Inhibition of PbeXTH1 and PbeSEOB1 is required for the Valsa canker resistance contributed by Wall-associated kinase gene MbWAK1

Wall-associated kinases (WAKs) have been determined to recognize pathogenic signals and initiate plant immune responses. However, the roles of the family members in host resistance against Valsa canker, a serious fungal disease of apples and pears, are largely unknown. Here, we identified MbWAK1 in Malus baccata, a resistant germplasm differentially expressed during infection by Valsa mali (Vm). Over-expression of MbWAK1 enhanced the Valsa canker resistance of apple and pear fruits and 'Duli-G03' (Pyrus betulifolia) suspension cells. A large number of phloem, cell wall, and lipid metabolic process-related genes were differentially expressed in overexpressed suspension cell lines in response to Valsa pyri (Vp) signals. Among these, the expression of xyloglucan endotransglucosylase/hydrolase (XTH) gene PbeXTH1 and sieve element occlusion B-like (SEOB) gene PbeSEOB1 were significantly inhibited. Transient expression of PbeXTH1 or PbeSEOB1 compromised the expressional induction of MbWAK1 and the resistance contributed by MbWAK1. In addition, PbeXTH1 and PbeSEOB1 suppressed the immune response induced by MbWAK1. Our results enriched the molecular mechanisms for MbWAK1 against Valsa canker and resistant breeding.

The apple palmitoyltransferase MdPAT16 influences sugar content and salt tolerance via an MdCBL1-MdCIPK13-MdSUT2.2 pathway

Protein S-acyltransferases (PATs) are a category of eukaryotic transmembrane proteins that mediate the S-acylation of their target proteins. S-acylation, commonly known as palmitoylation, is a reversible protein modification that regulates the membrane association and function of target proteins. However, the functions and mechanisms of PATs in apple (Malus domestica) remain poorly understood. In this study, an MdPAT family member, MdPAT16, was identified and shown to have palmitoyltransferase activity. We demonstrated that this gene responds to salt stress and that its expression improves plant salt stress resistance. In addition, its overexpression significantly promotes the accumulation of soluble sugars. The same phenotypes were observed in transgenic tissue culture seedlings, transgenic roots, and Arabidopsis thaliana that ectopically expressed MdPAT16. MdPAT16 was shown to interact with MdCBL1 and stabilize MdCBL1 protein levels through palmitoylation. The N-terminal sequence of MdCBL1 contains a palmitoylation site, and its N-terminal deletion led to changes in MdCBL1 protein stability and subcellular localization. The phenotypes of MdCBL1 transgenic roots and transiently injected apple fruits were fully consistent with the sugar accumulation phenotype of MdPAT16. Mutation of the palmitoylation site interfered with this phenotype. These findings suggest that MdPAT16 palmitoylates its downstream target proteins, improving their stability. This may be a missing link in the plant salt stress response pathway and have an important impact on fruit quality.

The structural and functional characterization of Malus domestica double bond reductase MdDBR provides insights towards the identification of its substrates

In this study we describe the crystal structures of the apoform, the binary and the ternary complexes of a double bond reductase from Malus domestica L. (MdDBR) and explore a range of potential substrates. The overall fold of MdDBR is similar to that of the medium chain reductase/dehydrogenase/zinc-dependent alcohol dehydrogenase-like family. Structural comparison of MdDBR with Arabidopsis thaliana DBR (AtDBR), Nicotiana tabacum DBR (NtDBR) and Rubus idaeus DBR (RiDBR) allowed the identification of key amino acids involved in cofactor and ligands binding and shed light on how these residues may guide the orientation of the substrates. The enzyme kinetic for the substrate trans-4-phenylbuten-2-one has been analyzed, and MdDBR activity towards a variety of substrates was tested. This enzyme has been reported to be involved in the phenylpropanoid pathway where it would catalyze the NADPH-dependent reduction of the α, β-unsaturated double bond of carbonyl metabolites. Our study provides new data towards the identification of MdDBR natural substrate and the biosynthetic pathway where it belongs. Furthermore, the originally proposed involvement in dihydrochalcone biosynthesis in apple must be questioned.

Citramalate synthase yields a biosynthetic pathway for isoleucine and straight- and branched-chain ester formation in ripening apple fruit

A plant pathway that initiates with the formation of citramalate from pyruvate and acetyl-CoA by citramalate synthase (CMS) is shown to contribute to the synthesis of α-ketoacids and important odor-active esters in apple (Malus × domestica) fruit. Microarray screening led to the discovery of a gene with high amino acid similarity to 2-isopropylmalate synthase (IPMS). However, functional analysis of recombinant protein revealed its substrate preference differed substantially from IPMS and was more typical of CMS. MdCMS also lacked the regulatory region present in MdIPMS and was not sensitive to feedback inhibition. 13C-acetate feeding of apple tissue labeled citramalate and α-ketoacids in a manner consistent with the presence of the citramalate pathway, labeling both straight- and branched-chain esters. Analysis of genomic DNA (gDNA) revealed the presence of two nearly identical alleles in "Jonagold" fruit (MdCMS_1 and MdCMS_2), differing by two nonsynonymous single-nucleotide polymorphisms (SNPs). The mature proteins differed only at amino acid 387, possessing either glutamine387 (MdCMS_1) or glutamate387 (MdCMS_2). Glutamate387 was associated with near complete loss of activity. MdCMS expression was fruit-specific, increasing severalfold during ripening. The translated protein product was detected in ripe fruit. Transient expression of MdCMS_1 in Nicotiana benthamiana induced the accumulation of high levels of citramalate, whereas MdCMS_2 did not. Domesticated apple lines with MdCMS isozymes containing only glutamate387 produced a very low proportion of 2-methylbutanol- and 2-methylbutanoate (2MB) and 1-propanol and propanoate (PROP) esters. The citramalate pathway, previously only described in microorganisms, is shown to function in ripening apple and contribute to isoleucine and 2MB and PROP ester biosynthesis without feedback regulation.

The apple RING-H2 protein MdCIP8 regulates anthocyanin accumulation and hypocotyl elongation by interacting with MdCOP1

COP1, an important RING ubiquitin ligase E3, is a molecular switch for light regulation in plant development. As an interacting protein of COP1, CIP8 contains a RING-H2 domain, but its biological function is unclear. Here, the apple MdCIP8 was identified based on its homology with AtCIP8 in Arabidopsis. MdCIP8 was constitutively expressed at different levels in various apple tissues, and the expression level of MdCIP8 was not affected by light and dark conditions. MdCIP8 reversed the short hypocotyl phenotype of the cip8 mutant under light conditions. Furthermore, the yeast two-hybrid experiment showed that MdCIP8 interacted with the RING domain of MdCOP1 through its RING-H2 domain. MdCIP8-OX/cop1-4 exhibited the phenotype of the cop1-4 mutant, indicating that CIP8 acts upstream of COP1. In addition, an apple transient injection experiment showed that MdCIP8 inhibited anthocyanin accumulation in an MdCOP1-dependent pathway. Overall, our findings reveal that CIP8 plays an inhibitory role in the light-regulation responses of plants.

Overexpression of MdCPK1a gene, a calcium dependent protein kinase in apple, increase tobacco cold tolerance via scavenging ROS accumulation

Calcium-dependent protein kinases (CDPKs) are important calcium receptors, which play a crucial part in the process of sensing and decoding intracellular calcium signals during plant development and adaptation to various environmental stresses. In this study, a CDPK gene MdCPK1a, was isolated from apple (Malus×domestica) which contains 1701bp nucleotide and encodes a protein of 566 amino acid residues, and contains the conserved domain of CDPKs. The transient expression and western blot experiment showed that MdCPK1a protein was localized in the nucleus and cell plasma membrane. Ectopic expression of MdCPK1a in Nicotiana benthamiana increased the resistance of the tobacco plants to salt and cold stresses. The mechanism of MdCPK1a regulating cold resistance was further investigated. The overexpressed MdCPK1a tobacco plants had higher survival rates and longer root length than wild type (WT) plants under cold stress, and the electrolyte leakages (EL), the content of malondialdehyde (MDA) and reactive oxygen species (ROS) were lower, and accordingly, antioxidant enzyme activities, such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were higher, suggesting the transgenic plants suffered less chilling injury than WT plants. Moreover, the transcript levels of ROS-scavenging and stress-related genes were higher in the transgenic plants than those in WT plants whether under normal conditions or cold stress. The above results suggest that the improvement of cold tolerance in MdCPK1a-overexpressed plants was due to scavenging ROS accumulation and modulating the expression of stress-related genes.

Melatonin treatment reduces ethylene production and maintains fruit quality in apple during postharvest storage

The effects of treatment with melatonin on ripening of 'Fuji' apples during storage at 1 °C for 56 d were investigated. The apples were harvested at the commercial ripening stage and treated with 1 mmol L^-1 melatonin. Compared with the control, melatonin treated apples had significant reduced ethylene production (28 d-56 d) and weight loss (14 d-56 d) during storage (p < 0.05). Also, the melatonin treatment maintained better apple skin structure throughout storage. The reduced ethylene production was regulated by the decreased expressions of MdACO1, MdACS1, MdAP2.4 and MdERF109, based on RNA-Seq analysis, which was validated using qRT-PCR analysis. Moreover, the activity of 3 enzymes, including peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT), were significantly increased in melatonin treated fruit (p < 0.05). Taken together, this study highlights the inhibitory effects of melatonin in ethylene biosynthesis and factors influencing postharvest quality in apple.

An apple long-chain acyl-CoA synthetase, MdLACS4, induces early flowering and enhances abiotic stress resistance in Arabidopsis

The aerial parts of apple are protected against environmental stress by cuticular wax. Although it has been suggested that several long-chain acyl-CoA synthetases are involved in wax biosynthesis, the molecular pathway of apple cuticular wax biosynthesis remains unclear. In this study, an MdLACS4 protein with long-chain acyl-CoA synthetase activity was isolated from apple. The MdLACS4 gene was highly expressed in pericarp, stem, and mature leaf tissues. Ectopic expression of MdLACS4 in Arabidopsis induced early flowering. Compared with wild-type plants, MdLACS4 transgenic Arabidopsis exhibited lower water loss rates, reduced epidermal permeability, increased cuticular wax in stems and leaves, and altered cuticular ultrastructure. Furthermore, the accumulation of cuticular wax enhanced the resistance of MdLACS4 transgenic plants to drought and salt stress. Finally, predicted protein functional interaction networks for LACS4 suggested that the molecular regulation pathway of MdLACS4 mediates wax biosynthesis in apple.

Inhibitory Effect of Azamacrocyclic Ligands on Polyphenol Oxidase in Model and Food Systems

Enzymatic browning is one of the main problems faced by the food industry due to the enzyme polyphenol oxidase (PPO) provoking an undesirable color change in the presence of oxygen. Here, we report the evaluation of 10 different azamacrocyclic compounds with diverse morphologies as potential inhibitors against the activity of PPO, both in model and real systems. An initial screening of 10 ligands shows that all azamacrocyclic compounds inhibit to some extent the enzymatic browning, but the molecular structure plays a crucial role on the power of inhibition. Kinetic studies of the most active ligand (L2) reveal a S-parabolic I-parabolic noncompetitive inhibition mechanism and a remarkable inhibition at micromolar concentration (IC₅₀ = 10 μM). Furthermore, L2 action has been proven on apple juice to significantly reduce the enzymatic browning.

Overexpression of MsGH3.5 inhibits shoot and root development through the auxin and cytokinin pathways in apple plants

Phytohormonal interactions are crucial for plant development. Auxin and cytokinin (CK) both play critical roles in regulating plant growth and development; however, the interaction between these two phytohormones is complex and not fully understood. Here, we isolated a wild apple (Malus sieversii Roem) GRETCHEN HAGEN3 (GH3) gene, MsGH3.5, encoding an indole-3-acetic acid (IAA)-amido synthetase. Overexpression of MsGH3.5 significantly reduced the free IAA content and increased the content of some IAA-amino acid conjugates, and MsGH3.5-overexpressing lines were dwarfed and produced fewer adventitious roots (ARs) than the control. This phenotype is consistent with the role of GH3 in conjugating excess free active IAA to amino acids in auxin homeostasis. Surprisingly, overexpression of MsGH3.5 significantly increased CK concentrations in the whole plant, and altered the expression of genes involved in CK biosynthesis, metabolism and signaling. Furthermore, exogenous CK application induced MsGH3.5 expression through the activity of the CK type-B response regulator, MsRR1a, which mediates the CK primary response. MsRR1a activated MsGH3.5 expression by directly binding to its promoter, linking auxin and CK signaling. Plants overexpressing MsRR1a also displayed fewer ARs, in agreement with the regulation of MsGH3.5 expression by MsRR1a. Taken together, we reveal that MsGH3.5 affects apple growth and development by modulating auxin and CK levels and signaling pathways. These findings provide insight into the interaction between the auxin and CK pathways, and might have substantial implications for efforts to improve apple architecture.

Genome wide analysis and functional identification of MdKCS genes in apple

Apple fruit is covered by cuticle wax, which plays important roles protecting fruits from adverse environmental conditions. β-Ketoacyl-CoA synthase (KCS) is the key rate-limiting enzyme in plant wax synthesis. In this study, we identified 28 KCS gene family members from apple (Malus × domestica Borkh.) by homology analysis. Multi-sequence alignment and phylogenetic analyses revealed that the 28 MdKCS genes were divided into four subgroups, including KCS1-like, FAE1-like, FDH-like, and CER6. A chromosomal localization analysis revealed that 27 apple KCS genes were located on 11 chromosomes, while MdKCS28 was localized to the unassembled genomic scaffold. Most of the MdKCS proteins were hydrophilic proteins and they had similar secondary and tertiary structures. The prediction of cis-acting elements of the MdKCS gene promoters suggested that the MdKCS genes may be widely involved in hormone signaling and the stress response. Furthermore, the quantitative real-time polymerase chain reaction results showed that eight MdKCS genes were highly expressed in the apple pericarp, and were significantly induced by drought, abscisic acid (ABA), and NaCl treatments. We transformed the MdKCS21 gene into apple calli, and found the MdKCS21 overexpressing transgenic apple calli exhibited higher tolerance to ABA treatment. Finally, the MdKCS proteins were localized to the endoplasmic reticulum and vacuolar membrane by confocal laser microscopy. This study established a foundation to further analyze the function of KCS genes and provided candidate genes for molecular improvement of wax content in apple.

Arbuscular mycorrhizal fungi enhanced drought resistance in apple by regulating genes in the MAPK pathway

Arbuscular mycorrhizal fungi (AMF) can form a symbiotic relationships with most terrestrial plants and play an important role in plant growth and adaptation to various stresses. To study the role of AMF in regulating drought resistance in apple, the effects of drought stress on Malus hupehensis inoculated with AMF were investigated. Inoculation of AMF enhanced apple plants growth. Mycorrhizal plants had higher total chlorophyll concentrations but lower relative electrolyte leakage under drought stress. Mycorrhizal plants increased net photosynthetic rate, stomatal conductance, and transpiration rate under drought stress, however, they showed lower inhibition in the quantum yield of PSII photochemistry. Mycorrhizal plants also had higher superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities under drought conditions. Thus, mycorrhizal plants had lower accumulated MDA, H₂O₂, and O₂^- than non-mycorrhizal seedlings. Total sugar and proline concentrations also significantly increased, helping maintain the osmotic balance. Furthermore, mitogen-activated protein kinase (MAPK) cascades, which participate in the regulation of responses of plants and microorganisms to biotic and abiotic stress, were up-regulated in apple plants and AMF during drought. We saw that there were at least two motifs that were identical in MAPK proteins and many elements that responded to hormones and stress from these MAPK genes. In summary, our results showed that mycorrhizal colonization enhanced apple drought tolerance by improving gas exchange capacity, increasing chlorophyll fluorescence parameters, creating a greater osmotic adjustment capacity, increasing scavenging of reactive oxygen species (ROS), and using MAPK signals for interactions between AMF and their apple plant hosts.

An apple MYB transcription factor regulates cold tolerance and anthocyanin accumulation and undergoes MIEL1-mediated degradation

MYB transcription factors (TFs) have been demonstrated to play diverse roles in plant growth and development through interaction with basic helix-loop-helix (bHLH) TFs. MdbHLH33, an apple bHLH TF, has been identified as a positive regulator in cold tolerance and anthocyanin accumulation by activating the expressions of MdCBF2 and MdDFR. In the present study, a MYB TF MdMYB308L was found to also positively regulate cold tolerance and anthocyanin accumulation in apple. We found that MdMYB308L interacted with MdbHLH33 and enhanced its binding to the promoters of MdCBF2 and MdDFR. In addition, an apple RING E3 ubiquitin ligase MYB30-INTERACTING E3 LIGASE 1 (MdMIEL1) was identified to be an MdMYB308L-interacting protein and promoted the ubiquitination degradation of MdMYB308L, thus negatively regulated cold tolerance and anthocyanin accumulation in apple. These results suggest that MdMYB308L acts as a positive regulator in cold tolerance and anthocyanin accumulation in apple by interacting with MdbHLH33 and undergoes MdMIEL1-mediated protein degradation. The dynamic change in MYB-bHLH protein complex seems to play a key role in the regulation of plant growth and development.

Nectar- and stigma exudate-specific expression of an acidic chitinase could partially protect certain apple cultivars against fire blight disease

MAIN CONCLUSION

Certain apple cultivars accumulate to high levels in their nectar and stigma exudate an acidic chitinase III protein that can protect against pathogens including fire blight disease causing Erwinia amylovora. To prevent microbial infections, flower nectars and stigma exudates contain various antimicrobial compounds. Erwinia amylovora, the causing bacterium of the devastating fire blight apple disease, is the model pathogen that multiplies in flower secretions and infects through the nectaries. Although Erwinia-resistant apples are not available, certain cultivars are tolerant. It was reported that in flower infection assay, the 'Freedom' cultivar was Erwinia tolerant, while the 'Jonagold' cultivar was susceptible. We hypothesized that differences in the nectar protein compositions lead to different susceptibility. Indeed, we found that an acidic chitinase III protein (Machi3-1) selectively accumulates to very high levels in the nectar and the stigma exudate of the 'Freedom' cultivar. We show that three different Machi3-1 alleles exist in apple cultivars and that only the 5B-Machi3-1 allele expresses the Machi3-1 protein in the nectar and the stigma exudate. We demonstrate that the 5B-Machi3-1 allele was introgressed from the Malus floribunda 821 clone into different apple cultivars including the 'Freedom'. Our data suggest that MYB-binding site containing repeats of the 5B-Machi3-1 promoter is responsible for the strong nectar- and stigma exudate-specific expression. As we found that in vitro, the Machi3-1 protein impairs growth and biofilm formation of Erwinia at physiological concentration, we propose that the Machi3-1 protein could partially protect 5B-Machi3-1 allele containing cultivars against Erwinia by inhibiting the multiplication and biofilm formation of the pathogen in the stigma exudate and in the nectar.

BTB-BACK Domain E3 Ligase MdPOB1 Suppresses Plant Pathogen Defense against Botryosphaeria dothidea by Ubiquitinating and Degrading MdPUB29 Protein in Apple

Apple ring rot is a severe disease that affects the yield and quality of apple fruits worldwide. However, the underlying molecular mechanism that involved in this process still remains largely unexplored. Here, we report that apple POZ/BTB CONTAINING-PROTEIN 1 (MdPOB1), a BTB-BACK domain E3 ligase protein, functions to suppress apple pathogen defense against Botryosphaeria dothidea (B. dothidea). Both in vitro and in vivo assays indicated that MdPOB1 interacted directly with and degraded apple U-box E3 ligase MdPUB29, a well-established positive regulator of plant innate immunity, through the ubiquitin/26S proteasome pathway. A series of transgenic analyses in apple fruits demonstrated that MdPOB1 affected apple pathogen defense against B. dothidea at least partially, if not completely, via regulating MdPUB29. Additionally, it was found that the apple pathogen defense against B. dothidea was correlated with the H2O2 contents and the relative expression of salicylic acid (SA) synthesis- and SA signaling-related genes, which might be regulated via degradation of MdPUB29 by MdPOB1. Overall, our findings provide new insights into the mechanism of the MdPOB1 modulation of apple ring rot resistance, which occur by directly regulating potential downstream target protein MdPUB29 for proteasomal degradation in apple.

Identification of leucoanthocyanidin reductase and anthocyanidin reductase genes involved in proanthocyanidin biosynthesis in Malus crabapple plants

Proanthocyanidins (PAs) from plants are a nutritionally valuable component of the human diet and play important roles in defense against pests and diseases. PAs are products of the flavonoid pathway, which also leads to the production of anthocyanins and flavonols. The enzymes leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR) are involved in PA biosynthesis. The PA biosynthetic pathway has been characterized in several plant species, but the relationship between its expression and PA accumulation in Malus crabapple remains unclear. Here, we cloned the LAR genes MrLAR1, 2, and the ANR genes MrANR1, 2, from the red leaved Malus crabapple cultivar 'Royalty'. The contents of PAs and the expression levels of the LAR and ANR genes were investigated in different organs of the two crabapple cultivars. The transcript levels of two LAR genes and two ANR genes correlated with the contents of the catechin and epicatechin, which are proanthocyanidin precursors. Over-expression of the MrLAR1, 2 and MrANR1, 2 in tobacco (Nicotiana tabacum) promoted the accumulation of PAs, while transient silencing of their expression in crabapple resulted in reduced PA levels. In addition, a negative correlation between quercetin, anthocyanin, and PA biosynthesis was also found during crabapple leaf and fruit peel development. We also found that MrLAR1 and 2 may contribute to epicatechin biosynthesis. In summary, the LAR and ANR genes are critical factors in PA biosynthesis, and there is competition between the quercetin, anthocyanin, and PA biosynthetic pathways during leaf and fruit peel development in Malus crabapple.

A Peptide-Induced Self-Cleavage Reaction Initiates the Activation of Tyrosinase

The conversion of inactive pro-polyphenol oxidases (pro-PPOs) into the active enzyme results from the proteolytic cleavage of its C-terminal domain. Herein, a peptide-mediated cleavage process that activates pro-MdPPO1 (Malus domestica) is reported. Mass spectrometry, mutagenesis studies, and X-ray crystal-structure analysis of pro-MdPPO1 (1.35 Å) and two separated C-terminal domains, one obtained upon self-cleavage of pro-MdPPO1 and the other one produced independently, were applied to study the observed self-cleavage. The sequence Lys 355-Val 370 located in the linker between the active and the C-terminal domain is indispensable for the self-cleavage. Partial introduction (Lys 352-Ala 360) of this peptide into the sequence of two other PPOs, MdPPO2 and aurone synthase (CgAUS1), triggered self-cleavage in the resulting mutants. This is the first experimental proof of a self-cleavage-inducing peptide in PPOs, unveiling a new mode of activation for this enzyme class that is independent of any external protease.

The apple U-box E3 ubiquitin ligase MdPUB29 contributes to activate plant immune response to the fungal pathogen Botryosphaeria dothidea

MdPUB29 is a positive regulator of the defense response to the fungal pathogen Botryosphaeria dothidea possibly by directly regulating the salicylic acid (SA) content as well as SA synthesis-related and signaling-related gene transcription. In plants, ubiquitin E3 ligases containing a U-box domain (PUBs, Plant U-box E3 ubiquitin ligase) have been identified as key regulators of fundamental cellular processes, such as cellular growth, development, and apoptosis, as well as biotic and abiotic stress responses. However, the function of PUBs in apple ring rot remains elusive. Here, we isolated the U-box E3 ligase MdPUB29 from the apple cultivar 'Royal Gala' and characterized its function in plant pathogen defense against Botryosphaeria dothidea. qRT-PCR showed that the expression of MdPUB29 was significantly induced in apple fruits after B. dothidea infection. Overexpression of the MdPUB29 gene in apple calli increased the resistance to B. dothidea infection. In contrast, silencing MdPUB29 in apple calli resulted in reduced resistance. Ectopic expression of MdPUB29 in Arabidopsis also exhibited enhanced resistance to B. dothidea infection compared to that of the wild-type (Col) control. In addition, it was found that the increase of plant pathogen defense was correlated with the increased salicylic acid (SA) content, as well as SA synthesis-related and signaling-related gene transcription in comparison to the wild type. We elucidated the mechanism by which MdPUB29 elevates plant pathogen defense against B. dothidea possibly by regulating the SA pathway.

Changes in the sucrose metabolism in apple fruit following postharvest acibenzolar-S-methyl treatment

BACKGROUND

Apple (cv. Ralls) fruit were treated with 0.1 g L^-1 acibenzolar-S-methyl (ASM) for 10 min to evaluate the changes in enzyme activity and gene expression in the sucrose metabolism during storage at 20 °C with 30%-40% relative humidity.

RESULTS

The results showed that sucrose phosphate synthase (SPS) and sucrose synthase synthesis (SS-s) activity was enhanced by ASM in apple fruit during the entire storage period. Sucrose synthase-cleavage (SS-c) and neutral invertase (NI) activity was suppressed by ASM treatment but acid invertase (AI) activity was increased in the middle period after ASM treatment. Acibenzolar-S-methyl treatment also significantly inhibited SPS and NI gene expression in apple fruit during storage. However, SS gene expression increased in the ASM-treated apple fruit. High levels of expression of the fructokinase (FK) and hexokinase (HK) genes were observed during the middle storage period in the ASM-treated fruit.

CONCLUSION

Taken together, these results suggest that ASM delays the senescence of apple fruit by regulating the sugar metabolism. © 2018 Society of Chemical Industry.

Application of sodium silicate retards apple softening by suppressing the activity of enzymes related to cell wall degradation

BACKGROUND

During the storage of apples, apple softening is one of the main problems. Sodium silicate has been used to enhance disease resistance and maintain quality of fruits. In the present study, apple fruit (cv. Golden delicious) were treated with 100 mmol L^-1 sodium silicate for 10 min and stored at 20 °C to investigate its effects on weight loss, flesh firmness, and the activity of cell wall-degrading enzymes.

RESULTS

The results indicated that 100 mmol L^-1 of sodium silicate treatment delayed the increase of weight loss and decrease of the flesh firmness in apples. Sodium silicate treatment also suppressed the activity of polygalacturonic acid transeliminase and pectin methyltranseliminase, pectin methylgalacturonase, polygalacturonase, cellulase and β-galactosidase in the fruit.

CONCLUSIONS

Delaying apple softening by sodium silicate treatment is closely related to the inhibition of the activity of cell wall-degrading enzymes and weight loss. © 2018 Society of Chemical Industry.

A CIPK protein kinase targets sucrose transporter MdSUT2.2 at Ser254 for phosphorylation to enhance salt tolerance

Soil salinity is one of the major abiotic stressors that negatively affect crop growth and yield. Salt stress can regulate antioxidants and the accumulation of osmoprotectants. In the study, a sucrose transporter MdSUT2.2 was identified in apple. Overexpression of MdSUT2.2 gene increased salt tolerance in the transgenic apple, compared with the WT control "Gala." In addition, it was found that protein MdSUT2.2 was phosphorylated at Ser²⁵⁴ site in response to salt. A DUAL membrane yeast hybridization system through an apple cDNA library demonstrated that a protein kinase MdCIPK13 interacted with MdSUT2.2. A series of transgenic analysis in apple calli showed that MdCIPK13 was required for the salt-induced phosphorylation of MdSUT2.2 protein and enhanced its stability and transport activity. Finally, it was found that MdCIPK13 improved salt resistance in an MdSUT2.2-dependent manner. These findings had enriched our understanding of the molecular mechanisms underlying abiotic stress.

Molecular cloning and functional analysis of a biphenyl phytoalexin-specific O-methyltransferase from apple cell suspension cultures

This manuscript describes the cloning and functional characterization of a biphenyl phytoalexin biosynthetic gene, 3,5-dihydroxybiphenyl O-methyltransferase from elicitor-treated cell cultures of scab resistant apple cultivar 'Florina'. Apples belong to the subtribe Malinae of the Rosaceae family. Biphenyls and dibenzofurans are the specialized phytoalexins of Malinae, of which aucuparin is the most widely distributed biphenyl. The precursor of aucuparin, 3,5-dihydroxybiphenyl, is a benzoate-derived polyketide, which is formed by the sequential condensation of three molecules of malonyl-CoA and one molecule of benzoyl-CoA in a reaction catalyzed by biphenyl synthase (BIS). This 3,5-dihydroxybiphenyl then undergoes sequential 5-O-methylation, 4-hydroxylation, and finally 3-O-methylation to form aucuparin. A cDNA encoding O-methyltransferase (OMT) was isolated and functionally characterized from the cell cultures of scab-resistant apple cultivar 'Florina' (Malus domestica cultivar 'Florina'; MdOMT) after treatment with elicitor prepared from the apple scab causing fungus Venturia inaequalis. MdOMT catalyzed the regiospecific O-methylation of 3,5-dihydroxybiphenyl at the 5-position to form 3-hydroxy-5-methoxybiphenyl. The enzyme showed absolute substrate preference for 3,5-dihydroxybiphenyl. The elicitor-treated apple cell cultures showed transient increases in the MdOMT (GenBank ID MF740747) and MdBIS3 (GenBank ID JQ390523) transcript levels followed by the accumulation of biphenyls (aucuparin and noraucuparin) and dibenzofuran (eriobofuran) phytoalexins. MdOMT fused with N- and C-terminal yellow fluorescent protein showed cytoplasmic localization in the epidermis of Nicotiana benthamiana leaves. In scab inoculated greenhouse-grown 'Florina' plants, the expression of MdOMT was transiently induced in the stem followed by the accumulation of biphenyl phytoalexins.

MhMAPK4 from Malus hupehensis Rehd. decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

The SUMO E3 Ligase MdSIZ1 Targets MdbHLH104 to Regulate Plasma Membrane H+-ATPase Activity and Iron Homeostasis

SIZ1 (a SIZ/PIAS-type SUMO E3 ligase)-mediated small ubiquitin-like modifier (SUMO) modification of target proteins is important for various biological processes related to abiotic stress resistance in plants; however, little is known about its role in resistance toward iron (Fe) deficiency. Here, the SUMO E3 ligase MdSIZ1 was shown to be involved in the plasma membrane (PM) H⁺-ATPase-mediated response to Fe deficiency. Subsequently, a basic helix-loop-helix transcription factor, MdbHLH104 (a homolog of Arabidopsis bHLH104 in apple), which acts as a key component in regulating PM H⁺-ATPase-mediated rhizosphere acidification and Fe uptake in apples (Malus domestica), was identified as a direct target of MdSIZ1. MdSIZ1 directly sumoylated MdbHLH104 both in vitro and in vivo, especially under conditions of Fe deficiency, and this sumoylation was required for MdbHLH104 protein stability. Double substitution of K139R and K153R in MdbHLH104 blocked MdSIZ1-mediated sumoylation in vitro and in vivo, indicating that the K139 and K153 residues were the principal sites of SUMO conjugation. Moreover, the transcript level of the MdSIZ1 gene was substantially induced following Fe deficiency. MdSIZ1 overexpression exerted a positive influence on PM H⁺-ATPase-mediated rhizosphere acidification and Fe uptake. Our findings reveal an important role for sumoylation in the regulation of PM H⁺-ATPase-mediated rhizosphere acidification and Fe uptake during Fe deficiency in plants.

Genome-wide identification and biochemical characterization of the UGT88F subfamily in Malus x domestica Borkh

The UDP-glycosyltransferase UGT88F subfamily has been described first in Malus x domestica with the characterization of UGT88F1. Up to now UGT88F1 was one of the most active UGT glycosylating dihydrochalcones in vitro. The involvement of UGT88F1 in phloridzin (phloretin 2'-O-glucoside) synthesis, the main apple tree dihydrochalcone, was further confirmed in planta. Since the characterization of UGT88F1, this new UGT subfamily has been poorly studied probably because it seemed restricted to Maloideae. In the present study, we investigate the apple tree genome to identify and biochemically characterize the whole UGT88F subfamily. The apple tree genome contains five full-length UGT88F genes out of which three newly identified members (UGT88F6, UGT88F7 and UGT88F8) and a pseudogene. These genes are organized into two genomic clusters resulting from the recent global genomic duplication event in the apple tree. We show that recombinant UGT88F8 protein specifically glycosylates phloretin in the 2'OH position to synthetize phloridzin in vitro and was therefore named UDP-glucose: phloretin 2'-O-glycosyltransferase. The Km values of UGT88F8 are 7.72 μM and 10.84 μM for phloretin and UDP-glucose respectively and are in the same range as UGT88F1 catalytic parameters thus constituting two isoforms. Co-expression patterns of both UGT88F1 and UGT88F8 argue for a redundant function in phloridzin biosynthesis in planta. Contrastingly, recombinant UGT88F6 protein is able to glycosylate in vitro a wide range of flavonoids including flavonols, flavones, flavanones, chalcones and dihydrochalcones, although flavonols are the preferred substrates, e.g. Km value for kaempferol is 2.1 μM. Depending on the flavonoid, glycosylation occurs at least on the 3-OH and 7-OH positions. Therefore UGT88F6 corresponds to an UDP-glucose: flavonoid 3/7-O-glycosyltransferase. Finally, a molecular modeling study highlights a very high substitution rate of residues in the acceptor binding pocket between UGT88F8 and UGT88F6 which is responsible for the enzymes divergence in substrate and regiospecificity, despite an overall high protein homology.

Apple SUMO E3 ligase MdSIZ1 is involved in the response to phosphate deficiency

In plants, SIZ1 regulates abiotic and biotic stress responses by promoting the SUMOylation of proteins. The apple MdSIZ1 protein has conserved domains similar to those of Arabidopsis AtSIZ1. Real-time fluorescent quantitative analysis showed that MdSIZ1 gene expression was induced by phosphate-deficient conditions. In addition, the level of SUMOylation was also significantly increased under these conditions. The MYB transcription factor MdPHR1 might be a target for the SUMO protein, which is a phosphorus starvation-dependent protein. Subsequently, an MdSIZ1 expression vector was constructed and transformed in Arabidopsis mutant siz1-2 and apple callus. The MdSIZ1 transgenic Arabidopsis partially complemented the defect phenotype of siz1-2 under phosphate-deficient conditions. The survival rate, length of primary root, and number or density of lateral roots were similar between the transgenic lines and wild type (WT). Under phosphate-deficient conditions, the SUMO conjugate and fresh weight of the MdSIZ1 transgenic apple callus were improved compared with WT. The MdSIZ1 transgenic apple callus grew under phosphate-deficient conditions, whereas the MdSIZ1 sense apple callus did not. Therefore, MdSIZ1 is involved in the regulation of the phosphate-deficiency response in apple.

The SUMO E3 Ligase MdSIZ1 Targets MdbHLH104 to Regulate Plasma Membrane H+-ATPase Activity and Iron Homeostasis

SIZ1 (a SIZ/PIAS-type SUMO E3 ligase)-mediated small ubiquitin-like modifier (SUMO) modification of target proteins is important for various biological processes related to abiotic stress resistance in plants; however, little is known about its role in resistance toward iron (Fe) deficiency. Here, the SUMO E3 ligase MdSIZ1 was shown to be involved in the plasma membrane (PM) H+-ATPase-mediated response to Fe deficiency. Subsequently, a basic helix-loop-helix transcription factor, MdbHLH104 (a homolog of Arabidopsis bHLH104 in apple), which acts as a key component in regulating PM H+-ATPase-mediated rhizosphere acidification and Fe uptake in apples (Malus domestica), was identified as a direct target of MdSIZ1. MdSIZ1 directly sumoylated MdbHLH104 both in vitro and in vivo, especially under conditions of Fe deficiency, and this sumoylation was required for MdbHLH104 protein stability. Double substitution of K139R and K153R in MdbHLH104 blocked MdSIZ1-mediated sumoylation in vitro and in vivo, indicating that the K139 and K153 residues were the principal sites of SUMO conjugation. Moreover, the transcript level of the MdSIZ1 gene was substantially induced following Fe deficiency. MdSIZ1 overexpression exerted a positive influence on PM H+-ATPase-mediated rhizosphere acidification and Fe uptake. Our findings reveal an important role for sumoylation in the regulation of PM H+-ATPase-mediated rhizosphere acidification and Fe uptake during Fe deficiency in plants.

Exogenous γ-Aminobutyric Acid Treatment That Contributes to Regulation of Malate Metabolism and Ethylene Synthesis in Apple Fruit during Storage

Organic acid is an important indicator of fruit quality, and malate is the predominant organic acid in apple fruit. However, the regulation of malate metabolism in postharvest fruit is rarely reported. Here, we found that, compared with a control treatment, a 10 mM γ-aminobutyric acid (GABA) treatment remarkably delayed the loss of tiftratable acidity and malate and increased the succinate and oxalate contents in "Cripps Pink" fruit stored in polyethylene bags at room temperature. The higher malate levels in GABA-treated fruit were accompanied by higher activities of cytosolic nicotinamide adenine dinucleotide-dependent malate dehydrogenase (cyNAD-MDH) and phosphoenolpyruvate carboxylase (PEPC) but lower cytosolic NAD phosphate-dependent malic enzyme (cyNADP-ME) and phosphoenolpyruvate carboxykinase (PEPCK) activities than those seen in control fruit. Notably, ethylene production was significantly reduced by GABA treatment, paralleling the downregulation of MdACS, MdACO, and MdERF expression. Meanwhile, GABA treatment also enhanced the activity of the GABA shunt and promoted the accumulation of GABA. This study provides new insights into the regulation of malate metabolism and reports for the first time the possible interplay between GABA and ethylene signaling pathways in apple fruit during postharvest storage.

Comprehensive genomic analysis of the TYROSINE AMINOTRANSFERASE (TAT) genes in apple (Malus domestica) allows the identification of MdTAT2 conferring tolerance to drought and osmotic stresses in plants

Tyrosine aminotransferase (TAT, EC 2.6.1.5) is the first key enzyme that catalyzes the reversible interconversion of tyrosine and 4-hydroxyphenylpyruvate in the tyrosine-derived pathway for syntheses of important secondary metabolites and compounds. Although plant TAT genes have been proposed to be important in response to abiotic stress, there is little information about TAT genes in woody perennial tree species, especially in economic fruit trees. Based on TAT domain searching, sequence homology screening and phylogenetic analysis, we identified four TATs in apple genome. Then, we carried out a detailed phylogenetic analysis of TAT genes from multi-species, focusing on apple (Malus domestica). The result showed that the TAT family comprises three major classes corresponding to genes from angiosperms, mammals, and bacteria. Angiosperm TAT genes could be further divided into six subclasses. Analysis of intron-exon structure revealed that the typical TAT gene contains six introns and seven exons, with exons of similar size at each exon location. Promoter analysis showed that the 5'-flanking region of apple MdTATs contain multiple cis-acting elements including those implicated in light, biotic stress, abiotic stress, and hormone response. MdTATs were expressed to various levels in all apple structures and organs evaluated, and showed distinct expression patterns under water deficit stress. Ectopic expression of MdTAT2 in Arabidopsis or over-expression of MdTAT2 in apple callus tissue conferred enhanced tolerance to drought and osmotic stress. Collectively, these results suggest a role for TAT genes in drought and osmotic stresses and provide valuable information for further research of TAT genes and their function in plants.

Genome-wide identification and characterization of apple long-chain Acyl-CoA synthetases and expression analysis under different stresses

Long-chain acyl-CoA synthetases (LACSs) are members of the acyl-activating enzyme superfamily that have important roles in lipid synthesis and storage, fatty acid catabolism, vectorial acylation, and synthesis of cutin and wax. Here, 11 apple MdLACS genes were identified based on the Malus × domestica reference genome, clustered into six groups and mapped to ten chromosomes. Multiple sequence alignment and conserved motifs analyses showed that the sequences of the AtLACS and MdLACS proteins were highly conserved. A cis-element analysis in the promoter regions of the MdLACS genes revealed various elements related to stress responsiveness and plant hormones. Subsequently, expression analysis demonstrated that the MdLACS genes had different expression profiles in different tissues in response to various abiotic stresses. To further study the function of MdLACS genes in apple, MdLACS1 was isolated to identify its basic function, which the function of MdLACS1 in response to apple abiotic stress resistance was determined by the transgenic method. The results showed the MdLACS1 enhanced tolerance to polyethylene glycol, salt, and abscisic acid in the apple callus, suggesting that MdLACS1 is an important regulator in response to abiotic stresses. Finally, the functional interoperability network among the MdLACS proteins was predicted and analyzed, which could the understanding of the possible interactions among proteins and genes regulatory networks concerned with wax biosynthesis and regulatory mechanisms in response to abiotic stresses in apple.

Genome-wide Identification, Classification, Molecular Evolution and Expression Analysis of Malate Dehydrogenases in Apple

Malate dehydrogenase plays crucial roles in energy homeostasis, plant development and cold and salt tolerance, as it mediates the reversible conversion of malate to oxaloacetate. However, the evolutionary pattern of MDH genes in apple remains elusive. In this study, a total of 20 MDH genes were identified from the "Golden Delicious" apple draft genome. We revealed the physiological and biochemical properties, gene structure, and conserved motifs of MdMDH genes. Chromosomal localization and Ka/Ks ratio analysis of MdMDH genes revealed different selective pressures acted on duplicated MdMDH genes. Exploration of the phylogenetic relationships revealed six clades and similar frequencies between old and recent duplications, and significant differences in the evolutionary rates of the MDH gene family were observed. One MdMDH gene, MDP0000807458, which was highly expressed during apple fruit development and flower bud differentiation, was under positive selection. Thus, we speculated that MDP0000807458 is a likely candidate gene involved in regulation of flower bud differentiation and organic acid metabolism in apple fruits. This study provides a foundation for improved understanding of the molecular evolution of MdMDH genes and further facilitates the functional analysis of MDP0000807458 to unravel its exact role in flower bud differentiation and organic acid metabolism.

Apple S-RNase triggers inhibition of tRNA aminoacylation by interacting with a soluble inorganic pyrophosphatase in growing self-pollen tubes in vitro

Apple exhibits S-RNase-based self-incompatibility (SI), in which S-RNase plays a central role in rejecting self-pollen. It has been proposed that the arrest of pollen growth in SI of Solanaceae plants is a consequence of the degradation of pollen rRNA by S-RNase; however, the underlying mechanism in Rosaceae is still unclear. Here, we used S₂ -RNase as a bait to screen an apple pollen cDNA library and characterized an apple soluble inorganic pyrophosphatase (MdPPa) that physically interacted with S-RNases. When treated with self S-RNases, apple pollen tubes showed a marked growth inhibition, as well as a decrease in endogenous soluble pyrophosphatase activity and elevated levels of inorganic pyrophosphate (PPi). In addition, S-RNase was found to bind to two variable regions of MdPPa, resulting in a noncompetitive inhibition of its activity. Silencing of MdPPa expression led to a reduction in pollen tube growth. Interestingly, tRNA aminoacylation was inhibited in self S-RNase-treated or MdPPa-silenced pollen tubes, resulting in the accumulation of uncharged tRNA. Furthermore, we provide evidence showing that this disturbance of tRNA aminoacylation is independent of RNase activity. We propose an alternative mechanism differing from RNA degradation to explain the cytotoxicity of the S-RNase apple SI process.

Evolutionary diversification of galactinol synthases in Rosaceae: adaptive roles of galactinol and raffinose during apple bud dormancy

Galactinol synthase (GolS) is a key enzyme in the biosynthetic pathway of raffinose family oligosaccharides (RFOs), which play roles in carbon storage, signal transduction, and osmoprotection. The present work assessed the evolutionary history of GolS genes across the Rosaceae using several bioinformatic tools. Apple (Malus × domestica) GolS genes were transcriptionally characterized during bud dormancy, in parallel with galactinol and raffinose measurements. Additionally, MdGolS2, a candidate to regulate seasonal galactinol and RFO content during apple bud dormancy, was functionally characterized in Arabidopsis. Evolutionary analyses revealed that whole genome duplications have driven GolS gene evolution and diversification in Rosaceae speciation. The strong purifying selection identified in duplicated GolS genes suggests that differential gene expression might define gene function better than protein structure. Interestingly, MdGolS2 was differentially expressed during bud dormancy, concomitantly with the highest galactinol and raffinose levels. One of the intrinsic adaptive features of bud dormancy is limited availability of free water; therefore, we generated transgenic Arabidopsis plants expressing MdGolS2. They showed higher galactinol and raffinose contents and increased tolerance to water deficit. Our results suggest that MdGolS2 is the major GolS responsible for RFO accumulation during apple dormancy, and these carbohydrates help to protect dormant buds against limited water supply.

Genome-wide analysis of carotenoid cleavage oxygenase genes and their responses to various phytohormones and abiotic stresses in apple (Malus domestica)

Carotenoid cleavage oxygenases (CCOs) are able to cleave carotenoids to produce apocarotenoids and their derivatives, which are important for plant growth and development. In this study, 21 apple CCO genes were identified and divided into six groups based on their phylogenetic relationships. We further characterized the apple CCO genes in terms of chromosomal distribution, structure and the presence of cis-elements in the promoter. We also predicted the cellular localization of the encoded proteins. An analysis of the synteny within the apple genome revealed that tandem, segmental, and whole-genome duplication events likely contributed to the expansion of the apple carotenoid oxygenase gene family. An additional integrated synteny analysis identified orthologous carotenoid oxygenase genes between apple and Arabidopsis thaliana, which served as references for the functional analysis of the apple CCO genes. The net photosynthetic rate, transpiration rate, and stomatal conductance of leaves decreased, while leaf stomatal density increased under drought and saline conditions. Tissue-specific gene expression analyses revealed diverse spatiotemporal expression patterns. Finally, hormone and abiotic stress treatments indicated that many apple CCO genes are responsive to various phytohormones as well as drought and salinity stresses. The genome-wide identification of apple CCO genes and the analyses of their expression patterns described herein may provide a solid foundation for future studies examining the regulation and functions of this gene family.

Browning in apples: Exploring the biochemical basis of an easily-observable phenotype

Many fruits and vegetables undergo browning when they are cut and the tissue is exposed to the air. This is due to the activity of the enzyme polyphenol oxidase (PPO, EC 1.14.18.1) with endogenous substrates. In this laboratory experiment, students prepare slices of different varieties of apples and assess the rate of browning. They make a simple extract of the apple tissue and measure the activity of PPO using 3,4-dihydroxy-l-phenylalanine (l-DOPA) as substrate. They determine the protein concentration of the extract with the Bradford Coomassie Blue reagent and calculate the specific activity of PPO. Finally, the students measure the total concentration of the potential substrates for PPO with the Folin-Ciocalteau phenol reagent using a gallic acid standard curve. By comparing the tendency of the apples to turn brown, the specific activity of PPO, and the concentration of potential substrates, they can assess the biochemical basis of the browning phenotype. This experiment can be done as a series of weekly laboratory exercises, as an intensive 1-week laboratory project, or as the basis of an extended student research investigation. © 2017 by The International Union of Biochemistry and Molecular Biology, 46(1):76-82, 2018.

Iron deficiency stress can induce MxNAS1 protein expression to facilitate iron redistribution in Malus xiaojinensis

Iron (Fe) is a vital trace element in plants, and deficiency of this element in apple trees can reduce fruit quality. Nicotianamine (NA) is known to play an important role in Fe transport and endogenous hormone balance. In the present study, we investigated the role of a nicotianamine synthase 1 gene (MxNas1) in an apple species, Malus xiaojinensis, that has a more Fe-efficient genotype than other apple species and ecotypes. To characterise the response of M. xiaojinensis to Fe deficiency, we used quantitative Q-PCR to determine the level of expression of MxNas1 and Western blot to measure protein levels. Immunohistochemical staining and GFP fluorescence localisation of the MxNAS1 protein were also carried out. HPLC and polarised absorption spectrophotometry were performed to investigate the effects of overexpression of MxNas1 in order to elucidate the role of MxNAS1 in the cellular uptake of active Fe in tobacco suspension cells. We found that MxNas1 expression and protein levels were higher under Fe deficiency stress than under Fe sufficiency. Immunohistochemical staining showed that MxNAS1 was localised mainly in the epidermal and vascular tissues of the roots, vascular tissues of the stem and palisade cells of mature leaves, and in parenchyma cells of young leaves. MxNAS1 was mainly localised in the plasma membranes and vesicles of protoplasts. In addition, overexpression of MxNas1 in stable transgenic tobacco cells increased NA and active Fe content under Fe sufficiency. The results suggest that MxNas1 expression in M. xiaojinensis is induced in response to Fe deficiency stress, resulting in higher levels of the protein. MxNAS1 may be involved in the redistribution of Fe in M. xiaojinensis under Fe deficiency.

Molecular architectures of benzoic acid-specific type III polyketide synthases

Biphenyl synthase and benzophenone synthase constitute an evolutionarily distinct clade of type III polyketide synthases (PKSs) that use benzoic acid-derived substrates to produce defense metabolites in plants. The use of benzoyl-CoA as an endogenous substrate is unusual for type III PKSs. Moreover, sequence analyses indicate that the residues responsible for the functional diversification of type III PKSs are mutated in benzoic acid-specific type III PKSs. In order to gain a better understanding of structure-function relationships within the type III PKS family, the crystal structures of biphenyl synthase from Malus × domestica and benzophenone synthase from Hypericum androsaemum were compared with the structure of an archetypal type III PKS: chalcone synthase from Malus × domestica. Both biphenyl synthase and benzophenone synthase contain mutations that reshape their active-site cavities to prevent the binding of 4-coumaroyl-CoA and to favor the binding of small hydrophobic substrates. The active-site cavities of biphenyl synthase and benzophenone synthase also contain a novel pocket associated with their chain-elongation and cyclization reactions. Collectively, these results illuminate structural determinants of benzoic acid-specific type III PKSs and expand the understanding of the evolution of specialized metabolic pathways in plants.

An apple CIPK protein kinase targets a novel residue of AREB transcription factor for ABA-dependent phosphorylation

Phytohormone abscisic acid (ABA) regulates many important processes in plants. It is a major molecule facilitating signal transduction during the abiotic stress response. In this study, an ABA-inducible transcription factor gene, MdAREB2, was identified in apple. Transgenic analysis was performed to characterize its function in ABA sensitivity. Overexpression of the MdAREB2 gene increased ABA sensitivity in the transgenic apple compared with the wild-type (WT) control. In addition, it was found that the protein MdAREB2 was phosphorylated at a novel site Thr⁴¹¹ in response to ABA. A yeast two-hybridization screen of an apple cDNA library demonstrated that a protein kinase, MdCIPK22, interacted with MdAREB2. Their interaction was further verified with Pull Down and Co-IP assays. A series of transgenic analyses in apple calli and plantlets showed that MdCIPK22 was required for ABA-induced phosphorylation at Thr⁴¹¹ of the MdAREB2 protein and enhanced its stability and transcriptional activity. Finally, it was found that MdCIPK22 increased ABA sensitivity in an MdAREB2-dependent manner. Our findings indicate a novel phosphorylation site in CIPK-AREB regulatory module for the ABA signalling pathway, which would be helpful for researchers to identify the functions of uncharacterized homologs in the future.

Plant mitochondria synthesize melatonin and enhance the tolerance of plants to drought stress

Synthesis of melatonin in mitochondria was reported in animals. However, there is no report on whether plant mitochondria also produce melatonin. Herein, we show that plant mitochondria are a major site for melatonin synthesis. In an in vitro study, isolated apple mitochondria had the capacity to generate melatonin. Subcellular localization analysis documented that an apple SNAT isoform, MzSNAT5, was localized in the mitochondria of both Arabidopsis protoplasts and apple callus cells. The kinetic analysis revealed that the recombinant MzSNAT5 protein exhibited high enzymatic activity to catalyze serotonin to N-acetylserotonin with the K_m and V_max of 55 μmol/L and 0.909 pmol/min/mg protein at 35°C, respectively; this pathway functioned over a wide range of temperatures from 5 to 75°C. In an in vivo study, MzSNAT5 was drought inducible. The transgenic Arabidopsis ectopically expressing MzSNAT5 elevated the melatonin level and, hence, enhanced drought tolerance. The mechanistic study indicated that the ectopically expressing MzSNAT5 allows plant mitochondria to increase melatonin synthesis. As a potent free radical scavenger, melatonin reduces the oxidative stress caused by the elevated reactive oxygen species which are generated under drought stress in plants. Our findings provide evidence that engineered melatonin-enriched plants exhibit enhanced oxidative tolerance.

In crystallo activity tests with latent apple tyrosinase and two mutants reveal the importance of the mutated sites for polyphenol oxidase activity

Tyrosinases are type 3 copper enzymes that belong to the polyphenol oxidase (PPO) family and are able to catalyze both the ortho-hydroxylation of monophenols and their subsequent oxidation to o-quinones, which are precursors for the biosynthesis of colouring substances such as melanin. The first plant pro-tyrosinase from Malus domestica (MdPPO1) was recombinantly expressed in its latent form (56.4 kDa) and mutated at four positions around the catalytic pocket which are believed to influence the activity of the enzyme. Mutating the amino acids, which are known as activity controllers, yielded the mutants MdPPO1-Ala239Thr and MdPPO1-Leu243Arg, whereas mutation of the so-called water-keeper and gatekeeper residues resulted in the mutants MdPPO1-Glu234Ala and MdPPO1-Phe259Ala, respectively. The wild-type enzyme and two of the mutants, MdPPO1-Ala239Thr and MdPPO1-Phe259Ala, were successfully crystallized, leading to single crystals that diffracted to 1.35, 1.55 and 1.70 Å resolution, respectively. All crystals belonged to space group P212121, exhibiting similar unit-cell parameters: a = 50.70, b = 80.15, c = 115.96 Å for the wild type, a = 50.58, b = 79.90, c = 115.76 Å for MdPPO1-Ala239Thr and a = 50.53, b = 79.76, c = 116.07 Å for MdPPO1-Phe259Ala. In crystallo activity tests with the crystals of the wild type and the two mutants were performed by adding the monophenolic substrate tyramine and the diphenolic substrate dopamine to crystal-containing drops. The effects of the mutation on the activity of the enzyme were observed by colour changes of the crystals owing to the conversion of the substrates to dark chromophore products.

Characterization of three chalcone synthase-like genes from apple (Malus x domestica Borkh.)

Apple (Malus x domestica Brokh.) is a widely cultivated deciduous tree species of significant economic importance. Apple leaves accumulate high levels of flavonoids and dihydrochalcones, and their formation is dependent on enzymes of the chalcone synthase family. Three CHS genes were cloned from apple leaves and expressed in Escherichia coli. The encoded recombinant enzymes were purified and functionally characterized. In-vitro activity assays indicated that MdCHS1, MdCHS2 and MdCHS3 code for proteins exhibiting polyketide synthase activity that accepted either p-dihydrocoumaroyl-CoA, p-coumaroyl-CoA, or cinnamoyl-CoA as starter CoA substrates in the presence of malonyl-CoA, leading to production of phloretin, naringenin chalcone, and pinocembrin chalcone. MdCHS3 coded a chalcone-dihydrochalcone synthase enzyme with narrower substrate specificity than the previous ones. The apparent Km values of MdCHS3 for p-dihydrocoumaryl-CoA and p-coumaryl-CoA were both 5.0 μM. Expression analyses of MdCHS genes varied according to tissue type. MdCHS1, MdCHS2 and MdCHS3 expression levels were associated with the levels of phloretin accumulate in the respective tissues.

Transcription Factor AREB2 Is Involved in Soluble Sugar Accumulation by Activating Sugar Transporter and Amylase Genes

Sugars play important roles in plant growth and development, crop yield and quality, as well as responses to abiotic stresses. Abscisic acid (ABA) is a multifunctional hormone. However, the exact mechanism by which ABA regulates sugar accumulation is largely unknown in plants. Here, we tested the expression profile of several sugar transporter and amylase genes in response to ABA treatment. MdSUT2 and MdAREB2 were isolated and genetically transformed into apple (Malus domestica) to investigate their roles in ABA-induced sugar accumulation. The MdAREB2 transcription factor was found to bind to the promoters of the sugar transporter and amylase genes and activate their expression. Both MdAREB2 and MdSUT2 transgenic plants produced more soluble sugars than controls. Furthermore, MdAREB2 promoted the accumulation of sucrose and soluble sugars in an MdSUT2-dependent manner. Our results demonstrate that the ABA-responsive transcription factor MdAREB2 directly activates the expression of amylase and sugar transporter genes to promote soluble sugar accumulation, suggesting a mechanism by which ABA regulates sugar accumulation in plants.

Fine-tuning of the flavonoid and monolignol pathways during apple early fruit development

A coordinated regulation of different branches of the flavonoid pathway was highlighted that may contribute to elucidate the role of this important class of compounds during the early stages of apple fruit development. Apple (Malus × domestica Borkh.) is an economically important fruit appreciated for its organoleptic characteristics and its benefits for human health. The first stages after fruit set represent a very important and still poorly characterized developmental process. To enable the profiling of genes involved in apple early fruit development, we combined the suppression subtractive hybridization (SSH) protocol to next-generation sequencing. We identified and characterized genes induced and repressed during fruit development in the apple cultivar 'Golden Delicious'. Our results showed an opposite regulation of genes coding for enzymes belonging to flavonoid and monolignol pathways, with a strong induction of the former and a simultaneous repression of the latter. Two isoforms of phenylalanine ammonia-lyase and 4-coumarate:CoA ligase, key enzymes located at the branching point between flavonoid and monolignol pathways, showed opposite expression patterns during the period in analysis, suggesting a possible regulation mechanism. A targeted metabolomic analysis supported the SSH results and revealed an accumulation of the monomers catechin and epicatechin as well as several forms of procyanidin oligomers in apple fruitlets starting early after anthesis, together with a decreased production of other classes of flavonoids such as some flavonols and the dihydrochalcone phlorizin. Moreover, gene expression and metabolites accumulation of 'Golden Delicious' were compared to a wild apple genotype of Manchurian crabapple (Malus mandshurica (Maxim.) Kom.). Significant differences in both gene expression and metabolites accumulation were found between the two genotypes.

Delayed response to cold stress is characterized by successive metabolic shifts culminating in apple fruit peel necrosis

BACKGROUND

Superficial scald is a physiological disorder of apple fruit characterized by sunken, necrotic lesions appearing after prolonged cold storage, although initial injury occurs much earlier in the storage period. To determine the degree to which the transition to cell death is an active process and specific metabolism involved, untargeted metabolic and transcriptomic profiling was used to follow metabolism of peel tissue over 180 d of cold storage.

RESULTS

The metabolome and transcriptome of peel destined to develop scald began to diverge from peel where scald was controlled using antioxidant (diphenylamine; DPA) or rendered insensitive to ethylene using 1-methylcyclopropene (1-MCP) beginning between 30 and 60 days of storage. Overall metabolic and transcriptomic shifts, representing multiple pathways and processes, occurred alongside α-farnesene oxidation and, later, methanol production alongside symptom development.

CONCLUSIONS

Results indicate this form of peel necrosis is a product of an active metabolic transition involving multiple pathways triggered by chilling temperatures at cold storage inception rather than physical injury. Among multiple other pathways, enhanced methanol and methyl ester levels alongside upregulated pectin methylesterases are unique to peel that is developing scald symptoms similar to injury resulting from mechanical stress and herbivory in other plants.

Investigating Proteome and Transcriptome Defense Response of Apples Induced by Yarrowia lipolytica

A better understanding of the mode of action of postharvest biocontrol agents on fruit surfaces is critical for the advancement of successful implementation of postharvest biocontrol products. This is due to the increasing importance of biological control of postharvest diseases over chemical and other control methods. However, most of the mechanisms involved in biological control remain unknown and need to be explored. Yarrowia lipolytica significantly inhibited blue mold decay of apples caused by Penicillium expansum. The findings also demonstrated that Y. lipolytica stimulated the activities of polyphenoloxidase, peroxidase, chitinase, l-phenylalanine ammonia lyase involved in enhancing defense responses in apple fruit tissue. Proteomic and transcriptomic analysis revealed a total of 35 proteins identified as up- and down-regulated in response to the Y. lipolytica inducement. These proteins were related to defense, biotic stimulus, and stress responses, such as pathogenesis-related proteins and dehydrin. The analysis of the transcriptome results proved that the induced resistance was mediated by a crosstalk between salicylic acid (SA) and ethylene/jasmonate (ET/JA) pathways. Y. lipolytica treatment activated the expression of isochorismate synthase gene in the SA pathway, which up-regulates the expression of PR4 in apple. The expression of 1-aminocyclopropane-1-carboxylate oxidase gene and ET-responsive transcription factors 2 and 4, which are involved in the ET pathway, were also activated. In addition, cytochrome oxidase I, which plays an important role in JA signaling for resistance acquisition, was also activated. However, not all of the genes had a positive effect on the SA and ET/JA signal pathways. As transcriptional repressors in JA signaling, TIFY3B and TIFY11B were triggered by the yeast, but the gene expression levels were relatively low. Taken together, Y. lipolytica induced the SA and ET/JA signal mediating the defense pathways by stimulating defense response genes, such as peroxidase, thaumatin-like protein, and chitinase 4-like, which are involved in defense response in apple. [Formula: see text]

Green tea extract as an anti-browning agent for cloudy apple juice

BACKGROUND

Enzymatic browning of fruits and vegetables and their products is an important factor worsening their quality. The influence of five green tea extracts at the concentrations of 1 g L^-1 , 2 g L^-1 and 3 g L^-1 on polyphenol oxidase (PPO) activity in fresh cloudy apple juice was investigated. Moreover, PPO inhibition by tea extract and colour stability of juice during short-time refrigerated storage was studied. The changes of juice colour during storage was expressed as the total colour differences (ΔE*), browning index (BI), yellowness index (YI), and the absorbance at 420 nm (A₄₂₀ ).

RESULTS

All extracts inhibited PPO activity in fresh apple juice in concentration-dependent manner. PPO activity in pure apple juice decreased by 7% after 48 h, whereas PPO activity in samples with 1 g L^-1 , 2 g L^-1 and 3 g L^-1 tea extract decreased by 53%, 74%, and 96%, respectively. Browning of apple juice during storage decreased with increased concentration of green tea extract. After 48 h, extract at 1 g L^-1 , 2 g L^-1 and 3 g L^-1 inhibited browning of juice expressed as BI by 48%, 60%, and 86%, respectively, comparing to pure apple juice.

CONCLUSION

Green tea extract may be an effective anti-browning agent for short-time stored cloudy apple juices. © 2016 Society of Chemical Industry.

Analysis of the Xyloglucan Endotransglucosylase/Hydrolase Gene Family during Apple Fruit Ripening and Softening

Ethylene and xyloglucan endotransglucosylase/hydrolase (XTH) genes were important for fruit ripening and softening in 'Taishanzaoxia' apple. In this study, we found it was ACS1-1/-1 homozygotes in 'Taishanzaoxia' apple, which determined the higher transcription activity of ACS1. XTH1, XTH3, XTH4, XTH5, and XTH9 were mainly involved in the early fruit softening independent of ethylene, while XTH2, XTH6, XTH7, XTH8, XTH10, and XTH11 were predominantly involved in the late fruit softening dependent on ethylene. Overexpression of XTH2 and XTH10 in tomato resulted in the elevated expression of genes involved in ethylene biosynthesis (ACS2, ACO1), signal transduction (ERF2), and fruit softening (XTHs, PG2A, Cel2, and TBG4). In summary, the burst of ethylene in 'Taishanzaoxia' apple was predominantly determined by ACS1-1/-1 genotype, and the differential expression of XTH genes dependent on and independent of ethylene played critical roles in the fruit ripening and softening. XTH2 and XTH10 may act as a signal switch in the feedback regulation of ethylene signaling and fruit softening.

The enhancement of tolerance to salt and cold stresses by modifying the redox state and salicylic acid content via the cytosolic malate dehydrogenase gene in transgenic apple plants

In this study, we characterized the role of an apple cytosolic malate dehydrogenase gene (MdcyMDH) in the tolerance to salt and cold stresses and investigated its regulation mechanism in stress tolerance. The MdcyMDH transcript was induced by mild cold and salt treatments, and MdcyMDH-overexpressing apple plants possessed improved cold and salt tolerance compared to wild-type (WT) plants. A digital gene expression tag profiling analysis revealed that MdcyMDH overexpression largely altered some biological processes, including hormone signal transduction, photosynthesis, citrate cycle and oxidation-reduction. Further experiments verified that MdcyMDH overexpression modified the mitochondrial and chloroplast metabolisms and elevated the level of reducing power, primarily caused by increased ascorbate and glutathione, as well as the increased ratios of ascorbate/dehydroascorbate and glutathione/glutathione disulphide, under normal and especially stress conditions. Concurrently, the transgenic plants produced a high H2 O2 content, but a low O2·- production rate was observed compared to the WT plants. On the other hand, the transgenic plants accumulated more free and total salicylic acid (SA) than the WT plants under normal and stress conditions. Taken together, MdcyMDH conferred the transgenic apple plants a higher stress tolerance by producing more reductive redox states and increasing the SA level; MdcyMDH could serve as a target gene to genetically engineer salt- and cold-tolerant trees.

Multifunctional oxidosqualene cyclases and cytochrome P450 involved in the biosynthesis of apple fruit triterpenic acids

Apple (Malus × domestica) accumulates bioactive ursane-, oleanane-, and lupane-type triterpenes in its fruit cuticle, but their biosynthetic pathway is still poorly understood. We used a homology-based approach to identify and functionally characterize two new oxidosqualene cyclases (MdOSC4 and MdOSC5) and one cytochrome P450 (CYP716A175). The gene expression patterns of these enzymes and of previously described oxidosqualene cyclases were further studied in 20 apple cultivars with contrasting triterpene profiles. MdOSC4 encodes a multifunctional oxidosqualene cyclase producing an oleanane-type triterpene, putatively identified as germanicol, as well as β-amyrin and lupeol, in the proportion 82 : 14 : 4. MdOSC5 cyclizes 2,3-oxidosqualene into lupeol and β-amyrin at a ratio of 95 : 5. CYP716A175 catalyses the C-28 oxidation of α-amyrin, β-amyrin, lupeol and germanicol, producing ursolic acid, oleanolic acid, betulinic acid, and putatively morolic acid. The gene expression of MdOSC1 was linked to the concentrations of ursolic and oleanolic acid, whereas the expression of MdOSC5 was correlated with the concentrations of betulinic acid and its caffeate derivatives. Two new multifuntional triterpene synthases as well as a multifunctional triterpene C-28 oxidase were identified in Malus × domestica. This study also suggests that MdOSC1 and MdOSC5 are key genes in apple fruit triterpene biosynthesis.

Functional identification of MdSIZ1 as a SUMO E3 ligase in apple

SUMOylation, the conjugation of target proteins with SUMO (small ubiquitin-related modifier), is a type of post-translational modification in eukaryotes and involves the sequential action of activation (E1), conjugation (E2) and ligation (E3) enzymes. In Arabidopsis, the AtSIZ1 protein is a SUMO E3 ligase that promotes the conjugation of SUMO proteins to target substrates. Here, we isolated and identified a SUMO E3 ligase, MdSIZ1, in apple, which was similar to AtSIZ1. SUMOylation analysis showed that MdSIZ1 had SUMO E3 ligase activity in vitro and in vivo. SUMO conjugation was increased by high temperatures, low temperatures, and abscisic acid (ABA). The ectopic expression of MdSIZ1 in Arabidopsis siz1-2 mutant plants partially complemented the morphological mutant phenotype and enhanced the levels of SUMO conjugation. Taken together, these results suggest that MdSIZ1-mediated SUMO conjugation of target proteins is an important process that regulates the adaptation of apple plants to various environmental stresses.