Sulfoxidation Regulation of Musa acuminata Calmodulin (MaCaM) Influences the Functions of MaCaM-Binding Proteins

Calmodulin-like protein CML15 interacts with PP2C46/65 to regulate papaya fruit ripening via integrating calcium, ABA and ethylene signals

It is well known that calcium, ethylene and abscisic acid (ABA) can regulate fruit ripening, however, their interaction in the regulation of fruit ripening has not yet been fully clarified. The present study found that the expression of the papaya calcium sensor CpCML15 was strongly linked to fruit ripening. CpCML15 could bind Ca2+ and served as a true calcium sensor. CpCML15 interacted with CpPP2C46 and CpPP2C65, the candidate components of the ABA signalling pathways. CpPP2C46/65 expression was also related to fruit ripening and regulated by ethylene. CpCML15 was located in the nucleus and CpPP2C46/65 were located in both the nucleus and membrane. The interaction between CpCML15 and CpPP2C46/65 was calcium dependent and further repressed the activity of CpPP2C46/65 in vitro. The transient overexpression of CpCML15 and CpPP2C46/65 in papaya promoted fruit ripening and gene expression related to ripening. The reduced expression of CpCML15 and CpPP2C46/65 by virus-induced gene silencing delayed fruit colouring and softening and repressed the expression of genes related to ethylene signalling and softening. Moreover, ectopic overexpression of CpCML15 in tomato fruit also promoted fruit softening and ripening by increasing ethylene production and enhancing gene expression related to ripening. Additionally, CpPP2C46 interacted with CpABI5, and CpPP2C65 interacted with CpERF003-like, two transcriptional factors in ABA and ethylene signalling pathways that are closely related to fruit ripening. Taken together, our results showed that CpCML15 and CpPP2Cs positively regulated fruit ripening, and their interaction integrated the cross-talk of calcium, ABA and ethylene signals in fruit ripening through the CpCML15-CpPP2Cs-CpABI5/CpERF003-like pathway.

WUSCHEL-related homeobox transcription factor SlWOX13 regulates tomato fruit ripening

Fruit ripening is a complex, genetically programmed process involving the action of critical transcription factors (TFs). Despite the established importance of WUSCHEL-related homeobox (WOX) TFs in plant development, the involvement of WOX and its underlying mechanism in the regulation of fruit ripening remain unclear. Here, we demonstrate that SlWOX13 regulates fruit ripening in tomato (Solanum lycopersicum). Overexpression of SlWOX13 accelerates fruit ripening, whereas loss-of-function mutation in SlWOX13 delays this process. Moreover, ethylene synthesis and carotenoid accumulation are significantly inhibited in slwox13 mutant fruit but accelerated in SlWOX13 transgenic fruit. Integrated analyses of RNA-seq and chromatin immunoprecipitation (ChIP)-seq identified 422 direct targets of SlWOX13, of which 243 genes are negatively regulated and 179 are positively regulated by SlWOX13. Electrophoretic mobility shift assay, RT-qPCR, dual-luciferase reporter assay, and ChIP-qPCR analyses demonstrated that SlWOX13 directly activates the expression of several genes involved in ethylene synthesis and signaling and carotenoid biosynthesis. Furthermore, SlWOX13 modulates tomato fruit ripening through key ripening-related TFs, such as RIPENING INHIBITOR (RIN), NON-RIPENING (NOR), and NAM, ATAF1, 2, and CUC2 4 (NAC4). Consequently, these effects promote fruit ripening. Taken together, these results demonstrate that SlWOX13 positively regulates tomato fruit ripening via both ethylene synthesis and signaling and by transcriptional regulation of key ripening-related TFs.

PpMYB105 inhibits chilling injury by regulating PpMsrA1 in peach fruit

KEY MESSAGE

MeJA supplementation enhanced the chilling tolerance and gene expression of PpMsrA1. PpMYB105 protein positively regulated the PpMsrA1 promoter. PpMYB105 mediated the MeJA-boosted chilling tolerance by regulating PpMsrA1. Cold storage can maintain the quality of postharvest fruit. However, peaches easily suffer from chilling injury (CI) during cold storage, leading to economic loss. Results showed that methyl jasmonate (MeJA) supplementation reduced the CI severity, and enhanced the gene expression of methionine sulfoxide reductase A1 (PpMsrA1). It was found that MeJA application elevated the MsrA activity and methionine (Met) content, and reduced the methionine-S-sulfoxide (Met-S-SO) content and reactive oxygen species (ROS) production afterwards. Moreover, PpMYB105 could activate the transcription of PpMsrA1 by binding to the MYB binding element in its promoter. The gene expression of PpMYB105 was up-regulated by MeJA application. Overexpression of PpMYB105 in tomatoes enhanced the chilling tolerance and gene expression of SlMsrA1. Virus-induced gene silencing of PpMYB105 in peaches resulted in the increase in CI severity and the decrease in gene expression of PpMsrA1. Thus, PpMYB105 was involved in the MeJA-boosted chilling tolerance by regulating PpMsrA1.

Interaction of methionine sulfoxide reductase B5 with SlMYC2 stimulates the transcription of MeJA-mediated autophagy-related genes in tomato fruit

Methyl jasmonate (MeJA) has been shown to induce autophagy in various plant stress responses and metabolic pathways. MYC2 is involved in MeJA-mediated postharvest fruit biological metabolism, but it is unclear how it affects MeJA-induced fruit autophagy. In this study, we noticed that silencing SlMYC2 significantly reduced the increase in autophagy-related genes (SlATGs) expression induced by MeJA. SlMYC2 could also bind to the promoters of several SlATGs, including SlATG13a, SlATG13b, SlATG18a, and SlATG18h, and activate their transcript levels. Moreover, SlMsrB5, a methionine sulfoxide reductase, could interact with SlMYC2. Methionine oxidation in SlMYC2 and mimicking sulfoxidation in SlMYC2 by mutation of methionine-542 to glutamine reduced the DNA-binding ability and transcriptional activity of SlMYC2, respectively. SlMsrB5 partially repaired oxidized SlMYC2 and restored its DNA-binding ability. On the other hand, silencing SlMsrB5 inhibited the transcript levels of SlMYC2-targeted genes (SlATG13a, SlATG13b, SlATG18a, and SlATG18h). Similarly, dual-luciferase reporter (DLR) analysis revealed that SlMsrB5-SlMYC2 interaction significantly increased the ability of SlMYC2-mediated transcriptional activation of SlATG13a, SlATG13b, SlATG18a, and SlATG18h. These findings demonstrate that SlMsrB5-mediated cyclic oxidation/reduction of methionine in SlMYC2 influences SlATGs expression. Collectively, these findings reveal the mechanism of SlMYC2 in SlATGs transcriptional regulation, providing insight into the mechanism of MeJA-mediated postharvest fruit quality regulation.

CaCl2 treatment effectively delays postharvest senescence of passion fruit

This study aimed to systematically investigate the changes in peel color, physicochemical characteristics, textural properties, and peel ultrastructure between CaCl₂-treated and water-soaked passion fruit under short-term storage at room temperature (20 °C) for eight days. The fruit peel was further analyzed and compared for the differences in calmodulin (CaM) gene expression between the two groups. The data were analyzed using principal component analysis. The results confirmed that CaCl₂ treatment effectively maintained the appearance and color of passion fruit, inhibited peel browning, and improved fruit quality. The treatment had an effect on maintaining the physiological properties of passion fruit parenchyma, effectively delayed the passion fruit senescence, and kept the structural integrity of the fruit peel. The relative expression of PeCaM gene in the CaCl₂-treated fruit peels was higher than that of the control peels. The Ca²⁺ stimulated the relative expression of the PeCaM gene, which delayed the senescence of passion fruit.

Methionine oxidation and reduction of the ethylene signaling component MaEIL9 are involved in banana fruit ripening

The ethylene insensitive 3/ethylene insensitive 3-like (EIN3/EIL) plays an indispensable role in fruit ripening. However, the regulatory mechanism that links post-translational modification of EIN3/EIL to fruit ripening is largely unknown. Here, we studied the expression of 13 MaEIL genes during banana fruit ripening, among which MaEIL9 displayed higher enhancement particularly in the ripening stage. Consistent with its transcript pattern, abundance of MaEIL9 protein gradually increased during the ripening process, with maximal enhancement in the ripening. DNA affinity purification (DAP)-seq analysis revealed that MaEIL9 directly targets a subset of genes related to fruit ripening, such as the starch hydrolytic genes MaAMY3D and MaBAM1. Stably overexpressing MaEIL9 in tomato fruit hastened fruit ripening, whereas transiently silencing this gene in banana fruit retarded the ripening process, supporting a positive role of MaEIL9 in fruit ripening. Moreover, oxidation of methionines (Met-129, Met-130, and Met-282) in MaEIL9 resulted in the loss of its DNA-binding capacity and transcriptional activation activity. Importantly, we identified MaEIL9 as a potential substrate protein of methionine sulfoxide reductase A MaMsrA4, and oxidation of Met-129, Met-130, and Met-282 in MaEIL9 could be restored by MaMsrA4. Collectively, our findings reveal a novel regulatory network controlling banana fruit ripening, which involves MaMsrA4-mediated redox regulation of the ethylene signaling component MaEIL9.

The selenoprotein methionine sulfoxide reductase B1 (MSRB1)

Methionine (Met) can be oxidized to methionine sulfoxide (MetO), which exist as R- and S-diastereomers. Present in all three domains of life, methionine sulfoxide reductases (MSR) are the enzymes that reduce MetO back to Met. Most characterized among them are MSRA and MSRB, which are strictly stereospecific for the S- and R-diastereomers of MetO, respectively. While the majority of MSRs use a catalytic Cys to reduce their substrates, some employ selenocysteine. This is the case of mammalian MSRB1, which was initially discovered as selenoprotein SELR or SELX and later was found to exhibit an MSRB activity. Genomic analyses demonstrated its occurrence in most animal lineages, and biochemical and structural analyses uncovered its catalytic mechanism. The use of transgenic mice and mammalian cell culture revealed its physiological importance in the protection against oxidative stress, maintenance of neuronal cells, cognition, cancer cell proliferation, and the immune response. Coincident with the discovery of Met oxidizing MICAL enzymes, recent findings of MSRB1 regulating the innate immunity response through reversible stereospecific Met-R-oxidation of cytoskeletal actin opened up new avenues for biological importance of MSRB1 and its role in disease. In this review, we discuss the current state of research on MSRB1, compare it with other animal Msrs, and offer a perspective on further understanding of biological functions of this selenoprotein.

Melatonin treatment improves postharvest quality and regulates reactive oxygen species metabolism in "Feizixiao" litchi based on principal component analysis

Postharvest quality of litchi reduces rapidly during storage at room temperature. This study aimed to investigate the effect of melatonin treatment on postharvest quality and oxidative stress markers of litchi fruit during cold storage. The "Feizixiao" litchi was treated with melatonin solution concentrations of 0.2 and 0.6 mmol·L^-1 and then stored at 4°C for 12 days. The results confirmed that the melatonin treatment effectively maintained the appearance and color of the litchi fruit, suppressed the peel browning, and improved the litchi quality. The treatment also significantly enhanced the levels of endogenous melatonin, antioxidant components (total phenolics, flavonoids, and anthocyanin), and antioxidant enzyme activities of the fruit. It also inhibited the other oxidative stress markers, such as O 2 - , H₂O₂, MDA, and protein carbonyl content, and upregulated the expressions of antioxidant and Msr-related genes. Correlation and principal component analyses further confirmed that the melatonin treatment effectively delayed the fruit senescence by enhancing the antioxidant enzyme activities and modulating peel browning and reactive oxygen species metabolism of the litchi fruit via regulating gene expression of the related enzymes (SOD and PPO). These findings suggested that the exogenous application of melatonin to litchi during the postharvest is an ideal way to preserve the fruit quality and delay fruit senescence.

An ethylene-hypersensitive methionine sulfoxide reductase regulated by NAC transcription factors increases methionine pool size and ethylene production during kiwifruit ripening

Ethylene plays an important role in regulating fruit ripening by triggering dynamic changes in expression of ripening-associated genes, but the functions of many of these genes are still unknown. Here, a methionine sulfoxide reductase gene (AdMsrB1) was identified by transcriptomics-based analysis as the gene most responsive to ethylene treatment in ripening kiwifruit. The AdMsrB1 protein exhibits a stereospecific activity toward the oxidative stress-induced R enantiomer of methionine sulfoxide (MetSO), reducing it to methionine (Met). Stable overexpression of AdMsrB1 in kiwifruit significantly increased the content of free Met and 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, and increased ethylene production. Dual-luciferase assays indicated that the AdMsrB1 promoter was not directly upregulated by ethylene treatment but was modulated by two ethylene-inducible NAM/ATAF/CUC transcription factors (AdNAC2 and AdNAC72) that bind directly to the AdMsrB1 promoter. Overexpression of AdNAC72 in kiwifruit not only enhanced AdMsrB1 expression, but also increased free Met and ACC content and ethylene production rates. This finding establishes an unexpected regulatory loop that enhances ethylene production and the concentration of its biosynthetic intermediates.

Alternative splicing of MaMYB16L regulates starch degradation in banana fruit during ripening

The alternative splicing of select genes is an important mechanism to regulate responses to endogenous and environmental signals in plants. However, the role of alternative splicing in regulating fruit ripening remains unclear. Here, we discovered that MaMYB16L, an R1-type MYB transcription factor, undergoes alternative splicing and generates two transcripts, the full-length isoform MaMYB16L and a truncated form MaMYB16S, in banana fruit. During banana fruit ripening, the alternative splicing process intensifies with downregulated MaMYB16L and upregulated MaMYB16S. Moreover, MaMYB16L is a transcriptional repressor that directly binds with the promoters of many genes associated with starch degradation and MaDREB2, a positive ripening regulator, and represses their expression. In contrast, MaMBY16S lacks a DNA-binding domain but competitively combines and forms non-functional heterodimers with functional MaMYB16L. MaMYB16L-MaMYB16S heterodimers decrease the binding capacity and transrepression activity of MaMYB16L. The downregulation of MaMYB16L and the upregulation of MaMYB16S, that is, a decreased ratio of active to non-active isoforms, facilitates the activation of ripening-related genes and thereby promotes fruit ripening. Furthermore, the transient overexpression of MaMYB16S promotes banana fruit ripening, whereas the overexpression of MaMYB16L delays this process. Therefore, the alternative splicing of MaMYB16L might generate a self-controlled regulatory loop to regulate banana fruit ripening.

Methionine Sulfoxide Reductase B Regulates the Activity of Ascorbate Peroxidase of Banana Fruit

Ascorbate peroxidase (APX) is a key antioxidant enzyme that is involved in diverse developmental and physiological process and stress responses by scavenging H₂O₂ in plants. APX itself is also subjected to multiple posttranslational modifications (PTMs). However, redox-mediated PTM of APX in plants remains poorly understood. Here, we identified and confirmed that MaAPX1 interacts with methionine sulfoxide reductase B2 (MsrB2) in bananas. Ectopic overexpression of MaAPX1 delays the detached leaf senescence induced by darkness in Arabidopsis. Sulfoxidation of MaAPX1, i.e., methionine oxidation, leads to loss of the activity, which is repaired partially by MaMsrB2. Moreover, mimicking sulfoxidation by mutating Met36 to Gln also decreases its activity in vitro and in vivo, whereas substitution of Met36 with Val36 to mimic the blocking of sulfoxidation has little effect on APX activity. Spectral analysis showed that mimicking sulfoxidation of Met36 hinders the formation of compound I, the first intermediate between APX and H₂O₂. Our findings demonstrate that the redox state of methionine in MaAPX1 is critical to its activity, and MaMsrB2 can regulate the redox state and activity of MaAPX1. Our results revealed a novel post-translational redox modification of APX.

Sulfoxidation regulation of transcription factor NAC42 influences its functions in relation to stress-induced fruit ripening in banana

Redox modification of functional or regulatory proteins has emerged as an important mechanism of post-translational modification. However, the role of redox modifications of transcription factors mediated by methionine sulfoxide reductase (Msr) in regulating physiological processes in plants remains unclear, especially in fruit ripening. In this study, we determined that MaNAC42, a transcriptional activator, is involved in the regulation of fruit ripening in banana under oxidative stress. Integrated analysis of ChIP-qPCR and EMSA data showed that MaNAC42 directly binds to promoters of genes related to oxidative stress and ripening. Ectopic overexpression of MaNAC42 in Arabidopsis delays dark-induced senescence in leaves, indicating that MaNAC42 plays a negative role in senescence. Furthermore, we found that MaNAC42 is a target of MaMsrB2, a methionine sulfoxide reductase B. Methionine oxidation in MaNAC42 (i.e. sulfoxidation) or mimicking sulfoxidation by mutating methionine to glutamine both lead to decreased DNA-binding capacity and transcriptional activity. On the other hand, MaMsrB2 can partially repair oxidized MaNAC42 and restore its DNA-binding capacity. Thus, our results suggest a novel regulatory mechanism of fruit ripening in banana involving MaMsrB2-mediated redox regulation of the ripening-related transcription factor MaNAC42.

Localization and Dynamics of the Methionine Sulfoxide Reductases MsrB1 and MsrB2 in Beech Seeds

Beech seeds are produced irregularly, and there is a need for long-term storage of these seeds for forest management practices. Accumulated reactive oxygen species broadly oxidize molecules, including amino acids, such as methionine, thereby contributing to decreased seed viability. Methionine oxidation can be reversed by the activity of methionine sulfoxide reductases (Msrs), which are enzymes involved in the regulation of many developmental processes and stress responses. Two types of Msrs, MsrB1 and MsrB2, were investigated in beech seeds to determine their abundance and localization. MsrB1 and MsrB2 were detected in the cortical cells and the outer area of the vascular cylinder of the embryonic axes as well as in the epidermis and parenchyma cells of cotyledons. The abundances of MsrB1 and MsrB2 decreased during long-term storage. Ultrastructural analyses have demonstrated the accumulation of these proteins in protein storage vacuoles and in the cytoplasm, especially in close proximity to the cell membrane. In silico predictions of possible Msr interactions supported our findings. In this study, we investigate the contribution of MsrB1 and MsrB2 locations in the regulation of seed viability and suggest that MsrB2 is linked with the longevity of beech seeds via association with proper utilization of storage material.

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.

Genome-wide identification, characterization and expression profile of glutaredoxin gene family in relation to fruit ripening and response to abiotic and biotic stresses in banana (Musa acuminata)

Glutaredoxins (GRXs) are disulfide oxidoreductases that are involved in various biological processes. However, little information on the role of GRXs in the regulation of fruit ripening and the response to stress is available. In this study, we isolated 64 GRX genes from banana genome. Their encoded GRX proteins could be classified into four classes: CC, CGFS, CPYC and GRL types. The distribution and synteny of these GRXs on chromosomes, the gene structures, the promoter sequences, and the possible protein subcellular localizations were characterized. Molecular interaction network analysis suggested that MaGRX might interact with glutathione reductase (GR), sulfiredoxin, peroxiredoxin (Prx), and NADPH-dependent thioredoxin reductase C (NTRC), contributing to the antioxidative defense of banana fruit. MicroRNA prediction showed that MaGRX genes might be targeted by different miRNAs. Transcriptome analysis characterized the expression profiles of different MaGRX genes during banana fruit ripening, and in response to different storage stresses. The results suggested that CC-type, CPYC-type and GRL-type MaGRXs might be more active than CGFS-type MaGRXs during banana fruit ripening and the response to stress. Moreover, MaGRX6/7/9/11/17/23/28 and MaGRL3/16/19 might play important roles in regulating fruit ripening or in response to low and high temperature, or Fusarium proliferatum infection.

Histone demethylase SlJMJ6 promotes fruit ripening by removing H3K27 methylation of ripening-related genes in tomato

Fruit ripening is governed by a complex regulatory network. Reversible histone methylation and demethylation regulate chromatin structure and gene expression. However, little is known about the involvement of histone demethylases in regulating fruit ripening. Here, we found that the tomato (Solanum lycopersicum) SlJMJ6 encodes a histone lysine demethylase that specifically demethylates H3K27 methylation. Overexpression of SlJMJ6 accelerates tomato fruit ripening, which is associated with the upregulated expression of a large number of ripening-related genes. Integrated analysis of RNA-seq and chromatin immunoprecipitation followed by sequencing identified 32 genes directly targeted by SlJMJ6 and transcriptionally upregulated with decreased H3K27m3 in SlJMJ6-overexpressed fruit. Numerous SlJMJ6-regulated genes are involved in transcription regulation, ethylene biosynthesis, cell wall degradation and hormone signaling. Eleven ripening-related genes including RIPENING INHIBITOR (RIN), 1-aminocyclopropane 1-carboxylate synthase-4 (ACS4), 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1), pectate lyase (PL) and beta-galactosidase 4 (TBG4), and a DNA demethylase DML2, were confirmed to be regulated directly by SlJMJ6 through removing H3K27me3. Our results demonstrate that SlJMJ6 is a ripening-prompting H3K27me3 demethylase that activates the expression of the ripening-related genes by modulating H3K27me3, thereby facilitating tomato fruit ripening. Our work also reveals a novel link between histone demethylation and DNA demethylation in regulating fruit ripening. To our knowledge, this is the first report of the involvement of a histone lysine demethylase in the regulation of fruit ripening.

Oxidation of Methionine 77 in Calmodulin Alters Mouse Growth and Behavior

Methionine 77 in calmodulin can be stereospecifically oxidized to methionine sulfoxide by mammalian methionine sulfoxide reductase A. Whether this has in vivo significance is unknown. We therefore created a mutant mouse in which wild type calmodulin-1 was replaced by a calmodulin containing a mimic of methionine sulfoxide at residue 77. Total calmodulin levels were unchanged in the homozygous M77Q mutant, which is viable and fertile. No differences were observed on learning tests, including the Morris water maze and associative learning. Cardiac stress test results were also the same for mutant and wild type mice. However, young male and female mice were 20% smaller than wild type mice, although food intake was normal for their weight. Young M77Q mice were notably more active and exploratory than wild type mice. This behavior difference was objectively documented on the treadmill and open field tests. The mutant mice ran 20% longer on the treadmill than controls and in the open field test, the mutant mice explored more than controls and exhibited reduced anxiety. These phenotypic differences bore a similarity to those observed in mice lacking calcium/calmodulin kinase IIα (CaMKIIα). We then showed that MetO77 calmodulin was less effective in activating CaMKIIα than wild type calmodulin. Thus, characterization of the phenotype of a mouse expressing a constitutively active mimic of calmodulin led to the identification of the first calmodulin target that can be differentially regulated by the oxidation state of Met77. We conclude that reversible oxidation of methionine 77 in calmodulin by MSRA has the potential to regulate cellular function.

Physiological Roles of Plant Methionine Sulfoxide Reductases in Redox Homeostasis and Signaling

Oxidation of methionine (Met) leads to the formation of two S- and R-diastereoisomers of Met sulfoxide (MetO) that are reduced back to Met by methionine sulfoxide reductases (MSRs), A and B, respectively. Here, we review the current knowledge about the physiological functions of plant MSRs in relation with subcellular and tissue distribution, expression patterns, mutant phenotypes, and possible targets. The data gained from modified lines of plant models and crop species indicate that MSRs play protective roles upon abiotic and biotic environmental constraints. They also participate in the control of the ageing process, as shown in seeds subjected to adverse conditions. Significant advances were achieved towards understanding how MSRs could fulfil these functions via the identification of partners among Met-rich or MetO-containing proteins, notably by using redox proteomic approaches. In addition to a global protective role against oxidative damage in proteins, plant MSRs could specifically preserve the activity of stress responsive effectors such as glutathione-S-transferases and chaperones. Moreover, several lines of evidence indicate that MSRs fulfil key signaling roles via interplays with Ca²⁺- and phosphorylation-dependent cascades, thus transmitting ROS-related information in transduction pathways.