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.

Untargeted Metabolomic Liquid Chromatography High-Resolution Mass Spectrometry Fingerprinting of Apple Cultivars for the Identification of Biomarkers Related to Resistance to Rosy Apple Aphid.

Liquid chromatography high-resolution mass spectrometry fingerprinting together with pattern recognition techniques was used to determine the metabolites involved in the susceptibility of apple cultivars to rosy apple aphid (RAA). Preprocessing of ultra-high-performance liquid chromatography coupled to electrospray ionization and quadrupole time-of-flight mass spectrometry raw data of resistant and susceptible apple cultivars was carried out with XCMS and CAMERA packages. Univariate statistical tools and multivariate data analysis highlighted significant different profiles of the apple metabolomes according to their tolerance to RAA. Optimized and cross-validated Partial least squares discriminant analysis and orthogonal projections to latent structures discriminant analysis models confirmed trans-4-caffeoylquinic acid and 4-p-coumaroylquinic acid as biomarkers for the identification of resistant and susceptible apple cultivars to RAA and disclosed that only hydroxycinnamic acids are involved in the disease susceptibility of cultivars. In this sense, the final steps of the biosynthesis of caffeoylquinic acid (CQA) and p-coumaroylquinic acid (p-CoQA) become decisive because the isomerization of 5-CQA to 4-CQA is favored in resistant cultivars, whereas the isomerization of 5-p-CoQA to 4-p-CoQA is favored in susceptible cultivars.

Susceptibility of Cider Apple Accessions to European Canker-Comparison between Evaluations in Field Planted Trees and Rapid Screening Tests.

European canker, caused by Neonectria ditissima Bres., is an economically damaging fungal disease of apple. Breeding new cultivars with a high level of resistance to European canker is the main aim of apple breeding programs. Observations of symptoms in naturally infected trees were carried out in 400 apple accessions in Asturias (north-western Spain). Young and mature field planted trees were assessed under conditions highly conducive for N. ditissima development. The results demonstrated that juvenile trees (4-year-old) barely showed noticeable symptoms whereas a wide variability in the levels of resistance among accession was observed in mature trees (14-year-old). Around 28% of the locally maintained collection resulted to be highly resistant to this disease in the region. Field observations on mature trees were also compared to four rapid screening tests based on artificially induced lesions. Spearman correlation analysis using two resistance parameters revealed that none of the methods resulted in similar rankings of cultivar susceptibility as some accessions that were ranked as resistant for a given test turned out to be susceptible in the field. This study might suggest that whilst conventional resistance phenotyping techniques are time-consuming, the outcomes of this approach still seem the preferred option to assess the response to N. ditissima of apple accessions.

Apple pomaces derived from mono-varietal Asturian ciders production are potential source of pectins with appealing functional properties.

Comprehensive chemical characterization of nine mono-varietal apple pomaces obtained from the production of ciders with PDO is described. They were rich in essential minerals, fibers (35-52.9 %), and polyphenols. High levels in GalA (11.8-21.6 %), revealed the suitability of these apple pomaces as efficient sources of pectins. Extracted pectins showed high variability in monomer composition, with degrees of methylesterification, strongly associated with pectins functional properties, ranging from 58 to 88 %. For a subset of apple pomace varieties, pectin extraction was accomplished by conventional acid heat treatment or ultrasound. Despite ultrasound-assisted extraction did not improve pectin yield, it minimized levels of "non-pectin" components as revealed by the low content of Glc/Man, leading to the obtainment of high-purity pectin. Our work highlights the key role played by the selection of the apple variety to streamline the potential food applications (gelling/thickening agents or prebiotics) of the extracted pectins that largely depend on their structural features.

Relationship between hydroxycinnamic acids and the resistance of apple cultivars to rosy apple aphid.

The phenolic profiles of apple cultivars from the SERIDA Asturian cider apple breeding program, including parents and progenies, were determined by ultrahigh-performance liquid chromatography-diode array detector-electrospray ionization-quadrupole time of flight/mass spectrometer in order to study the relationship between phenols and the resistance of apple tree cultivars to rosy apple aphid (RAA). A pattern recognition technique named partial least square discriminant analysis (PLS-DA) was used to classify apple cultivars based on resistance to RAA, resistant and susceptible, reaching scores with accuracy higher than 97% and 91% respectively. Hydroxycinnamic acids, particularly 4-caffeoylquinic acid (4-CQA) and 4-p-coumaroylquinic acid (4-pCoQA), were identified as the major player in RAA resistance by the PLS-DA model. Indeed, the isomerisation 5-CQA → 4-CQA is favoured in resistant cultivars, whereas the isomerisation 5-pCoQA → 4-pCoQA is favoured in susceptible cultivars. As a result, resistant cultivars accumulate higher amounts of 4-CQA than susceptible ones, and the opposite occurs for 4-pCoQA. Also, minor isomerisations of 5-CQA to 1-CQA or 3-CQA show opposite behaviour for resistant and susceptible cultivars. Cultivar resistance to RAA is concluded to be related with the phenylpropanoid pathway, the isomerisation reactions being the key metabolic reaction for a cultivar to be resistant or susceptible to RAA.