An early ethylene up-regulated gene encoding a calmodulin-binding protein involved in plant senescence and death

The Vital Role of the CAMTA Gene Family in Phoebe bournei in Response to Drought, Heat, and Light Stress

The calmodulin-binding transcriptional activator (CAMTA) is a small, conserved gene family in plants that plays a crucial role in regulating growth, development, and responses to various abiotic stress. Given the significance of the CAMTA gene family, various studies have been dedicated to uncovering its functional characteristics. In this study, genome-wide identification and bioinformatics analysis were conducted to explore CAMTAs in Phoebe bournei. A total of 17 CAMTA genes, each containing at least one domain from CG-1, TIG, ANK, or IQ, were identified in the P. bournei genome. The diversity of PbCAMTAs could be varied depending on their subcellular localization. An analysis of protein motifs, domains, and gene structure revealed that members within the same subgroup exhibited similar organization, supporting the results of the phylogenetic analysis. Gene duplications occurred among members of the PbCAMTA gene family. According to the cis-regulatory element prediction and protein-protein interaction network analysis, eight genes were subjected to qRT-PCR under drought, heat, and light stresses. The expression profiles indicated that PbCAMTAs, particularly PbCAMTA2, PbCAMTA12, and PbCAMTA16, were induced by abiotic stress. This study provides profound insights into the functions of CAMTAs in P. bournei.

Calmodulin and calmodulin-like protein-mediated plant responses to biotic stresses

Plants have evolved a set of finely regulated mechanisms to respond to various biotic stresses. Transient changes in intracellular calcium (Ca2+ ) concentration have been well documented to act as cellular signals in coupling environmental stimuli to appropriate physiological responses with astonishing accuracy and specificity in plants. Calmodulins (CaMs) and calmodulin-like proteins (CMLs) are extensively characterized as important classes of Ca2+ sensors. The spatial-temporal coordination between Ca2+ transients, CaMs/CMLs and their target proteins is critical for plant responses to environmental stresses. Ca2+ -loaded CaMs/CMLs interact with and regulate a broad spectrum of target proteins, such as ion transporters (including channels, pumps, and antiporters), transcription factors, protein kinases, protein phosphatases, metabolic enzymes and proteins with unknown biological functions. This review focuses on mechanisms underlying how CaMs/CMLs are involved in the regulation of plant responses to diverse biotic stresses including pathogen infections and herbivore attacks. Recent discoveries of crucial functions of CaMs/CMLs and their target proteins in biotic stress resistance revealed through physiological, molecular, biochemical, and genetic analyses have been described, and intriguing insights into the CaM/CML-mediated regulatory network are proposed. Perspectives for future directions in understanding CaM/CML-mediated signalling pathways in plant responses to biotic stresses are discussed. The application of accumulated knowledge of CaM/CML-mediated signalling in biotic stress responses into crop cultivation would improve crop resistance to various biotic stresses and safeguard our food production in the future.

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.

Comprehensive identification and expression analysis of CAMTA gene family in Phyllostachys edulis under abiotic stress

Background

Calmodulin-binding transcription factor (CAMTA) is a major transcription factor regulated by calmodulin (CaM) that plays an essential role in plant growth, development and response to biotic and abiotic stresses. The CAMTA gene family has been identified in Arabidopsis thaliana, rice (Oryza sativa) and other model plants, and its gene function in moso bamboo (Phyllostachys edulis) has not been identified.

Results

In this study, a total of 11 CAMTA genes were identified in P. edulis genome. Conserved domain and multiplex sequence alignment analysis showed that the structure between these genes was highly similar, with all members having CG-1 domains and some members having TIG and IQ domains. Phylogenetic relationship analysis showed that the CAMTA genes were divided into five subfamilies, and gene fragment replication promoted the evolution of this gene family. Promoter analysis revealed a large number of drought stress-related cis-acting elements in PeCAMTAs, and similarly high expression of the CAMTA gene family was found in drought stress response experiments, indicating the involvement of this gene family in drought stress. Gene expression pattern according to transcriptome data revealed participation of the PeCAMTA genes in tissue development.

Conclusions

Our results present new findings for the P. edulis CAMTA gene family and provide partial experimental evidence for further validation of the function of PeCAMTAs.

Calcium decoders and their targets: The holy alliance that regulate cellular responses in stress signaling

Calcium (Ca²⁺) signaling is versatile communication network in the cell. Stimuli perceived by cells are transposed through Ca²⁺-signature, and are decoded by plethora of Ca²⁺ sensors present in the cell. Calmodulin, calmodulin-like proteins, Ca²⁺-dependent protein kinases and calcineurin B-like proteins are major classes of proteins that decode the Ca²⁺ signature and serve in the propagation of signals to different parts of cells by targeting downstream proteins. These decoders and their targets work together to elicit responses against diverse stress stimuli. Over a period of time, significant attempts have been made to characterize as well as summarize elements of this signaling machinery. We begin with a structural overview and amalgamate the newly identified Ca²⁺ sensor protein in plants. Their ability to bind Ca²⁺, undergo conformational changes, and how it facilitates binding to a wide variety of targets is further embedded. Subsequently, we summarize the recent progress made on the functional characterization of Ca²⁺ sensing machinery and in particular their target proteins in stress signaling. We have focused on the physiological role of Ca²⁺, the Ca²⁺ sensing machinery, and the mode of regulation on their target proteins during plant stress adaptation. Additionally, we also discuss the role of these decoders and their mode of regulation on the target proteins during abiotic, hormone signaling and biotic stress responses in plants. Finally, here, we have enumerated the limitations and challenges in the Ca²⁺ signaling. This article will greatly enable in understanding the current picture of plant response and adaptation during diverse stimuli through the lens of Ca²⁺ signaling.

The Role of Calmodulin Binding Transcription Activator in Plants under Different Stressors: Physiological, Biochemical, Molecular Mechanisms of Camellia sinensis and Its Current Progress of CAMTAs

Low temperatures have a negative effect on plant development. Plants that are exposed to cold temperatures undergo a cascade of physiological, biochemical, and molecular changes that activate several genes, transcription factors, and regulatory pathways. In this review, the physiological, biochemical, and molecular mechanisms of Camellia sinensis have been discussed. Calmodulin binding transcription activator (CAMTAs) by molecular means including transcription is one of the novel genes for plants' adaptation to different abiotic stresses, including low temperatures. Therefore, the role of CAMTAs in different plants has been discussed. The number of CAMTAs genes discussed here are playing a significant role in plants' adaptation to abiotic stress. The illustrated diagrams representing the mode of action of calcium (Ca2+) with CAMTAs have also been discussed. In short, Ca2+ channels or Ca2+ pumps trigger and induce the Ca2+ signatures in plant cells during abiotic stressors, including low temperatures. Ca2+ signatures act with CAMTAs in plant cells and are ultimately decoded by Ca2+sensors. To the best of our knowledge, this is the first review reporting CAMAT's current progress and potential role in C. sinensis, and this study opens a new road for researchers adapting tea plants to abiotic stress.

Genome-wide identification and expression analysis of the calmodulin-binding transcription activator (CAMTA) family genes in tea plant

Background

As a type of calmodulin binding protein, CAMTAs are widely involved in vegetative and reproductive processes as well as various hormonal and stress responses in plants. To study the functions of CAMTA genes in tea plants, we investigated bioinformatics analysis and performed qRT-PCR analysis of the CAMTA gene family by using the genomes of ‘ShuChaZao’ tea plant cultivar.

Results

In this study, 6 CsCAMTAs were identified from tea plant genome. Bioinformatics analysis results showed that all CsCAMTAs contained six highly conserved functional domains. Tissue-specific analysis results found that CsCAMTAs played great roles in mediating tea plant aging and flowering periods. Under hormone and abiotic stress conditions, most CsCAMTAs were upregulated at different time points under different treatment conditions. In addition, the expression levels of CsCAMTA1/3/4/6 were higher in cold-resistant cultivar ‘LongJing43’ than in the cold-susceptible cultivar ‘DaMianBai’ at cold acclimation stage, while CsCAMTA2/5 showed higher expression levels in ‘DaMianBai’ than in ‘LongJing43’ during entire cold acclimation periods.

Conclusions

In brief, the present results revealed that CsCAMTAs played great roles in tea plant growth, development and stress responses, which laid the foundation for deeply exploring their molecular regulation mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08894-x.

Genome-Wide Analysis of Calmodulin Binding Transcription Activator (CAMTA) Gene Family in Peach (Prunus persica L. Batsch) and Ectopic Expression of PpCAMTA1 in Arabidopsis camta2,3 Mutant Restore Plant Development

Calmodulin-binding transcription activator (CAMTA) is a transcription factor family containing calmodulin (CaM) binding sites and is involved in plant development. Although CAMTAs in Arabidopsis have been extensively investigated, the functions of CAMTAs remain largely unclear in peaches. In this study, we identified five peach CAMTAs which contained conserved CG-1 box, ANK repeats, CaM binding domain (CaMBD) and IQ motifs. Overexpression in tobacco showed that PpCAMTA1/2/3 were located in the nucleus, while PpCAMTA4 and PpCAMTA5 were located in the plasma membrane. Increased expression levels were observed for PpCAMTA1 and PpCAMTA3 during peach fruit ripening. Expression of PpCAMTA1 was induced by cold treatment and was inhibited by ultraviolet B irradiation (UV-B). Driven by AtCAMTA3 promoter, PpCAMTA1/2/3 were overexpressed in Arabidopsis mutant. Here, we characterized peach PpCAMTA1, representing an ortholog of AtCAMTA3. PpCAMTA1 expression in Arabidopsis complements the developmental deficiencies of the camta2,3 mutant, and restored the plant size to the wild type level. Moreover, overexpressing PpCAMTA1 in camta2,3 mutant inhibited salicylic acid (SA) biosynthesis and expression of SA-related genes, resulting in a susceptibility phenotype to Pst DC3000. Taken together, our results provide new insights for CAMTAs in peach fruit and indicate that PpCAMTA1 is associated with response to stresses during development.

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.

Calcium Mediated Cold Acclimation in Plants: Underlying Signaling and Molecular Mechanisms

Exposure of plants to low temperatures adversely affects plant growth, development, and productivity. Plant response to cold stress is an intricate process that involves the orchestration of various physiological, signaling, biochemical, and molecular pathways. Calcium (Ca²⁺) signaling plays a crucial role in the acquisition of several stress responses, including cold. Upon perception of cold stress, Ca²⁺ channels and/or Ca²⁺ pumps are activated, which induces the Ca²⁺ signatures in plant cells. The Ca²⁺ signatures spatially and temporally act inside a plant cell and are eventually decoded by specific Ca²⁺ sensors. This series of events results in the molecular regulation of several transcription factors (TFs), leading to downstream gene expression and withdrawal of an appropriate response by the plant. In this context, calmodulin binding transcription activators (CAMTAs) constitute a group of TFs that regulate plant cold stress responses in a Ca²⁺ dependent manner. The present review provides a catalog of the recent progress made in comprehending the Ca²⁺ mediated cold acclimation in plants.

Systematic Analysis and Identification of Drought-Responsive Genes of the CAMTA Gene Family in Wheat (Triticum aestivum L.)

The calmodulin-binding transcription activator (CAMTA) is a Ca²⁺/CaM-mediated transcription factor (TF) that modulates plant stress responses and development. Although the investigations of CAMTAs in various organisms revealed a broad range of functions from sensory mechanisms to physiological activities in crops, little is known about the CAMTA family in wheat (Triticum aestivum L.). Here, we systematically analyzed phylogeny, gene expansion, conserved motifs, gene structure, cis-elements, chromosomal localization, and expression patterns of CAMTA genes in wheat. We described and confirmed, via molecular evolution and functional verification analyses, two new members of the family, TaCAMTA5-B.1 and TaCAMTA5-B.2. In addition, we determined that the expression of most TaCAMTA genes responded to several abiotic stresses (drought, salt, heat, and cold) and ABA during the seedling stage, but it was mainly induced by drought stress. Our study provides considerable information about the changes in gene expression in wheat under stress, notably that drought stress-related gene expression in TaCAMTA1b-B.1 transgenic lines was significantly upregulated under drought stress. In addition to providing a comprehensive view of CAMTA genes in wheat, our results indicate that TaCAMTA1b-B.1 has a potential role in the drought stress response induced by a water deficit at the seedling stage.

Evolution Analyses of CAMTA Transcription Factor in Plants and Its Enhancing Effect on Cold-tolerance

The calmodulin binding transcription activator (CAMTA) is a transcription factor that is widely present in eukaryotes with conserved structure. It contributes to the response to biotic and abiotic stresses and promotes the growth and development of plants. Although previous studies have investigated the number and function of CAMTAs in some species, there is still a lack of comprehensive understanding of the evolutionary process, phylogenetic relationship, expression patterns, and functions of CAMTAs in plants. Here we identified 465 CMATA genes from 112 plants and systematically studied the origin of CAMTA family, gene expansion, functional differentiation, gene structure, and conservative motif distribution. Based on these analyses, we presented the evidence that CAMTA family was originated from chlorophyta, and we speculated that CAMTA might experience obvious structure variation during its early evolution, and that the number of CAMTA genes might gradually increase in higher plants. To reveal potential functions of CAMTA genes, we analyzed the expression patterns of 12 representative species and found significant species specificity, tissue specificity, and developmental stage specificity of CAMTAs. The results also indicated that the CAMTA genes might promote the maturation and senescence. The expression levels and regulatory networks of CAMTAs revealed that CAMTAs could enhance cold tolerance of rice by regulating carbohydrate metabolism-related genes to accumulate carbohydrates or by modulating target genes together with other transcription factors. Our study provides an insight into the molecular evolution of CAMTA family and lays a foundation for further study of related biological functions.

Comprehensive genome-wide analysis of calmodulin-binding transcription activator (CAMTA) in Durio zibethinus and identification of fruit ripening-associated DzCAMTAs

Background

Fruit ripening is an intricate developmental process driven by a highly coordinated action of complex hormonal networks. Ethylene is considered as the main phytohormone that regulates the ripening of climacteric fruits. Concomitantly, several ethylene-responsive transcription factors (TFs) are pivotal components of the regulatory network underlying fruit ripening. Calmodulin-binding transcription activator (CAMTA) is one such ethylene-induced TF implicated in various stress and plant developmental processes.

Results

Our comprehensive analysis of the CAMTA gene family in Durio zibethinus (durian, Dz) identified 10 CAMTAs with conserved domains. Phylogenetic analysis of DzCAMTAs, positioned DzCAMTA3 with its tomato ortholog that has already been validated for its role in the fruit ripening process through ethylene-mediated signaling. Furthermore, the transcriptome-wide analysis revealed DzCAMTA3 and DzCAMTA8 as the highest expressing durian CAMTA genes. These two DzCAMTAs possessed a distinct ripening-associated expression pattern during post-harvest ripening in Monthong, a durian cultivar native to Thailand. The expression profiling of DzCAMTA3 and DzCAMTA8 under natural ripening conditions and ethylene-induced/delayed ripening conditions substantiated their roles as ethylene-induced transcriptional activators of ripening. Similarly, auxin-suppressed expression of DzCAMTA3 and DzCAMTA8 confirmed their responsiveness to exogenous auxin treatment in a time-dependent manner. Accordingly, we propose that DzCAMTA3 and DzCAMTA8 synergistically crosstalk with ethylene during durian fruit ripening. In contrast, DzCAMTA3 and DzCAMTA8 antagonistically with auxin could affect the post-harvest ripening process in durian. Furthermore, DzCAMTA3 and DzCAMTA8 interacting genes contain significant CAMTA recognition motifs and regulated several pivotal fruit-ripening-associated pathways.

Conclusion

Taken together, the present study contributes to an in-depth understanding of the structure and probable function of CAMTA genes in the post-harvest ripening of durian.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08022-1.

A calmodulin-binding transcription factor links calcium signaling to antiviral RNAi defense in plants

RNA interference (RNAi) is an across-kingdom gene regulatory and defense mechanism. However, little is known about how organisms sense initial cues to mobilize RNAi. Here, we show that wounding to Nicotiana benthamiana cells during virus intrusion activates RNAi-related gene expression through calcium signaling. A rapid wound-induced elevation in calcium fluxes triggers calmodulin-dependent activation of calmodulin-binding transcription activator-3 (CAMTA3), which activates RNA-dependent RNA polymerase-6 and Bifunctional nuclease-2 (BN2) transcription. BN2 stabilizes mRNAs encoding key components of RNAi machinery, notably AGONAUTE1/2 and DICER-LIKE1, by degrading their cognate microRNAs. Consequently, multiple RNAi genes are primed for combating virus invasion. Calmodulin-, CAMTA3-, or BN2-knockdown/knockout plants show increased susceptibility to geminivirus, cucumovirus, and potyvirus. Notably, Geminivirus V2 protein can disrupt the calmodulin-CAMTA3 interaction to counteract RNAi defense. These findings link Ca2+ signaling to RNAi and reveal versatility of host antiviral defense and viral counter-defense.

Genome-Wide Characterization and Expression Analysis of CAMTA Gene Family Under Salt Stress in Cucurbita moschata and Cucurbita maxima

Cucurbita Linn. vegetables have a long history of cultivation and have been cultivated all over the world. With the increasing area of saline–alkali soil, Cucurbita Linn. is affected by salt stress, and calmodulin-binding transcription activator (CAMTA) is known for its important biological functions. Although the CAMTA gene family has been identified in several species, there is no comprehensive analysis on Cucurbita species. In this study, we analyzed the genome of Cucurbita maxima and Cucurbita moschata. Five C. moschata calmodulin-binding transcription activators (CmoCAMTAs) and six C. maxima calmodulin-binding transcription activators (CmaCAMTAs) were identified, and they were divided into three subfamilies (Subfamilies I, II, and III) based on the sequence identity of amino acids. CAMTAs from the same subfamily usually have similar exon–intron distribution and conserved domains (CG-1, TIG, IQ, and Ank_2). Chromosome localization analysis showed that CmoCAMTAs and CmaCAMTAs were unevenly distributed across four and five out of 21 chromosomes, respectively. There were a total of three duplicate gene pairs, and all of which had experienced segmental duplication events. The transcriptional profiles of CmoCAMTAs and CmaCAMTAs in roots, stems, leaves, and fruits showed that these CAMTAs have tissue specificity. Cis-acting elements analysis showed that most of CmoCAMTAs and CmaCAMTAs responded to salt stress. By analyzing the transcriptional profiles of CmoCAMTAs and CmaCAMTAs under salt stress, it was shown that both C. moschata and C. maxima shared similarities against salt tolerance and that it is likely to contribute to the development of these species. Finally, quantitative real-time polymerase chain reaction (qRT-PCR) further demonstrated the key role of CmoCAMTAs and CmaCAMTAs under salt stress. This study provided a theoretical basis for studying the function and mechanism of CAMTAs in Cucurbita Linn.

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.

Interactions between calcium and ABA signaling pathways in the regulation of fruit ripening

Fruit ripening and senescence are finely controlled by plant hormones such as ethylene and abscisic acid (ABA) but also by calcium ions and by calcium-dependent signaling pathways. Although there are extensive data supporting an interaction between ethylene and calcium in fruit ripening, the regulatory mechanisms resulting from the interaction between ABA and calcium have not yet been fully clarified. In this article, we have reviewed the full roles of calcium and its sensors (CaM, CMLs, CDPKs, CBLs) as well as ABA and the interactions between the two signaling pathways in the regulation of stress responses and in fruit ripening. To illustrate the possible interaction between calcium sensors and ABA signaling components in the control of fruit ripening, we propose an interaction model between the calcium and ABA signaling pathways.

Calmodulin binding transcription activators: An interplay between calcium signalling and plant stress tolerance

In plants, next to the secondary messengers lies an array of signal relaying molecules among which Calmodulins convey the unequivocal alarms of calcium influxes to Calmodulin-Binding Transcription Activators (CAMTA). Upon reception, CAMTA transcription factors decode the calcium signatures by transcribing the genes corresponding to the specific stimulus, thus have direct/indirect engagement in the complex signalling crosstalk. CAMTA transcription factors make an important contribution to the genome of all eukaryotes, including plants, from Brassica napus (18) to Carica papaya (2), the number of CAMTA genes varies across the plant species, however they exhibit a similar evolutionarily conserved domain organization including a DNA-Binding Domain (CG-1), a Transcription Factor Immunoglobulin Binding Domain (TIG), a Calmodulin-Binding Domain (CaMBD/IQ) and several Ankyrin repeats. The regulatory region of CAMTA genes possess multiple stress-responsive cis motifs including ABRE, SARE, G-box, W-box, AuXRE, DRE and others. CAMTA TFs in Arabidopsis have been studied extensively, however in other plants (with a few exceptions), the evidence merely bases upon expression analyses. CAMTAs are reported to orchestrate biotic as well as abiotic stresses including those occurring due to water and temperature fluctuations as well as heavy metals, light and salinity. Through CG-1 domain, CAMTA TFs bind the CG-box in the promoter of their target genes and modulate their expression under adverse conditions. Here we present a glimpse of how calcium signatures are coded and decoded and translated into necessary responses. In addition, we have emphasized on exploitation of the multiple-stress responsive nature of CAMTAs in engineering plants with desired traits.

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.

CAMTA3 negatively regulates disease resistance through modulating immune response and extensive transcriptional reprogramming in cassava

As one of the important crops in the world, cassava production is seriously threatened by Xanthomonas axonopodis pv. manihotis (Xam) all year round. Calmodulin-binding transcription activators (CAMTAs) play key roles in biotic stress and abiotic stress in plants, however, their roles in cassava remain elusive. In this study, six MeCAMTAs were identified, and MeCAMTA3 with the highest induction upon Xam infection was confirmed as a transcription factor that binds to the vCGCGb motif. MeCAMTA3 negatively regulates plant disease resistance against Xam. On the one hand, MeCAMTA3 negatively regulated endogenous salicylic acid and reactive oxygen species accumulation, pathogenesis-related genes MePRs' transcripts and callose deposition during cassava-Xam interaction but not under control conditions. On the other hand, RNA sequencing showed extensive transcriptional reprogramming by MeCAMTA3, especially 18 genes with a vCGCGb motif in the promoter region in hormone signaling, antioxidant signaling and other disease resistance signaling. Notably, chromatin immunoprecipitation-polymerase chain reaction showed that eight of these genes might be directly regulated by MeCAMTA3 through transcriptional repression. In summary, MeCAMTA3 negatively regulates plant disease resistance against cassava bacterial blight through modulation of multiple immune responses during cassava-Xam interaction and extensive transcriptional reprogramming.

Calmodulin-binding transcription activator (CAMTA) genes family: Genome-wide survey and phylogenetic analysis in flax (Linum usitatissimum)

Flax (Linum usitatissimum) is a member of family linaceae with annual growth habit. It is included among those crops which were domesticated very early and has been used in development related studies as a model plant. In plants, Calmodulin-binding transcription activators (CAMTAs) comprise a unique set of Calmodulin-binding proteins. To elucidate the transport mechanism of secondary metabolites in flax, a genome-based study on these transporters was performed. The current investigation identified nine CAMTAs proteins, classified into three categories during phylogenetic analysis. Each group had significant evolutionary role as illustrated by the conservation of gene structures, protein domains and motif organizations over the distinctive phylogenetic classes. GO annotation suggested a link to sequence-specific DNA and protein binding, response to low temperature and transcription regulation by RNA polymerase II. The existence of different hormonal and stress responsive cis-regulatory elements in promotor region may directly correlate with the variation of their transcripts. MicroRNA target analysis revealed that various groups of miRNA families targeted the LuCAMTAs genes. Identification of CAMTA genes, miRNA studies and phylogenetic analysis may open avenues to uncover the underlying functional mechanism of this important family of genes in flax.

Overexpression of GmCAMTA12 Enhanced Drought Tolerance in Arabidopsis and Soybean

Fifteen transcription factors in the CAMTA (calmodulin binding transcription activator) family of soybean were reported to differentially regulate in multiple stresses; however, their functional analyses had not yet been attempted. To characterize their role in stresses, we first comprehensively analyzed the GmCAMTA family in silico and thereafter determined their expression pattern under drought. The bioinformatics analysis revealed multiple stress-related cis-regulatory elements including ABRE, SARE, G-box and W-box, 10 unique miRNA (microRNA) targets in GmCAMTA transcripts and 48 proteins in GmCAMTAs' interaction network. We then cloned the 2769 bp CDS (coding sequence) of GmCAMTA12 in an expression vector and overexpressed in soybean and Arabidopsis through Agrobacterium-mediated transformation. The T3 (Transgenic generation 3) stably transformed homozygous lines of Arabidopsis exhibited enhanced tolerance to drought in soil as well as on MS (Murashige and Skoog) media containing mannitol. In their drought assay, the average survival rate of transgenic Arabidopsis lines OE5 and OE12 (Overexpression Line 5 and Line 12) was 83.66% and 87.87%, respectively, which was ~30% higher than that of wild type. In addition, the germination and root length assays as well as physiological indexes such as proline and malondialdehyde contents, catalase activity and leakage of electrolytes affirmed the better performance of OE lines. Similarly, GmCAMTA12 overexpression in soybean promoted drought-efficient hairy roots in OE chimeric plants as compare to that of VC (Vector control). In parallel, the improved growth performance of OE in Hoagland-PEG (polyethylene glycol) and on MS-mannitol was revealed by their phenotypic, physiological and molecular measures. Furthermore, with the overexpression of GmCAMTA12, the downstream genes including AtAnnexin5, AtCaMHSP, At2G433110 and AtWRKY14 were upregulated in Arabidopsis. Likewise, in soybean hairy roots, GmELO, GmNAB and GmPLA1-IId were significantly upregulated as a result of GmCAMTA12 overexpression and majority of these upregulated genes in both plants possess CAMTA binding CGCG/CGTG motif in their promoters. Taken together, we report that GmCAMTA12 plays substantial role in tolerance of soybean against drought stress and could prove to be a novel candidate for engineering soybean and other plants against drought stress. Some research gaps were also identified for future studies to extend our comprehension of Ca-CaM-CAMTA-mediated stress regulatory mechanisms.

Evolutionary and expression analysis of CAMTA gene family in Nicotiana tabacum yielded insights into their origin, expansion and stress responses

Calmodulin-binding transcription activators (CAMTAs) represent the novel gene family of transcriptional regulators, which play important biological functions. Though, the first ever plant CAMTA gene was evidenced in Nicotiana tabacum in 2002. But, the systematic identification, origin and function of this gene family has not been performed due to the lack of reference genome information until now. Here, we identified 29 CAMTA genes in four Nicotiana species, including thirteen NtabCAMTAs, six NsylCAMTAs, and five NtomCAMTAs and NbenCAMTAs. These CAMTA families were classified into five phylogenetic groups (I-V), among which, the group-IV CAMTAs probably emerged the earliest. The NtabCAMTA family genes have diverse structures, and are randomly localized on five chromosomes and scaffolds. N. tabacum acquired 11 copies of homolog CAMATA genes from the parental genomes of N. tomentosiformis and N. sylvestris, followed by expansion through polyploidization and duplication. The NtabCAMTA genes were differentially expressed in different plant parts, and showed sensitivity towards different abiotic and biotic stresses. Co-expression network analysis revealed that some NtabCAMTA subunits interact with each other, and co-expressed. The current study is the first report presenting a comprehensive overview of Nicotiana CAMTA families, and opens a new avenue for the improvement of the cultivated tobacco.

Genome-wide comparative and evolutionary analysis of Calmodulin-binding Transcription Activator (CAMTA) family in Gossypium species

The CAMTA gene family is crucial in managing both biotic and abiotic stresses in plants. Our comprehensive analysis of this gene family in cotton resulted in the identification of 6, 7 and 9 CAMTAs in three sequenced cotton species, i.e., Gossypium arboreum, Gossypium raimondii, and Gossypium hirsutum, respectively. All cotton CAMTAs were localized in the nucleus and possessed calmodulin-binding domain (CaMBD) as identified computationally. Phylogenetically four significant groups of cotton CAMTAs were identified out of which, Group II CAMTAs experienced higher evolutionary pressure, leading to a faster evolution in diploid cotton. The expansion of cotton CAMTAs in the genome was mainly due to segmental duplication. Purifying selection played a significant role in the evolution of cotton CAMTAs. Expression profiles of GhCAMTAs revealed that GhCAMTA2A.2 and GhCAMTA7A express profoundly in different stages of cotton fiber development. Positive correlation between expression of these two CAMTAs and fiber strength confirmed their functional relevance in fiber development. The promoter region of co-expressing genes network of GhCAMTA2A.2 and GhCAMTA7A showed a higher frequency of occurrence of CAMTA binding motifs. Our present study thus contributes to broad probing into the structure and probable function of CAMTA genes in Gossypium species.

Beauvericin synthetase contains a calmodulin binding motif in the entomopathogenic fungus Beauveria bassiana

Beauvericin is a mycotoxin which has insecticidal, anti-microbial, anti-viral and anti-cancer activities. Beauvericin biosynthesis is rapidly catalyzed by the beauvericin synthetase (BEAS) in Beauveria bassiana. Ca²⁺ plays crucial roles in multiple signaling pathways in eukaryotic cells. These Ca²⁺ signals are partially decoded by Ca²⁺ sensor calmodulin (CaM). In this report, we describe that B. bassiana BEAS (BbBEAS) can interact with CaM in a Ca²⁺-dependent manner. A synthetic BbBEAS peptide, corresponding to the putative CaM-binding motif, formed a stable complex with CaM in the presence of Ca²⁺. In addition, in vitro CaM-binding assay revealed that the His-tagged BbBEAS (amino acids 2421-2538) binds to CaM in a Ca²⁺-dependent manner. Therefore, this work suggests that BbBEAS is a novel CaM-binding protein in B. bassiana.

Calmodulin-mediated suppression of 2-ketoisovalerate reductase in Beauveria bassiana beauvericin biosynthetic pathway

Ketoisovalerate reductase (KIVR, E.C. 1.2.7.7) mediates the specific reduction of 2-ketoisovalerate (2-Kiv) to d-hydroxyisovalerate (d-Hiv), a precursor for beauvericin biosynthesis. Beauvericin, a famous mycotoxin produced by many fungi, is a cyclooligomer depsipeptide, which has insecticidal, antimicrobial, antiviral, and cytotoxic activities. In this report, we demonstrated that Beauveria bassiana 2-ketoisovalerate reductase (BbKIVR) acts as a typical KIVR enzyme in the entomopathogenic fungus B. bassiana. In addition, we found that BbKIVR interacts with calmodulin (CaM) in vitro and in vivo. The functional role of CaM-binding to BbKIVR was to negatively regulate the BbKIVR activity in B. bassiana. Environmental stimuli such as light and salt stress suppressed BbKIVR activity in B. bassiana. Interestingly, this negative effect of BbKIVR activity by light and salt stress was recovered by CaM inhibitors, suggesting that the inhibitory mechanism is mediated through stimulation of CaM activity. Therefore, this work suggests that BbKIVR plays an important role in the beauvericin biosynthetic pathway mediated by environmental stimuli such as light and salt stress via the CaM signaling pathway.

Regulation of MAP kinase Hog1 by calmodulin during hyperosmotic stress

Mitogen-activated protein kinase (Hog1 in yeast and ortholog p38 in human cells) plays a critical role in the signal transduction pathway that is rapidly activated under multiple stress conditions. Environmental stress stimuli such as hyperosmotic stress cause changes in cellular ATP metabolism required for hyperosmotic stress tolerance. Furthermore, hyperosmotic stress induces rapid Ca²⁺ signals in eukaryotic cells. These Ca²⁺ signals can be decoded by Ca²⁺ sensor calmodulin (CaM). By using genetic and biochemical approaches, we demonstrate that Hog1 is a novel CaM-binding protein, and that CaM-binding to Hog1 is involved in the mediation of the hyperosmotic stress signaling pathway. In addition, we show that p38α, a human ortholog of Hog1, interacts with CaM, suggesting that the CaM-binding feature of Hog1/p38α is evolutionarily conserved in eukaryotic cells. Hog1 is likely involved in cellular ATP regulation through CaM signaling during hyperosmotic stress. Therefore, this work suggests that Hog1 plays an important role in connecting CaM signaling with the hyperosmotic stress pathway by directly interacting with CaM in Saccharomyces cerevisiae.

Global gene expression analysis using RNA-seq uncovered a new role for SR1/CAMTA3 transcription factor in salt stress

Abiotic and biotic stresses cause significant yield losses in all crops. Acquisition of stress tolerance in plants requires rapid reprogramming of gene expression. SR1/CAMTA3, a member of signal responsive transcription factors (TFs), functions both as a positive and a negative regulator of biotic stress responses and as a positive regulator of cold stress-induced gene expression. Using high throughput RNA-seq, we identified ~3000 SR1-regulated genes. Promoters of about 60% of the differentially expressed genes have a known DNA binding site for SR1, suggesting that they are likely direct targets. Gene ontology analysis of SR1-regulated genes confirmed previously known functions of SR1 and uncovered a potential role for this TF in salt stress. Our results showed that SR1 mutant is more tolerant to salt stress than the wild type and complemented line. Improved tolerance of sr1 seedlings to salt is accompanied with the induction of salt-responsive genes. Furthermore, ChIP-PCR results showed that SR1 binds to promoters of several salt-responsive genes. These results suggest that SR1 acts as a negative regulator of salt tolerance by directly repressing the expression of salt-responsive genes. Overall, this study identified SR1-regulated genes globally and uncovered a previously uncharacterized role for SR1 in salt stress response.

Genome-wide identification, characterization and expression analysis of calmodulin-like (CML) proteins in tomato (Solanum lycopersicum)

Calcium (Ca(2+)) has emerged as a significant secondary messenger that regulates the activities of hormonal and environmental signals that are associated with biotic and abiotic stresses. Ca(2+) binding proteins typically contain a Ca(2+) binding EF-hand (a helix-loop-helix structure) motif. In this study, tomato genes encoding calmodulin-like (CML) proteins that possess EF-hand motifs and no other identifiable functional domains were analyzed. Using genome analysis and BLAST searches in database, 52 CML genes were identified in tomato. Comprehensive analyses, including evolutionary relationships, gene structures, chromosomal locations, functional annotations, and gene duplications, were performed. Distribution mapping exhibited that 52 SlCML proteins containing different intron/exon patterns were unevenly distributed among ten chromosomes. In addition, 24 SlCML proteins were predicted as segmentally duplicated. Conserved motifs, promoter cis-regulatory elements, organ-based expression patterns and expression analyses indicated the potential responsiveness of SlCML proteins to abiotic stresses and phytohormones. These results illustrate the complexity of the CML gene family and indicate a potential vital role for these molecules in tomato growth and development as Ca(2+) signal transducers.

Quantitative and Functional Phosphoproteomic Analysis Reveals that Ethylene Regulates Water Transport via the C-Terminal Phosphorylation of Aquaporin PIP2;1 in Arabidopsis

Ethylene participates in the regulation of numerous cellular events and biological processes, including water loss, during leaf and flower petal wilting. The diverse ethylene responses may be regulated via dynamic interplays between protein phosphorylation/dephosphorylation and ubiquitin/26S proteasome-mediated protein degradation and protease cleavage. To address how ethylene alters protein phosphorylation through multi-furcated signaling pathways, we performed a (15)N stable isotope labelling-based, differential, and quantitative phosphoproteomics study on air- and ethylene-treated ethylene-insensitive Arabidopsis double loss-of-function mutant ein3-1/eil1-1. Among 535 non-redundant phosphopeptides identified, two and four phosphopeptides were up- and downregulated by ethylene, respectively. Ethylene-regulated phosphorylation of aquaporin PIP2;1 is positively correlated with the water flux rate and water loss in leaf. Genetic studies in combination with quantitative proteomics, immunoblot analysis, protoplast swelling/shrinking experiments, and leaf water loss assays on the transgenic plants expressing both the wild-type and S280A/S283A-mutated PIP2;1 in the both Col-0 and ein3eil1 genetic backgrounds suggest that ethylene increases water transport rate in Arabidopsis cells by enhancing S280/S283 phosphorylation at the C terminus of PIP2;1. Unknown kinase and/or phosphatase activities may participate in the initial up-regulation independent of the cellular functions of EIN3/EIL1. This finding contributes to our understanding of ethylene-regulated leaf wilting that is commonly observed during post-harvest storage of plant organs.

Phylogeny of Plant CAMTAs and Role of AtCAMTAs in Nonhost Resistance to Xanthomonas oryzae pv. oryzae

Calmodulin-binding transcription activator (CAMTA) constitutes one of the most important Ca(2+)/CaM-regulated transcription factor families in plants. Nevertheless, the phylogeny, protein interaction network, and role in nonhost resistance of plant CAMTAs are not well understood. In this study, 200 CAMTA genes were identified from 35 species representing four major plant lineages. The CAMTA genes were conserved in multicellular land plants but absent in unicellular eukaryotes, and were likely to emerge from the fusion of two separate genes encoding a CAMTA-like protein and an IQ/CaM binding motif containing protein, respectively, in the embryophyta lineage ancestor. Approximately one fourth of plant CAMTAs did not contain a TIG domain. This non-TIG class of CAMTAs seems to have newly evolved through mutation of some key amino acids in the TIG domain of flowering land plants after divergence from the non-flowering plants. Phylogenetic analysis classified CAMTA proteins into three major groups and nine distinct subgroups, a result supported by protein domain and motif conservation analyses. Most (59.0 and 21.5%) of the identified CAMTA genes contained 12 or 11 introns, respectively. Gene duplication, intron invasion, enlargement and turnover, as well as exon rearrangements and skipping have apparently occurred during evolution of the CAMTA family. Moreover, 38 potential interactors of six Arabidopsis CAMTAs were predicted and 10 predicted target genes of AtCAMTA3 exhibited changes in expression between Atcamta3 mutants and wild-type plants. The majority of predicted interactors are transcription factors and/or Ca(2+)/CaM-regulated proteins, suggesting that transcriptional regulation of the target genes might be the dominant functional mechanism of AtCAMTAs, and AtCAMTAs might act together with other Ca(2+) signaling components to regulate Ca(2+)-related biological processes. Furthermore, functional analyses employing Atcamta mutants revealed that AtCAMTA3 negatively regulated the immunity triggered by flg22 and nonhost resistance to Xanthomonas oryzae pv. oryzae via repressing accumulation of reactive oxygen species probably by targeting CBP60G, EDS1, and NDR1 and involving SA pathway.

Cellular and Molecular Changes Associated with Onion Skin Formation Suggest Involvement of Programmed Cell Death

Skin formation of onion (Allium cepa L.) bulb involves scale desiccation accompanied by scale senescence, resulting in cell death and tissue browning. Understanding the mechanism of skin formation is essential to improving onion skin and bulb qualities. Although onion skin plays a crucial role in postharvest onion storage and shelf life, its formation is poorly understood. We investigated the mode of cell death in the outermost scales that are destined to form the onion skin. Surprisingly, fluorescein diacetate staining and scanning electron microscopy indicated that the outer scale desiccates from the inside out. This striking observation suggests that cell death in the outer scales, during skin formation, is an internal and organized process that does not derive only from air desiccation. DNA fragmentation, a known hallmark of programmed cell death (PCD), was revealed in the outer scales and gradually decreased toward the inner scales of the bulb. Transmission electron microscopy further revealed PCD-related structural alterations in the outer scales which were absent from the inner scales. De novo transcriptome assembly for three different scales: 1st (outer), 5th (intermediate) and 8th (inner) fleshy scales identified 2,542 differentially expressed genes among them. GO enrichment for cluster analysis revealed increasing metabolic processes in the outer senescent scale related to defense response, PCD processes, carbohydrate metabolism and flavonoid biosynthesis, whereas increased metabolism and developmental growth processes were identified in the inner scales. High expression levels of PCD-related genes were identified in the outer scale compared to the inner ones, highlighting the involvement of PCD in outer-skin development. These findings suggest that a program to form the dry protective skin exists and functions only in the outer scales of onion.

Calmodulin-binding transcription activators and perspectives for applications in biotechnology

In recent years, a novel family of calmodulin-binding transcription activators (CAMTAs) has been reported in various species. The CAMTAs share a conserved domain organization, with a CG-1 DNA-binding domain, a transcription factor immunoglobulin domain, several ankyrin repeats, a calmodulin-binding domain, and a varying number of IQ motifs. CAMTAs participate in transcriptional regulation by recognizing and binding to a specific cis-element: (G/A/C)CGCG(C/G/T). Plants suffer from the environmental challenges, including abiotic and biotic stresses. Investigations in various plant species indicate a broad range of CAMTA functions involved in developmental regulation, environmental stress response, and hormone cross talk. In this review, we focus on the expression patterns and biological functions of CAMTAs to explore their probable applications in biotechnology. Furthermore, the identification and phylogenetic analysis of CAMTAs in crops could open new perspectives for enhancing stress tolerance, which could lead to improved crop production.

Involvement of calmodulin and calmodulin-like proteins in plant responses to abiotic stresses

Transient changes in intracellular Ca(2+) concentration have been well recognized to act as cell signals coupling various environmental stimuli to appropriate physiological responses with accuracy and specificity in plants. Calmodulin (CaM) and calmodulin-like proteins (CMLs) are major Ca(2+) sensors, playing critical roles in interpreting encrypted Ca(2+) signals. Ca(2+)-loaded CaM/CMLs interact and regulate a broad spectrum of target proteins such as channels/pumps/antiporters for various ions, transcription factors, protein kinases, protein phosphatases, metabolic enzymes, and proteins with unknown biochemical functions. Many of the target proteins of CaM/CMLs directly or indirectly regulate plant responses to environmental stresses. Basic information about stimulus-induced Ca(2+) signal and overview of Ca(2+) signal perception and transduction are briefly discussed in the beginning of this review. How CaM/CMLs are involved in regulating plant responses to abiotic stresses are emphasized in this review. Exciting progress has been made in the past several years, such as the elucidation of Ca(2+)/CaM-mediated regulation of AtSR1/CAMTA3 and plant responses to chilling and freezing stresses, Ca(2+)/CaM-mediated regulation of CAT3, MAPK8 and MKP1 in homeostasis control of reactive oxygen species signals, discovery of CaM7 as a DNA-binding transcription factor regulating plant response to light signals. However, many key questions in Ca(2+)/CaM-mediated signaling warrant further investigation. Ca(2+)/CaM-mediated regulation of most of the known target proteins is presumed based on their interaction. The downstream targets of CMLs are mostly unknown, and how specificity of Ca(2+) signaling could be realized through the actions of CaM/CMLs and their target proteins is largely unknown. Future breakthroughs in Ca(2+)/CaM-mediated signaling will not only improve our understanding of how plants respond to environmental stresses, but also provide the knowledge base to improve stress-tolerance of crops.

Characterization of a Calmodulin-binding Transcription Factor from Strawberry (Fragaria × ananassa)

Calmodulin-binding transcription activator (CAMTA) is a calmodulin-binding transcription factor that has a broad range of functions from sensory mechanisms to regulating many growth and developmental processes. In this study, we isolated four strawberry CAMTA (FaCAMTA) genes using HMMER and BLAST analysis. The chromosome scaffold locations of these CAMTA genes in the strawberry genome were determined and the protein domain and motif organization [CG-1, transcription factor immunoglobulin, ankyrin (ANK) repeats, calmodulin-binding IQ motif) of FaCAMTAs were also assessed. All FaCAMTAs were predicted to be Ca- and calmodulin-binding proteins. The expression profiles of FaCAMTA genes were measured in different tissues and revealed distinct FaCAMTA gene expression patterns under heat, cold, and salt stress. These data not only contribute to a better understanding of the complex regulation of the FaCAMTA gene family but also provide evidence supporting the role of CAMTAs in multiple signaling pathways involved in stress responses. This investigation can provide useful information for further study.

Involvement of secondary messengers and small organic molecules in auxin perception and signaling

Auxin is a major phytohormone involved in most aspects of plant growth and development. Generally, auxin is perceived by three distinct receptors: TRANSPORT INHIBITOR RESISTANT1-Auxin/INDOLE ACETIC ACID, S-Phase Kinase-Associated Protein 2A and AUXIN-BINDING PROTEIN1. The auxin perception is regulated by a variety of secondary messenger molecules, including nitric oxide, reactive oxygen species, calcium, cyclic GMP, cyclic AMP, inositol triphosphate, diacylglycerol and by physiological pH. In addition, some small organic molecules, including inositol hexakisphosphate, yokonolide B, p-chlorophenoxyisobutyric acid, toyocamycin and terfestatin A, are involved in auxin signaling. In this review, we summarize and discuss the recent progress in understanding the functions of these secondary messengers and small organic molecules, which are now thoroughly demonstrated to be pervasive and important in auxin perception and signal transduction.

Genome-wide identification of CAMTA gene family members in Medicago truncatula and their expression during root nodule symbiosis and hormone treatments

Calmodulin-binding transcription activators (CAMTAs) are well-characterized calmodulin-binding transcription factors in the plant kingdom. Previous work shows that CAMTAs play important roles in various biological processes including disease resistance, herbivore attack response, and abiotic stress tolerance. However, studies that address the function of CAMTAs during the establishment of symbiosis between legumes and rhizobia are still lacking. This study undertook comprehensive identification and analysis of CAMTA genes using the latest updated M. truncatula genome. All the MtCAMTA genes were expressed in a tissues-specific manner and were responsive to environmental stress-related hormones. The expression profiling of MtCAMTA genes during the early phase of Sinorhizobium meliloti infection was also analyzed. Our data showed that the expression of most MtCAMTA genes was suppressed in roots by S. meliloti infection. The responsiveness of MtCAMTAs to S. meliloti infection indicated that they may function as calcium-regulated transcription factors in the early nodulation signaling pathway. In addition, bioinformatics analysis showed that CAMTA binding sites existed in the promoter regions of various early rhizobial infection response genes, suggesting possible MtCAMTAs-regulated downstream candidate genes during the early phase of S. meliloti infection. Taken together, these results provide basic information about MtCAMTAs in the model legume M. truncatula, and the involvement of MtCAMTAs in nodule organogenesis. This information furthers our understanding of MtCAMTA protein functions in M. truncatula and opens new avenues for continued research.

Identification and expression profiling analysis of calmodulin-binding transcription activator genes in maize (Zea mays L.) under abiotic and biotic stresses

The calmodulin-binding transcription activators (CAMTA) play critical roles in plant growth and responses to environmental stimuli. However, how CAMTAs function in responses to abiotic and biotic stresses in maize (Zea mays L.) is largely unknown. In this study, we first identified all the CAMTA homologous genes in the whole genome of maize. The results showed that nine ZmCAMTA genes showed highly diversified gene structures and tissue-specific expression patterns. Many ZmCAMTA genes displayed high expression levels in the roots. We then surveyed the distribution of stress-related cis-regulatory elements in the -1.5 kb promoter regions of ZmCAMTA genes. Notably, a large number of stress-related elements present in the promoter regions of some ZmCAMTA genes, indicating a genetic basis of stress expression regulation of these genes. Quantitative real-time PCR was used to test the expression of ZmCAMTA genes under several abiotic stresses (drought, salt, and cold), various stress-related hormones [abscisic acid, auxin, salicylic acid (SA), and jasmonic acid] and biotic stress [rice black-streaked dwarf virus (RBSDV) infection]. Furthermore, the expression pattern of ZmCAMTA genes under RBSDV infection was analyzed to investigate their potential roles in responses of different maize cultivated varieties to RBSDV. The expression of most ZmCAMTA genes responded to both abiotic and biotic stresses. The data will help us to understand the roles of CAMTA-mediated Ca(2+) signaling in maize tolerance to environmental stresses.

To die or not to die? Lessons from lesion mimic mutants

Programmed cell death (PCD) is a ubiquitous genetically regulated process consisting in an activation of finely controlled signaling pathways that lead to cellular suicide. Although some aspects of PCD control appear evolutionary conserved between plants, animals and fungi, the extent of conservation remains controversial. Over the last decades, identification and characterization of several lesion mimic mutants (LMM) has been a powerful tool in the quest to unravel PCD pathways in plants. Thanks to progress in molecular genetics, mutations causing the phenotype of a large number of LMM and their related suppressors were mapped, and the identification of the mutated genes shed light on major pathways in the onset of plant PCD such as (i) the involvements of chloroplasts and light energy, (ii) the roles of sphingolipids and fatty acids, (iii) a signal perception at the plasma membrane that requires efficient membrane trafficking, (iv) secondary messengers such as ion fluxes and ROS and (v) the control of gene expression as the last integrator of the signaling pathways.

Tomato SR/CAMTA transcription factors SlSR1 and SlSR3L negatively regulate disease resistance response and SlSR1L positively modulates drought stress tolerance

BACKGROUND

The SR/CAMTA proteins represent a small family of transcription activators that play important roles in plant responses to biotic and abiotic stresses. Seven SlSR/CAMTA genes were identified in tomato as tomato counterparts of SR/CAMTA; however, the involvement of SlSRs/CAMTAs in biotic and abiotic stress responses is not clear. In this study, we performed functional analysis of the SlSR/CAMTA family for their possible functions in defense response against pathogens and tolerance to drought stress.

RESULTS

Expression of SlSRs was induced with distinct patterns by Botrytis cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000. Virus-induced gene silencing (VIGS)-based knockdown of either SlSR1 or SlSR3L in tomato resulted in enhanced resistance to B. cinerea and Pst DC3000 and led to constitutive accumulation of H2O2, elevated expression of defense genes, marker genes for pathogen-associated molecular pattern-triggered immunity, and regulatory genes involved in the salicylic acid- and ethylene-mediated signaling pathways. Furthermore, the expression of SlSR1L and SlSR2L in detached leaves and whole plants was significantly induced by drought stress. Silencing of SlSR1L led to decreased drought stress tolerance, accelerated water loss in leaves, reduced root biomass and attenuated expression of drought stress responsive genes in tomato. The SlSR1 and SlSR3L proteins were localized in the nucleus of plant cells when transiently expressed in Nicotiana benthamiana and had transcriptional activation activity in yeast.

CONCLUSIONS

VIGS-based functional analyses demonstrate that both SlSR1 and SlSR3L in the tomato SlSR/CAMTA family are negative regulators of defense response against B. cinerea and Pst DC3000 while SlSR1L is a positive regulator of drought stress tolerance in tomato.

Regulation of plant immunity through ubiquitin-mediated modulation of Ca(2+) -calmodulin-AtSR1/CAMTA3 signaling

Transient changes in intracellular Ca(2+) concentration are essential signals for activation of plant immunity. It has also been reported that Ca(2+) signals suppress salicylic acid-mediated plant defense through AtSR1/CAMTA3, a member of the Ca(2+) /calmodulin-regulated transcription factor family that is conserved in multicellular eukaryotes. How plants overcome this negative regulation to mount an effective defense response during a stage of intracellular Ca(2+) surge is unclear. Here we report the identification and functional characterization of an important component of ubiquitin ligase, and the associated AtSR1 turnover. The AtSR1 interaction protein 1 (SR1IP1) was identified by CytoTrap two-hybrid screening. The loss-of-function mutant of SR1IP1 is more susceptible to bacterial pathogens, and over-expression of SR1IP1 confers enhanced resistance, indicating that SR1IP1 acts as a positive regulator of plant defense. SR1IP1 and AtSR1 act in the same signaling pathway to regulate plant immunity. SR1IP1 contains the structural features of a substrate adaptor in cullin 3-based E3 ubiquitin ligase, and was shown to serve as a substrate adaptor that recruits AtSR1 for ubiquitination and degradation when plants are challenged with pathogens. Hence, SR1IP1 positively regulates plant immunity by removing the defense suppressor AtSR1. These findings provide a mechanistic insight into how Ca(2+) -mediated actions are coordinated to achieve effective plant immunity.

Identification and bioinformatic analysis of signal responsive/calmodulin-binding transcription activators gene models in Vitis vinifera

In this study, 10 grapevine (Vitis vinifera) SR/CAMTA (Signal Responsive/Calmodulin-binding Transcription Activators) gene models were identified from three grapevine genome protein datasets. They belong to four gene groups: VvCAMTA1, VvCAMTA3, VvCAMTA4 and VvCAMTA5, which were located on chromosome 5, 7_random, 1 and 5, respectively. Alternative splicing could explain the multiple gene models in one gene group. Subcellular localization using the WoLF tool showed that most of the VvCAMTAs were located in the nucleus, except for VvCAMTA3.1, VvCAMTA3.2 and VvCAMTA5.2, which were located in the chloroplast, chloroplast and cytosol, respectively. Subcellular localization using TargetP showed that most of the VvCAMTAs were not located in the chloroplast, mitochondrion and secretory pathway in cells. VvCAMTA1.1 and VvCAMTA1.2 were located in the mitochondria. The digital gene expression profile showed that VvCAMTAs play important roles in Ca2+ signal transduction. The gene expression patterns of VvCAMTAs were different; for example, VvCAMTA1 was expressed mainly in the bud, while VvCAMTA3 was expressed mainly in fruit and inflorescence, with low expression in the bud. The results of this study make a substantial contribution to our knowledge concerning genes, genome annotation, and cell signal transduction in grapevine.

Calcium signaling and biotic defense responses in plants

Calcium (Ca(2+)) acts as an important second messenger in plant cells. Cytosolic free Ca(2+) concentration in plant cells changes rapidly and dynamically in response to various endogenous or environmental cues. Elevation in calcium concentration in plant cells is an essential early event during plant defense responses. Alterations in the Ca(2+) concentration are sensed by Ca(2+)-binding proteins, including calmodulin, calcium-dependent protein kinases and calcineurin B-like proteins, which relay or decode the encoded Ca(2+) signals into specific cellular and physiological responses in order to survive challenges by pathogens. Genetic and functional studies have revealed that Ca(2+) signaling plays both positive and negative roles in regulating the establishment of defense responses. Furthermore, recent studies revealed that actions of Ca(2+)-mediated signaling could be regulated by other cell signaling systems such as the ubiquitin-proteasome system to mount precise and prompt plant defense responses.

Stable isotope metabolic labeling-based quantitative phosphoproteomic analysis of Arabidopsis mutants reveals ethylene-regulated time-dependent phosphoproteins and putative substrates of constitutive triple response 1 kinase

Ethylene is an important plant hormone that regulates numerous cellular processes and stress responses. The mode of action of ethylene is both dose- and time-dependent. Protein phosphorylation plays a key role in ethylene signaling, which is mediated by the activities of ethylene receptors, constitutive triple response 1 (CTR1) kinase, and phosphatase. To address how ethylene alters the cellular protein phosphorylation profile in a time-dependent manner, differential and quantitative phosphoproteomics based on (15)N stable isotope labeling in Arabidopsis was performed on both one-minute ethylene-treated Arabidopsis ethylene-overly-sensitive loss-of-function mutant rcn1-1, deficient in PP2A phosphatase activity, and a pair of long-term ethylene-treated wild-type and loss-of-function ethylene signaling ctr1-1 mutants, deficient in mitogen-activated kinase kinase kinase activity. In total, 1079 phosphopeptides were identified, among which 44 were novel. Several one-minute ethylene-regulated phosphoproteins were found from the rcn1-1. Bioinformatic analysis of the rcn1-1 phosphoproteome predicted nine phosphoproteins as the putative substrates for PP2A phosphatase. In addition, from CTR1 kinase-enhanced phosphosites, we also found putative CTR1 kinase substrates including plastid transcriptionally active protein and calcium-sensing receptor. These regulatory proteins are phosphorylated in the presence of ethylene. Analysis of ethylene-regulated phosphosites using the group-based prediction system with a protein-protein interaction filter revealed a total of 14 kinase-substrate relationships that may function in both CTR1 kinase- and PP2A phosphatase-mediated phosphor-relay pathways. Finally, several ethylene-regulated post-translational modification network models have been built using molecular systems biology tools. It is proposed that ethylene regulates the phosphorylation of arginine/serine-rich splicing factor 41, plasma membrane intrinsic protein 2A, light harvesting chlorophyll A/B binding protein 1.1, and flowering bHLH 3 proteins in a dual-and-opposing fashion.

Recent advances in calcium/calmodulin-mediated signaling with an emphasis on plant-microbe interactions

Calcium/calmodulin-mediated signaling contributes in diverse roles in plant growth, development, and response to environmental stimuli .

Differential expression of calcium/calmodulin-regulated SlSRs in response to abiotic and biotic stresses in tomato fruit

Calcium has been shown to enhance stress tolerance, maintain firmness and reduce decay in fruits. Previously we reported that seven tomato SlSRs encode calcium/calmodulin-regulated proteins, and that their expressions are developmentally regulated during fruit development and ripening, and are also responsive to ethylene. To study their expressions in response to stresses encountered during postharvest handling, tomato fruit at the mature-green stage was subjected to chilling and wounding injuries, infected with Botrytis cinerea and treated with salicylic acid or methyl jasmonate. Gene expression studies revealed that the seven SlSRs differentially respond to different stress signals. SlSR2 was the only gene upregulated by all the treatments. SlSR4 acted as a late pathogen-induced gene; it was upregulated by salicylic acid and methyl jasmonate, but downregulated by cold treatment. SlSR3L was cold- and wound-responsive and was also induced by salicylic acid. SlSR1 and SlSR1L were repressed by cold, wounding and pathogen infection, but were upregulated by salicylic acid and methyl jasmonate. Overall, results of these expression studies indicate that individual SlSRs have distinct roles in responses to the specific stress signals, and SlSRs may act as a coordinator(s) connecting calcium-mediated signaling with other stress signal transduction pathways during fruit ripening and storage.