Ca2+/Calmodulin-Dependent AtSR1/CAMTA3 Plays Critical Roles in Balancing Plant Growth and Immunity

Duplicated Copy Number Variant of the Maize 9-Lipoxygenase ZmLOX5 Improves 9,10-KODA-Mediated Resistance to Fall Armyworms

Extensive genome structure variations, such as copy number variations (CNVs) and presence/absence variations, are the basis for the remarkable genetic diversity of maize; however, the effect of CNVs on maize herbivory defense remains largely underexplored. Here, we report that the naturally occurring duplication of the maize 9-lipoxygenase gene ZmLOX5 leads to increased resistance of maize to herbivory by fall armyworms (FAWs). Previously, we showed that ZmLOX5-derived oxylipins are required for defense against chewing insect herbivores and identified several inbred lines, including Yu796, that contained duplicated CNVs of ZmLOX5, referred to as Yu796-2×LOX5. To test whether introgression of the Yu796-2×LOX5 locus into a herbivore-susceptible B73 background that contains a single ZmLOX5 gene is a feasible approach to increase resistance, we generated a series of near-isogenic lines that contained either two, one, or zero copies of the Yu796-2×LOX5 locus in the B73 background via six backcrosses (BC6). Droplet digital PCR (ddPCR) confirmed the successful introgression of the Yu796-2×LOX5 locus in B73. The resulting B73-2×LOX5 inbred line displayed increased resistance against FAW, associated with increased expression of ZmLOX5, increased wound-induced production of its primary oxylipin product, the α-ketol, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA), and the downstream defense hormones regulated by this molecule, 12-oxo-phytodienoic acid (12-OPDA) and abscisic acid (ABA). Surprisingly, wound-induced JA-Ile production was not increased in B73-2×LOX5, resulting from the increased JA catabolism. Furthermore, B73-2×LOX5 displayed reduced water loss in response to drought stress, likely due to increased ABA and 12-OPDA content. Taken together, this study revealed that the duplicated CNV of ZmLOX5 quantitively contributes to maize antiherbivore defense and presents proof-of-concept evidence that the introgression of naturally occurring duplicated CNVs of a defensive gene into productive but susceptible crop varieties is a feasible breeding approach for enhancing plant resistance to herbivory and tolerance to abiotic stress.

Phytophthora sojae Effector PsCRN108 Targets CAMTA2 to Suppress HSP40 Expression and ROS Burst

Oomycete pathogens secrete numerous crinkling and necrosis proteins (CRNs) to manipulate plant immunity and promote infection. However, the functional mechanism of CRN effectors is still poorly understood. Previous research has shown that the Phytophthora sojae effector PsCRN108 binds to the promoter of HSP90s and inhibits their expression, resulting in impaired plant immunity. In this study, we found that in addition to HSP90, PsCRN108 also suppressed other Heat Shock Protein (HSP) family genes, including HSP40. Interestingly, PsCRN108 inhibited the expression of NbHSP40 through its promoter, but did not directly bind to its promoter. Instead, PsCRN108 interacted with NbCAMTA2, a negative regulator of plant immunity. NbCAMTA2 was a negative regulator of NbHSP40 expression, and PsCRN108 could promote such inhibition activity of NbCAMTA2. Our results elucidated the multiple roles of PsCRN108 in the suppression of plant immunity and revealed a new mechanism by which the CRN effector hijacked transcription factors to affect immunity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

DNA-Binding Activity of CAMTA3 Is Essential for Its Function: Identification of Critical Amino Acids for Its Transcriptional Activity

Calmodulin-binding transcription activators (CAMTAs), a small family of highly conserved transcription factors, function in calcium-mediated signaling pathways. Of the six CAMTAs in Arabidopsis, CAMTA3 regulates diverse biotic and abiotic stress responses. A recent study has shown that CAMTA3 is a guardee of NLRs (Nucleotide-binding, Leucine-rich repeat Receptors) in modulating plant immunity, raising the possibility that CAMTA3 transcriptional activity is dispensable for its function. Here, we show that the DNA-binding activity of CAMTA3 is essential for its role in mediating plant immune responses. Analysis of the DNA-binding (CG-1) domain of CAMTAs in plants and animals showed strong conservation of several amino acids. We mutated six conserved amino acids in the CG-1 domain to investigate their role in CAMTA3 function. Electrophoretic mobility shift assays using these mutants with a promoter of its target gene identified critical amino acid residues necessary for DNA-binding activity. In addition, transient assays showed that these residues are essential for the CAMTA3 function in activating the Rapid Stress Response Element (RSRE)-driven reporter gene expression. In line with this, transgenic lines expressing the CG-1 mutants of CAMTA3 in the camta3 mutant failed to rescue the mutant phenotype and restore the expression of CAMTA3 downstream target genes. Collectively, our results provide biochemical and genetic evidence that the transcriptional activity of CAMTA3 is indispensable for its function.

DORN1 Is Involved in Drought Stress Tolerance through a Ca2+-Dependent Pathway

Water shortages caused by climate change seriously threaten the survival and production of plants and are also one of the major environmental pressures faced by plants. DORN1 was the first identified purinoceptor for the plant response to extracellular ATP. It has been established that DORN1 could play key roles in a series of biological activities in plants. However, the biological roles of DORN1 and the mechanism remain unclear under drought stress conditions in plants. Here, DORN1 was targeted for knockout by using the CRISPR/Cas 9 system. It was found that the loss function of DORN1 resulted in a significant decrease in the effective quantum yield of PSII [Y(II)], the photochemical quenching coefficient (qP), and the rate of photosynthetic electron transport through PSII (ETR), which reflected plants' photochemical efficiency. Whereas Y_(NO) values showed obvious enhancement under drought stress conditions. Further experimental results showed that the Y_(II), q_P, and ETR, which reflect plants' photochemical efficiency, increased significantly with CaCl₂ treatment. These results indicated that the drought tolerance of the mutant was decreased, and the exogenous application of calcium ions could effectively promote the drought tolerance of the dorn1 mutant. Transpiration loss controlled by stomata is closely related to drought tolerance, further, we examined the transpirational water loss in dorn1 and found that it was greater than wild-type (WT). Besides, the dorn1 mutant's stomatal aperture significantly increased compared with the WT and the stomata of dorn1 mutant plants tend to close after CaCl₂ treatment. Taken together, our results show that DORN1 plays a key role in drought stress tolerance in plants, which may depend on calcium and calcium-related signaling pathways.

Physiology and Molecular Breeding in Sustaining Wheat Grain Setting and Quality under Spring Cold Stress

Spring cold stress (SCS) compromises the reproductive growth of wheat, being a major constraint in achieving high grain yield and quality in winter wheat. To sustain wheat productivity in SCS conditions, breeding cultivars conferring cold tolerance is key. In this review, we examine how grain setting and quality traits are affected by SCS, which may occur at the pre-anthesis stage. We have investigated the physiological and molecular mechanisms involved in floret and spikelet SCS tolerance. It includes the protective enzymes scavenging reactive oxygen species (ROS), hormonal adjustment, and carbohydrate metabolism. Lastly, we explored quantitative trait loci (QTLs) that regulate SCS for identifying candidate genes for breeding. The existing cultivars for SCS tolerance were primarily bred on agronomic and morphophysiological traits and lacked in molecular investigations. Therefore, breeding novel wheat cultivars based on QTLs and associated genes underlying the fundamental resistance mechanism is urgently needed to sustain grain setting and quality under SCS.

Calcium/calmodulin-mediated microbial symbiotic interactions in plants

Cytoplasmic calcium (Ca²⁺) transients and nuclear Ca²⁺ oscillations act as hubs during root nodulation and arbuscular mycorrhizal symbioses. Plants perceive bacterial Nod factors or fungal signals to induce the Ca²⁺ oscillation in the nucleus of root hair cells, and subsequently activate calmodulin (CaM) and Ca²⁺/CaM-dependent protein kinase (CCaMK). Ca²⁺ and CaM-bound CCaMK phosphorylate transcription factors then initiate down-stream signaling events. In addition, distinct Ca²⁺ signatures are activated at different symbiotic stages: microbial colonization and infection; nodule formation; and mycorrhizal development. Ca²⁺ acts as a key signal that regulates a complex interplay of downstream responses in many biological processes. This short review focuses on advances in Ca²⁺ signaling-regulated symbiotic events. It is meant to be an introduction to readers in and outside the field of bacterial and fungal symbioses. We summarize the molecular mechanisms underlying Ca²⁺/CaM-mediated signaling in fine-tuning both local and systemic symbiotic events.

Functional Characterization of the M36 Metalloprotease FgFly1 in Fusarium graminearum

Fungalysin metallopeptidase (M36), a hydrolase, catalyzes the hydrolysis of alanine, glycine, etc. Normally, it is considered to play an important role in the progress of fungal infection. However, the function of fungalysin metallopeptidase (M36) in Fusarium graminearum has not been reported. In this study, we explored the biological functions of FgFly1, a fungalysin metallopeptidase (M36) of F. graminearum. We found that ΔFgFly1 did not affect the ability to produce DON toxin, although it inhibited spore germination during asexual reproduction and reduction in pathogenicity compared with PH-1. Therefore, we speculated that FgFly1 affects the pathogenicity of F.graminearum by affecting pathways related to wheat disease resistance. Target protein TaCAMTA (calmodulin-binding transcription activator) was selected by a yeast two-hybrid (Y2H) system. Then, the interaction between FgFly1 and TaCAMTA was verified by bimolecular fluorescent complimentary (BiFC) and luciferase complementation assay (LCA). Furthermore, compared with wild-type Arabidopsis thaliana, the morbidity level of ΔAtCAMTA was increased after infection with F.graminearum, and the expression level of NPR1 was significantly reduced. Based on the above results, we concluded that FgFly1 regulated F. graminearum pathogenicity by interacting with host cell CAMTA protein.

Genome-Wide Identification and Characterization of the Calmodulin-Binding Transcription Activator (CAMTA) Gene Family in Plants and the Expression Pattern Analysis of CAMTA3/SR1 in Tomato under Abiotic Stress

Calmodulin-binding transcription activator (CAMTA) plays an important regulatory role in plant growth, development, and stress response. This study identified the phylogenetic relationships of the CAMTA family in 42 plant species using a genome-wide search approach. Subsequently, the evolutionary relationships, gene structures, and conservative structural domain of CAMTA3/SR1 in different plants were analyzed. Meanwhile, in the promoter region, the cis-acting elements, protein clustering interaction, and tissue-specific expression of CAMTA3/SR1 in tomato were identified. The results show that SlCAMTA3/SR1 genes possess numerous cis-acting elements related to hormones, light response, and stress in the promoter regions. SlCAMTA3 might act together with other Ca²⁺ signaling components to regulate Ca²⁺-related biological processes. Then, the expression pattern of SlCAMTA3/SR1 was also investigated by quantitative real-time PCR (qRT-PCR) analysis. The results show that SlCAMTA3/SR1 might respond positively to various abiotic stresses, especially Cd stress. The expression of SlCAMTA3/SR1 was scarcely detected in tomato leaf at the seedling and flowering stages, whereas SlCAMTA3/SR1 was highly expressed in the root at the seedling stage. In addition, SlCAMTA3/SR1 had the highest expression levels in flowers at the reproductive stage. Here, we provide a basic reference for further studies about the functions of CAMTA3/SR1 proteins in plants.

Arabidopsis CAMTA3/SR1 is involved in drought stress tolerance and ABA signaling

Calcium/calmodulin signals are important for various cellular and physiological activities in plants. Calmodulin binding transcription activators also named Signal Responsive (SR) proteins belong to an important calcium/calmodulin-dependent transcription factor family that plays critical roles in stress responses. However, the role of SRs in abscisic acid (ABA) regulated plant responses to drought stress is largely unknown. Here, we characterized the role of Arabidopsis SR1 in drought stress tolerance and ABA response by analyzing the phenotypes of SR1 knockout and SR1-overexpression plants. sr1 mutants which accumulate salicylic acid (SA) were found more sensitive to drought stress and showed a higher water loss rate as compared with wild-type. By contrast, SR1-overexpression lines exhibited increased drought tolerance and less water loss than wild-type. Furthermore, sr1 mutants showed reduced ABA response in seed germination, root elongation, and stomatal closure, while SR1-overexpression lines displayed more sensitive to ABA than wild-type. In addition, the drought-sensitive and ABA-insensitive phenotypes of sr1 mutants were recovered by diminishing SA accumulation via knockouts of SA synthesizer ICS1 or activator PAD4, or through expression of SA-degrading enzyme NahG. Some drought/ABA-responsive genes exhibited differentially expressed in sr1 mutants and SR1-overexpression plants. These results suggest that SR1 plays a positive role in drought stress tolerance and ABA response, and drought/ABA responses are antagonized by SA accumulation that is negatively regulated by SR1.

Heterologous Expression of Arabidopsis AtARA6 in Soybean Enhances Salt Tolerance

Salt damage is an important abiotic stress affecting the agronomic traits of soybean. Soybeans rapidly sense and transmit adverse signals when salt-damaged, inducing a set of response mechanisms to resist salt stress. AtARA6 encodes a small GTPase, which plays an important role in Arabidopsis vesicle transport and salt tolerance. In this study, we transformed the Arabidopsis gene AtARA6 into the cultivated soybean Shen Nong 9 (SN9). To investigate the salt tolerance pathways affected by AtARA6 in soybean, we performed transcriptome sequencing using transgenic soybean and wild-type (SN9) under salt treatment and water treatment. Our results suggest that AtARA6 is involved in the regulation of soybean SNARE complexes in the vesicle transport pathway, which may directly strengthen salt tolerance. In addition, we comprehensively analyzed the RNA-seq data of transgenic soybean and SN9 under different treatments and obtained 935 DEGs. GO analysis showed that these DEGs were significantly enriched in transcription factor activity, sequence-specific DNA binding, and the inositol catabolic process. Three salt-responsive negative regulator transcription factors, namely MYC2, WRKY6, and WRKY86, were found to be significantly downregulated after salt treatment in transgenic soybeans. Moreover, four genes encoding inositol oxygenase were significantly enriched in the inositol catabolic process pathway, which could improve the salt tolerance of transgenic soybeans by reducing their reactive oxygen species content. These are unique salt tolerance effects produced by transgenic soybeans. Our results provide basic insights into the function of AtARA6 in soybeans and its role in abiotic stress processes in plants.

ORA47 is a transcriptional regulator of a general stress response hub

Transcriptional regulators of the general stress response (GSR) reprogram the expression of selected genes to transduce informational signals into cellular events, ultimately manifested in a plant's ability to cope with environmental challenges. Identification of the core GSR regulatory proteins will uncover the principal modules and their mode of action in the establishment of adaptive responses. To define the GSR regulatory components, we employed a yeast-one-hybrid assay to identify the protein(s) binding to the previously established functional GSR motif, termed the rapid stress response element (RSRE). This led to the isolation of octadecanoid-responsive AP2/ERF-domain transcription factor 47 (ORA47), a methyl jasmonate inducible protein. Subsequently, ORA47 transcriptional activity was confirmed using the RSRE-driven luciferase (LUC) activity assay performed in the ORA47 loss- and gain-of-function lines introgressed into the 4xRSRE::Luc background. In addition, the prime contribution of CALMODULIN-BINDING TRANSCRIPTIONAL ACTIVATOR3 (CAMTA3) protein in the induction of RSRE was reaffirmed by genetic studies. Moreover, exogenous application of methyl jasmonate led to enhanced levels of ORA47 and CAMTA3 transcripts, as well as the induction of RSRE::LUC activity. Metabolic analyses illustrated the reciprocal functional inputs of ORA47 and CAMTA3 in increasing JA levels. Lastly, transient assays identified JASMONATE ZIM-domain1 (JAZ1) as a repressor of RSRE::LUC activity. Collectively, the present study provides fresh insight into the initial features of the mechanism that transduces informational signals into adaptive responses. This mechanism involves the functional interplay between the JA biosynthesis/signaling cascade and the transcriptional reprogramming that potentiates GSR. Furthermore, these findings offer a window into the role of intraorganellar communication in the establishment of adaptive responses.

Interplay between Ca2+/Calmodulin-Mediated Signaling and AtSR1/CAMTA3 during Increased Temperature Resulting in Compromised Immune Response in Plants

Changing temperatures are known to affect plant–microbe interactions; however, the molecular mechanism involved in plant disease resistance is not well understood. Here, we report the effects of a moderate change in temperature on plant immune response through Ca²⁺/calmodulin-mediated signaling. At 30 °C, Pst DC3000 triggered significantly weak and relatively slow Ca²⁺ influx in plant cells, as compared to that at 18 °C. Increased temperature contributed to an enhanced disease susceptibility in plants; the enhanced disease susceptibility is the result of the compromised stomatal closure induced by pathogens at high temperature. A Ca²⁺ receptor, AtSR1, contributes to the decreased plant immunity at high temperatures and the calmodulin-binding domain (CaMBD) is required for its function. Furthermore, both salicylic acid biosynthesis (ICS) and salicylic acid receptor (NPR1) are involved in this process. In addition to stomatal control, AtSR1 is involved in high temperature-compromised apoplastic immune response through the salicylic acid signaling pathway. The qRT-PCR data revealed that AtSR1 contributed to increased temperatures-mediated susceptible immune response by regulating SA-related genes in atsr1, such as PR1, ICS1, NPR1, as well as EDS1. Our results indicate that Ca²⁺ signaling has broad effects on the molecular interplay between changing temperatures as well as plant defense during plant–pathogen interactions.

Calcium/Calmodulin-Mediated Defense Signaling: What Is Looming on the Horizon for AtSR1/CAMTA3-Mediated Signaling in Plant Immunity

Calcium (Ca2+) signaling in plant cells is an essential and early event during plant-microbe interactions. The recognition of microbe-derived molecules activates Ca2+ channels or Ca2+ pumps that trigger a transient increase in Ca2+ in the cytoplasm. The Ca2+ binding proteins (such as CBL, CPK, CaM, and CML), known as Ca2+ sensors, relay the Ca2+ signal into down-stream signaling events, e.g., activating transcription factors in the nucleus. For example, CaM and CML decode the Ca2+ signals to the CaM/CML-binding protein, especially CaM-binding transcription factors (AtSRs/CAMTAs), to induce the expressions of immune-related genes. In this review, we discuss the recent breakthroughs in down-stream Ca2+ signaling as a dynamic process, subjected to continuous variation and gradual change. AtSR1/CAMTA3 is a CaM-mediated transcription factor that represses plant immunity in non-stressful environments. Stress-triggered Ca2+ spikes impact the Ca2+-CaM-AtSR1 complex to control plant immune response. We also discuss other regulatory mechanisms in which Ca2+ signaling activates CPKs and MAPKs cascades followed by regulating the function of AtSR1 by changing its stability, phosphorylation status, and subcellular localization during plant defense.

Genomic and Transcriptomic Analysis Provide Insights Into Root Rot Resistance in Panax notoginseng

Panax notoginseng (Panax notoginseng (Burk.) F.H. Chen), a plant of high medicinal value, is severely affected by root rot during cultivation. Here, we generated a reference genome of P. notoginseng, with a contig N50 size of 241.268 kb, and identified 66 disease-resistance genes (R-genes) as candidate genes for breeding disease-resistant varieties. We then investigated the molecular mechanism underlying the responses of resistant and susceptible P. notoginseng genotypes to Fusarium oxysporum infection at six time points by RNA-seq. Functional analysis of the genes differentially expressed between the two genotypes indicated that genes involved in the defense response biological process like hormone transduction and plant-pathogen interaction are continuously and highly expressed in resistant genotype during infection. Moreover, salicylic acid and jasmonic acid levels gradually increased during infection in the resistant genotype. Coexpression analysis showed that PnWRKY22 acts as a hub gene in the defense response of the resistant genotype. Finally, transiently overexpressing PnWRKY22 increased salicylic acid levels in P. notoginseng leaves. Our findings provide a theoretical basis for studying root rot resistance in P. notoginseng.

Chromatin accessibility landscapes activated by cell-surface and intracellular immune receptors

Activation of cell-surface and intracellular receptor-mediated immunity results in rapid transcriptional reprogramming that underpins disease resistance. However, the mechanisms by which co-activation of both immune systems lead to transcriptional changes are not clear. Here, we combine RNA-seq and ATAC-seq to define changes in gene expression and chromatin accessibility. Activation of cell-surface or intracellular receptor-mediated immunity, or both, increases chromatin accessibility at induced defence genes. Analysis of ATAC-seq and RNA-seq data combined with publicly available information on transcription factor DNA-binding motifs enabled comparison of individual gene regulatory networks activated by cell-surface or intracellular receptor-mediated immunity, or by both. These results and analyses reveal overlapping and conserved transcriptional regulatory mechanisms between the two immune systems.

Calmodulin-binding transcription activator AtSR1/CAMTA3 fine-tunes plant immune response by transcriptional regulation of the salicylate receptor NPR1

Calcium (Ca²⁺ ) signalling regulates salicylic acid (SA)-mediated immune response through calmodulin-meditated transcriptional activators, AtSRs/CAMTAs, but its mechanism is not fully understood. Here, we report an AtSR1/CAMTA3-mediated regulatory mechanism involving the expression of the SA receptor, NPR1. Results indicate that the transcriptional expression of NPR1 was regulated by AtSR1 binding to a CGCG box in the NPR1 promotor. The atsr1 mutant exhibited resistance to the virulent strain of Pseudomonas syringae pv. tomato (Pst), however, was susceptible to an avirulent Pst strain carrying avrRpt2, due to the failure of the induction of hypersensitive responses. These resistant/susceptible phenotypes in the atsr1 mutant were reversed in the npr1 mutant background, suggesting that AtSR1 regulates NPR1 as a downstream target during plant immune response. The virulent Pst strain triggered a transient elevation in intracellular Ca²⁺ concentration, whereas the avirulent Pst strain triggered a prolonged change. The distinct Ca²⁺ signatures were decoded into the regulation of NPR1 expression through AtSR1's IQ motif binding with Ca²⁺ -free-CaM2, while AtSR1's calmodulin-binding domain with Ca²⁺ -bound-CaM2. These observations reveal a role for AtSR1 as a Ca²⁺ -mediated transcription regulator in controlling the NPR1-mediated plant immune response.

The transcriptional landscape of Arabidopsis thaliana pattern-triggered immunity

Plants tailor their metabolism to environmental conditions, in part through the recognition of a wide array of self and non-self molecules. In particular, the perception of microbial or plant-derived molecular patterns by cell-surface-localized pattern recognition receptors (PRRs) induces pattern-triggered immunity, which includes massive transcriptional reprogramming1. An increasing number of plant PRRs and corresponding ligands are known, but whether plants tune their immune outputs to patterns of different biological origins or of different biochemical natures remains mostly unclear. Here, we performed a detailed transcriptomic analysis in an early time series focused to study rapid-signalling transcriptional outputs induced by well-characterized patterns in the model plant Arabidopsis thaliana. This revealed that the transcriptional responses to diverse patterns (independent of their origin, biochemical nature or type of PRR) are remarkably congruent. Moreover, many of the genes most rapidly and commonly upregulated by patterns are also induced by abiotic stresses, suggesting that the early transcriptional response to patterns is part of the plant general stress response (GSR). As such, plant cells' response is in the first instance mostly to danger. Notably, the genetic impairment of the GSR reduces pattern-induced antibacterial immunity, confirming the biological relevance of this initial danger response. Importantly, the definition of a small subset of 'core immunity response' genes common and specific to pattern response revealed the function of previously uncharacterized GLUTAMATE RECEPTOR-LIKE (GLR) calcium-permeable channels in immunity. This study thus illustrates general and unique properties of early immune transcriptional reprogramming and uncovers important components of plant immunity.

EgJUB1 and EgERF113 transcription factors as potential master regulators of defense response in Elaeis guineensis against the hemibiotrophic Ganoderma boninense

BACKGROUND

Hemibiotrophic pathogen such as the fungal pathogen Ganoderma boninense that is destructive to oil palm, manipulates host defense mechanism by strategically switching from biotrophic to necrotrophic phase. Our previous study revealed two distinguishable expression profiles of oil palm genes that formed the basis in deducing biotrophic phase at early interaction which switched to necrotrophic phase at a later stage of infection.

RESULTS

The present report is a continuing study from our previous published transcriptomic profiling of oil palm seedlings against G. boninense. We focused on identifying differentially expressed genes (DEGs) encoding transcription factors (TFs) from the same RNA-seq data; resulting in 106 upregulated and 108 downregulated TFs being identified. The DEGs are involved in four established defense-related pathways responsible for cell wall modification, reactive oxygen species (ROS)-mediated signaling, programmed cell death (PCD) and plant innate immunity. We discovered upregulation of JUNGBRUNNEN 1 (EgJUB1) during the fungal biotrophic phase while Ethylene Responsive Factor 113 (EgERF113) demonstrated prominent upregulation when the palm switches to defense against necrotrophic phase. EgJUB1 was shown to have a binding activity to a 19 bp palindromic SNBE1 element, WNNYBTNNNNNNNAMGNHW found in the promoter region of co-expressing EgHSFC-2b. Further in silico analysis of promoter regions revealed co-expression of EgJUB1 with TFs containing SNBE1 element with single nucleotide change at either the 5th or 18th position. Meanwhile, EgERF113 binds to both GCC and DRE/CRT elements promoting plasticity in upregulating the downstream defense-related genes. Both TFs were proven to be nuclear-localized based on subcellular localization experiment using onion epidermal cells.

CONCLUSION

Our findings demonstrated unprecedented transcriptional reprogramming of specific TFs potentially to enable regulation of a specific set of genes during different infection phases of this hemibiotrophic fungal pathogen. The results propose the intricacy of oil palm defense response in orchestrating EgJUB1 during biotrophic and EgERF113 during the subsequent transition to the necrotrophic phase. Binding of EgJUB1 to SNBE motif instead of NACBS while EgERF113 to GCC-box and DRE/CRT motifs is unconventional and not normally associated with pathogen infection. Identification of these phase-specific oil palm TFs is important in designing strategies to tackle or attenuate the progress of infection.

Signal Integration by Cyclin-Dependent Kinase 8 (CDK8) Module and Other Mediator Subunits in Biotic and Abiotic Stress Responses

Environmental stresses have driven plants to develop various mechanisms to acclimate in adverse conditions. Extensive studies have demonstrated that a significant reprogramming occurs in the plant transcriptome in response to biotic and abiotic stresses. The highly conserved and large multi-subunit transcriptional co-activator of eukaryotes, known as the Mediator, has been reported to play a substantial role in the regulation of important genes that help plants respond to environmental perturbances. CDK8 module is a relatively new component of the Mediator complex that has been shown to contribute to plants' defense, development, and stress responses. Previous studies reported that CDK8 module predominantly acts as a transcriptional repressor in eukaryotic cells by reversibly associating with core Mediator. However, growing evidence has demonstrated that depending on the type of biotic and abiotic stress, the CDK8 module may perform a contrasting regulatory role. This review will summarize the current knowledge of CDK8 module as well as other previously documented Mediator subunits in plant cell signaling under stress conditions.

Ca2+/Calmodulin Complex Triggers CAMTA Transcriptional Machinery Under Stress in Plants: Signaling Cascade and Molecular Regulation

Calcium (Ca2+) ion is a critical ubiquitous intracellular second messenger, acting as a lead currency for several distinct signal transduction pathways. Transient perturbations in free cytosolic Ca2+ ([Ca2+]cyt) concentrations are indispensable for the translation of signals into adaptive biological responses. The transient increase in [Ca2+]cyt levels is sensed by an array of Ca2+ sensor relay proteins such as calmodulin (CaM), eventually leading to conformational changes and activation of CaM. CaM, in a Ca2+-dependent manner, regulates several transcription factors (TFs) that are implicated in various molecular, physiological, and biochemical functions in cells. CAMTA (calmodulin-binding transcription activator) is one such member of the Ca2+-loaded CaM-dependent family of TFs. The present review focuses on Ca2+ as a second messenger, its interaction with CaM, and Ca2+/CaM-mediated CAMTA transcriptional regulation in plants. The review recapitulates the molecular and physiological functions of CAMTA in model plants and various crops, confirming its probable involvement in stress signaling pathways and overall plant development. Studying Ca2+/CaM-mediated CAMTA TF will help in answering key questions concerning signaling cascades and molecular regulation under stress conditions and plant growth, thus improving our knowledge for crop improvement.

Distinct Molecular Pattern-Induced Calcium Signatures Lead to Different Downstream Transcriptional Regulations via AtSR1/CAMTA3

Plants encrypt the perception of different pathogenic stimuli into specific intracellular calcium (Ca²⁺) signatures and subsequently decrypt the signatures into appropriate downstream responses through various Ca²⁺ sensors. Two microbe-associated molecular patterns (MAMPs), bacterial flg22 and fungal chitin, and one damage-associated molecular pattern (DAMP), AtPep1, were used to study the differential Ca²⁺ signatures in Arabidopsis leaves. The results revealed that flg22, chitin, and AtPep1 induced distinct changes in Ca²⁺ dynamics in both the cytosol and nucleus. In addition, Flg22 and chitin upregulated the expression of salicylic acid-related genes, ICS1 and EDS1, whereas AtPep1 upregulated the expression of jasmonic acid-related genes, JAZ1 and PDF1.2, in addition to ICS1 and EDS1. These data demonstrated that distinct Ca²⁺ signatures caused by different molecular patterns in leaf cells lead to specific downstream events. Furthermore, these changes in the expression of defense-related genes were disrupted in a knockout mutant of the AtSR1/CAMTA3 gene, encoding a calmodulin-binding transcription factor, in which a calmodulin-binding domain on AtSR1 was required for deciphering the Ca²⁺ signatures into downstream transcription events. These observations extend our knowledge regarding unique and intrinsic roles for Ca²⁺ signaling in launching and fine-tuning plant immune response, which are mediated by the AtSR1/CAMTA3 transcription factor.

PI4Kγ2 Interacts with E3 Ligase MIEL1 to Regulate Auxin Metabolism and Root Development

Root development is important for normal plant growth and nutrient absorption. Studies have revealed the involvement of various factors in this complex process, improving our understanding of the relevant regulatory mechanisms. Here, we functionally characterize the role of Arabidopsis (Arabidopsis thaliana) phosphatidylinositol 4-kinase γ2 (PI4Kγ2) in root elongation regulation, which functions to modulate stability of the RING-type E3 ligase MYB30-INTERACTING E3 LIGASE1 (MIEL1) and auxin metabolism. Mutant plants deficient in PI4Kγ2 (pi4kγ2) exhibited a shortened root length and elongation zone due to reduced auxin level. PI4Kγ2 was shown to interact with MIEL1, regulating its degradation and furthering the stability of transcription factor MYB30 (which suppresses auxin metabolism by directly binding to promoter regions of GH3 2 and GH3 6). Interestingly, pi4kγ2 plants presented altered hypersensitive response, indicating that PI4Kγ2 regulates synergetic growth and defense of plants through modulating auxin metabolism. These results reveal the importance of protein interaction in regulating ubiquitin-mediated protein degradation in eukaryotic cells, and illustrate a mechanism coordinating plant growth and biotic stress response.

Phenazine-Producing Rhizobacteria Promote Plant Growth and Reduce Redox and Osmotic Stress in Wheat Seedlings Under Saline Conditions

Application of plant growth promoting bacteria may induce plant salt stress tolerance, however the underpinning microbial and plant mechanisms remain poorly understood. In the present study, the specific role of phenazine production by rhizosphere-colonizing Pseudomonas in mediating the inhibitory effects of salinity on wheat seed germination and seedling growth in four different varieties was investigated using Pseudomonas chlororaphis 30-84 (wild type) and isogenic derivatives deficient or enhanced in phenazine production. The results showed that varieties differed in how they responded to the salt stress treatment and the benefits derived from colonization by P. chlororaphis 30-84. In all varieties, the salt stress treatment significantly reduced seed germination, and in seedlings, reduced relative water content, increased reactive oxygen species (ROS) levels in leaves, and in three of four varieties, reduced shoot and root production compared to the no salt stress treatment. Inoculation of seeds with Pseudomonas chlororaphis 30-84 wild type or derivatives promoted salt-stress tolerance in seedlings of the four commercial winter wheat varieties tested, but the salt-stress tolerance phenotype was not entirely due to phenazine production. For example, all P. chlororaphis derivatives (including the phenazine-producing mutant) significantly improved relative water content in two varieties, Iba and CV 1, for which the salt stress treatment had a large impact. Importantly, all P. chlororaphis derivatives enabled the salt inhibited wheat varieties studied to maintain above ground productivity in saline conditions. However, only phenazine-producing derivatives enhanced the shoot or root growth of seedlings of all varieties under nonsaline conditions. Notably, ROS accumulation was reduced, and antioxidant enzyme (catalase) activity enhanced in the leaves of seedlings grown in saline conditions that were seed-treated with phenazine-producing P. chlororaphis derivatives as compared to noninoculated seedlings. The results demonstrate the capacity of P. chlororaphis to improve salt tolerance in wheat seedlings by promoting plant growth and reducing osmotic stress and a role for bacterial phenazine production in reducing redox stress.

Downy Mildew effector HaRxL21 interacts with the transcriptional repressor TOPLESS to promote pathogen susceptibility

Hyaloperonospora arabidopsidis (Hpa) is an oomycete pathogen causing Arabidopsis downy mildew. Effector proteins secreted from the pathogen into the plant play key roles in promoting infection by suppressing plant immunity and manipulating the host to the pathogen's advantage. One class of oomycete effectors share a conserved 'RxLR' motif critical for their translocation into the host cell. Here we characterize the interaction between an RxLR effector, HaRxL21 (RxL21), and the Arabidopsis transcriptional co-repressor Topless (TPL). We establish that RxL21 and TPL interact via an EAR motif at the C-terminus of the effector, mimicking the host plant mechanism for recruiting TPL to sites of transcriptional repression. We show that this motif, and hence interaction with TPL, is necessary for the virulence function of the effector. Furthermore, we provide evidence that RxL21 uses the interaction with TPL, and its close relative TPL-related 1, to repress plant immunity and enhance host susceptibility to both biotrophic and necrotrophic pathogens.

Salt-Induced Stability of SR1/CAMTA3 mRNA Is Mediated by Reactive Oxygen Species and Requires the 3' End of Its Open Reading Frame

Soil salinity, a prevalent abiotic stress, causes enormous losses in global crop yields annually. Previous studies have shown that salt stress-induced reprogramming of gene expression contributes to the survival of plants under this stress. However, mechanisms regulating gene expression in response to salt stress at the posttranscriptional level are not well understood. In this study, we show that salt stress increases the level of Signal Responsive 1 (SR1) mRNA, a member of signal-responsive Ca2+/calmodulin-regulated transcription factors, by enhancing its stability. We present multiple lines of evidence indicating that reactive oxygen species generated by NADPH oxidase activity mediate salt-induced SR1 transcript stability. Using mutants impaired in either nonsense-mediated decay, XRN4 or mRNA decapping pathways, we show that neither the nonsense-mediated mRNA decay pathway, XRN4 nor the decapping of SR1 mRNA is required for its decay. We analyzed the salt-induced accumulation of eight truncated versions of the SR1 coding region (∼3 kb) in the sr1 mutant background. This analysis identified a 500-nt region at the 3' end of the SR1 coding region to be required for the salt-induced stability of SR1 mRNA. Potential mechanisms by which this region confers SR1 transcript stability in response to salt are discussed.

Calcium Signaling-Mediated Plant Response to Cold Stress

Low temperatures have adverse impacts on plant growth, developmental processes, crop productivity and food quality. It is becoming clear that Ca²⁺ signaling plays a crucial role in conferring cold tolerance in plants. However, the role of Ca²⁺ involved in cold stress response needs to be further elucidated. Recent studies have shown how the perception of cold signals regulate Ca²⁺ channels to induce Ca²⁺ transients. In addition, studies have shown how Ca²⁺ signaling and its cross-talk with nitric oxide (NO), reactive oxygen species (ROS) and mitogen-activated protein kinases (MAPKs) signaling pathways ultimately lead to establishing cold tolerance in plants. Ca²⁺ signaling also plays a key role through Ca²⁺/calmodulin-mediated Arabidopsis signal responsive 1 (AtSR1/CAMTA3) when temperatures drop rapidly. This review highlights the current status in Ca²⁺ signaling-mediated cold tolerance in plants.