MYB transcription factors that colour our fruit

Anthocyanin concentration is a primary determinant of plant colour. Fruit anthocyanin biosynthesis is controlled by a distinct clade of R2R3 MYB transcription factors. In apple, three recent papers describe the discovery of MYB genes activating skin, flesh and foliage anthocyanic colour. These findings lead the way to new approaches in the breeding and biotechnological development of fruit with new colour patterns.

Melatonin mediates the regulation of ABA metabolism, free-radical scavenging, and stomatal behaviour in two Malus species under drought stress

Melatonin pre-treatment significantly increases the tolerance of both drought-tolerant Malus prunifolia and drought-sensitive M. hupehensis plants. Its beneficial effects include better water conservation in leaves, less electrolyte leakage, steady chlorophyll contents, and greater photosynthetic performance under stress conditions. Melatonin selectively down-regulates MdNCED3, an abscisic acid (ABA) synthesis gene, and up-regulates its catabolic genes, MdCYP707A1 and MdCYP707A2, thereby reducing ABA contents in drought-stressed plants. Melatonin also directly scavenges H2O2 and enhances the activities of antioxidant enzymes to detoxify H2O2 indirectly. These two mechanisms work synergistically to improve the functions of stomata, i.e. causing them to re-open. Plants can effectively regulate their water balance under drought conditions by up-regulating the expression of melatonin synthesis genes MdTDC1, MdAANAT2, MdT5H4, and MdASMT1. Therefore, inducing melatonin production is an important mechanism by which plants can counteract the influence of this abiotic stressor.

High temperature reduces apple fruit colour via modulation of the anthocyanin regulatory complex

The biosynthesis of anthocyanin in many plants is affected by environmental conditions. In apple (Malus × domestica Borkh.), concentrations of fruit anthocyanins are lower under hot climatic conditions. We examined the anthocyanin accumulation in the peel of maturing 'Mondial Gala' and 'Royal Gala' apples, grown in both temperate and hot climates, and using artificial heating of on-tree fruit. Heat caused a dramatic reduction of both peel anthocyanin concentration and transcripts of the genes of the anthocyanin biosynthetic pathway. Heating fruit rapidly reduced expression of the R2R3 MYB transcription factor (MYB10) responsible for coordinative regulation for red skin colour, as well as expression of other genes in the transcriptional activation complex. A single night of low temperatures is sufficient to elicit a large increase in transcription of MYB10 and consequently the biosynthetic pathway. Candidate genes that can repress anthocyanin biosynthesis did not appear to be responsible for reductions in anthocyanin content. We propose that temperature-induced regulation of anthocyanin biosynthesis is primarily caused by altered transcript levels of the activating anthocyanin regulatory complex.

Low temperature, but not photoperiod, controls growth cessation and dormancy induction and release in apple and pear

In contrast to most temperate woody species, apple and pear and some other woody species of the Rosaceae family are insensitive to photoperiod, and no alternative environmental seasonal signal is known to control their dormancy. We studied growth and dormancy induction in micropropagated plants of four apple (Malus pumila Mill.) and one pear (Pyrus communis L.) commercial rootstock cultivars in controlled environments. The results confirm that growth cessation and dormancy induction in apple and pear are not influenced by photoperiod, and demonstrate that low temperature (< 12 degrees C) consistently induces both processes, regardless of photoperiodic conditions. Successive stages of the autumn syndrome (growth cessation, formation of bud scales and winter buds, leaf senescence and abscission, and dormancy induction) occurred in response to low temperature. Long days increased internode length at higher temperatures, but had no significant effect on leaf production in any of the cultivars. Chilling at 6 or 9 degrees C for at least 6 weeks (about 1000 h) was required for dormancy release and growth resumption, whereas treatment at 12 degrees C was marginally effective, even after 14 weeks of exposure. We are thus faced with the paradox that the same low temperature conditions that induce dormancy are also required for dormancy release in these species.

An atypical R2R3 MYB transcription factor increases cold hardiness by CBF-dependent and CBF-independent pathways in apple

Apple (Malus × domestica) trees are vulnerable to freezing temperatures. However, there has been only limited success in developing cold-hardy cultivars. This lack of progress is due at least partly to lack of understanding of the molecular mechanisms of freezing tolerance in apple. In this study, we evaluated the potential roles for two R2R3 MYB transcription factors (TFs), MYB88 and the paralogous FLP (MYB124), in cold stress in apple and Arabidopsis. We found that MYB88 and MYB124 positively regulate freezing tolerance and cold-responsive gene expression in both apple and Arabidopsis. Chromatin-Immunoprecipitation-qPCR and electrophoretic mobility shift assays showed that MdMYB88/MdMYB124 act as direct regulators of the COLD SHOCK DOMAIN PROTEIN 3 (MdCSP3) and CIRCADIAN CLOCK ASSOCIATED 1 (MdCCA1) genes. Dual luciferase reporter assay indicated that MdCCA1 but not MdCSP3 activated the expression of MdCBF3 under cold stress. Moreover, MdMYB88 and MdMYB124 promoted anthocyanin accumulation and H₂ O₂ detoxification in response to cold. Taken together, our results suggest that MdMYB88 and MdMYB124 positively regulate cold hardiness and cold-responsive gene expression under cold stress by C-REPEAT BINDING FACTOR (CBF)-dependent and CBF-independent pathways.

The ERF transcription factor MdERF38 promotes drought stress-induced anthocyanin biosynthesis in apple

Drought stress induces anthocyanin biosynthesis in many plant species, but the underlying molecular mechanism remains unclear. Ethylene response factors (ERFs) play key roles in plant growth and various stress responses, including affecting anthocyanin biosynthesis. Here, we characterized an ERF protein, MdERF38, which is involved in drought stress-induced anthocyanin biosynthesis. Biochemical and molecular analyses showed that MdERF38 interacted with MdMYB1, a positive modulator of anthocyanin biosynthesis, and facilitated the binding of MdMYB1 to its target genes. Therefore, MdERF38 promoted anthocyanin biosynthesis in response to drought stress. Furthermore, we found that MdBT2, a negative modulator of anthocyanin biosynthesis, decreased MdERF38-promoted anthocyanin biosynthesis by accelerating the degradation of the MdERF38 protein. In summary, our data provide a mechanism for drought stress-induced anthocyanin biosynthesis that involves dynamic modulation of MdERF38 at both transcriptional and post-translational levels.

Neonicotinoid pesticide exposure impairs crop pollination services provided by bumblebees

Recent concern over global pollinator declines has led to considerable research on the effects of pesticides on bees. Although pesticides are typically not encountered at lethal levels in the field, there is growing evidence indicating that exposure to field-realistic levels can have sublethal effects on bees, affecting their foraging behaviour, homing ability and reproductive success. Bees are essential for the pollination of a wide variety of crops and the majority of wild flowering plants, but until now research on pesticide effects has been limited to direct effects on bees themselves and not on the pollination services they provide. Here we show the first evidence to our knowledge that pesticide exposure can reduce the pollination services bumblebees deliver to apples, a crop of global economic importance. Bumblebee colonies exposed to a neonicotinoid pesticide provided lower visitation rates to apple trees and collected pollen less often. Most importantly, these pesticide-exposed colonies produced apples containing fewer seeds, demonstrating a reduced delivery of pollination services. Our results also indicate that reduced pollination service delivery is not due to pesticide-induced changes in individual bee behaviour, but most likely due to effects at the colony level. These findings show that pesticide exposure can impair the ability of bees to provide pollination services, with important implications for both the sustained delivery of stable crop yields and the functioning of natural ecosystems.

Apple skin patterning is associated with differential expression of MYB10

BACKGROUND

Some apple (Malus × domestica Borkh.) varieties have attractive striping patterns, a quality attribute that is important for determining apple fruit market acceptance. Most apple cultivars (e.g. 'Royal Gala') produce fruit with a defined fruit pigment pattern, but in the case of 'Honeycrisp' apple, trees can produce fruits of two different kinds: striped and blushed. The causes of this phenomenon are unknown.

RESULTS

Here we show that striped areas of 'Honeycrisp' and 'Royal Gala' are due to sectorial increases in anthocyanin concentration. Transcript levels of the major biosynthetic genes and MYB10, a transcription factor that upregulates apple anthocyanin production, correlated with increased anthocyanin concentration in stripes. However, nucleotide changes in the promoter and coding sequence of MYB10 do not correlate with skin pattern in 'Honeycrisp' and other cultivars differing in peel pigmentation patterns. A survey of methylation levels throughout the coding region of MYB10 and a 2.5 Kb region 5' of the ATG translation start site indicated that an area 900 bp long, starting 1400 bp upstream of the translation start site, is highly methylated. Cytosine methylation was present in all three contexts, with higher methylation levels observed for CHH and CHG (where H is A, C or T) than for CG. Comparisons of methylation levels of the MYB10 promoter in 'Honeycrisp' red and green stripes indicated that they correlate with peel phenotypes, with an enrichment of methylation observed in green stripes.

CONCLUSIONS

Differences in anthocyanin levels between red and green stripes can be explained by differential transcript accumulation of MYB10. Different levels of MYB10 transcript in red versus green stripes are inversely associated with methylation levels in the promoter region. Although observed methylation differences are modest, trends are consistent across years and differences are statistically significant. Methylation may be associated with the presence of a TRIM retrotransposon within the promoter region, but the presence of the TRIM element alone cannot explain the phenotypic variability observed in 'Honeycrisp'. We suggest that methylation in the MYB10 promoter is more variable in 'Honeycrisp' than in 'Royal Gala', leading to more variable color patterns in the peel of this cultivar.

Phloridzin: biosynthesis, distribution and physiological relevance in plants

The phenolic compound phloridzin (phloretin 2'-O-glucoside, phlorizin, phlorrhizin, phlorhizin or phlorizoside) is a prominent member of the chemical class of dihydrochalcones, which are phenylpropanoids. The apple tree (Malus sp.) accumulates high amounts of phloridzin, whereas few other species contain this compound only in low amounts. Additionally, Malus sp. show a species- and tissue-specific distribution of phloridzin and its derivatives. Whereas the physiological role of phloridzin in planta is not fully understood, the effect on human health - especially diabetes - and membrane permeability is well documented. The biosynthesis of phloridzin was investigated only recently with recombinant enzymes and plant protein extracts and involved a NADPH-dependent dehydrogenase, chalcone synthase and UDP-glucose:phloretin 2'-O-glycosyltransferase.

R2R3-MYB transcription factor MdMYB23 is involved in the cold tolerance and proanthocyanidin accumulation in apple

Cold stress severely affects plant growth and yield. C-repeat binding factors (CBFs) play important roles in the response to cold stress. In the present study, we identified an R2R3-MYB transcription factor (TF) MdMYB23 from apple (Malus × domestic) using transcriptome analyses, which was notably induced in response to cold stress. Transgenic apple calli and Arabidopsis with overexpression of MdMYB23 exhibited increased cold tolerance. Electrophoretic mobility shift assay (EMSA) and transient expression assays indicated that MdMYB23 directly bound to the promoters of MdCBF1 and MdCBF2 and activated their expression. MdMYB23 interacted with the promoter of MdANR, a key modulator of proanthocyanidin biosynthesis, and activated its expression to promote proanthocyanidin accumulation and reactive oxygen species (ROS) scavenging. MdBT2 was identified as an MdMYB23-interacting protein using yeast two-hybrid (Y2H), pull-down, and bimolecular fluorescence complementation (BiFC) assays. MdBT2 repressed cold tolerance and proanthocyanidin accumulation by promoting the degradation of MdMYB23 protein. Our findings shed light on the functions of MYB TFs and underlying mechanism in the modulation of plant cold tolerance.

The apple WD40 protein MdTTG1 interacts with bHLH but not MYB proteins to regulate anthocyanin accumulation

The abundance of anthocyanins and proanthocyanins in apples is tightly regulated by three classes of regulatory factors, MYB, bHLH and WD40 proteins, only some of which have been previously identified. In this study, we identified an apple WD40 protein (MdTTG1) that promotes the accumulation of anthocyanins. The biosynthetic genes required downstream in the flavonoid pathway were up-regulated when MdTTG1 was over-expressed in Arabidopsis. Consistent with its role as a transcriptional regulator, an MdTTG1-GFP fusion protein was observed only in the nucleus. We assayed the expression patterns of this gene in different organs and found that they were positively correlated with anthocyanin accumulation in the apple. Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that MdTTG1 interacted with bHLH transcription factors (TFs) but not MYB protein, whereas bHLH was known to interact with MYB in apples. However, based on a ChIP assay, MdTTG1 does not appear to bind to the promoter of the anthocyanin biosynthetic genes MdDFR and MdUFGT. Taken together, these results suggest that the apple WD40 protein MdTTG1 interacts with bHLH but not MYB proteins to regulate anthocyanin accumulation.

Molecular characterization of FLOWERING LOCUS T-like genes of apple (Malus x domestica Borkh.)

The two FLOWERING LOCUS T (FT)-like genes of apple (Malus x domestica Borkh.), MdFT1 and MdFT2, have been isolated and characterized. MdFT1 and MdFT2 were mapped, respectively, on distinct linkage groups (LGs) with partial homoeology, LG 12 and LG 4. The expression pattern of MdFT1 and MdFT2 differed in that MdFT1 was expressed mainly in apical buds of fruit-bearing shoots in the adult phase, with little expression in the juvenile tissues, whereas MdFT2 was expressed mainly in reproductive organs, including flower buds and young fruit. On the other hand, both genes had the potential to induce early flowering since transgenic Arabidopsis, which ectopically expressed MdFT1 or MdFT2, flowered earlier than wild-type plants. Furthermore, overexpression of MdFT1 conferred precocious flowering in apple, with altered expression of other endogenous genes, such as MdMADS12. These results suggest that MdFT1 could function to promote flowering by altering the expression of those genes and that, at least, other genes may play an important role as well in the regulation of flowering in apple. The long juvenile period of fruit trees prevents early cropping and efficient breeding. Our findings will be useful information to unveil the molecular mechanism of flowering and to develop methods to shorten the juvenile period in various fruit trees, including apple.

Genome wide analysis of the apple MYB transcription factor family allows the identification of MdoMYB121 gene confering abiotic stress tolerance in plants

The MYB proteins comprise one of the largest families of transcription factors (TFs) in plants. Although several MYB genes have been characterized to play roles in secondary metabolism, the MYB family has not yet been identified in apple. In this study, 229 apple MYB genes were identified through a genome-wide analysis and divided into 45 subgroups. A computational analysis was conducted using the apple genomic database to yield a complete overview of the MYB family, including the intron-exon organizations, the sequence features of the MYB DNA-binding domains, the carboxy-terminal motifs, and the chromosomal locations. Subsequently, the expression of 18 MYB genes, including 12 were chosen from stress-related subgroups, while another 6 ones from other subgroups, in response to various abiotic stresses was examined. It was found that several of these MYB genes, particularly MdoMYB121, were induced by multiple stresses. The MdoMYB121 was then further functionally characterized. Its predicted protein was found to be localized in the nucleus. A transgenic analysis indicated that the overexpression of the MdoMYB121 gene remarkably enhanced the tolerance to high salinity, drought, and cold stresses in transgenic tomato and apple plants. Our results indicate that the MYB genes are highly conserved in plant species and that MdoMYB121 can be used as a target gene in genetic engineering approaches to improve the tolerance of plants to multiple abiotic stresses.

Down-regulation of POLYGALACTURONASE1 alters firmness, tensile strength and water loss in apple (Malus x domestica) fruit

BACKGROUND

While there is now a significant body of research correlating apple (Malus x domestica) fruit softening with the cell wall hydrolase ENDO-POLYGALACTURONASE1 (PG1), there is currently little knowledge of its physiological effects in planta. This study examined the effect of down regulation of PG1 expression in 'Royal Gala' apples, a cultivar that typically has high levels of PG1, and softens during fruit ripening.

RESULTS

PG1-suppressed 'Royal Gala' apples harvested from multiple seasons were firmer than controls after ripening, and intercellular adhesion was higher. Cell wall analyses indicated changes in yield and composition of pectin, and a higher molecular weight distribution of CDTA-soluble pectin. Structural analyses revealed more ruptured cells and free juice in pulled apart sections, suggesting improved integrity of intercellular connections and consequent cell rupture due to failure of the primary cell walls under stress. PG1-suppressed lines also had reduced expansion of cells in the hypodermis of ripe apples, resulting in more densely packed cells in this layer. This change in morphology appears to be linked with reduced transpirational water loss in the fruit.

CONCLUSIONS

These findings confirm PG1's role in apple fruit softening and suggests that this is achieved in part by reducing cellular adhesion. This is consistent with previous studies carried out in strawberry but not with those performed in tomato. In apple PG1 also appears to influence other fruit texture characters such as juiciness and water loss.

Significance of skin flavonoids for UV-B-protection in apple fruits

An attempt has been made to assess the UV-B-protective capacity of phenolic compounds accumulated in superficial structures of plants using apple fruit as a model. Two apple (Malus domestica Borkh.) cultivars (Braeburn and Granny Smith) differing in response to high fluxes of solar radiation were selected and exposed to increasing doses of UV-B radiation. The extent of UV-B-induced damage to photosystem II of apple skin correlated with its quercetin glycoside (but not anthocyanin) content. Granny Smith apples did not demonstrate a pronounced response to high sunlight in terms of the accumulation of phenolic substances in the skin and exhibited similar patterns of Fo, Fm, and Fv/Fm changes in the course of UV-B irradiation both on sun-exposed and shaded surfaces of a fruit. Unlike Granny Smith, Braeburn fruits were characterized by a significant accumulation of quercetin glycosides in sun-exposed skin, however, shaded skin contained these compounds in similar amounts to those in Granny Smith. Accordingly, photosystem II in sun-exposed skin of Braeburn apples was resistant to high doses of UV-B radiation (up to 97 kJ m-2), whereas the susceptibility of the photosynthetic apparatus in shaded skin of Braeburn to UV-B-induced damage was much higher and similar to that of both sun-exposed and shaded skin of Granny Smith fruits. Anthocyanins, at least in the amounts found in Braeburn, did not show an additional effect in UV-B protection.

MdMYB46 could enhance salt and osmotic stress tolerance in apple by directly activating stress-responsive signals

To expand the cultivation area of apple (Malus×domestica Borkh.) and select resistant varieties by genetic engineering, it is necessary to clarify the mechanism of salt and osmotic stress tolerance in apple. The MdMYB46 transcription factor was identified, and the stress treatment test of MdMYB46-overexpressing and MdMYB46-RNAi apple lines indicated that MdMYB46 could enhance the salt and osmotic stress tolerance in apple. In transgenic Arabidopsis and apple, MdMYB46 promoted the biosynthesis of secondary cell wall and deposition of lignin by directly binding to the promoter of lignin biosynthesis-related genes. To explore whether MdMYB46 could coordinate stress signal transduction pathways to cooperate with the formation of secondary walls to enhance the stress tolerance of plants, MdABRE1A, MdDREB2A and dehydration-responsive genes MdRD22 and MdRD29A were screened out for their positive correlation with osmotic stress, salt stress and the transcriptional level of MdMYB46. The further verification test demonstrated that MdMYB46 could activate their transcription by directly binding to the promoters of these genes. The above results indicate that MdMYB46 could enhance the salt and osmotic stress tolerance in apple not only by activating secondary cell wall biosynthesis pathways, but also by directly activating stress-responsive signals.

MdMYB88 and MdMYB124 Enhance Drought Tolerance by Modulating Root Vessels and Cell Walls in Apple

Water deficit is one of the main limiting factors in apple (Malus × domestica Borkh.) cultivation. Root architecture plays an important role in the drought tolerance of plants; however, research efforts to improve drought tolerance of apple trees have focused on aboveground targets. Due to the difficulties associated with visualization and data analysis, there is currently a poor understanding of the genetic players and molecular mechanisms involved in the root architecture of apple trees under drought conditions. We previously observed that MdMYB88 and its paralog MdMYB124 regulate apple tree root morphology. In this study, we found that MdMYB88 and MdMYB124 play important roles in maintaining root hydraulic conductivity under long-term drought conditions and therefore contribute toward adaptive drought tolerance. Further investigation revealed that MdMYB88 and MdMYB124 regulate root xylem development by directly binding MdVND6 and MdMYB46 promoters and thus influence expression of their target genes under drought conditions. In addition, MdMYB88 and MdMYB124 were shown to regulate the deposition of cellulose and lignin root cell walls in response to drought. Taken together, our results provide novel insights into the importance of MdMYB88 and MdMYB124 in root architecture, root xylem development, and secondary cell wall deposition in response to drought in apple trees.

Stomatal penetration by aqueous solutions--an update involving leaf surface particles

The recent visualization of stomatal nanoparticle uptake ended a 40-yr-old paradigm. Assuming clean, hydrophobic leaf surfaces, the paradigm considered stomatal liquid water transport to be impossible as a result of water surface tension. However, real leaves are not clean, and deposited aerosols may change hydrophobicity and water surface tension. Droplets containing NaCl, NaClO(3), (NH(4))(2) SO(4), glyphosate, an organosilicone surfactant or various combinations thereof were evaporated on stomatous abaxial and astomatous adaxial surfaces of apple (Malus domestica) leaves. The effects on photosynthesis, necrosis and biomass were determined. Observed using an environmental scanning electron microscope, NaCl and NaClO(3) crystals on hydrophobic tomato (Solanum lycopersicum) cuticles underwent several humidity cycles, causing repeated deliquescence and efflorescence of the salts. All physiological parameters were more strongly affected by abaxial than adaxial treatments. Spatial expansion and dendritic crystallization of the salts occurred and cuticular hydrophobicity was decreased more rapidly by NaClO(3) than NaCl. The results confirmed the stomatal uptake of aqueous solutions. Humidity fluctuations promote the spatial expansion of salts into the stomata. The ion-specific effects point to the Hofmeister series: chaotropic ions reduce surface tension, probably contributing to the defoliant action of NaClO(3), whereas the salt spray tolerance of coastal plants is probably linked to the kosmotropic nature of chloride ions.

Apple B-box protein BBX37 regulates jasmonic acid mediated cold tolerance through the JAZ-BBX37-ICE1-CBF pathway and undergoes MIEL1-mediated ubiquitination and degradation

The plant hormone jasmonic acid (JA) is involved in the cold stress response, and the inducer of CBF expression 1 (ICE1)- C-repeat binding factor (CBF) regulatory cascade plays a key role in the regulation of cold stress tolerance. In this study, we showed that a novel B-box (BBX) protein MdBBX37 positively regulates JA-mediated cold-stress resistance in apple. We found that MdBBX37 bound to the MdCBF1 and MdCBF4 promoters to activate their transcription, and also interacted with MdICE1 to enhance the transcriptional activity of MdICE1 on MdCBF1, thus promoting its cold tolerance. Two JA signaling repressors, MdJAZ1 and MdJAZ2 (JAZ, JAZMONATE ZIM-DOMAIN), interacted with MdBBX37 to repress the transcriptional activity of MdBBX37 on MdCBF1 and MdCBF4, and also interfered with the interaction between MdBBX37 and MdICE1, thus negatively regulating JA-mediated cold tolerance. E3 ligase MdMIEL1 (MIEL1, MYB30-Interacting E3 Ligase1) reduced MdBBX37-improved cold resistance by mediating ubiquitination and degradation of the MdBBX37 protein. The data reveal that MIEL1 and JAZ proteins co-regulate JA-mediated cold stress tolerance through the BBX37-ICE1-CBF module in apple. These results will aid further examination of the post-translational modification of BBX proteins and the regulatory mechanism of JA-mediated cold stress tolerance.

Genomic selection for fruit quality traits in apple (Malus×domestica Borkh.)

The genome sequence of apple (Malus×domestica Borkh.) was published more than a year ago, which helped develop an 8K SNP chip to assist in implementing genomic selection (GS). In apple breeding programmes, GS can be used to obtain genomic breeding values (GEBV) for choosing next-generation parents or selections for further testing as potential commercial cultivars at a very early stage. Thus GS has the potential to accelerate breeding efficiency significantly because of decreased generation interval or increased selection intensity. We evaluated the accuracy of GS in a population of 1120 seedlings generated from a factorial mating design of four females and two male parents. All seedlings were genotyped using an Illumina Infinium chip comprising 8,000 single nucleotide polymorphisms (SNPs), and were phenotyped for various fruit quality traits. Random-regression best liner unbiased prediction (RR-BLUP) and the Bayesian LASSO method were used to obtain GEBV, and compared using a cross-validation approach for their accuracy to predict unobserved BLUP-BV. Accuracies were very similar for both methods, varying from 0.70 to 0.90 for various fruit quality traits. The selection response per unit time using GS compared with the traditional BLUP-based selection were very high (>100%) especially for low-heritability traits. Genome-wide average estimated linkage disequilibrium (LD) between adjacent SNPs was 0.32, with a relatively slow decay of LD in the long range (r² = 0.33 and 0.19 at 100 kb and 1,000 kb respectively), contributing to the higher accuracy of GS. Distribution of estimated SNP effects revealed involvement of large effect genes with likely pleiotropic effects. These results demonstrated that genomic selection is a credible alternative to conventional selection for fruit quality traits.

HEAT SHOCK FACTOR A8a Modulates Flavonoid Synthesis and Drought Tolerance

Drought is an important environmental factor affecting the growth and production of agricultural crops and fruits worldwide, including apple (Malus domestica). Heat shock factors (HSFs) have well-documented functions in stress responses, but their roles in flavonoid synthesis and the flavonoid-mediated drought response mechanism remain elusive. In this study, we demonstrated that a drought-responsive HSF, designated MdHSFA8a, promotes the accumulation of flavonoids, scavenging of reactive oxygen species, and plant survival under drought conditions. A chaperone, HEAT SHOCK PROTEIN90 (HSP90), interacted with MdHSFA8a to inhibit its binding activity and transcriptional activation. However, under drought stress, the MdHSP90-MdHSFA8a complex dissociated and the released MdHSFA8a further interacted with the APETALA2/ETHYLENE RESPONSIVE FACTOR family transcription factor RELATED TO AP2.12 to activate downstream gene activity. In addition, we demonstrated that MdHSFA8a participates in abscisic acid-induced stomatal closure and promotes the expression of abscisic acid signaling-related genes. Collectively, these findings provide insight into the mechanism by which stress-inducible MdHSFA8a modulates flavonoid synthesis to regulate drought tolerance.

Transcriptomic Analysis of Red-Fleshed Apples Reveals the Novel Role of MdWRKY11 in Flavonoid and Anthocyanin Biosynthesis

In plants, flavonoids are important secondary metabolites that contribute to the nutritional quality of many foods. Apple is a popular and frequently consumed food because of its high flavonoid content. In this study, flavonoid composition and content were detected and compared between red- and white-fleshed apples in a BC₁ hybrid population using ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry. Transcriptomic analysis of the red- and white-fleshed apples was then performed using RNA-seq technology. By screening differentially expressed genes encoding transcription factors, we unearthed a WRKY-family transcription factor designated MdWRKY11. Overexpression of MdWRKY11 promoted the expression of F3H, FLS, DFR, ANS, and UFGT and increased the accumulation of flavonoids and anthocyanin in apple calli. Our findings explored the novel role of MdWRKY11 in flavonoid biosynthesis and suggest several other genes that may also be potentially involved. This provides valuable information on flavonoid synthesis for the breeding of elite red-fleshed apples.

Osmyb4 expression improves adaptive responses to drought and cold stress in transgenic apples

Constitutive expression of the rice cold-inducible Osmyb4 gene in transgenic Arabidopsis (Arabidopsis thaliana) plants improves adaptive responses to cold and drought stress, most likely due to the constitutive activation of several stress-inducible pathways and to the accumulation of several compatible solutes (e.g., glucose, fructose, sucrose, proline, glycine betaine and some aromatic compounds). Although the Osmyb4 gene seems able to activate stress responsive pathways in different species, we previously reported that its specific effect on stress tolerance depends on the transformed species. In the present work, we report the effects of the Osmyb4 expression for improving the stress response in apple (Malus pumila Mill.) plants. Namely, we found that the ectopic expression of the Myb4 transcription factor improved physiological and biochemical adaptation to cold and drought stress and modified metabolite accumulation. Based on these results it may be of interest to use Osmyb4 as a tool for improving the productivity of woody perennials under environmental stress conditions.

How rainfall, relative humidity and temperature influence volatile emissions from apple trees in situ

Headspace volatiles from apple-bearing twigs were collected in the field with a Radiello sampler during three different diurnal periods over the complete fruit growing season. Analyses by thermal desorption-GC-MS identified a total of 62 compounds in changing quantities, including the terpenoids alpha-pinene, camphene, beta-pinene, limonene, beta-caryophyllene and (E,E)-alpha-farnesene, the aldehydes (E)-2-hexenal, benzaldehyde and nonanal, and the alcohol (Z)-3-hexen-1-ol. The variations in emission of these plant odours were statistically related to temperature, humidity and rainfall in the field. Remarkably, rainfall had a significant positive influence on changes in volatile release during all three diurnal periods, and further factors of significance were temperature and relative humidity around noon, relative humidity in the late afternoon, and temperature and relative humidity during the night. Rainfall was associated consistently with an increase in the late afternoon in terpene and aldehyde volatiles with a known repellent effect on the codling moth, one of the key pests of apple fruit. During the summer of 2003, a season characterized by below-average rainfall, some postulated effects of drought on trees were tested by establishing correlations with rainfall. Emissions of the wood terpenes alpha-pinene, beta-pinene and limonene were negatively correlated with rainfall. Another monoterpene, camphene, was only detected in this summer but not in the previous years, and its emissions were negatively correlated with rainfall, further supporting the theory that drought can result in higher formation of secondary metabolites. Finally, the two green leaf volatiles (E)-2-hexenal and (Z)-3-hexen-1-ol were negatively correlated with rainfall, coinciding well with the expectation that water deficit stress increases activity of lipoxygenase. To our knowledge, this work represents the first empirical study concerning the influence of abiotic factors on volatile emissions from apple trees in situ.

Optical properties and contribution of cuticle to UV protection in plants: experiments with apple fruit

To assess the UV-screening capacity of plant surface structures, the optical properties of isolated cuticle and detached peel of apple fruit (Malus domestica Borkh., cv. Antonovka) have been studied. It was found that the cuticle exhibits considerable scattering of UV radiation, negligible absorption between 500-800 nm and attenuates UV radiation: on average, cuticular transmittance of non-reflected light amounts to 35.7 +/- 20.2 and 14.2 +/- 7.1% at 375 and 300 nm, respectively. The principal UV-A absorbers in the cuticle were identified as quercetin glycosides with an in vivo absorption maximum near 375 nm and content ranging from 10 to 70 nmol cm(-2). On the shaded side of apple fruit, both UV-A and UV-B absorption by the peel is, to a large extent, governed by cuticular phenolics, whereas on the sunlit surface, the absorption of the peel in the UV-A range is determined mainly by vacuolar peel flavonoids. It is concluded that a massive build-up of flavonoids in the peel cells located just below the cuticle, resulting in trapping of radiation in a broad spectral range, plays a dominant role in the long-term adaptation of apple fruit to elevated levels of solar radiation.