redox, and MAPK signaling was observed in transgenic lines expressing MSP1 in the apoplast. Taken together, our results identified potential PRR candidates involved in MSP1 recognition and suggested the overview mechanism of the MSP1-induced PTI signaling in rice leaves. SIGNIFICANCE: In plants, recognition of pathogen pathogen-derived molecules, such as PAMPs, by plant plant-derived PRRs has an essential role for in the activation of PTI against pathogen invasion. Typically, PAMPs are recognized by plasma membrane (PM) localized PRRs, however, identifying the PM-localized PRR proteins is challenging due to their low abundance. In this study, we performed an integrated membrane protein enrichment by microsomal membrane extraction (MME) method and subsequent TMT-labeling-based quantitative proteomic analysis using MSP1 overexpressed rice. Based on these results, we successfully identified various intracellular and membrane membrane-localized proteins that participated in the MSP1-induced immune response and characterized the potential PM-localized PRR candidates in rice.

Effect of Qiangdi 863 Nanosynergids Treated Water, Nitrogen, Phosphorous and Potassium Fertilizers on Rice Growth Physiology and Grain Quality

Nanotechnology is an emerging technique that helps in solving the biotic and abiotic agricultural issues leading to enhance crop productivity. Therefore, it was hypothesized to check the effect of Qiangdi 863 nano synergids biological-assisted growth apparatus and nitrogen, phosphorous, and potassium (NPK) fertilizers improving rice germination, early growth, physiology, and yield. An experiment was performed on five rice varieties for three consecutive years (2017-2019). The nanosynergids-treated water (NTW) significantly improved the speed of germination (25.3, 35.6, and 32.3%), final emergence percentage (100%) and seed emergence energy percentage (80, 95, and 90%), radical (1.25, 1.7, and 2.35 cm) and plumule growth (1.29, 1.24, and 1.66 cm), soil plant analysis development (46, 45, and 47), antioxidant enzymatic activities, such as catalase activity (34,376 μg-1FW h-1, 33,264 μg-1FW h-1, and 34,453 μg-1F W h-1), superoxide dismutase (18,456 μg-1F W h-1, 19,445 μg-1F W h-1, and 19,954 μg-1F W h-1), peroxide (745 Ug-1F W, 734 Ug-1F W, and 752 Ug-1F W), production and declined malondialdehyde (4.5 μmolg-1F W, 5.1 μmolg-1F W, and 4.2 μmolg-1F W) for all years respectively in KSK 133. The application of nano-treated irrigated water enriched the biomass of rice seedlings. The overall nano synergid treatments successfully enhanced the endogenous hormones as salicylic acid (6,016.27 p mol/L, 5823.22 p mol/L, and 5922.12 p mol/L), jasmonates (JA) (5,175.6 p mol/L, 4231 p mol/L, and 5014.21 p mol/L) brassinosteroids (BR) (618.2 p mol/L, 546.83 p mol/L, and 582.1 p mol/L) quantification and yield 1000 grain weight (22.3, 22, and 23.2 g) of KSK 133. Hence, the overall results proved that NTW could effectively enhance the early growth and yield of rice varieties.

Brassinosteroid-regulated bHLH transcription factor CESTA induces the gibberellin 2-oxidase GA2ox7

Brassinosteroids (BRs) are plant steroids that have growth-promoting capacities, which are partly enabled by an ability to induce biosynthesis of gibberellins (GAs), a second class of plant hormones. In addition, BRs can also activate GA catabolism; here we show that in Arabidopsis (Arabidopsis thaliana) the basic helix-loop-helix transcription factor CESTA (CES) and its homologues BRASSINOSTEROID-ENHANCED EXPRESSION (BEE) 1 and 3 contribute to this activity. CES and the BEEs are BR-regulated at the transcriptional and posttranslational level and participate in different physiological processes, including vegetative and reproduction development, shade avoidance, and cold stress responses. We show that CES/BEEs can induce the expression of the class III GA 2-oxidase GA2ox7 and that this activity is increased by BRs. In BR signaling - and CES/BEE-deficient mutants, GA2ox7 expression decreased, yielding reduced levels of GA110, a product of GA2ox7 activity. In plants that over-express CES, GA2ox7 expression is hyper-responsive to BR, GA110 levels are elevated and amounts of bioactive GA are reduced. We provide evidence that CES directly binds to the GA2ox7 promoter and is activated by BRs, but can also act by BR-independent means. Based on these results, we propose a model for CES activity in GA catabolism where CES can be recruited for GA2ox7 induction not only by BR, but also by other factors.

OsAPX1 Positively Contributes to Rice Blast Resistance

Ascorbate peroxidases (APXs) maintain cellular reactive oxygen species (ROS) homeostasis through their peroxidase activity. Here, we report that OsAPX1 also promotes ROS production such that a delicate cellular ROS homeostasis is achieved temporally after Magnaporthe oryzae infection. OsAPX1 specifically induces ROS production through increasing respiratory burst oxidase homologs (OsRBOHs) expression and can be inhibited by DPI, a ROS inhibitor. The time-course experiment data show that the simultaneous induction of OsAPX1 and OsRBOHs leads to ROS accumulation at an early stage; whereas a more durable expression of OsAPX1 leads to ROS scavenging at a later stage. By the temporal switching between ROS inducer and eliminator, OsAPX1 triggers an instant ROS burst upon M. oryzae infection and then a timely elimination of ROS toxicity. We find that OsAPX1 is under the control of the miR172a-OsIDS1 regulatory module. OsAPX1 also affects salicylic acid (SA) synthesis and signaling, which contribute to blast resistance. In conclusion, we show that OsAPX1 is a key factor that connects the upstream gene silencing and transcription regulatory routes with the downstream phytohormone and redox pathway, which provides an insight into the sophisticated regulatory network of rice innate immunity.

Interactions of Gibberellins with Phytohormones and Their Role in Stress Responses

Gibberellins are amongst the main plant growth regulators. Discovered over a century ago, the interest in gibberellins research is growing due to their current and potential applications in crop production and their role in the responses to environmental stresses. In the present review, the current knowledge on gibberellins’ homeostasis and modes of action is outlined. Besides this, the complex interrelations between gibberellins and other plant growth regulators are also described, providing an intricate network of interactions that ultimately drives towards precise and specific gene expression. Thus, genes and proteins identified as being involved in gibberellin responses in model and non-model species are highlighted. Furthermore, the molecular mechanisms governing the gibberellins’ relation to stress responses are also depicted. This review aims to provide a comprehensive picture of the state-of-the-art of the current perceptions of the interactions of gibberellins with other phytohormones, and their responses to plant stresses, thus allowing for the identification of the specific mechanisms involved. This knowledge will help us to improve our understanding of gibberellins’ biology, and might help increase the biotechnological toolbox needed to refine plant resilience, particularly under a climate change scenario.

Salicylic Acid and N-Hydroxypipecolic Acid at the Fulcrum of the Plant Immunity-Growth Equilibrium

Salicylic acid (SA) and N-hydroxypipecolic acid (NHP) are two central plant immune signals involved in both resistance at local sites of pathogen infection (basal resistance) and at distal uninfected sites after primary infection (systemic acquired resistance). Major discoveries and advances have led to deeper understanding of their biosynthesis and signaling during plant defense responses. In addition to their well-defined roles in immunity, recent research is emerging on their direct mechanistic impacts on plant growth and development. In this review, we will first provide an overview of how SA and NHP regulate local and systemic immune responses in plants. We will emphasize how these two signals are mutually potentiated and are convergent on multiple aspects-from biosynthesis to homeostasis, and from signaling to gene expression and phenotypic responses. We will then highlight how SA and NHP are emerging to be crucial regulators of the growth-defense balance, showcasing recent multi-faceted studies on their metabolism, receptor signaling and direct growth/development-related host targets. Overall, this article reflects current advances and provides future outlooks on SA/NHP biology and their functional significance as central signals for plant immunity and growth. Because global climate change will increasingly influence plant health and resilience, it is paramount to fundamentally understand how these two tightly linked plant signals are at the nexus of the growth-defense balance.

Salicylic Acid in Root Growth and Development

In plants, salicylic acid (SA) is a hormone that mediates a plant’s defense against pathogens. SA also takes an active role in a plant’s response to various abiotic stresses, including chilling, drought, salinity, and heavy metals. In addition, in recent years, numerous studies have confirmed the important role of SA in plant morphogenesis. In this review, we summarize data on changes in root morphology following SA treatments under both normal and stress conditions. Finally, we provide evidence for the role of SA in maintaining the balance between stress responses and morphogenesis in plant development, and also for the presence of SA crosstalk with other plant hormones during this process.

Salicylic Acid, Jasmonate, and Ethylene Contribute to Rice Defense Against White Tip Nematodes Aphelenchoides besseyi

Plant hormones have a prominent place in the plant immune and defense mechanism. To gain more information about the plant hormone pathways involved in rice defense against nematodes, here, we studied the roles of three core hormones, namely, salicylic acid (SA), jasmonate (JA), and ethylene (ET) in rice defense to Aphelenchoides besseyi by using the susceptible variety, Nipponbare as well as the resistant variety Tetep. The data showed that Tetep exhibited pre- and post-invasion with suppression of nematode infection, development, and reproduction. The quantitative real-time (qRT)-PCR analysis of plant hormone marker genes in the two cultivars clearly revealed that all the SA-related genes were downregulated in susceptible Nipponbare plants but were significantly upregulated in resistant Tetep plants at the flowering stage. The exogenous application of the SA analog, benzo-1,2,3-thiadiazole-7-carbothioic acid S-methyl ester (BTH), methyl jasmonate (MeJA), and ethephon did induce rice resistance to A. besseyi, and the rice plants treated by hormone inhibitors increased susceptibility to A. besseyi. Similarly, corresponding transgenic biosynthesis or signaling mutants of those hormones also showed an increased susceptibility. Collectively, these results suggest that SA, JA, and ET play important defense roles in rice against A. besseyi.

The interplay of plant hormonal pathways and geminiviral proteins: partners in disease development

Viruses belonging to the family Geminiviridae infect plants and are responsible for a number of diseases of crops in the tropical and sub-tropical regions of the World. The innate immune response of the plant assists in its defense against such viral pathogens by the recognition of pathogen/microbe-associated molecular patterns through pattern-recognition receptors. Phytohormone signalling pathways play a vital role in plant defense responses against these devastating viruses. Geminiviruses, however, have developed counter-defense strategies that prevail over the above defense pathways. The proteins encoded by geminiviruses act as suppressors of plant immunity by interacting with the signalling components of several hormones. In this review we focus on the molecular interplay of phytohormone pathways and geminiviral infection and try to find interesting parallels with similar mechanisms known in other plant-infecting viruses and strengthen the argument that this interplay is necessary for disease development.

Biosynthesis and beneficial effects of microbial gibberellins on crops for sustainable agriculture

Soil microbes promote plant growth through several mechanisms such as secretion of chemical compounds including plant growth hormones. Among the phytohormones, auxins, ethylene, cytokinins, abscisic acid and gibberellins are the best understood compounds. Gibberellins were first isolated in 1935 from the fungus Gibberella fujikuroi and are synthesized by several soil microbes. The effect of gibberellins on plant growth and development has been studied, as has the biosynthesis pathways, enzymes, genes and their regulation. This review revisits the history of gibberellin research highlighting microbial gibberellins and their effects on plant health with an emphasis on the early discoveries and current advances that can find vital applications in agricultural practices.

Characterization of evolutionarily distinct rice BAHD-Acyltransferases provides insight into their plausible role in rice susceptibility to Rhizoctonia solani

Plants produce diverse secondary metabolites in response to different environmental cues including pathogens. The modification of secondary metabolites, including acylation, modulates their biological activity, stability, transport, and localization. A plant-specific BAHD-acyltransferase (BAHD-AT) gene family members catalyze the acylation of secondary metabolites. Here we characterized the rice (Oryza sativa L.) BAHD-ATs at the genome-wide level and endeavor to define their plausible role in the tolerance against Rhizoctonia solani AG1-IA. We identified a total of 85 rice OsBAHD-AT genes and classified them into five canonical clades based on their phylogenetic relationship with characterized BAHD-ATs from other plant species. The time-course RNA sequencing (RNA-seq) analysis of OsBAHD-AT genes and qualitative real-time polymerase chain reaction (qRT-PCR) validation showed higher expression in sheath blight susceptible rice genotype. Furthermore, the DNA methylation analysis revealed higher hypomethylation of OsBAHD-AT genes that corresponds to their higher expression in susceptible rice genotype, indicating epigenetic regulation of OsBAHD-AT genes in response to R. solani AG1-IA inoculation. The results shown here indicate that BAHD-ATs may have a negative role in rice tolerance against R. solani AG1-IA possibly mediated through the brassinosteroid (BR) signaling pathway. Altogether, the present analysis suggests the putative functions of several OsBAHD-AT genes, which will provide a blueprint for their functional characterization and to understand the rice-R. solani AG1-IA interaction.

Supraoptimal Brassinosteroid Levels Inhibit Root Growth by Reducing Root Meristem and Cell Elongation in Rice

Root growth depends on cell proliferation and cell elongation at the root meristem, which are controlled by plant hormones and nutrient availability. As a foraging strategy, rice (Oryza sativa L.) grows longer roots when nitrogen (N) is scarce. However, how the plant steroid hormone brassinosteroid (BR) regulates rice root meristem development and responses to N deficiency remains unclear. Here, we show that BR has a negative effect on meristem size and a dose-dependent effect on cell elongation in roots of rice seedlings treated with exogenous BR (24-epicastasterone, ECS) and the BR biosynthesis inhibitor propiconazole (PPZ). A genome-wide transcriptome analysis identified 4110 and 3076 differentially expressed genes in response to ECS and PPZ treatments, respectively. The gene ontology (GO) analysis shows that terms related to cell proliferation and cell elongation were enriched among the ECS-repressed genes. Furthermore, microscopic analysis of ECS- and PPZ-treated roots grown under N-sufficient and N-deficient conditions demonstrates that exogenous BR or PPZ application could not enhance N deficiency-mediated root elongation promotion as the treatments could not promote root meristem size and cell elongation simultaneously. Our study demonstrates that optimal levels of BR in the rice root meristem are crucial for optimal root growth and the foraging response to N deficiency.

Brassinosteroids as a multidimensional regulator of plant physiological and molecular responses under various environmental stresses

Biotic and abiotic stresses, especially heavy metal toxicity, are becoming a big problem in agriculture, which pose serious threats to crop production. Plant hormones have recently been used to develop stress tolerance in a variety of plants. Brassinosteroids (BRs) are the sixth class of plant steroid hormones, with pleiotropic effects on plants. Exogenous application of BRs to boost plant tolerance mechanisms to various stresses has been a major research focus. Numerous studies have revealed the role of these steroidal hormones in the up-regulation of stress-related resistance genes, as well as their interactions with other metabolic pathways. BRs interact with other phytohormones such as auxin, cytokinin, ethylene, gibberellin, jasmonic acid, abscisic acid, salicylic acid, and polyamines to regulate a variety of physiological and developmental processes in plants. BRs regulate expressions of many BR-inducible genes by activating the brassinazole-resistant 1 (BZR1)/BRI1-EMS suppressor 1 (BES1) complex. Moreover, to improve plant development under a variety of stresses, BRs regulate antioxidant enzyme activity, chlorophyll concentration, photosynthetic capability, and glucose metabolism. This review will provide insights into the mechanistic role and actions of brassinosteroids in plants in response to various stresses.

Orchestration of plant development and defense by indirect crosstalk of salicylic acid and brassinosteorid signaling via transcription factor GhTINY2

Salicylic acid (SA) and brassinosteroids (BRs) are well known to regulate diverse processes of plant development and stress responses, but the mechanisms by which these phytohormones mediate the growth and defense trade-off are largely unclear. In addition, little is known about the roles of DEHYDRATION RESPONSIVE ELEMENT BINDING transcription factors, especially in biotic stress and plant growth. Here, we identified a cotton (Gossypium hirsutum) APETALA2/ETHYLENE RESPONSIVE FACTOR gene GhTINY2 that is strongly induced by Verticillium dahliae. Overexpression of GhTINY2 in cotton and Arabidopsis enhanced tolerance to V. dahliae, while knockdown of expression increased the susceptibility of cotton to the pathogen. GhTINY2 was found to promote SA accumulation and SA signaling transduction by directly activating expression of WRKY51. Moreover, GhTINY2-overexpressing cotton and Arabidopsis showed retardation of growth, increased sensitivity to inhibitors of BR biosynthesis, down-regulation of several BR-induced genes, and up-regulation of BR-repressed genes, while GhTINY2-RNAi cotton showed the opposite effects. We further determined that GhTINY2 negatively regulates BR signaling by interacting with BRASSINAZOLE-RESISTANT 1 (BZR1) and restraining its transcriptional activation of the expression of INDOLE-3-ACETIC ACID INDUCIBLE 19 (IAA19). These findings indicate that GhTINY2 fine-tunes the trade-off between immunity and growth via indirect crosstalk between WRKY51-mediated SA biosynthesis and BZR1-IAA19-regulated BR signaling.

Nucleotide-Binding Leucine-Rich Repeat Genes CsRSF1 and CsRSF2 Are Positive Modulators in the Cucumis sativus Defense Response to Sphaerotheca fuliginea

Cucumber powdery mildew caused by Sphaerotheca fuliginea is a leaf disease that seriously affects cucumber's yield and quality. This study aimed to report two nucleotide-binding site-leucine-rich repeats (NBS-LRR) genes CsRSF1 and CsRSF2, which participated in regulating the resistance of cucumber to S. fuliginea. The subcellular localization showed that the CsRSF1 protein was localized in the nucleus, cytoplasm, and cell membrane, while the CsRSF2 protein was localized in the cell membrane and cytoplasm. In addition, the transcript levels of CsRSF1 and CsRSF2 were different between resistant and susceptible cultivars after treatment with exogenous substances, such as abscisic acid (ABA), methyl jasmonate (MeJA), salicylic acid (SA), ethephon (ETH), gibberellin (GA) and hydrogen peroxide (H₂O₂). The expression analysis showed that the transcript levels of CsRSF1 and CsRSF2 were correlated with plant defense response against S. fuliginea. Moreover, the silencing of CsRSF1 and CsRSF2 impaired host resistance to S. fuliginea, but CsRSF1 and CsRSF2 overexpression improved resistance to S. fuliginea in cucumber. These results showed that CsRSF1 and CsRSF2 genes positively contributed to the resistance of cucumber to S. fuliginea. At the same time, CsRSF1 and CsRSF2 genes could also regulate the expression of defense-related genes. The findings of this study might help enhance the resistance of cucumber to S. fuliginea.

The Critical Role of Small RNAs in Regulating Plant Innate Immunity

Plants, due to their sessile nature, have an innate immune system that helps them to defend against different pathogen infections. The defense response of plants is composed of a highly regulated and complex molecular network, involving the extensive reprogramming of gene expression during the presence of pathogenic molecular signatures. Plants attain proper defense against pathogens through the transcriptional regulation of genes encoding defense regulatory proteins and hormone signaling pathways. Small RNAs are emerging as versatile regulators of plant development and act in different tiers of plant immunity, including pathogen-triggered immunity (PTI) and effector-triggered immunity (ETI). The versatile regulatory functions of small RNAs in plant growth and development and response to biotic and abiotic stresses have been widely studied in recent years. However, available information regarding the contribution of small RNAs in plant immunity against pathogens is more limited. This review article will focus on the role of small RNAs in innate immunity in plants.

Gibberellin Metabolism and Signaling: Targets for Improving Agronomic Performance of Crops

Gibberellins (GAs) are a class of tetracyclic diterpenoid phytohormones that regulate many aspects of plant development, including seed germination, stem elongation, leaf expansion, pollen maturation, and the development of flowers, fruits and seeds. During the past decades, the primary objective of crop breeding programs has been to increase productivity or yields. 'Green Revolution' genes that can produce semidwarf, high-yielding crops were identified as GA synthesis or response genes, confirming the value of research on GAs in improving crop productivity. The manipulation of GA status either by genetic alteration or by exogenous application of GA or GA biosynthesis inhibitors is often used to optimize plant growth and yields. In this review, we summarize the roles of GAs in major aspects of crop growth and development and present the possible targets for the fine-tuning of GA metabolism and signaling as a promising strategy for crop improvement.

It takes two to tango - molecular links between plant immunity and brassinosteroid signalling

In response to the invasion of microorganisms, plants actively balance their resources for growth and defence, thus ensuring their survival. The regulatory mechanisms underlying plant immunity and growth operate through complex networks, in which the brassinosteroid phytohormone is one of the central players. In the past decades, a growing number of studies have revealed a multi-layered crosstalk between brassinosteroid-mediated growth and plant immunity. In this Review, by means of the tango metaphor, we immerse ourselves into the intimate relationship between brassinosteroid and plant immune signalling pathways that is tailored by the lifestyle of the pathogen and modulated by other phytohormones. The plasma membrane is the unique stage where brassinosteroid and immune signals are dynamically integrated and where compartmentalization into nanodomains that host distinct protein consortia is crucial for the dance. Shared downstream signalling components and transcription factors relay the tango play to the nucleus to activate the plant defence response and other phytohormonal signalling pathways for the finale. Understanding how brassinosteroid and immune signalling pathways are integrated in plants will help develop strategies to minimize the growth-defence trade-off, a key challenge for crop improvement.

Powdery Mildew-Induced Hormonal and Photosynthetic Changes in Barley Near Isogenic Lines Carrying Various Resistant Genes

The present work focused on the characterization of some physiological mechanisms activated upon powdery mildew inoculation of the susceptible barley cultivar Ingrid and its near-isogenic lines (NILs) carrying various resistant genes (Mla, Mlg and mlo). After inoculation with Blumeria graminis f. sp. hordei (Bgh), measurements of leaf reflectance and chlorophyll a fluorescence were performed 3 and 7 day post-inoculation (dpi), while hormone assays were made 7 dpi. Bgh-inoculated resistant genotypes were characterized by lowered leaf reflectance parameters that correlated with carotenoids (CRI) and water content (WBI) in comparison to inoculated Ingrid. The PSII activity (i.e., Fv/Fm, ETo/CSm and P.I._ABS) strongly decreased in susceptible Ingrid leaves when the disease symptoms became visible 7 dpi. In Mla plants with visible hypersensitive spots the PSII activity decreased to a lesser extent. Inoculation resulted in a very slight decrease of photosynthesis at later stage of infection in Mlg plants, whereas in resistant mlo plants the PSII activity did not change. Chlorophyll a fluorescence measurements allowed presymptomatic detection of infection in Ingrid and Mla. Changes in the homeostasis of 22 phytohormones (cytokinins, auxins, gibberellins and the stress hormones JA, SA and ABA) in powdery mildew inoculated barley are discussed in relation to resistance against this biotrophic pathogen.

Regulation of ovule initiation by gibberellins and brassinosteroids in tomato and Arabidopsis: two plant species, two molecular mechanisms

Ovule primordia formation is a complex developmental process with a strong impact on the production of seeds. In Arabidopsis this process is controlled by a gene network, including components of the signalling pathways of auxin, brassinosteroids (BRs) and cytokinins. Recently, we have shown that gibberellins (GAs) also play an important role in ovule primordia initiation, inhibiting ovule formation in both Arabidopsis and tomato. Here we reveal that BRs also participate in the control of ovule initiation in tomato, by promoting an increase on ovule primordia formation. Moreover, molecular and genetic analyses of the co-regulation by GAs and BRs of the control of ovule initiation indicate that two different mechanisms occur in tomato and Arabidopsis. In tomato, GAs act downstream of BRs. BRs regulate ovule number through the downregulation of GA biosynthesis, which provokes stabilization of DELLA proteins that will finally promote ovule primordia initiation. In contrast, in Arabidopsis both GAs and BRs regulate ovule number independently of the activity levels of the other hormone. Taken together, our data strongly suggest that different molecular mechanisms could operate in different plant species to regulate identical developmental processes even, as for ovule primordia initiation, if the same set of hormones trigger similar responses, adding a new level of complexity.

Salicylic Acid Steers the Growth-Immunity Tradeoff

Plants possess an effective immune system to combat most microbial attackers. The activation of immune responses to biotrophic pathogens requires the hormone salicylic acid (SA). Accumulation of SA triggers a plethora of immune responses (like massive transcriptional reprogramming, cell wall strengthening, and production of secondary metabolites and antimicrobial proteins). A tradeoff of strong immune responses is the active suppression of plant growth and development. The tradeoff also works the opposite way, where active growth and developmental processes suppress SA production and immune responses. Here, we review research on the role of SA in the growth-immunity tradeoff and examples of how the tradeoff can be bypassed. This knowledge will be instrumental in resistance breeding of crops with optimal growth and effective immunity.

Genetic Network between Leaf Senescence and Plant Immunity: Crucial Regulatory Nodes and New Insights

Leaf senescence is an essential physiological process that is accompanied by the remobilization of nutrients from senescent leaves to young leaves or other developing organs. Although leaf senescence is a genetically programmed process, it can be induced by a wide variety of biotic and abiotic factors. Accumulating studies demonstrate that senescence-associated transcription factors (Sen-TFs) play key regulatory roles in controlling the initiation and progression of leaf senescence process. Interestingly, recent functional studies also reveal that a number of Sen-TFs function as positive or negative regulators of plant immunity. Moreover, the plant hormone salicylic acid (SA) and reactive oxygen species (ROS) have been demonstrated to be key signaling molecules in regulating leaf senescence and plant immunity, suggesting that these two processes share similar or common regulatory networks. However, the interactions between leaf senescence and plant immunity did not attract sufficient attention to plant scientists. Here, we review the regulatory roles of SA and ROS in biotic and abiotic stresses, as well as the cross-talks between SA/ROS and other hormones in leaf senescence and plant immunity, summarize the transcriptional controls of Sen-TFs on SA and ROS signal pathways, and analyze the cross-regulation between senescence and immunity through a broad literature survey. In-depth understandings of the cross-regulatory mechanisms between leaf senescence and plant immunity will facilitate the cultivation of high-yield and disease-resistant crops through a molecular breeding strategy.

Therapeutic Potential of Brassinosteroids in Biomedical and Clinical Research

Steroids are a pivotal class of hormones with a key role in growth modulation and signal transduction in multicellular organisms. Synthetic steroids are widely used to cure large array of viral, fungal, bacterial, and cancerous infections. Brassinosteroids (BRs) are a natural collection of phytosterols, which have structural similarity with animal steroids. BRs are dispersed universally throughout the plant kingdom. These plant steroids are well known to modulate a plethora of physiological responses in plants leading to improvement in quality as well as yield of food crops. Moreover, they have been found to play imperative role in stress-fortification against various stresses in plants. Over a decade, BRs have conquered worldwide interest due to their diverse biological activities in animal systems. Recent studies have indicated anticancerous, antiangiogenic, antiviral, antigenotoxic, antifungal, and antibacterial bioactivities of BRs in the animal test systems. BRs inhibit replication of viruses and induce cytotoxic effects on cancerous cell lines. Keeping in view the biological activities of BRs, this review is an attempt to update the information about prospects of BRs in biomedical and clinical application.

Exploring the Brassinosteroid Signaling in Monocots Reveals Novel Components of the Pathway and Implications for Plant Breeding

Brassinosteroids (BRs) are a class of steroidal phytohormones which are key regulators of diverse processes during whole life cycle of plants. Studies conducted in the dicot model species Arabidopsis thaliana have allowed identification and characterization of various components of the BR signaling. It is currently known that the BR signaling is interconnected at various stages with other phytohormonal and stress signaling pathways. It enables a rapid and efficient adaptation of plant metabolism to constantly changing environmental conditions. However, our knowledge about mechanism of the BR signaling in the monocot species is rather limited. Thus, identification of new components of the BR signaling in monocots, including cereals, is an ongoing process and has already led to identification of some monocot-specific components of the BR signaling. It is of great importance as disturbances in the BR signaling influence architecture of mutant plants, and as a consequence, the reaction to environmental conditions. Currently, the modulation of the BR signaling is considered as a target to enhance yield and stress tolerance in cereals, which is of particular importance in the face of global climate change.

Directing Trophic Divergence in Plant-Pathogen Interactions: Antagonistic Phytohormones With NO Doubt?

A fundamental process culminating in the mechanisms of plant-pathogen interactions is the regulation of trophic divergence into biotrophic, hemibiotrophic, and necrotrophic interactions. Plant hormones, of almost all types, play significant roles in this regulatory apparatus. In plant-pathogen interactions, two classical mechanisms underlying hormone-dependent trophic divergence are long recognized. While salicylic acid dominates in the execution of host defense response against biotrophic and early-stage hemibiotrophic pathogens, jasmonic acid, and ethylene are key players facilitating host defense response against necrotrophic and later-stage hemibiotrophic pathogens. Evidence increasingly suggests that trophic divergence appears to be modulated by more complex signaling networks. Acting antagonistically or agonistically, other hormones such as auxins, cytokinins, abscisic acid, gibberellins, brassinosteroids, and strigolactones, as well as nitric oxide, are emerging candidates in the regulation of trophic divergence. In this review, the latest advances in the dynamic regulation of trophic divergence are summarized, emphasizing common and contrasting hormonal and nitric oxide signaling strategies deployed in plant-pathogen interactions.

Stressed Out About Hormones: How Plants Orchestrate Immunity

Plants are under relentless challenge by pathogenic bacteria, fungi, and oomycetes, for whom they provide a resource of living space and nutrients. Upon detection of pathogens, plants carry out multiple layers of defense response, orchestrated by a tightly organized network of hormones. In this review, we provide an overview of the phytohormones involved in immunity and the ways pathogens manipulate their biosynthesis and signaling pathways. We highlight recent developments, including the discovery of a defense signaling molecule, new insights into hormone biosynthesis, and the increasing importance of signaling hubs at which hormone pathways intersect.

Transcriptome Analysis of the Hierarchical Response of Histone Deacetylase Proteins That Respond in an Antagonistic Manner to Salinity Stress

Acetylation in histone and non-histone proteins is balanced by histone acetyltransferase and histone deacetylase (HDAC) enzymatic activity, an essential aspect of fine-tuning plant response to environmental stresses. HDACs in Arabidopsis are composed of three families (RPD3-like, SIRT, and HD-tuins). A previous study indicated that class I (HDA19) and class II (HDA5/14/15/18) RPD3-like family HDACs control positive and negative responses to salinity stress, respectively. Furthermore, quintuple hda5/14/15/18/19 mutants (quint) exhibit salinity stress tolerance, suggesting that hda19 suppresses the sensitivity to salinity stress present in quadruple hda5/14/15/18 mutants (quad). In the present study, transcriptome analysis of the quint mutant was conducted to elucidate the hierarchical control of salinity stress response operated by RPD3-like family HDACs (HDA5/14/15/18/19). The analysis identified 4,832 salt-responsive genes in wild-type (Col-0), hda19-3, quad, and quint plants and revealed that 56.7% of the salt-responsive genes exhibited a similar expression pattern in both the hda19-3 and quint plants. These results indicate that deficiency in HDA19 has a bigger impact on salinity stress response than in class II HDACs. Furthermore, the expression pattern of genes encoding enzymes that metabolize phytohormones raises the possibility that a drastic change in the homeostasis of phytohormones, such as abscisic acid, brassinosteroid, and gibberellin, may contribute to increasing stress tolerance in hda19-3 and quint plants. Among these phytohormones, abscisic acid accumulation actually increased in hda19-3 and quint plants, and decreased in quad, compared with wild-type plants. Importantly, 7.8% of the salt-responsive genes in quint plants exhibited a similar expression pattern in quad plants, suggesting that some gene sets are regulated in an HDA5/14/15/18-dependent manner. The transcriptome analysis conducted in the present study revealed the hierarchical and independent regulation of salt stress response that is mediated through HDA19 and class II HDACs.

Alternative Splicing in the Regulation of Plant-Microbe Interactions

As sessile organisms, plants are continuously exposed to a wide range of biotic interactions. While some biotic interactions are beneficial or even essential for the plant (e.g. rhizobia and mycorrhiza), others such as pathogens are detrimental and require fast adaptation. Plants partially achieve this growth and developmental plasticity by modulating the repertoire of genes they express. In the past few years, high-throughput transcriptome sequencing have revealed that, in addition to transcriptional control of gene expression, post-transcriptional processes, notably alternative splicing (AS), emerged as a key mechanism for gene regulation during plant adaptation to the environment. AS not only can increase proteome diversity by generating multiple transcripts from a single gene but also can reduce gene expression by yielding isoforms degraded by mechanisms such as nonsense-mediated mRNA decay. In this review, we will summarize recent discoveries detailing the contribution of AS to the regulation of plant-microbe interactions, with an emphasis on the modulation of immunity receptor function and other components of the signaling pathways that deal with pathogen responses. We will also discuss emerging evidences that AS could contribute to dynamic reprogramming of the plant transcriptome during beneficial interactions, such as the legume-symbiotic interaction.

The Dual Effect of the Brassinosteroid Pathway on Rice Black-Streaked Dwarf Virus Infection by Modulating the Peroxidase-Mediated Oxidative Burst and Plant Defense

The phytohormone brassinosteroid (BR) not only plays key roles in regulating plant growth and development but is also involved in modulating the plant defense system in response to pathogens. We previously found that BR application made rice plants more susceptible to the devastating pathogen rice black-streaked dwarf virus (RBSDV), but the mechanism of BR-mediated susceptibility remains unclear. We now show that both BR-deficient and -insensitive mutants are resistant to RBSDV infection. High-throughput sequencing showed that the defense hormone salicylic acid and jasmonic acid pathways were activated in the RBSDV-infected BR mutant. Meanwhile, a number of class III peroxidases (OsPrx) were significantly changed and basal reactive oxygen species (ROS) accumulated in BR mutants. Treatment with exogenous hormones and other chemicals demonstrated that the BR pathway could suppress the levels of OsPrx and the ROS burst by directly binding the promoters of OsPrx genes. Together, our findings indicate that BR-mediated susceptibility is at least partly caused by inhibition of the action of defense hormones, preventing the accumulation of the peroxidase-mediated oxidative burst.

Plasmopara viticola effector PvRXLR131 suppresses plant immunity by targeting plant receptor-like kinase inhibitor BKI1

The grapevine downy mildew pathogen Plasmopara viticola secretes a set of RXLR effectors (PvRXLRs) to overcome host immunity and facilitate infection, but how these effectors function is unclear. Here, the biological function of PvRXLR131 was investigated via heterologous expression. Constitutive expression of PvRXLR131 in Colletotrichum gloeosporioides significantly enhanced its pathogenicity on grapevine leaves. Constitutive expression of PvRXLR131 in Arabidopsis promoted Pseudomonas syringae DC3000 and P. syringae DC3000 (hrcC^- ) growth as well as suppressed defence-related callose deposition. Transient expression of PvRXLR131 in Nicotiana benthamiana leaves could also suppress different elicitor-triggered cell death and inhibit plant resistance to Phytophthora capsici. Further analysis revealed that PvRXLR131 interacted with host Vitis vinifera BRI1 kinase inhibitor 1 (VvBKI1), and its homologues in N. benthamiana (NbBKI1) and Arabidopsis (AtBKI1). Moreover, bimolecular fluorescence complementation analysis revealed that PvRXLR131 interacted with VvBKI1 in the plasma membrane. Deletion assays showed that the C-terminus of PvRXLR131 was responsible for the interaction and mutation assays showed that phosphorylation of a conserved tyrosine residue in BKI1s disrupted the interaction. BKI1 was a receptor inhibitor of growth- and defence-related brassinosteroid (BR) and ERECTA (ER) signalling. When silencing of NbBKI1 in N. benthamiana, the virulence function of PvRXLR131 was eliminated, demonstrating that the effector activity is mediated by BKI1. Moreover, PvRXLR131-transgenic plants displayed BKI1-overexpression dwarf phenotypes and suppressed BR and ER signalling. These physiological and genetic data clearly demonstrate that BKI1 is a virulence target of PvRXLR131. We propose that P. viticola secretes PvRXLR131 to target BKI1 as a strategy for promoting infection.

Rice miR394 suppresses leaf inclination through targeting an F-box gene, LEAF INCLINATION 4

Rice leaf inclination is an important agronomic trait, closely related to plant architecture and yield. Identification of genes controlling leaf inclination would assist in crop improvement. Although various factors, including the plant hormones auxin and brassinosteroids, have been shown to regulate lamina joint development, the role of microRNAs in regulating leaf inclination remains largely unknown. Here, we functionally characterize the role of rice miR394 and its target, LEAF INCLINCATION 4 (LC4), which encodes an F-box protein, in the regulation of leaf inclination. We show that miR394 and LC4 work, antagonistically, to regulate leaf lamina joint development and rice architecture, by modulating expansion and elongation of adaxial parenchyma cells. Suppressed expression of miR394, or enhanced expression of LC4, results in enlarged leaf angles, whereas reducing LC4 expression by CRISPR/Cas9 leads to reduced leaf inclination, suggesting LC4 as candidate for use in rice architecture improvement. LC4 interacts with SKP1, a component of the SCF E3 ubiquitin ligase complex, and transcription of both miR394 and LC4 are regulated by auxin. Rice plants with altered expression of miR394 or LC4 have altered auxin responses, indicating that the miR394-LC4 module mediates auxin effects important for determining rice leaf inclination and architecture.

Overexpression of SoCYP85A1 Increases the Accumulation of Castasterone and Confers Enhanced Black Shank Tolerance in Tobacco Through Modulation of the Antioxidant Enzymes' Activities

Black shank caused by Phytophthora nicotianae is one of the most devastating diseases in tobacco production. In this study, we characterized a novel cytochromic resistance gene, SoCYP85A1, from spinach, which was upregulated in response to P. nicotianae infection. Overexpression of SoCYP85A1 in tobacco resulted in remarkable resistance to pathogen inoculation, with diverse resistance levels in different transgenic lines. Meanwhile, a significant accumulation of castasterone (CS) was detected in transgenic plants when challenged with the pathogen. Moreover, activities of antioxidant enzymes were enhanced by SoCYP85A1 in the transgenic lines as compared to those in the wild types inoculated with P. nicotianae. In addition, the alteration of CS content resulted in interference of phytohormone homeostasis. Overall, these results demonstrate that SoCYP85A1 can participate in the defense response to P. nicotianae through the involvement of defense enzymes and by interaction with certain phytohormones. Our findings suggest that SoCYP85A1 could be used as a potential candidate gene for improving resistance to black shank disease in tobacco and other economic crops.

In-situ localization and biochemical analysis of bio-molecules reveals Pb-stress amelioration in Brassica juncea L. by co-application of 24-Epibrassinolide and Salicylic Acid

Lead (Pb) toxicity is a major environmental concern affirming the need of proper mitigation strategies. In the present work, potential of combined treatment of 24-Epibrassinolide (24-EBL) and Salicylic acid (SA) against Pb toxicity to Brassica juncea L. seedlings were evaluated. Seedlings pre-imbibed in EBL (0.1 mM) and SA (1 mM) individually and in combination, were sown in Pb supplemented petri-plates (0.25, 0.50 and 0.75 mM). Various microscopic observations and biochemical analysis were made on 10 days old seedlings of B. juncea. The toxic effects of Pb were evident with enhancement in in-situ accumulation of Pb, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), nuclear damage, membrane damage, cell death and polyamine. Furthermore, free amino acid were lowered in response to Pb toxicity. The levels of osmoprotectants including total carbohydrate, reducing sugars, trehalose, proline and glycine betaine were elevated in response to Pb treatment. Soaking treatment with combination of 24-EBL and SA led to effective amelioration of toxic effects of Pb. Reduction in Pb accumulation, reactive oxygen content (ROS), cellular damage and GSH levels were noticed in response to treatment with 24-EBL and SA individual and combined levels. The contents of free amino acid, amino acid profiling as well as in-situ localization of polyamine (spermidine) was recorded to be enhanced by co-application of 24-EBLand SA. Binary treatment of 24-EBL and SA, further elevated the content of osmoprotectants. The study revealed that co-application of combined treatment of 24-EBL and SA led to dimination of toxic effects of Pb in B. juncea seedlings.

Diffusible signal factor (DSF)-mediated quorum sensing modulates expression of diverse traits in Xanthomonas citri and responses of citrus plants to promote disease

Background

The gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide. The diffusible signal factor (DSF) - mediated quorum sensing (QS) system coordinates expression of virulence factors in plant pathogenic Xanthomonas spp. However, the regulatory effects of this system during the Xanthomonas- plant interactions remain unclear from both the pathogen and host aspects.

Results

In this study, we investigated the in planta DSF- mediated QS regulon of X. citri subsp. citri (Xac), the causal agent of citrus canker. We also characterized the transcriptional responses of citrus plants to DSF-mediated Xac infection via comparing the gene expression patterns of citrus trigged by wild type Xac strain 306 with those trigged by its DSF- deficient (∆rpfF) mutant using the dual RNA-seq approach. Comparative global transcript profiles of Xac strain 306 and the ∆rpfF mutant during host infection revealed that DSF- mediated QS specifically modulates bacterial adaptation, nutrition uptake and metabolisms, stress tolerance, virulence, and signal transduction to favor host infection. The transcriptional responses of citrus to DSF-mediated Xac infection are characterized by downregulation of photosynthesis genes and plant defense related genes, suggesting photosynthetically inactive reactions and repression of defense responses. Alterations of phytohormone metabolism and signaling pathways were also triggered by DSF-mediated Xac infection to benefit the pathogen.

Conclusions

Collectively, our findings provide new insight into the DSF- mediated QS regulation during plant-pathogen interactions and advance the understanding of traits used by Xanthomonas to promote infection on host plants.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5384-4) contains supplementary material, which is available to authorized users.