Apple Germplasm Research Articles

Expression of the polyphenol oxidase gene MdPPO7 is modulated by MdWRKY3 to regulate browning in sliced apple fruit.

Browning is a pervasive problem in horticultural products, substantially diminishing the appearance, flavor and nutritional value of fruit, including important fruits like apple (Malus × domestica Borkh.). In this study, we compared the physiological characteristics of the browning-resistant line 'Rb-18' with the susceptible variety 'Fuji' and found that the polyphenol oxidase (PPO) enzyme activity and phenolic content of Rb-18 were significantly lower than those in Fuji. In addition, the PPO enzyme in Fuji showed a stronger affinity for its substrate, catechol, compared to Rb-18. Through transcriptome and RT-qPCR analyses, MdPPO7 expression was identified as contributing to flesh browning after cutting. Subsequent fruit injection and stable genetic transformation of the MdPPO7 gene into apple fruit and calli determined that syringic acid, procyanidin, phloridzin, chlorogenic acid, gallic acid, catechin, and caffeic act as its catalytic substrates in the process involved in browning. Furthermore, luciferase reporter, yeast one-hybrid, β-glucuronidase reporter assays and chip-qPCR analysis demonstrated that a WRKY transcription factor(MdWRKY3) binds to the promoter region of polyphenol oxidase gene (MdPPO7) and positively regulates its expression to promote apple flesh browning. This study provides insights into the molecular regulatory mechanisms of fruit browning in fresh-cut apples and provides a theoretical basis for the generation of high-quality apple germplasm resources.

Comprehensive Evaluation of Apple Germplasm Genetic Diversity on the Basis of 26 Phenotypic Traits

We used 256 apple germplasm resources for a thorough examination of the genetic diversity associated with 26 phenotypic traits (i.e., genetic diversity analysis, cluster analysis, correlation analysis, principal component analysis, and membership function). The average coefficient of variation for 12 morphological traits was 66.39% (21.10–201.5%). The coefficient of variation was highest and lowest for the fruit arris and the width of the eye basin, respectively. Additionally, the diversity index ranged from 0.54 to 1.33. Moreover, the coefficient of variation for 14 numerical traits varied from 5.37% to 50%. The titratable acid content had the highest coefficient of variation, with a diversity index ranging from 2.01 to 2.08 (average of 2.045). A cluster analysis categorized 256 germplasms into four groups, among which Group I included germplasms with large fruits and the best comprehensive performance. Of the top 10 principal components revealed by the principal component analysis, principal component 1 was mainly related to fruit size and flavor. The top 10 germplasms were selected on the basis of comprehensive scores using the membership function method. Furthermore, a stepwise regression analysis identified 15 key traits for identifying apple germplasms, including the vegetative growth day, fruit weight, and the firmness of the fruit without skin. These results can serve as the foundation for future analyses of the phenotypic diversity of apple germplasms, while also providing a theoretical basis for screening, characterizing, and further improving excellent apple germplasms.

SNP genotyping Dutch heritage apple cultivars allows for germplasm characterization, curation, and pedigree reconstruction using genotypic data from multiple collection sites across the world

The curation and preservation of Dutch apple germplasm depends on reliable accession level information. However, many accessions of Dutch heirloom apple cultivars maintained publicly by the “Centre for Genetic Resources, the Netherlands” (CGN) and privately by Dutch pomological societies lack information regarding true-to-typeness and pedigree ancestry. The aim of this study was to address this knowledge gap by genotyping 652 apple accessions maintained in the CGN collection and Dutch private collections, compare their genotypic information to each other and to a large database of apple cultivars from around the world to identify genotypic duplicates and pedigree relationships for the Dutch apple cultivars. Towards this aim, accessions were genotyped on the 20 K Illumina Infinium(R) apple SNP array and with 15 SSR markers. Each accession was assigned to a genotypic profile code (MUNQ codes, as used in previous studies) facilitating communication regarding genotypic duplicates. There were 211 (51.1%) genotypic profiles in the Dutch germplasm which were not identified in other germplasm collections. Private collections maintained many of these unique accessions, including important pedigree ancestors. The study identified a number of common pedigree ancestors of Dutch cultivars, such as ‘Herfst Bloem Soete’, ‘Huismanszoet’ (2), and ‘Reinette Rouge Étoilée’. The duplicate and pedigree reconstruction results and relevant literature descriptions were used to pomologically verify the identity of relevant accessions. The results of this study resolved identity disputes, helped to decide which accessions should be retained or included in the CGN collection, and benefited ongoing pomological studies in ancestry and provenance of traditional Dutch cultivars.

FruitPhenoBox – a device for rapid and automated fruit phenotyping of small sample sizes

BackgroundFruit appearance of apple (Malus domestica Borkh.) is accession-specific and one of the main criteria for consumer choice. Consequently, fruit appearance is an important selection criterion in the breeding of new cultivars. It is also used for the description of older varieties or landraces. In commercial apple production, sorting devices are used to classify large numbers of fruit from a few cultivars. In contrast, the description of fruit from germplasm collections or breeding programs is based on only a few fruit from many accessions and is mostly performed visually by pomology experts. Such visual ratings are laborious, often difficult to compare and remain subjective.ResultsHere we report on a morphometric device, the FruitPhenoBox, for automated fruit weighing and appearance description using computer-based analysis of five images per fruit. Recording of approximately 100 fruit from each of 15 apple cultivars using the FruitPhenoBox was rapid, with an average handling and recording time of less than eleven seconds per fruit. Comparison of fruit images from the 15 apple cultivars identified significant differences in shape index, fruit width, height and weight. Fruit shape was characteristic for each cultivar, while fruit color showed larger variation within sample sets. Assessing a subset of 20 randomly selected fruit per cultivar, fruit height, width and weight were described with a relative margin of error of 2.6%, 2.2%, and 6.2%, respectively, calculated from the mean value of all available fruit.ConclusionsThe FruitPhenoBox allows for the rapid and consistent description of fruit appearance from individual apple accessions. By relating the relative margin of error for fruit width, height and weight description with different sample sizes, it was possible to determine an appropriate fruit sample size to efficiently and accurately describe the recorded traits. Therefore, the FruitPhenoBox is a useful tool for breeding and the description of apple germplasm collections.

A non-invasive method for phenotyping scab-tolerant apple plants using volatile organic compounds.

One of the most devastating diseases of apples is scab, caused by the fungus Venturia inaequalis. Most commercial apple varieties are susceptible to this disease; only a few are resistant. Breeding approaches are being used to develop better apple varieties that are resistant to scab. Volatile organic compounds (VOCs) contribute greatly to a plant's phenotype, and their emission profile largely depends on the genotype. In the non-destructive phenotyping of plants, VOCs can be used as biomarkers. In this study, we assessed non-destructively the scab tolerance potential of resistant (cv. 'Prima') and susceptible (cv. 'Oregon Spur') apple cultivars by comparing their major leaf VOC compositions and relative proportions. A comparison of the leaf VOC profiles of the two cultivars revealed 16 different VOCs, with cis-3-hexenyl acetate (3HA) emerging as a biomarker of cultivar differences. V. inaequalis growth was significantly inhibited in vitro by 3HA treatment. 3HA was significantly effective in reducing scab symptoms on V. inaequalis-inoculated leaves of 'Oregon Spur.' The resistant cultivar 'Prima' also exhibited higher lipoxygenase (LOX) activity and α-linolenic acid (ALA) levels, suggesting that V. inaequalis resistance is linked to LOX activity and 3HA biosynthesis. This study proposes 3HA as a potential biomarker for rapid non-destructive screening of scab-resistant apple germplasm of 'Prima' based on leaf VOCs.

Evaluation of Morphological Characteristics in Some Apple Varieties (Malus Pumila Mill.) from Northeastern Algeria

Abstract The current investigation is centered on the characterization of specific apple cultivars in the Garza region, within the Teniet El Abed municipality, situated 55 km east of Batna (NE-Algeria). Utilizing descriptors outlined by UPOV 2015 pertaining to apples, the study focuses on morphological parameters of leaves, fruits, and terminal shoots across three varieties: Golden Delicious, Royal Gala, and Starkrimson. Additionally, soil analysis was conducted to ascertain its suitability for apple cultivation. The examination revealed limited morphological diversity among the investigated apple cultivars, with some descriptors, particularly those related to fruit traits, showing insignificant differences between cultivar types. Overall, the studied varieties - Golden Delicious, Royal Gala, and Starkrimson - exhibit distinct physiological and morphological characteristics. Variations in size, shape, skin color, flesh attributes, as well as sugar and acid content, contribute to the uniqueness of each variety. Whether one prefers sweet or tart apples, or favors larger or smaller fruits, there exists an apple variety to cater to individual tastes. Our findings complement molecular analyses and underscore the need for further exploration of apple germplasm, alongside sustained conservation efforts.

Host-induced gene silencing in wild apple germplasm Malus hupehensis confers resistance to the fungal pathogen Botryosphaeria dothidea.

Host-induced gene silencing (HIGS) is an inherent mechanism of plant resistance to fungal pathogens, resulting from cross-kingdom RNA interference (RNAi) mediated by small RNAs (sRNAs) delivered from plants into invading fungi. Introducing artificial sRNA precursors into crops can trigger HIGS of selected fungal genes, and thus has potential applications in agricultural disease control. To investigate the HIGS of apple (Malus sp.) during the interaction with Botryosphaeria dothidea, the pathogenic fungus causing apple ring rot disease, we evaluated whether apple miRNAs can be transported into and target genes in B. dothidea. Indeed, miR159a from Malus hupehensis, a wild apple germplasm with B. dothidea resistance, silenced the fungal sugar transporter gene BdSTP. The accumulation of miR159a in extracellular vesicles (EVs) of both infected M. hupehensis and invading B. dothidea suggests that this miRNA of the host is transported into the fungus via the EV pathway. Knockout of BdSTP caused defects in fungal growth and proliferation, whereas knockin of a miR159a-insensitive version of BdSTP resulted in increased pathogenicity. Inhibition of miR159a in M. hupehensis substantially enhanced plant sensitivity to B. dothidea, indicating miR159a-mediated HIGS against BdSTP being integral to apple immunity. Introducing artificial sRNA precursors targeting BdSTP and BdALS, an acetolactate synthase gene, into M. hupehensis revealed that double-stranded RNAs were more potent than engineered MIRNAs in triggering HIGS alternative to those natural of apple and inhibiting infection. These results provide preliminary evidence for cross-kingdom RNAi in the apple-B. dothidea interaction and establish HIGS as a potential disease control strategy in apple.

Multi-omics analysis reveals the biosynthesis of flavonoids during the browning process of Malus sieversii explants.

Malus sieversii is a precious apple germplasm resource. Browning of explants is one of the most important factors limiting the survival rate of plant tissue culture. In order to explore the molecular mechanism of the browning degree of different strains of Malus sieversii, we compared the dynamic changes of Malus sieversii and Malus robusta Rehd. during the whole browning process using a multi-group method. A total of 44 048 differentially expressed genes (DEGs) were identified by transcriptome analysis on the DNBSEQ-T7 sequencing platform. KEGG enrichment analysis showed that the DEGs were significantly enriched in the flavonoid biosynthesis pathway. In addition, metabonomic analysis showed that (-)-epicatechin, astragalin, chrysin, irigenin, isoquercitrin, naringenin, neobavaisoflavone and prunin exhibited different degrees of free radical scavenging ability in the tissue culture browning process, and their accumulation in different varieties led to differences in the browning degree among varieties. Comprehensive transcriptome and metabonomics analysis of the data related to flavonoid biosynthesis showed that PAL, 4CL, F3H, CYP73A, CHS, CHI, ANS, DFR and PGT1 were the key genes for flavonoid accumulation during browning. In addition, WGCNA analysis revealed a strong correlation between the known flavonoid structure genes and the selected transcriptional genes. Protein interaction predictions demonstrated that 19 transcription factors (7 MYBs and 12 bHLHs) and 8 flavonoid structural genes had targeted relationships. The results show that the interspecific differential expression of flavonoid genes is the key influencing factor of the difference in browning degree between Malus sieversii and Malus robusta Rehd., providing a theoretical basis for further study on the regulation of flavonoid biosynthesis.

Pomological traits and profitability of selected velvet apple germplasm

The research was accomplished during February to August, 2021 at Germplasm Center (GC), Patuakhali Science and Technology University (PSTU) and farmers’ homesteads in Patuakhali district to find out different characteristics, fruit yield and profitability of four velvet apple germplasm. The experiment was laid out in Randomized Complete Block Design (RCBD) with four replications. Results manifested that there were significant variations among the germplasm, where local red germplasm exhibited the highest plant height (5.54 m), plant diameter (16.23 cm), number of main branches (12.75), length of the largest branch (3.89 m), plant canopy (8.80 m2), leaf area (135.24 cm2) but those traits were minimum in PSTU Bilati gab-2. PSTU Bilati gab-2 emerged floral bud for the longest duration (32.25 days). The measured length of inflorescence before first flowering varied from 5.03 – 6.60 cm where it required 24 – 28.25 days for development. A range of flowers per inflorescence (7.75 – 11) and duration of flowering (46.5 – 59.25 days) were observed in all the studied germplasm. The percentage of fruit set (18.28 – 30.81) per plant with individual fruit weight 126.70 to 202.43 g. The highest pulp weight (103.85 g) was obtained from PSTU Bilati gab-2 and the lowest (69.25 g) from local yellow germplasm. PSTU Bilati gab-2 was completely seedless. So, it contained the highest edible portion (81.92%). The highest fruit yield (3.5 t/ha) was recorded in PSTU Bilati gab-1 but maximum net profit (251160 Tk/ha) was obtained from PSTU Bilati gab-2 with the highest benefit cost ratio (BCR) of 3.81. Considering percent fruit set, percent edible portion, fruit yield and profitability, PSTU Bilati gab-2 was found to be the best among the four germplasm. Bangladesh J. Agri. 2023, 48(2): 1-12

The WRKY17-WRKY50 complex modulates anthocyanin biosynthesis to improve drought tolerance in apple

Drought stress is increasing worldwide due to global warming, which severely reduces apple (Malus domestica) yield. Clarifying the basis of drought tolerance in apple could accelerate the molecular breeding of drought-tolerant cultivars to maintain apple production. We identified a transcription factor MdWRKY50 by yeast two-hybrid (Y2H) assays as an interactor of the drought-tolerant protein MdWRKY17, and confirmed their interaction by bimolecular fluorescence complementation (BiFC) and pull-down assays. MdWRKY50 was induced by drought and when overexpressed in apple, conferred transgenic apple plants enhanced drought tolerance by directly binding to the promoter of anthocyanin synthetic gene Chalcone synthase (MdCHS) to upregulate its expression for higher anthocyanin. Increased anthocyanin relieves apple plants from oxidative damage under drought stress. MdWRKY50 RNA-interference transgenic apple plants showed opposite phenotypes. The dimerization of MdWRKY50 with mutated MdWRKY17DP mimicking drought-induced phosphorylation by the mitogen-activated protein kinase kinase 2 (MEK2)-MPK6 cascade, compared with MdWRKY17AP and MdWRKY17, further promoted anthocyanin biosynthesis, suggesting dimerization with MdWRKY17 makes MdWRKY50 more powerful in promoting anthocyanin biosynthesis under drought stress. Taken together, we isolated an entire MEK2-MAPK6-MdWRKY17-MdWRKY50-MdCHS pathway for drought tolerance and generated transgenic apple germplasm with enhanced drought tolerance and higher anthocyanin levels.

Susceptibility Evaluation to Fire Blight and Genome-Wide Associations within a Collection of Asturian Apple Accessions

Fire blight, caused by Erwinia amylovora, is one of the most devastating apple diseases. The selection of cultivars of low susceptibility and the study of the genetic mechanisms of the disease play important roles in fire blight management. The susceptibility level to fire blight was evaluated in 102 accessions originating from Asturias, a cider-producing region located in the north of Spain with a wide apple germplasm. Evaluations took place under quarantine conditions using artificial inoculations of grafted plants. The results revealed wide variation in susceptibility responses and low-susceptible cultivars were identified. In addition, 91 cultivars were genotyped using the Affymetrix Axiom® Apple 480 K SNP array to conduct genome-wide association studies (GWAS). A statistically significant signal was detected on chromosome 10 using the multi-locus mixed model (MLMM). Two genes were identified as major putative candidate genes: a TIR-NBS-LRR class disease protein and a protein containing a development and cell death (DCD) domain. The outcomes of this study provide a promising source of information, particularly in the context of cider apples, and set a starting point for future genetic and breeding approaches.

Profiling apple volatile organic compounds in a New Zealand collection of germplasm as a resource for breeding cultivars with desirable flavors

Flavor is a major component of the apple eating experience, along with taste, texture and appearance. Apples produce a wide array of volatile organic compounds that impart particular flavors. Cultivars producing increased or novel flavors may have more desirability for consumers and help to differentiate the fruit. Efficiently breeding more flavorful apples requires understanding the volatiles present in apple germplasm and their potential sensory impact. As an initial step towards the development of more flavorful apple cultivars, a New Zealand collection of germplasm was surveyed, and seventy-three volatiles were identified as being present in at least half the accessions. Substantial differences in the presence and relative abundance of specific volatiles were uncovered across the accessions and could sometimes be linked to an apple’s flavor profile. The large number of volatiles analyzed allowed relationships between and among molecule classes to be established. Esters were found to be the main drivers of volatile differentiation across accessions. Apples tended to produce either ethyl or acetate esters, suggesting there is different genetic control for these two ester types. Additionally, esters generally had larger broad-sense heritabilities, indicating they could be easier targets for modifying apple flavor. This volatile dataset is a valuable resource for apple breeding, and increases the understanding of an important consumer trait.

Analysis of Carotenoids and Gene Expression in Apple Germplasm Resources Reveals the Role of MdCRTISO and MdLCYE in the Accumulation of Carotenoids.

Carotenoids play an important role in the coloring and nutritional value of apple (Malus spp.) fruits. Here, six carotenoids, including lutein, zeaxanthin, β-carotene, β-cryptoxanthin, violaxanthin, and neoxanthin, were detected in 105 fruits of apple germplasm resources, which showed a skewed distribution in both the peel and pulp. There were more carotenoids in the peel than in the pulp, and lutein and β-carotene were the primary carotenoids that were present. The expression levels of most carotenoid pathway genes in germplasm fruits during fruit development were higher in the fruits that had an abundance of carotenoids. A linear relationship analysis showed that the expression levels of MdCRTISO and MdLCYE were highly correlated with the content of carotenoids. The leaves accumulated the greatest number of carotenoids, while the roots had the lowest amount. MdCRTISO and MdLCYE were highly expressed in the fruits compared to other tissues. Transgenic calli and transiently transformed fruits confirmed that MdCRTISO and MdLCYE affected the biosynthesis of carotenoids owing to their effects on the expression of other genes for enzymes in the carotenoid pathway. Our findings will extend the understanding of carotenoid biosynthesis in apple and excavate apple germplasm resources with rich carotenoids to breed high-quality apples.

A note on promising early colouring apple bud sports identified from commercial exotic apple germplasm

A note on promising early colouring apple bud sports identified from commercial exotic apple germplasm

Chemical properties and shelf life of velvet apple germplasm

The study was undertaken to evaluate bio-chemical properties, shelf-life determination and expansion of ripe fruits of four velvet apple germplasm. The selected germplasm were local red variety, local yellow variety, PSTU Bilati gab-1 and PSTU Bilati gab-2. Ripe fruits were collected from Germplasm Center, Patuakhali Science and Technology University (PSTU) and different locations of Patuakhali district of Bangladesh. Results expressed that maximum TSS (13.93%), vitamin-C (3.68 mg/100g), non-reducing sugar (5.15%) and total sugar (7.95%) were recorded from PSTU Bilati gab-2, the highest TA (1.30%) was found in PSTU Bilati gab-1 but highest pulp pH (7.50) and TSS to TA ratio (11.11) were recorded from local yellow variety whereas highest reducing sugar (3.17%) was recorded from local red variety. The shelf life of ripe fruit varied from 3.25 – 4.75 days where the longest shelf life was obtained from PSTU Bilati gab-2 and the shortest shelf life was recorded in local red variety. But the storability was extended more than 27 days when stored in “Refrigerator” followed by “Non-perforated polybag (NPP)” (10 days), perforated polybag (8 days) and paper bag (6 days). The firmness of fruits were decreased gradually with days but the reduction was minimized when stored in different storage condition where more firmness hold in refrigerator treatment followed by NPP. Similarly, the weight loss was controlled in refrigerator followed by NPP treatment. The results of the experiments revealed that PSTU Bilati gab-2 provides superior chemical properties among the germplasm and ‘Refrigerator’ treatment exhibited best performance among other.

Transcriptional repression of MdMa1 by MdMYB21 in Ma locus decreases malic acid content in apple fruit.

Malic acid is a major organic acid component of apples and a crucial determinant of fruit organoleptic quality. A candidate gene for malic acid content, designated MdMa1, was previously identified in the Ma locus, which is a major quantitative trait locus (QTL) for apple fruit acidity located on the linkage group 16. Region-based association mapping to detect candidate genes in the Ma locus identified MdMa1 and an additional MdMYB21 gene putatively associated with malic acid. MdMYB21 was significantly associated with fruit malic acid content, accounting for ~7.48% of the observed phenotypic variation in the apple germplasm collection. Analyses of transgenic apple calli, fruits and tomatoes demonstrated that MdMYB21 negatively regulated malic acid accumulation. The apple fruit acidity-related MdMa1 and its tomato ortholog, SlALMT9, exhibited lower expression profiles in apple calli, mature fruits and tomatoes in which MdMYB21 was overexpressed, compared with their corresponding wild-type variety. MdMYB21 directly binds to the MdMa1 promoter and represses its expression. Interestingly, a 2-bp variation in the MdMYB21 promoter region altered its expression and regulation of its target gene, MdMa1, expression. Our findings not only demonstrate the efficiency of integrating QTL and association mapping in the identification of candidate genes controlling complex traits in apples, but also provide insights into the complex regulatory mechanism of fruit malic acid accumulation.

Genetic Structure and Molecular Identities of 46 Apple Landraces (Malus Mill.) in China

In this study, we used genotyping to determine the genetic structure and molecular identities of apple landraces from six species of Malus Mill. in China, based on the fingerprints revealed by microsatellite markers with tailed primer M13. A total of 46 apple landraces of Malus Mill. selected from the National Apple and Pear Germplasm Repository in Xingcheng, China were genotyped with 14 SSR markers. The primers differentiated all the accessions. At least three SSR primers, CH04h02, CH01f07a and CH04g07, with a higher heterozygosity and Shannon’s information index than other combinations can distinguish all the accessions. All the alleles for these three primers were arranged in descending order, and they were assigned values beginning with 01. Character strings were constituted by combining all the codes of the three primers for every accession. By such means, separate and special molecular identities were obtained for every apple landrace, which could be expressed in the form of a bar code. Using such bar codes, trees can be labeled and scanned, which aids in the identification and tracking of genebank collections. The eight-step method for establishing the molecular identities of apple landraces reported here may serve as a reference when determining the molecular identities establishment of other apple germplasms of Malus Mill. This method might also be used for the establishment of a molecular database to aid the preservation of Malus Mill., which is in imminent danger in China. It may also be used to improve the gene bank management of Malus Mill.

Using High-Density SNP Array to Investigate Genetic Relationships and Structure of Apple Germplasm in Bosnia and Herzegovina

Apple accessions, currently maintained within the two main ex situ collections in Bosnia and Herzegovina (B&H), have previously been genotyped using microsatellite markers. The obtained molecular data provided insight into mislabeled accessions and redundancies, as well as the overall genetic structure of the germplasm. The available dataset enabled the creation of a core collection consisting of 52 accessions. The reliability and usefulness of microsatellites has made this low-density marker system a norm in studies on apple germplasm. However, the increased access to medium- and high-density SNP arrays, developed specifically for apples, has opened new avenues of research into apple genetic resources. In this study, 45 apple genotypes consisting of 33 diploid core collection accessions from B&H and 12 international reference cultivars were genotyped using an Axiom® Apple 480 K SNP array in order to examine their genetic relationships, population structure and diversity, as well as to compare the obtained results with those calculated on previously reported SSR profiles. The SNPs displayed a better ability to differentiate apple accessions based on their origin, as well as to cluster them according to their pedigree. Calculating identity by descent revealed 16 pairings with first-degree relationships and uncovered the introgression of ‘Delicious’ and ‘Golden Delicious’ into the core collection.

Population Structure and Association Mapping for Agronomical and Biochemical Traits of a Large Spanish Apple Germplasm.

A basic knowledge of linkage disequilibrium and population structure is necessary in order to determine the genetic control and identify significant associations with agronomical and phytochemical compounds in apple (Malus × domestica Borkh). In this study, 186 apple accessions (Pop1), representing both Spanish native accessions (94) and non-Spanish cultivars (92) from the EEAD-CSIC apple core collection, were assessed using 23 SSRs markers. Four populations were considered: Pop1, Pop2, Pop3, and Pop4. The initial Pop1 was divided into 150 diploid (Pop2) and 36 triploid accessions (Pop3), while for the inter-chromosomal linkage disequilibrium and the association mapping analysis, 118 phenotype diploid accessions were considered Pop4. Thus, the average number of alleles per locus and observed heterozygosity for the overall sample set (Pop1) were 15.65 and 0.75, respectively. The population structure analysis identified two subpopulations in the diploid accessions (Pop2 and Pop4) and four in the triploids (Pop3). Regarding the Pop4, the population structure with K = 2 subpopulations segregation was in agreement with the UPGMA cluster analysis according to the genetic pairwise distances. Moreover, the accessions seemed to be segregated by their origin (Spanish/non-Spanish) in the clustering analysis. One of the two subpopulations encountered was quite-exclusively formed by non-Spanish accessions (30 out of 33). Furthermore, agronomical and basic fruit quality parameters, antioxidant traits, individual sugars, and organic acids were assessed for the association mapping analysis. A high level of biodiversity was exhibited in the phenotypic characterization of Pop4, and a total of 126 significant associations were found between the 23 SSR markers and the 21 phenotypic traits evaluated. This study also identified many new marker-locus trait associations for the first time, such as in the antioxidant traits or in sugars and organic acids, which may be useful for predictions and for a better understanding of the apple genome.

Autochthonous Apple Cultivars from the Campania Region (Southern Italy): Bio-Agronomic and Qualitative Traits

Apple (Malus × domestica Borkh.) is an important fruit crop widely spread in the cold and mild climates of temperate regions in the world, with more than 93 million tons harvested worldwide in 2021. The object of this work was to analyze thirty-one local apple cultivars of the Campania region (Southern Italy) using agronomic, morphological (UPOV descriptors) and physicochemical (solid soluble content, texture, pH and titratable acidity, skin color, Young's modulus and browning index) traits. UPOV descriptors highlighted similarities and differences among apple cultivars with a depth phenotypic characterization. Apple cultivars showed significant differences in fruit weight (31.3-236.02 g) and physicochemical trait ranging from 8.0 to 14.64° Brix for solid soluble content, 2.34-10.38 g malic acid L-1 for titratable acidity, and 15-40% for browning index. Furthermore, different percentages in apple shape and skin color have been detected. Similarities among the cultivars based on their bio-agronomic and qualitative traits have been evaluated by cluster analyses and principal component analyses. This apple germplasm collection represents an irreplaceable genetic resource with considerable morphological and pomological variabilities among several cultivars. Nowadays, some local cultivars, widespread only in restricted geographical areas, could be reintroduced in cultivation contribution to improving the diversity of our diets and contemporary to preserve knowledge on traditional agricultural systems.