Leaf senescence signaling: the Ca2+-conducting Arabidopsis cyclic nucleotide gated channel2 acts through nitric oxide to repress senescence programming

GhCNGC31 is critical for conferring resistance to Verticillium wilt in cotton

In the past decades, cyclic nucleotide-gated ion channels (CNGCs) have been extensively studied in diploid species Arabidopsis thaliana. However, the functional diversification of CNGCs in crop plants, mostly polyploid, remains poorly understood. In allotetraploid Upland cotton (Gossypium hirsutum), GhCNGC31 is one of the multiple orthologs of AtCNGC2, being present in the plasma membrane, capable of interacting with itself and binding to calmodulins and cyclic nucleotides. GhCNGC31 knockdown plants exhibited slight growth inhibition, and became more susceptible to Verticillium dahliae infection, which was associated with the reduced lignin and flavonoid accumulation, impaired ROS (reactive oxygen species) burst, and down-regulation of defense-related genes PR1, JAZ2, LOX2, and RBOH10. RNA-Seq analysis identified 1817 differentially expressed genes from GhCNGC31 knockdown, of which 1184 (65%) were responsive to V. dahliae infection and accounted for 57% among a total of 2065 V. dahliae-responsive genes identified in this study. These GhCNGC31-regulated genes mainly function with cell wall organization and biogenesis, cellular carbohydrate metabolic or biosynthetic process, cellular component macromolecule biosynthetic process, and rhythmic process. They are significantly enriched in the pathways of plant MAPK signaling, plant-pathogen interaction, phenylpropanoid biosynthesis, and plant hormone signal transduction. A set of transcription factors (TFs) and resistance (R) genes are among the GhCNGC31-regulated genes, which are significantly over-represented with the TCP and WRKY TFs families, as well as with the R genes of T (TIR) and TNL (TIR-NB-LRR) classes. Together, our results unraveled a critical role of GhCNGC31 for conferring resistance to Verticillium wilt in cotton.

TRX h2-PP2AC2 module serves as a convergence node for aluminum stress and leaf senescence signals, regulating cell death via ABA-mediated ROS pathway

ROS/redox signaling plays an important role in the regulation of signal transduction and acclimation pathways activated by multiple abiotic stresses and leaf senescence. However, the regulatory events that produce ROS under different stimuli are far from clear. Here, we report the elucidation of the molecular mechanism of an h type thioredoxin, AhTRX h2, positively regulates Al sensitivity and leaf senescence by promoting ROS. AhTRX h2 transcript levels increased greatly during both natural senescence and Al stress condition in peanut. Ectopic expression of AhTRX h2 in Arabidopsis conferred Al sensitivity as well as premature leaf senescence, manifested by multiple indices, including inhibiting root elongation, severe cell death, and accelerated expression of MC1 and CEX17. AhTRX h2 exhibited similar functions to AtTRX h2, as AhTRX h2 was able to restore the phenotypes of the AtTRX h2 defective mutant (trxh2-4) which showed Al tolerant and late senescence phenotypes. The knock down of AhTRX h2 markedly suppressed Al- and senescence-induced cell death in peanut. AhTRX h2 could recruit catalytic subunit of protein phosphatase 2A (PP2AC2) to form a stable complex. The interaction between AhTRX h2 and AtPP2AC2, as well as AhPP2AC2 and AtTRX h2 was also proved. Overexpression of AhPP2AC2 significantly enhanced Al sensitivity and leaf senescence in Arabidopsis. Protein stability assay revealed that AhTRX h2 was more stable during aging or aluminum stress. Moreover, PP2AC2 could greatly enhance the stability of AhTRX h2 in vivo. Consistent with these observations, overexpression of AhPP2AC2 effectively enhanced AhTRX h2-induced Al sensitivity and precocious leaf senescence. AhTRX h2 and AhPP2AC2 required ABA and ROS in response to cell death under Al stress and senescence, and it was evidence to suggest that ABA acted upstream of ROS in this process. Together, AhTRX h2 and AhPP2AC2 constitute a stable complex that promotes the accumulation of ABA and ROS, effectively regulate cell death. These findings suggest that TRX h2-PP2AC2-mediated pathway may be a widespread mechanism in regulating Al stress and leaf senescence.

OsCNGC7 modulates calcium dynamics and accelerates leaf senescence in rice

Calcium plays a crucial role in regulating plant senescence. However, the specific effects of increased intranuclear calcium versus cytoplasmic calcium on aging remain unclear. Cyclic nucleotide-gated channels (CNGCs), which manage Ca2⁺ levels in plant cells, are particularly significant in this context. These channels are known to relocate between the nuclear envelope and the plasma membrane in response to stress and developmental signals. Through this movement, CNGCs help regulate the balance of cytosolic and intranuclear Ca2⁺. In this study, we categorized the 16 CNGC genes in rice into five subgroups. OsCNGCs are notably expressed in leaves, especially during the reproductive stage. Both OsCNGC6 and OsCNGC7 exhibit dual localization to the plasma membrane and the nuclear envelope. Knockdown of OsCNGC7 led to reduced levels of Ca2⁺ and K⁺ in plants. Conversely, yeast expressing the OsCNGC7 gene showed increased sensitivity to Ca2⁺. Additionally, while the [Ca2⁺]cyt was maintained at relatively low levels in both wild-type and OsCNGC7-RNAi lines, the fluorescence intensity was significantly higher in OsCNGC7-overexpressing lines, particularly in the nucleus of root tips. Overexpression of OsCNGC7 resulted in enhanced stomatal opening and accelerated leaf senescence from the tillering stage to grain filling in rice. Treatment with MeJA rapidly induced OsCNGC7 expression, while knockdown of OsCNGC7 delayed both MeJA-induced and dark-induced leaf senescence. Further analysis revealed that OsCNGC7 interacts with OsKAT2 and OsALMT2. In conclusion, our findings highlight the distinct roles of OsCNGCs in regulating senescence. This knowledge could provide new strategies for manipulating plant senescence and enhancing crop productivity.

Altering cold-regulated gene expression decouples the salicylic acid-growth trade-off in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), overproduction of salicylic acid (SA) increases disease resistance and abiotic stress tolerance but penalizes growth. This growth-defense trade-off has hindered the adoption of SA-based disease management strategies in agriculture. However, investigation of how SA inhibits plant growth has been challenging because many SA-hyperaccumulating Arabidopsis mutants have developmental defects due to the pleiotropic effects of the underlying genes. Here, we heterologously expressed a bacterial SA synthase gene in Arabidopsis and observed that elevated SA levels decreased plant growth and reduced the expression of cold-regulated (COR) genes in a dose-dependent manner. Growth suppression was exacerbated at below-ambient temperatures. Severing the SA-responsiveness of individual COR genes was sufficient to overcome the growth inhibition caused by elevated SA at ambient and below-ambient temperatures while preserving disease- and abiotic-stress-related benefits. Our results show the potential of decoupling SA-mediated growth and defense trade-offs for improving crop productivity.

Transcriptome, hormonal, and secondary metabolite changes in leaves of DEFENSE NO DEATH 1 (DND1) silenced potato plants

DEFENSE NO DEATH 1 (DND1) is a cyclic nucleotide-gated ion channel protein. Earlier, it was shown that the silencing of DND1 in the potato (Solanum tuberosum L.) leads to resistance to late blight, powdery mildew, and gray mold diseases. At the same time, however, it can reduce plant growth and cause leaf necrosis. To obtain knowledge of the molecular events behind the pleiotropic effect of DND1 downregulation in the potato, metabolite and transcriptome analyses were performed on three DND1 silenced lines of the cultivar 'Désirée.' A massive increase in the salicylic acid content of leaves was detected. Concentrations of jasmonic acid and chlorogenic acid and their derivatives were also elevated. Expression of 1866 genes was altered in the same way in all three DND1 silenced lines, including those related to the synthesis of secondary metabolites. The activation of several alleles of leaf rust, late blight, and other disease resistance genes, as well as the induction of pathogenesis-related genes, was detected. WRKY and NAC transcription factor families were upregulated, whereas bHLHs were downregulated, indicating their central role in transcriptome changes. These results suggest that the maintenance of the constitutive defense state leads to the reduced growth of DND1 silenced potato plants.

GhCNGC13 and 32 Act as Critical Links between Growth and Immunity in Cotton

Cyclic nucleotide-gated ion channels (CNGCs) remain poorly studied in crop plants, most of which are polyploid. In allotetraploid Upland cotton (Gossypium hirsutum), silencing GhCNGC13 and 32 impaired plant growth and shoot apical meristem (SAM) development, while triggering plant autoimmunity. Both growth hormones (indole-3-acetic acid and gibberellin) and stress hormones (abscisic acid, salicylic acid, and jasmonate) increased, while leaf photosynthesis decreased. The silenced plants exhibited an enhanced resistance to Botrytis cinerea; however, Verticillium wilt resistance was weakened, which was associated with LIPOXYGENASE2 (LOX2) downregulation. Transcriptomic analysis of silenced plants revealed 4835 differentially expressed genes (DEGs) with functional enrichment in immunity and photosynthesis. These DEGs included a set of transcription factors with significant over-representation in the HSF, NAC, and WRKY families. Moreover, numerous members of the GhCNGC family were identified among the DEGs, which may indicate a coordinated action. Collectively, our results suggested that GhCNGC13 and 32 functionally link to photosynthesis, plant growth, and plant immunity. We proposed that GhCNGC13 and 32 play a critical role in the "growth-defense tradeoff" widely observed in crops.

CRISPR/Cas as a Genome-Editing Technique in Fruit Tree Breeding

CRISPR (short for "Clustered Regularly Interspaced Short Palindromic Repeats") is a technology that research scientists use to selectively modify the DNA of living organisms. CRISPR was adapted for use in the laboratory from the naturally occurring genome-editing systems found in bacteria. In this work, we reviewed the methods used to introduce CRISPR/Cas-mediated genome editing into fruit species, as well as the impacts of the application of this technology to activate and knock out target genes in different fruit tree species, including on tree development, yield, fruit quality, and tolerance to biotic and abiotic stresses. The application of this gene-editing technology could allow the development of new generations of fruit crops with improved traits by targeting different genetic segments or even could facilitate the introduction of traits into elite cultivars without changing other traits. However, currently, the scarcity of efficient regeneration and transformation protocols in some species, the fact that many of those procedures are genotype-dependent, and the convenience of segregating the transgenic parts of the CRISPR system represent the main handicaps limiting the potential of genetic editing techniques for fruit trees. Finally, the latest news on the legislation and regulations about the use of plants modified using CRISPR/Cas systems has been also discussed.

Identification of CNGCs in Glycine max and Screening of Related Resistance Genes after Fusarium solani Infection

Cyclic nucleotide-gated channels (CNGCs), non-selective cation channels localised on the plasmalemma, are involved in growth, development, and regulatory mechanisms in plants during adverse stress. To date, CNGC gene families in multiple crops have been identified and analysed. However, there have been no systematic studies on the evolution and development of CNGC gene families in legumes. Therefore, in the present study, via transcriptome analysis, we identified 143 CNGC genes in legumes, and thereafter, classified and named them according to the grouping method used for Arabidopsis thaliana. Functional verification for disease stress showed that four GmCNGCs were specifically expressed in the plasmalemma during the stress process. Further, functional enrichment analysis showed that their mode of participation and coordination included inorganic ion concentration regulation inside and outside the membrane via the transmembrane ion channel and participation in stress regulation via signal transduction. The CNGC family genes in G. max involved in disease stress were also identified and physiological stress response and omics analyses were also performed. Our preliminary results revealed the basic laws governing the involvement of CNGCs in disease resistance in G. max, providing important gene resources and a theoretical reference for the breeding of resistant soybean.

Phenotypes of cyclic nucleotide-gated cation channel mutants: probing the nature of native channels

Plant cyclic nucleotide gated channels (CNGCs) facilitate cytosolic Ca²⁺ influx as an early step in numerous signaling cascades. CNGC-mediated Ca²⁺ elevations are essential for plant immune defense and high temperature thermosensing. In the present study, we evaluated phenotypes of CNGC2, CNGC4, CNGC6, and CNGC12 null mutants in these two pathways. It is shown CNGC2, CNGC4, and CNGC6 physically interact in vivo, whereas CNGC12 does not. CNGC involvement in immune signaling was evaluated by monitoring mutant response to elicitor peptide Pep3. Pep3 response cascades involving CNGCs included mitogen-activated kinase activation mediated by Ca²⁺ -dependent protein kinase phosphorylation. Pep3-induced reactive oxygen species generation was impaired in cngc2, cngc4, and cngc6, but not in cngc12, suggesting that CNGC2, CNGC4, and CNGC6 (which physically interact) may be components of a multimeric CNGC channel complex for immune signaling. However, unlike cngc2 and cngc4, cngc6 is not sensitive to high Ca²⁺ and displays no pleiotropic dwarfism. All four cngc mutants showed thermotolerance compared to wild-type, although CNGC12 does not interact with the other three CNGCs. These results imply that physically interacting CNGCs may, in some cases, function in a signaling cascade as components of a heteromeric channel complex, although this may not be the case in other signaling pathways.

Genome-Wide Analysis and Expression of Cyclic Nucleotide-Gated Ion Channel (CNGC) Family Genes under Cold Stress in Mango (Mangifera indica)

The 'king of fruits' mango (Mangifera indica) is widely cultivated in tropical areas and has been threatened by frequent extreme cold weather. Cyclic nucleotide-gated ion channel (CNGC) genes have an important function in the calcium-mediated development and cold response of plants. However, few CNGC-related studies are reported in mango, regardless of the mango cold stress response. In this study, we identified 43 CNGC genes in mango showing tissue-specific expression patterns. Five MiCNGCs display more than 3-fold gene expression induction in the fruit peel and leaf under cold stress. Among these, MiCNGC9 and MiCNGC13 are significantly upregulated below 6 °C, suggesting their candidate functions under cold stress. Furthermore, cell membrane integrity was damaged at 2 °C in the mango leaf, as shown by the content of malondialdehyde (MDA), and eight MiCNGCs are positively correlated with MDA contents. The high correlation between MiCNGCs and MDA implies MiCNGCs might regulate cell membrane integrity by regulating MDA content. Together, these findings provide a valuable guideline for the functional characterization of CNGC genes and will benefit future studies related to cold stress and calcium transport in mango.

Melatonin delays ABA-induced leaf senescence via H2 O2 -dependent calcium signalling

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H₂ O₂ accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA-induced stress. Both melatonin and H₂ O₂ induced the accumulation of cytoplasmic-free Ca²⁺ ([Ca²⁺ ]_cyt ) in response to ABA, while blocking of Ca²⁺ influx channels attenuated melatonin- and H₂ O₂ -induced ABA tolerance. CmRBOHD overexpression induced [Ca²⁺ ]_cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca²⁺ ]_cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H₂ O₂  and Ca²⁺ attenuated ABA-induced K⁺ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K⁺ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H₂ O₂ production that triggers [Ca²⁺ ]_cyt accumulation and subsequently inhibits K⁺ efflux and delays cell death/leaf senescence in response to ABA.

Comparative genomics and evolutionary analysis of plant CNGCs

Comparative genomics and computational biology offer powerful research tools for studying evolutionary mechanisms of organisms, and the identification and characterization of conserved/distant genes and gene families. The plant CNGC gene family encodes evolutionary conserved ion channel proteins involved in important signaling pathways and biological functions. The fundamental ideas and standard procedures for genome-wide identification and evolutionary analysis of plant cyclic nucleotide-gated ion channels employing various software, tools, and online servers have been discussed. In particular, this developed method focused on practical procedures involving the comparative analysis of paralogs and orthologs of CNGC genes in different plant species at different levels including phylogenetic analysis, nomenclature and classification, gene structure, molecular protein evolution, and duplication events as mechanisms of gene family expansion and synteny.

Systematic analysis of CNGCs in cotton and the positive role of GhCNGC32 and GhCNGC35 in salt tolerance

BACKGROUND

Cyclic nucleotide-gated ion channels (CNGCs) are calcium-permeable channels that participate in a variety of biological functions, such as signaling pathways, plant development, and environmental stress and stimulus responses. Nevertheless, there have been few studies on CNGC gene family in cotton.

RESULTS

In this study, a total of 114 CNGC genes were identified from the genomes of 4 cotton species. These genes clustered into 5 main groups: I, II, III, IVa, and IVb. Gene structure and protein motif analysis showed that CNGCs on the same branch were highly conserved. In addition, collinearity analysis showed that the CNGC gene family had expanded mainly by whole-genome duplication (WGD). Promoter analysis of the GhCNGCs showed that there were a large number of cis-acting elements related to abscisic acid (ABA). Combination of transcriptome data and the results of quantitative RT-PCR (qRT-PCR) analysis revealed that some GhCNGC genes were induced in response to salt and drought stress and to exogenous ABA. Virus-induced gene silencing (VIGS) experiments showed that the silencing of the GhCNGC32 and GhCNGC35 genes decreased the salt tolerance of cotton plants (TRV:00). Specifically, physiological indexes showed that the malondialdehyde (MDA) content in gene-silenced plants (TRV:GhCNGC32 and TRV:GhCNGC35) increased significantly under salt stress but that the peroxidase (POD) activity decreased. After salt stress, the expression level of ABA-related genes increased significantly, indicating that salt stress can trigger the ABA signal regulatory mechanism.

CONCLUSIONS

we comprehensively analyzed CNGC genes in four cotton species, and found that GhCNGC32 and GhCNGC35 genes play an important role in cotton salt tolerance. These results laid a foundation for the subsequent study of the involvement of cotton CNGC genes in salt tolerance.

Role of Nitric Oxide in Plant Senescence

In plants senescence is the final stage of plant growth and development that ultimately leads to death. Plants experience age-related as well as stress-induced developmental ageing. Senescence involves significant changes at the transcriptional, post-translational and metabolomic levels. Furthermore, phytohormones also play a critical role in the programmed senescence of plants. Nitric oxide (NO) is a gaseous signalling molecule that regulates a plethora of physiological processes in plants. Its role in the control of ageing and senescence has just started to be elucidated. Here, we review the role of NO in the regulation of programmed cell death, seed ageing, fruit ripening and senescence. We also discuss the role of NO in the modulation of phytohormones during senescence and the significance of NO-ROS cross-talk during programmed cell death and senescence.

Transcriptome Analysis of the Molecular Patterns of Pear Plants Infected by Two Colletotrichum fructicola Pathogenic Strains Causing Contrasting Sets of Leaf Symptoms

Colletotrichum fructicola infects pear leaves, resulting in two major symptoms: tiny black spots (TS) followed by severe early defoliation and big necrotic lesions (BnL) without apparent damage depending on the pathotypes. How the same fungal species causes different symptoms remains unclear. To understand the molecular mechanism underlying the resulting diseases and the diverse symptoms, two C. fructicola pathogenetic strains (PAFQ31 and PAFQ32 responsible for TS and BnL symptoms, respectively) were inoculated on Pyrus pyrifolia leaves and subjected to transcriptome sequencing at the quiescent stage (QS) and necrotrophic stage (NS), respectively. In planta, the genes involved in the salicylic acid (SA) signaling pathway were upregulated at the NS caused by the infection of each strain. In contrast, the ethylene (ET), abscisic acid (ABA), and jasmonic acid (JA) signaling pathways were specifically related to the TS symptoms caused by the infection of strain PAFQ31, corresponding to the yellowish and early defoliation symptoms triggered by the strain infection. Correspondingly, SA was accumulated in similar levels in the leaves infected by each strain at NS, but JA was significantly higher in the PAFQ31-infected as measured using high-performance liquid chromatography. Weighted gene co-expression network analysis also reveals specific genes, pathways, phytohormones, and transcription factors (TFs) associated with the PAFQ31-associated early defoliation. Taken together, these data suggest that specific metabolic pathways were regulated in P. pyrifolia in response to the infection of two C. fructicola pathotypes resulting in the diverse symptoms: JA, ET, and ABA accumulated in the PAFQ31-infected leaves, which negatively affected the chlorophyll metabolism and photosynthesis pathways while positively affecting the expression of senescence-associated TFs and genes, resulted in leaf yellowing and defoliation; whereas SA inhibited JA-induced gene expression in the PAFQ32-infected leaves, which led to hypersensitive response-like reaction and BnL symptoms.

Dynamic Expression, Differential Regulation and Functional Diversity of the CNGC Family Genes in Cotton

Cyclic nucleotide-gated channels (CNGCs) constitute a family of non-selective cation channels that are primarily permeable to Ca2+ and activated by the direct binding of cyclic nucleotides (i.e., cAMP and cGMP) to mediate cellular signaling, both in animals and plants. Until now, our understanding of CNGCs in cotton (Gossypium spp.) remains poorly addressed. In the present study, we have identified 40, 41, 20, 20, and 20 CNGC genes in G. hirsutum, G. barbadense, G. herbaceum, G. arboreum, and G. raimondii, respectively, and demonstrated characteristics of the phylogenetic relationships, gene structures, chromosomal localization, gene duplication, and synteny. Further investigation of CNGC genes in G. hirsutum, named GhCNGC1-40, indicated that they are not only extensively expressed in various tissues and at different developmental stages, but also display diverse expression patterns in response to hormones (abscisic acid, salicylic acid, methyl jasmonate, ethylene), abiotic (salt stress) and biotic (Verticillium dahlia infection) stimuli, which conform with a variety of cis-acting regulatory elements residing in the promoter regions; moreover, a set of GhCNGCs are responsive to cAMP signaling during cotton fiber development. Protein–protein interactions supported the functional aspects of GhCNGCs in plant growth, development, and stress responses. Accordingly, the silencing of the homoeologous gene pair GhCNGC1&18 and GhCNGC12&31 impaired plant growth and development; however, GhCNGC1&18-silenced plants enhanced Verticillium wilt resistance and salt tolerance, whereas GhCNGC12&31-silenced plants had opposite effects. Together, these results unveiled the dynamic expression, differential regulation, and functional diversity of the CNGC family genes in cotton. The present work has laid the foundation for further studies and the utilization of CNGCs in cotton genetic improvement.

BrCNGC gene family in field mustard: genome-wide identification, characterization, comparative synteny, evolution and expression profiling

CNGCs are ligand-gated calcium signaling channels, which participate in important biological processes in eukaryotes. However, the CNGC gene family is not well-investigated in Brassica rapa L. (i.e., field mustard) that is economically important and evolutionary model crop. In this study, we systematically identified 29 member genes in BrCNGC gene family, and studied their physico-chemical properties. The BrCNGC family was classified into four major and two sub phylogenetic groups. These genes were randomly localized on nine chromosomes, and dispersed into three sub-genomes of B. rapa L. Both whole-genome triplication and gene duplication (i.e., segmental/tandem) events participated in the expansion of the BrCNGC family. Using in-silico bioinformatics approaches, we determined the gene structures, conserved motif compositions, protein interaction networks, and revealed that most BrCNGCs can be regulated by phosphorylation and microRNAs of diverse functionality. The differential expression patterns of BrCNGC genes in different plant tissues, and in response to different biotic, abiotic and hormonal stress types, suggest their strong role in plant growth, development and stress tolerance. Notably, BrCNGC-9, 27, 18 and 11 exhibited highest responses in terms of fold-changes against club-root pathogen Plasmodiophora brassicae, Pseudomonas syringae pv. maculicola, methyl-jasmonate, and trace elements. These results provide foundation for the selection of candidate BrCNGC genes for future breeding of field mustard.

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Identification and Expression Profiling Analysis of the Cation/Ca2+ Exchanger (CCX) Gene Family: Overexpression of SlCCX1-LIKE Regulates the Leaf Senescence in Tomato Flowering Phase

Cation gradients in plant cellular compartments are maintained by the synergistic actions of various ion exchangers, pumps, and channels. Cation/Ca2+ exchanger (CCX) is one of the clades of the Ca2+/cation antiporter super family. Here, five SlCCX genes were identified in tomato. Sequence analysis indicated that SlCCXs have the conserved motifs as the CCX domain. Analysis of the expression level of each member of tomato CCX gene family under cation (Mg2+, Mn2+, Na+, and Ca2+) treatment was determined by qRT-PCR. Tomato CCX demonstrated different degrees of responding to cation treatment. Changes in SlCCX1-LIKE expression was induced by Mg2+ and Mn2+ treatment. Analysis of the expression of SlCCX genes in different tissues demonstrated that constitutive high expression of a few genes, including SlCCX1-LIKE and SlCCX5, indicated their role in tomato organ growth and development. Overexpression of SlCCX1-LIKE dramatically induced leaf senescence. Transcriptome analysis showed that genes related to ROS and several IAA signaling pathways were significantly downregulated, whereas ETH and ABA signaling pathway-related genes were upregulated in overexpression of SlCCX1-LIKE (OE-SlCCX1-LIKE) plants, compared with the wild type (WT). Moreover, overexpression of SlCCX1-LIKE plants accumulated more ROS content but less Mg2+ content. Collectively, the findings of this study provide insights into the base mechanism through which CCXs regulate leaf senescence in tomato.

Effects of nitric oxide treatment on lignin biosynthesis and texture properties at wound sites of muskmelons

Lignin is an important component of the healing tissue in fruits. In this study, we treated muskmelon (Cucumis melo L. cv. "Manao") fruit with exogenous nitric oxide (NO) donor sodium nitroprusside (SNP) to observe and analyze its effect on lignin synthesis and accumulation during healing. Results showed that SNP treatment enhanced the contents of endogenous NO and H₂O₂, increased the activities of phenylalanine ammonia lyase, cinnamate 4 hydroxylase, cinnamyl alcohol dehydrogenase, and peroxidase, and raised the contents of sinapyl alcohol, coniferyl alcohol, coumaryl alcohol, and lignin. SNP augmented the hardness of the healing tissue and decreased its resilience, springiness, and cohesiveness. In addition, SNP treatment effectively reduced the weight loss and disease index of wounded muskmelons. All these results suggest that lignin metabolism mediated by NO play a crucial role in wound healing of muskmelons.

Valine-Glutamine Proteins in Plant Responses to Oxygen and Nitric Oxide

Multigene families coding for valine-glutamine (VQ) proteins have been identified in all kind of plants but chlorophytes. VQ proteins are transcriptional regulators, which often interact with WRKY transcription factors to regulate gene expression sometimes modulated by reversible phosphorylation. Different VQ-WRKY complexes regulate defense against varied pathogens as well as responses to osmotic stress and extreme temperatures. However, despite these well-known functions, new regulatory activities for VQ proteins are still to be explored. Searching public Arabidopsis thaliana transcriptome data for new potential targets of VQ-WRKY regulation allowed us identifying several VQ protein and WRKY factor encoding genes that were differentially expressed in oxygen-related processes such as responses to hypoxia or ozone-triggered oxidative stress. Moreover, some of those were also differentially regulated upon nitric oxide (NO) treatment. These subsets of VQ and WRKY proteins might combine into different VQ-WRKY complexes, thus representing a potential regulatory core of NO-modulated and O₂-modulated responses. Given the increasing relevance that gasotransmitters are gaining as plant physiology regulators, and particularly considering the key roles exerted by O₂ and NO in regulating the N-degron pathway-controlled stability of transcription factors, VQ and WRKY proteins could be instrumental in regulating manifold processes in plants.

The Complex Story of Plant Cyclic Nucleotide-Gated Channels

Plant cyclic nucleotide-gated channels (CNGCs) are tetrameric cation channels which may be activated by the cyclic nucleotides (cNMPs) adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP). The genome of Arabidopsis thaliana encodes 20 CNGC subunits associated with aspects of development, stress response and immunity. Recently, it has been demonstrated that CNGC subunits form heterotetrameric complexes which behave differently from the homotetramers produced by their constituent subunits. These findings have widespread implications for future signalling research and may help explain how specificity can be achieved by CNGCs that are known to act in disparate pathways. Regulation of complex formation may involve cyclic nucleotide-gated channel-like proteins.

Cyclic AMP: A Polyhedral Signalling Molecule in Plants

The cyclic nucleotide cAMP (3',5'-cyclic adenosine monophosphate) is nowadays recognised as an important signalling molecule in plants, involved in many molecular processes, including sensing and response to biotic and abiotic environmental stresses. The validation of a functional cAMP-dependent signalling system in higher plants has spurred a great scientific interest on the polyhedral role of cAMP, as it actively participates in plant adaptation to external stimuli, in addition to the regulation of physiological processes. The complex architecture of cAMP-dependent pathways is far from being fully understood, because the actors of these pathways and their downstream target proteins remain largely unidentified. Recently, a genetic strategy was effectively used to lower cAMP cytosolic levels and hence shed light on the consequences of cAMP deficiency in plant cells. This review aims to provide an integrated overview of the current state of knowledge on cAMP's role in plant growth and response to environmental stress. Current knowledge of the molecular components and the mechanisms of cAMP signalling events is summarised.

Loss of proton/calcium exchange 1 results in the activation of plant defense and accelerated senescence in Arabidopsis

Cytosolic Ca²⁺ increases in response to many stimuli. CAX1 (H⁺/Ca²⁺ exchanger 1) maintains calcium homeostasis by transporting calcium from the cytosol to vacuoles. Here, we determined that the cax1 mutant exhibits enhanced resistance against both an avirulent biotrophic pathogen Pst-avrRpm1 (Pseudomonas syringae pv tomato DC3000 avrRpm1), and a necrotrophic pathogen, B. cinerea (Botrytis cinerea). The defense hormone SA (salicylic acid) and phytoalexin scopoletin, which fight against biotrophs and necrotrophs respectively, accumulated more in cax1 than wild-type. Moreover, the cax1 mutant exhibited early senescence after exogenous Ca²⁺ application. The accelerated senescence in the cax1 mutant was dependent on SID2 (salicylic acid induction deficient 2) but not on NPR1 (nonexpressor of pathogenesis-related genes1). Additionally, the introduction of CAX1 into the cax1 mutant resulted in phenotypes similar to that of wild-type in terms of Ca²⁺-conditioned senescence and Pst-avrRpm1 and B. cinerea infections. However, disruption of CAX3, the homolog of CAX1, did not produce an obvious phenotype. Moreover, exogenous Ca²⁺ application on plants resulted in increased resistance to both Pst-avrRpm1 and B. cinerea. Therefore, we conclude that the disruption of CAX1, but not CAX3, causes the activation of pathogen defense mechanisms, probably through the manipulation of calcium homeostasis or other signals.

Arabidopsis cyclic nucleotide-gated channel 6 is negatively modulated by multiple calmodulin isoforms during heat shock

An increased concentration of cytosolic Ca2+ is an early response of plant cells to heat shock. Arabidopsis cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca2+ influx and is activated by cAMP. However, it remains unclear how the Ca2+ conductivity of CNGC6 is negatively regulated under the elevated cytosolic Ca2+ concentration. In this study, Arabidopsis calmodulin isoforms CaM1/4, CaM2/3/5, CaM6, and CaM7 were found to bind to CNGC6 to varying degrees, and this binding was dependent on the presence of Ca2+ and IQ6, an atypical isoleucine-glutamine motif in CNGC6. Knockout of CaM2, CaM3, CaM5, and CaM7 genes led to a marked increase in plasma membrane inward Ca2+ current under heat shock conditions; however, knockout of CaM1, CaM4, and CaM6 genes had no significant effect on plasma membrane Ca2+ current. Moreover, the deletion of IQ6 from CNGC6 led to a marked increase in plasma membrane Ca2+ current under heat shock conditions. Taken together, the data suggest that CNGC6-mediated Ca2+ influx is likely to be negatively regulated by CaM2/3/5 and CaM7 isoforms under heat shock conditions, and that IQ6 plays an important role in CaM binding and the feedback regulation of the channel.

CRISPR/Cas9-Mediated Mutagenesis of MdCNGC2 in Apple Callus and VIGS-Mediated Silencing of MdCNGC2 in Fruits Improve Resistance to Botryosphaeria dothidea

Cyclic nucleotide-gated ion channels (CNGCs) have been reported to be involved in multiple plant physiological processes. Their involvement in plant immunity has been studied in several herbal plant species. It remains unclear whether CNGCs in woody plants play a similar role in plant immunity. In the present study, we identified an apple CNGC (designated as MdCNGC2), which is the homolog of Arabidopsis CNGC2. Analysis of tissue distribution revealed that MdCNGC2 was expressed in all tested tissues. Abundant transcripts of MdCNGC2 were observed in leaves and shoot bark. Low expression was observed in fruits and roots. MdCNGC2 expression was induced in apple callus and shoot bark by Botryosphaeria dothidea. The induction of MdCNGC2 was significantly higher in susceptible cultivars "Fuji," "Ralls Janet," and "Gala" compared to the resistant cultivar "Jiguan," suggesting that MdCNGC2 may be a negative regulator of resistance to B. dothidea. MdCNGC2 mutagenesis mediated by gene editing based on the CRISPR/Cas9 system led to constitutive accumulation of SA in apple callus. A culture filtrate of B. dothidea (BCF) induced the expression of several defense-related genes including MdPR1, MdPR2, MdPR4, MdPR5, MdPR8, and MdPR10a. Moreover, the induction of these genes was significantly higher in mdcngc2 mutant (MUT) callus than in wild type (WT) callus. Further analysis showed that the spread of B. dothidea was significantly lower on MUT callus than on WT callus. Knockdown of the MdCNGC2 gene reduced lesions caused by B. dothidea in apple fruits. These results collectively indicate that MdCNGC2 is a negative regulator of resistance to B. dothidea in apple callus.

Cyclic nucleotide gated channels (CNGCs) in plant signalling-Current knowledge and perspectives

Cell signaling is an evolutionarily conserved mechanism that responds and adapts to various internal and external factors. Generally, a signal is mediated by various signaling molecules and is transferred to a cascade of effector proteins. To date, there is significant evidence that cyclic nucleotides (cNMPs), e.g., adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP), may represent important elements of many signaling pathways in plants. However, in contrast to the impressive progress made in understanding cyclic nucleotide signaling in mammalian hosts, only few studies have investigated this topic in plants. Existing evidence indicates that cNMPs participate in growth and developmental processes, as well as the response to various stresses. Once synthesized by adenylyl or guanylyl cyclases, these signals are transduced by acting through a number of cellular effectors. The regulatory effects of cNMPs in eukaryotes can be mediated via various downstream effector proteins, such as protein kinases, Exchange Protein directly Activated by cAMP (EPAC), and Cyclic Nucleotide-Gated ion Channels (CNGC). These proteins sense changes in intracellular cNMP levels and regulate numerous cellular responses. Moreover, the amplitude of cNMP levels and the duration of its signal in the cell is also governed by phosphodiesterases (PDEs), enzymes that are responsible for the breakdown of cNMPs. Data collected in recent years strongly suggest that cyclic nucleotide gated channels are the main cNMP effectors in plant cells. These channels are important cellular switches that transduce changes in intracellular concentrations of cyclic nucleotides into changes in membrane potential and ion concentrations. Structurally, these channels belong to the superfamily of pore-loop cation channels. In this review, we provide an overview of the molecular properties of CNGC structure, regulation and ion selectivity, and subcellular localization, as well as describing the signal transduction pathways in which these channels are involved. We will also summarize recent insights into the role of CNGC proteins in plant growth, development and response to stressors.

Nitric oxide alleviates cell death through protein S-nitrosylation and transcriptional regulation during the ageing of elm seeds

Seed ageing is a major problem in the conservation of germplasm resources. The involvement of possible signalling molecules during seed deterioration needs to be identified. In this study, we confirmed that nitric oxide (NO), a key signalling molecule in plants, plays a positive role in the resistance of elm seeds to deterioration. To explore which metabolic pathways were affected by NO, an untargeted metabolomic analysis was conducted, and 163 metabolites could respond to both NO and the ageing treatment. The primary altered pathways include glutathione, methionine, and carbohydrate metabolism. The genes involved in glutathione and methionine metabolism were up-regulated by NO at the transcriptional level. Using a biotin switch method, proteins with an NO-dependent post-translational modification were screened during seed deterioration, and 82 putative S-nitrosylated proteins were identified. Eleven of these proteins were involved in carbohydrate metabolism, and the activities of the three enzymes were regulated by NO. In combination, the results of the metabolomic and S-nitrosoproteomic studies demonstrated that NO could activate glycolysis and inhibit the pentose phosphate pathway. In summary, the combination of these results demonstrated that NO could modulate carbohydrate metabolism at the post-translational level and regulate glutathione and methionine metabolism at the transcriptional level. It provides initial insights into the regulatory mechanisms of NO in seed deterioration.

Changes in nitric oxide levels and their relationship with callose deposition during the interaction between soybean and Soybean mosaic virus

The present study aimed to investigate changes in nitric oxide (NO) level and its relationship with callose deposition during the interaction between soybean and Soybean mosaic virus (SMV). Soybean cv. 'Jidou 7' and SMV strains N3 and SC-8 were used to constitute incompatible and compatible combinations. Intracellular NO was labelled with the NO-specific fluorescence probe DAF-FM DA. Confocal laser scanning microscopy (CLSM) was then used to observe changes in NO production during SMV infection-induced defence responses in soybean. The results showed NO fluorescence increased rapidly at 2-72 h post-inoculation, peaked at 72 h and then decreased in the incompatible combination. However, in the compatible combination, extremely weak NO fluorescence appeared in the early stage (2-24 h) post-inoculation, but was not observed thereafter. Injections of the NO scavenger c-PTIO prior to inoculation postponed the onset of NO production to 48 or 72 h post-inoculation. The same occurred when injections of NR or NOS inhibitors were applied prior to inoculation. The observation of callose fluorescence in the incompatible combination revealed that either the elimination or reduction of NO in the early stage led to a delay in callose formation, enabling the virus to cause systemic infection. Together with our previous findings, this study indicates that viral infection could induce NO production and callose deposition during the incompatible interaction between soybean and SMV. The production of NO involves NR and NOS enzymatic pathways, and NO mediates the process of callose deposition at plasmodesmata.

The Integration of Electrical Signals Originating in the Root of Vascular Plants

Plants have developed different signaling systems allowing for the integration of environmental cues to coordinate molecular processes associated to both early development and the physiology of the adult plant. Research on systemic signaling in plants has traditionally focused on the role of phytohormones as long-distance signaling molecules, and more recently the importance of peptides and miRNAs in building up this communication process has also been described. However, it is well-known that plants have the ability to generate different types of long-range electrical signals in response to different stimuli such as light, temperature variations, wounding, salt stress, or gravitropic stimulation. Presently, it is unclear whether short or long-distance electrical communication in plants is linked to nutrient uptake. This review deals with aspects of sensory input in plant roots and the propagation of discrete signals to the plant body. We discuss the physiological role of electrical signaling in nutrient uptake and how nutrient variations may become an electrical signal propagating along the plant.

Involvement of NADPH oxidase isoforms in the production of O2- manipulated by ABA in the senescing leaves of early-senescence-leaf (esl) mutant rice (Oryza sativa)

In this study, the differences in reactive oxygen species (ROS) generation and abscisic acid (ABA) accumulation in senescing leaves were investigated by early-senescence-leaf (esl) mutant and its wild type, to clarify the relationship among ABA levels, ROS generation, and NADPH oxidase (Nox) in senescing leaves of rice (Oryza sativa). The temporal expression levels of OsNox isoforms in senescing leaves and their expression patterns in response to ABA treatment were determined through quantitative real-time reverse transcription PCR (qRT-PCR). Results showed that the flag leaf of the esl mutant generated more O2- concentrations and accumulated higher ABA levels than the wild-type cultivar did in the grain-filling stage. Exogenous ABA treatment induced O2- generation; however, it was depressed by diphenyleneiodonium chloride (DPI) pretreatment in the detached leaf segments. This finding suggested the involvement of NADPH oxidase in ABA-induced O2- generation. The esl mutant exhibited significantly higher expression of OsNox2, OsNox5, OsNox6, and OsNox7 in the initial of grain-filling stage, followed by sharply decrease. The transcriptional levels of OsNox1, OsNox3, and OsFR07 in the flag leaf of the esl mutant were significantly lower than those in the wild-type cultivar. The expression levels of OsNox2, OsNox5, OsNox6, and OsNox7 were significantly enhanced by exogenous ABA treatments. The enhanced expression levels of OsNox2 and OsNox6 were dependent on the duration of ABA treatment. The inducible expression levels of OsNox5 and OsNox7 were dependent on ABA concentrations. By contrast, exogenous ABA treatment severely repressed the transcripts of OsNox1, OsNox3, and OsFR07 in the detached leaf segments. Therefore, OsNox2, OsNox5, OsNox6, and OsNox7 were probably involved in the ABA-induced O2- generation in the initial stage of leaf senescence. Subsequently, other oxidases activated in deteriorating cells were associated with ROS generation and accumulation in the senescing leaves of the esl mutant. Conversely, OsNox1, OsNox3, and OsFR07 were not associated with ABA-induced O2- generation during leaf senescence.

The EPR Method for Detecting Nitric Oxide in Plant Senescence

Nitric oxide (NO) is gaining increasing attention as a central molecule with diverse signaling functions. It has been shown that NO acts as a negative regulator of leaf senescence. In this chapter, we describe a highly selective method, electron paramagnetic resonance ([EPR], also known as electron spin resonance [ESR]), for NO determination in leaf senescence. An iron complex of ferrous and mononitrosyl dithiocarbamate (Fe²⁺(DETC)₂) is used as a chelating agent for NO. Using ethyl acetate as extracting solvent, the NOFe²⁺(DETC)₂ complex is extracted and determined by EPR spectrometer.

Calmodulin 1 Regulates Senescence and ABA Response in Arabidopsis

Cellular calcium acts as a second messenger and regulates diverse developmental events and stress responses. Cytosolic calcium has long been considered as an important regulator of senescence, however, the role of Ca²⁺ in plant senescence has remained elusive. Here we show that the Calmodulin 1 (CaM1) gene, which encodes Ca²⁺-binding protein calmodulin 1, positively regulates leaf senescence in Arabidopsis. Yellowing of leaves, accumulation of reactive oxygen species (ROS), and expression of the senescence-associated gene 12 (SAG12) were significantly enhanced in CaM1 overexpression plants. In contrast, abscisic acid (ABA)-triggered ROS production and stomatal closure were reduced in amiRNA-CaM1 plants. We found a positive-feedback regulation loop among three signaling components, CaM1, RPK1, and RbohF, which physically associate with each other. RPK1 positively regulates the expression of the CaM1 gene, and the CaM1 protein, in turn, up-regulates RbohF gene expression. Interestingly, the expression of CaM1 was down-regulated in rbohD, rbohF, and rbohD/F mutants. We show that CaM1 positively regulates ROS production, leaf senescence, and ABA response in Arabidopsis.

Comprehensive genomic analysis of the CNGC gene family in Brassica oleracea: novel insights into synteny, structures, and transcript profiles

Background

The cyclic nucleotide-gated ion channel (CNGC) family affects the uptake of cations, growth, pathogen defence, and thermotolerance in plants. However, the systematic identification, origin and function of this gene family has not been performed in Brassica oleracea, an important vegetable crop and genomic model organism.

Results

In present study, we identified 26 CNGC genes in B. oleracea genome, which are non-randomly localized on eight chromosomes, and classified into four major (I-IV) and two sub-groups (i.e., IV-a and IV-b). The BoCNGC family is asymmetrically fractioned into the following three sub-genomes: least fractionated (14 genes), most fractionated-I (10), and most fractionated-II (2). The syntenic map of BoCNGC genes exhibited strong relationships with the model Arabidopsis thaliana and B. rapa CNGC genes and provided markers for defining the regions of conserved synteny among the three genomes. Both whole-genome triplication along with segmental and tandem duplications contributed to the expansion of this gene family. We predicted the characteristics of BoCNGCs regarding exon-intron organisations, motif compositions and post-translational modifications, which diversified their structures and functions. Using orthologous Arabidopsis CNGCs as a reference, we found that most CNGCs were associated with various protein–protein interaction networks involving CNGCs and other signalling and stress related proteins. We revealed that five microRNAs (i.e., bol-miR5021, bol-miR838d, bol-miR414b, bol-miR4234, and bol-miR_new2) have target sites in nine BoCNGC genes. The BoCNGC genes were differentially expressed in seven B. oleracea tissues including leaf, stem, callus, silique, bud, root and flower. The transcript abundance levels quantified by qRT-PCR assays revealed that BoCNGC genes from phylogenetic Groups I and IV were particularly sensitive to cold stress and infections with bacterial pathogen Xanthomonas campestris pv. campestris, suggesting their importance in abiotic and biotic stress responses.

Conclusion

Our comprehensive genome-wide analysis represents a rich data resource for studying new plant gene families. Our data may also be useful for breeding new B. oleracea cultivars with improved productivity, quality, and stress resistance.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4244-y) contains supplementary material, which is available to authorized users.

CNGC2 Is a Ca2+ Influx Channel That Prevents Accumulation of Apoplastic Ca2+ in the Leaf

Ca²⁺ is absorbed by roots and transported upward through the xylem to the apoplastic space of the leaf, after which it is deposited into the leaf cell. In Arabidopsis (Arabidopsis thaliana), the tonoplast-localized Ca²⁺/H⁺ transporters CATION EXCHANGER1 (CAX1) and CAX3 sequester Ca²⁺ from the cytosol into the vacuole, but it is not known what transporter mediates the initial Ca²⁺ influx from the apoplast to the cytosol. Here, we report that Arabidopsis CYCLIC NUCLEOTIDE-GATED CHANNEL2 (CNGC2) encodes a protein with Ca²⁺ influx channel activity and is expressed in the leaf areas surrounding the free endings of minor veins, which is the primary site for Ca²⁺ unloading from the vasculature and influx into leaf cells. Under hydroponic growth conditions, with 0.1 mm Ca²⁺, both Arabidopsis cngc2 and cax1cax3 loss-of-function mutants grew normally. Increasing the Ca²⁺ concentration to 10 mm induced H₂O₂ accumulation, cell death, and leaf senescence and partially suppressed the hypersensitive response to avirulent pathogens in the mutants but not in the wild type. In vivo apoplastic Ca²⁺ overaccumulation was found in the leaves of cngc2 and cax1cax3 but not the wild type under the 10 mm Ca²⁺ condition, as monitored by Oregon Green BAPTA 488 5N, a low-affinity and membrane-impermeable Ca²⁺ probe. Our results indicate that CNGC2 likely has no direct roles in leaf development or the hypersensitive response but, instead, that CNGC2 could mediate Ca²⁺ influx into leaf cells. Finally, the in vivo extracellular Ca²⁺ imaging method developed in this study provides a new tool for investigating Ca²⁺ dynamics in plant cells.

CCX1, a Putative Cation/Ca2+ Exchanger, Participates in Regulation of Reactive Oxygen Species Homeostasis and Leaf Senescence

The major developmental significance of leaf senescence is the massive recycling of nutrients from senescing leaves to nascent organs, including seeds, to meet the requirement of their rapid development, so-called nutrient remobilization. The efficiency of nutrient remobilization is associated with the activity of diverse transporters. A large number of transporters are up-regulated during leaf senescence in Arabidopsis, many of which participate in regulating leaf senescence via different signaling pathways. Here, we report that a member of the cation/Ca²⁺ exchanger family, CCX1, is highly induced during leaf senescence in Arabidopsis. Although single mutation of CCX1 did not change the senescence phenotype, double mutation of CCX1 and CCX4 resulted in a subtle but significant stay-green phenotype during natural and dark-induced leaf senescence, suggesting that some members of the cation/Ca²⁺ exchanger family act redundantly in mediating leaf senescence. Consistently, overexpression of CCX1 accelerated leaf senescence. Moreover, the ccx1ccx4 double mutant was more tolerant to H₂O₂, whereas CCX1-overexpressing lines showed an elevated response to H₂O₂ treatment, presumably due to an overaccumulation of reactive oxygen species (ROS), indicating that CCX1 may promote leaf senescence via modulating ROS homeostasis. Notably, both ccx1-1 and ccx1ccx4 were sensitive to Ca²⁺ deprivation, implying that CCX1 may also be involved in modulating Ca²⁺ signaling and consequently affecting the initiation of leaf senescence.

Proteomic comparison of near-isogenic barley (Hordeum vulgare L.) germplasm differing in the allelic state of a major senescence QTL identifies numerous proteins involved in plant pathogen defense

Senescence is the last developmental phase of plant tissues, organs and, in the case of monocarpic senescence, entire plants. In monocarpic crops such as barley, it leads to massive remobilization of nitrogen and other nutrients to developing seeds. To further investigate this process, a proteomic comparison of flag leaves of near-isogenic late- and early-senescing barley germplasm was performed. Protein samples at 14 and 21 days past anthesis were analyzed using both two-dimensional gel-based and label-free quantitative mass spectrometry-based ('shotgun') proteomic techniques. This approach identified >9000 barley proteins, and one-third of them were quantified. Analysis focused on proteins that were significantly (p < 0.05; difference ≥1.5-fold) upregulated in early-senescing line '10_11' as compared to late-senescing variety 'Karl', as these may be functionally important for senescence. Proteins in this group included family 1 pathogenesis-related proteins, intracellular and membrane receptors or co-receptors (NBS-LRRs, LRR-RLKs), enzymes involved in attacking pathogen cell walls (glucanases), enzymes with possible roles in cuticle modification, and enzymes involved in DNA repair. Additionally, proteases and elements of the ubiquitin-proteasome system were upregulated in line '10_11', suggesting involvement of nitrogen remobilization and regulatory processes. Overall, the proteomic data highlight a correlation between early senescence and upregulated defense functions. This correlation emerges more clearly from the current proteomic data than from a previously performed transcriptomic comparison of 'Karl' and '10_11'. Our findings stress the value of studying biological systems at both the transcript and protein levels, and point to the importance of pathogen defense functions during developmental leaf senescence.

NORE1/SAUL1 integrates temperature-dependent defense programs involving SGT1b and PAD4 pathways and leaf senescence in Arabidopsis

Leaf senescence is not only primarily governed by developmental age but also influenced by various internal and external factors. Although some genes that control leaf senescence have been identified, the detailed regulatory mechanisms underlying integration of diverse senescence-associated signals into the senescence programs remain to be elucidated. To dissect the regulatory pathways involved in leaf senescence, we isolated the not oresara1-1 (nore1-1) mutant showing accelerated leaf senescence phenotypes from an EMS-mutagenized Arabidopsis thaliana population. We found that altered transcriptional programs in defense response-related processes were associated with the accelerated leaf senescence phenotypes observed in nore1-1 through microarray analysis. The nore1-1 mutation activated defense program, leading to enhanced disease resistance. Intriguingly, high ambient temperature effectively suppresses the early senescence and death phenotypes of nore1-1. The gene responsible for the phenotypes of nore1-1 contains a missense mutation in SENESCENCE-ASSOCIATED E3 UBIQUITIN LIGASE 1 (SAUL1), which was reported as a negative regulator of premature senescence in the light intensity- and PHYTOALEXIN DEFICIENT 4 (PAD4)-dependent manner. Through extensive double mutant analyses, we recently identified suppressor of the G2 Allele of SKP1b (SGT1b), one of the positive regulators for disease resistance conferred by many resistance (R) proteins, as a downstream signaling component in NORE1-mediated senescence and cell death pathways. In conclusion, NORE1/SAUL1 is a key factor integrating signals from temperature-dependent defense programs and leaf senescence in Arabidopsis. These findings provide a new insight that plants might utilize defense response program in regulating leaf senescence process, possibly through recruiting the related genes during the evolution of the leaf senescence program.

CaM/BAG5/Hsc70 signaling complex dynamically regulates leaf senescence

Calcium signaling plays an essential role in plant cell physiology, and chaperone-mediated protein folding directly regulates plant programmed cell death. The Arabidopsis thaliana protein AtBAG5 (Bcl-2-associated athanogene 5) is unique in that it contains both a BAG domain capable of binding Hsc70 (Heat shock cognate protein 70) and a characteristic IQ motif that is specific for Ca(2+)-free CaM (Calmodulin) binding and hence acts as a hub linking calcium signaling and the chaperone system. Here, we determined crystal structures of AtBAG5 alone and in complex with Ca(2+)-free CaM. Structural and biochemical studies revealed that Ca(2+)-free CaM and Hsc70 bind AtBAG5 independently, whereas Ca(2+)-saturated CaM and Hsc70 bind AtBAG5 with negative cooperativity. Further in vivo studies confirmed that AtBAG5 localizes to mitochondria and that its overexpression leads to leaf senescence symptoms including decreased chlorophyll retention and massive ROS production in dark-induced plants. Mutants interfering the CaM/AtBAG5/Hsc70 complex formation leads to different phenotype of leaf senescence. Collectively, we propose that the CaM/AtBAG5/Hsc70 signaling complex plays an important role in regulating plant senescence.

The arabidopsis cyclic nucleotide interactome

BACKGROUND

Cyclic nucleotides have been shown to play important signaling roles in many physiological processes in plants including photosynthesis and defence. Despite this, little is known about cyclic nucleotide-dependent signaling mechanisms in plants since the downstream target proteins remain unknown. This is largely due to the fact that bioinformatics searches fail to identify plant homologs of protein kinases and phosphodiesterases that are the main targets of cyclic nucleotides in animals.

METHODS

An affinity purification technique was used to identify cyclic nucleotide binding proteins in Arabidopsis thaliana. The identified proteins were subjected to a computational analysis that included a sequence, transcriptional co-expression and functional annotation analysis in order to assess their potential role in plant cyclic nucleotide signaling.

RESULTS

A total of twelve cyclic nucleotide binding proteins were identified experimentally including key enzymes in the Calvin cycle and photorespiration pathway. Importantly, eight of the twelve proteins were shown to contain putative cyclic nucleotide binding domains. Moreover, the identified proteins are post-translationally modified by nitric oxide, transcriptionally co-expressed and annotated to function in hydrogen peroxide signaling and the defence response. The activity of one of these proteins, GLYGOLATE OXIDASE 1, a photorespiratory enzyme that produces hydrogen peroxide in response to Pseudomonas, was shown to be repressed by a combination of cGMP and nitric oxide treatment.

CONCLUSIONS

We propose that the identified proteins function together as points of cross-talk between cyclic nucleotide, nitric oxide and reactive oxygen species signaling during the defence response.

AtCNGC2 is involved in jasmonic acid-induced calcium mobilization

Calcium (Ca(2+)) mobilization is a central theme in various plant signal transduction pathways. We demonstrate that Arabidopsis thaliana cyclic nucleotide-gated channel 2 (AtCNGC2) is involved in jasmonic acid (JA)-induced apoplastic Ca(2+) influx in Arabidopsis epidermal cells. Ca(2+) imaging results showed that JA can induce an elevation in the cytosolic cAMP concentration ([cAMP]cyt), reaching a maximum within 3 min. Dibutyryl cAMP (db-cAMP), a cell membrane-permeable analogue of cAMP, induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)]cyt), with a peak at 4 min. This [Ca(2+)]cyt increase was triggered by the JA-induced increase in [cAMP]cyt. W-7[N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], an antagonist of calmodulin, positively modulated the JA-induced increase in [Ca(2+)]cyt, while W-5[N-(6-aminohexyl)-1-naphthalenesulfonamide], an inactive antagonist of calmodulin, had no apparent effect. db-cAMP and JA positively induced the expression of primary (i.e. JAZ1 and MYC2) and secondary (i.e. VSP1) response genes in the JA signalling pathway in wild-type Arabidopsis thaliana, whereas they had no significant effect in the AtCNGC2 mutant 'defense, no death (dnd1) plants. These data provide evidence that JA first induces the elevation of cAMP, and cAMP, as an activating ligand, activates the AtCNGC2 channel, resulting in apoplastic Ca(2+) influx through AtCNGC2.

Nitric oxide mitigates arsenic-induced oxidative stress and genotoxicity in Vicia faba L

The protective effects of nitric oxide (NO) against arsenic (As)-induced structural disturbances in Vicia faba have been investigated. As treatment (0.25, 0.50, and 1 mM) resulted in a declined growth of V. faba seedlings. Arsenic treatment stimulates the activity of SOD and CAT while the activities of APX and GST content were decreased. The oxidative stress markers such as superoxide radical, hydrogen peroxide and malondialdehyde (lipid peroxidation) contents were enhanced by As. Overall results revealed that significant accumulation of As suppressed growth, photosynthesis, antioxidant enzymes (SOD, CAT, APX, and GST activity), mitotic index, and induction of different chromosomal abnormalities, hence led to oxidative stress. The concentration of SNP (0.02 mM) was very effective in counteracting the adverse effect of As toxicity. These abnormalities use partially or fully reversed by a simultaneous application of As and NO donor and sodium nitroprusside and has an ameliorating effect against As-induced oxidative stress and genotoxicity in V. faba roots.

Crossroads of stress responses, development and flowering regulation--the multiple roles of Cyclic Nucleotide Gated Ion Channel 2

The Arabidopsis autoimmune mutant, defense-no death 1 (dnd1) is a null mutant of CYCLIC NUCLEOTIDE-GATED ION CHANNEL2 (AtCNGC2). dnd1 exhibits constitutive pathogen resistance responses including higher levels of endogenous salicylic acid (SA), which is an important signaling molecule for pathogen defense responses. Recently we have reported that dnd1 exhibits a significantly delayed flowering phenotype, indicating the involvement of AtCNGC2 in flowering transition. However, since SA has been known to influence flowering timing as a positive regulator, the delayed flowering phenotype in dnd1 was unexpected. In this study, we have asked whether SA is involved in the dnd1-mediated delayed flowering phenotype. In addition, in order to gain insight into the involvement of SA and CNGCs in flowering transition, we analyzed the flowering transition of cpr22, another CNGC mutant with a similar autoimmune phenotype as dnd1 (including high SA accumulation), and null mutants of several other CNGCs. Our data suggest that dnd1 does not require SA or SA signaling for its delayed flowering phenotype, while SA was responsible for the early flowering phenotype of cpr22. None of the other CNGC mutants besides AtCNGC4 (1) displayed an alteration in flowering transition. This indicates that AtCNGC2 and AtCNGC4 have a unique role controlling flowering timing and this function is independent from its role in pathogen defense.

Salicylic acid signaling inhibits apoplastic reactive oxygen species signaling

BACKGROUND

Reactive oxygen species (ROS) are used by plants as signaling molecules during stress and development. Given the amount of possible challenges a plant face from their environment, plants need to activate and prioritize between potentially conflicting defense signaling pathways. Until recently, most studies on signal interactions have focused on phytohormone interaction, such as the antagonistic relationship between salicylic acid (SA)-jasmonic acid and cytokinin-auxin.

RESULTS

In this study, we report an antagonistic interaction between SA signaling and apoplastic ROS signaling. Treatment with ozone (O3) leads to a ROS burst in the apoplast and induces extensive changes in gene expression and elevation of defense hormones. However, Arabidopsis thaliana dnd1 (defense no death1) exhibited an attenuated response to O3. In addition, the dnd1 mutant displayed constitutive expression of defense genes and spontaneous cell death. To determine the exact process which blocks the apoplastic ROS signaling, double and triple mutants involved in various signaling pathway were generated in dnd1 background. Simultaneous elimination of SA-dependent and SA-independent signaling components from dnd1 restored its responsiveness to O3. Conversely, pre-treatment of plants with SA or using mutants that constitutively activate SA signaling led to an attenuation of changes in gene expression elicited by O3.

CONCLUSIONS

Based upon these findings, we conclude that plants are able to prioritize the response between ROS and SA via an antagonistic action of SA and SA signaling on apoplastic ROS signaling.

Biochemical, physiological and gene expression analysis reveals sex-specific differences in Populus tomentosa floral development

The productivity, distribution and population structure of poplar are affected by temperature transitions. Poplar floral buds develop in a fluctuating environment and the molecular basis of temperature-dependent flowering regulation has been extensively studied, but little is known about how sex-specific floral bud development responds to temperature transitions. Here, morphological observations indicated that floral bud growth rates were affected by maximum and minimum air temperature at the later stages of enlargement (stage 4) and later stage of dormancy (stage 8), respectively. We investigated the physiological, biochemical and gene expression changes in floral development and in response to temperature treatment (heat and chilling stress). Male floral buds showed more adverse effects than female floral buds under temperature treatment. Temperature treatment experiments revealed that temperature treatment significantly increased catalase, peroxidase, superoxide dismutase activities and transcription of related genes in female floral buds, whereas malondialdehyde (MDA) significantly increased only in males. Soluble sugars and protein increased both in female and male floral buds but were higher in males. Temperature treatment also caused significant increases in Ca(2+) content and transcription of genes related to calcium transport in female flowers. These results revealed sex-specific floral developmental responses to seasonal temperature transitions and suggest that in Populus tomentosa, female floral buds possess better mechanisms for environment adaptation than do males.

The Arabidopsis cyclic nucleotide-gated ion channels AtCNGC2 and AtCNGC4 work in the same signaling pathway to regulate pathogen defense and floral transition

Arabidopsis (Arabidopsis thaliana) cyclic nucleotide-gated ion channels (CNGCs) form a large family consisting of 20 members and have been implicated in Ca(2+) signaling related to various physiological processes, such as pathogen defense, development, and thermotolerance. The null mutant of AtCNGC2, defense, no death (dnd1), exhibits autoimmune phenotypes, while it is impaired in mounting the hypersensitive response, which is a hallmark of effector-triggered (i.e. RESISTANCE-gene mediated) resistance. It has been suggested that AtCNGC2 is involved in defense responses and likely other aspects of physiology through its role as a Ca(2+)-conducting channel. However, the downstream signaling components and its relation with AtCNGC4, which is the closest paralog of AtCNGC2, remain elusive. Despite the fact that cngc4 mutants display almost identical phenotypes to those seen in cngc2 mutants, not much is known about their relationship. Here, we report the identification and characterization of the Arabidopsis mutant repressor of defense no death1 (rdd1), obtained from a suppressor screen of a transfer DNA insertion knockout mutant of AtCNGC2 in order to identify downstream components of dnd1-mediated signal transduction. rdd1 suppressed the majority of dnd1-mediated phenotypes except Ca(2+) hypersensitivity. In addition, rdd1 also suppressed the dnd1-mediated late-flowering phenotype that was discovered in this study. Our genetic analysis conducted to elucidate the relationship between AtCNGC2 and AtCNGC4 indicates that RDD1 is also involved in AtCNGC4-mediated signal transduction. Lastly, bimolecular fluorescence complementation analysis suggests that AtCNGC2 and AtCNGC4 are likely part of the same channel complex.

Nitric oxide mediates cytokinin functions in cell proliferation and meristem maintenance in Arabidopsis

Cytokinin and nitric oxide (NO) have been characterized as signaling molecules to trigger cell division in tissue culture. Here, we show that the hypocotyl and root explants of Arabidopsis NO-deficient mutant nos1/noa1 exhibit severe defects in callus induction and shoot regeneration in response to cytokinin. Accordingly, depletion of NO caused by a NO scavenger leads to a severe inhibitory effect on callus induction. Moreover, cytokinin-induced NO production is impaired in nos1/noa1 in which cytokinin-triggered activation of cell cycle gene CYCD3;1 is inhibited, indicating that NO may act downstream of cytokinin in the control of cell proliferation through CYCD3;1. This hypothesis is further confirmed by the genetic evidence that constitutive expression of CYCD3;1 complements the defects of nos1/noa1 mutant in meristematic activity in shoot, root, and floral tissues as well as in cytokinin-induced callus initiation and shoot regeneration. Furthermore, we show that NO deficiency caused by loss of NOS1/NOA1 impairs cellular development such as the duration of the mitotic phase and timing of the transition to endocycles in nos1/noa1 mutant leaves, which can be reverted by constitutive expression of CYCD3;1. Taken together, these results demonstrate that NO mediates transcriptional activation of CYCD3;1 in regulating the mitotic cycles in response to cytokinins.

H2O2-induced leaf cell death and the crosstalk of reactive nitric/oxygen species

In plants, the chloroplast is the main reactive oxygen species (ROS) producing site under high light stress. Catalase (CAT), which decomposes hydrogen peroxide (H2 O2 ), is one of the controlling enzymes that maintains leaf redox homeostasis. The catalase mutants with reduced leaf catalase activity from different plant species exhibit an H2 O2 -induced leaf cell death phenotype. This phenotype was differently affected by light intensity or photoperiod, which may be caused by plant species, leaf redox status or growth conditions. In the rice CAT mutant nitric oxide excess 1 (noe1), higher H2 O2 levels induced the generation of nitric oxide (NO) and higher S-nitrosothiol (SNO) levels, suggesting that NO acts as an important endogenous mediator in H2 O2 -induced leaf cell death. As a free radical, NO could also react with other intracellular and extracellular targets and form a series of related molecules, collectively called reactive nitrogen species (RNS). Recent studies have revealed that both RNS and ROS are important partners in plant leaf cell death. Here, we summarize the recent progress on H2 O2 -induced leaf cell death and the crosstalk of RNS and ROS signals in the plant hypersensitive response (HR), leaf senescence, and other forms of leaf cell death triggered by diverse environmental conditions. [Formula: see text] [ Chengcai Chu (Corresponding author)].

Detection of reactive oxygen species downstream of cyclic nucleotide signals in plants

Cyclic nucleotides act in plant cell signal transduction cascades by activating cyclic nucleotide gated cation-conducting ion channels (CNGCs). Activation of CNGCs results in inward cation (including Ca(2+)) conductance across the plasma membrane. Elevation of cytosolic Ca(2+) is an early step in numerous plant cell signal transduction cascades, including plant immune responses to pathogens. CNGC involvement, along with cyclic nucleotides cAMP and cGMP, in pathogen defense programs is one relatively well-studied area of cyclic nucleotide signaling in plants. During plant immune responses, CNGC-dependent Ca(2+) elevations lead to a signaling cascade that results in the generation of defense molecules such as hydrogen peroxide and nitric oxide, and induction of defense gene expression. This pathogen defense response is discussed, and methods to detect some of the downstream signaling steps in the pathway are presented.

An Arabidopsis cell growth defect factor-related protein, CRS, promotes plant senescence by increasing the production of hydrogen peroxide

Arabidopsis thaliana Cell Growth Defect factor 1 (Cdf1) has been implicated in promotion of proapoptotic Bax-like cell death via the induction of reactive oxygen species (ROS). Here we report a conserved function of a chloroplast-targeting Cdf-related gene Responsive to Senescence (CRS) using CRS overexpression and loss of function in plants as well as CRS heterologous expression in yeast. CRS expression was strongly induced in senescent leaves, suggesting its main functions during plant senescence. CRS expression in yeast mitochondria increased the ROS level and led to cell death in a manner similar to Cdf1. In whole plants, overexpression of CRS caused the loss of chlorophylls (Chls) and the rapid onset of leaf senescence, while the lack of CRS led to the delay of leaf senescence in a loss-of-function mutant, crs. The higher and lower accumulation of H(2)O(2) was correlated with early and late senescence in CRS-overexpressing and crs mutant plants, respectively. Furthermore, expression of senescence-related marker genes and metacaspase genes was induced in CRS-overexpressing plants in response to dark. Our findings suggest that CRS plays a key role in the leaf senescence process that accompanies H(2)O(2) accumulation resulting in cell death promotion.