Additional amphivasal bundles in pedicel pith exacerbate central fruit dominance and induce self-thinning of lateral fruitlets in apple

Apple (Malus x domestica) trees naturally produce an excess of fruitlets that negatively affect the commercial value of fruits brought to maturity, and impact their capacity to develop flower buds the following season. Therefore, chemical thinning has become an important cultural practice allowing the selective removal of unwanted fruitlets. As the public pressure to limit the use of chemical agents increases, the control of thinning becomes a major issue. Here, we characterized the self-thinning capacity of an apple hybrid-genotype, from a tree scale to a molecular level. Additional amphivasal vascular bundles were identified in the pith of pedicels supporting the fruitlets with the lowest abscission potential (central fruitlet), indicating that these bundles might have a role in the acquisition of dominance over lateral fruitlets. Sugar content analysis revealed that central fruitlets were better supplied in sorbitol than laterals\u27. Transcriptomic profiles allowed us to identify genes potentially involved in the over-production of vascular tissues in central pedicels. In addition, histological and transcriptomic data permitted a detailed characterization of abscission zone (AZ) development and the identification of key genes involved in this process. Our data confirm the major role of ethylene, auxin, and cell wall remodeling enzymes in AZ formation. The shedding process in this hybrid appears to be triggered by a naturally exacerbated dominance of central fruitlets over lateral ones, brought about by an increased supply of sugars, possibly through additional amphivasal vascular bundles. The characterization of this genotype opens new perspectives for the selection of elite apple cultivars

QTL mapping for phenolic compounds in apple fruit and apple juice from a cider apple progeny

Polyphenols have favorable antioxidant potential on human health suggesting that their high content in apple is responsible for the beneficial effects of apple consumption. tehy are also related to the quality of ciders as they predominantly account for astringency, bitterness, color and aroma. Five groups of phenolic compounds are described in the apple fruit: flavanols, hydroxynnamic acids, dihydrochalcones, flavonols and anthocyanins. So far, only two studies have been published on the genetic basis of the phenolic content of dessert apples. As cider apples are commonly described to be much more concentrated in phenolic compounds than dessert varieties, the present study focuses on a cider apple progeny. 32 compounds belonging to the five groups were identified and quantified by HPLC-UV and UHPLC-UV-MS/MS in fruit extracts and juices. 53 QTL controlling phenolic compounds concentration were detected on nine linkage groups (LG) on the integrated linkage map, for all phenolic groups except anthocyanins. QTL clusters located on LG1, 12, 14, 15 and 17 were stable across the year or the studied material. QTL detected on LG1, 14 and 17 for quercitrin, p-coumaroylquinic acid, rutin and chlorogenic acid confirmed results of previous studies. However, no significant QTL was obtained on the LG16 where a major locus for flavanols was previously located. With the two previous studies, this study shows the diversity of genomic regions traits of interest in apple

The improved assembly of the European Pear

Apple and Pear diverged from each other between 5.4 and 21.5 MYA and are believed to share a common genome duplication event between 35 and 50 MYA (Velasco et al. 2010, Wu et al. 2012). Size differences have been observed between the Apple and Pear genomes which are estimated at 527Mb (Pyrus x Bretschneideri Rehd) and 700Mb (Malus x Domestica Borkh) respectively (Wu et al. 2013, Li et al. 2016). The difference in genome size has been accounted for primarily by the proliferation of transposable elements, with the gene space thought to be fairly similar between the two species (Wu et al. 2012). Comparative genomics of the lineage has however, been hampered by the fragmented nature of the reference assemblies. A new chromosome scale assembly was recently produced (Daccord et al. 2017) and now also a chromosome scale assmble of the European Pear (this study), which shows strong collinearity with Apple, greatly facilitating the comparative study of these genomes

Deciphering the genetic determinism of bud phenology in apple progenies: a new insight into chilling and heat requirement effects on flowering dates and positional candidate genes

The present study investigates the genetic determinism of bud phenological traits using two segregating F1 apple (Malus x domestica) progenies. Phenological trait variability was dissected into genetic and climatic components using mixed linear modeling, and estimated best linear unbiased predictors were used for quantitative trait locus (QTL) detection. For flowering dates, year effects were decomposed into chilling and heat requirements based on a previously developed model. QTL analysis permitted the identification of two major and population-specific genomic regions on LG08 and LG09. Both ‘chilling requirement’ and ‘heat requirement’ periods influenced flowering dates, although their relative impact was dependent on the genetic background. Using the apple genome sequence data, putative candidate genes underlying one major QTL were investigated. Numerous key genes involved in cell cycle control were identified in clusters within the confidence interval of the major QTL on LG09. Our results contribute towards a better understanding of the interaction between QTLs and climatic conditions, and provide a basis for the identification of genes involved in bud growth resumption

When virulence originates from nonagricultural hosts: evolutionary and epidemiological consequences of introgressions following secondary contacts in <i>Venturia inaequalis</i>

In pathogens, introgressions through secondary contacts between divergent populations from agricultural and nonagricultural disease reservoirs are expected to have crucial evolutionary and epidemiological implications. Despite the importance of this question for disease management, experimental demonstrations of these implications remain scarce. Recently, we identified a virulent population of the apple scab pathogen Venturia inaequalis that migrated from nonagricultural hosts to European domestic apple orchards. Here, we investigated the occurrence of gene flow between agricultural and nonagricultural populations sampled in two orchards, and thereafter its consequences on the pathogenicity of hybrids. Population genetic structure and demographic inferences based on the genotypes of 104 strains revealed a high amount of gene flow between the two populations in one orchard. In this site, mating between populations was made possible by the presence of a common host. Our results revealed an invasion of the virulent trait in the agricultural population; a main direction of introgression in hybrids from the agricultural to nonagricultural genetic backgrounds; and a population of hybrids with transgressive traits. We demonstrate a secondary contact with gene flow between divergent populations of pathogens. Our findings highlight evolutionary and epidemiological changes in pathogens and have concrete implications for sustainable disease management

QTL mapping for content of phenolic compounds extracted from fruit and juice in a cider apple progeny

Polyphenols have favorable antioxidant potential on human health suggesting that their high content in apple is responsible for the beneficial effects of apple consumption. They are also related to the quality of ciders as they predominantly account for astringency, bitterness, color and aroma. Five groups of phenolic compounds are described in the apple fruit: flavanols, hydroxycinnamic acids, dihydrochalcones, flavonols and anthocyanins. So far, only two studies have been published on the genetic basis of the phenolic content of dessert apples. As cider apples are commonly described to be much more concentrated in phenolic compounds than dessert varieties, the present study focuses on a cider apple progeny. 32 compounds belonging to the five groups were identified and quantified by HPLC‐UV and UHPLC‐UV‐MS/MS in fruit extracts and juices. 53 QTL controlling phenolic compounds concentration were detected on nine linkage groups (LG) on the integrated linkage map, for all p henolic groups except anthocyanins. QTL clusters located on LG1, 12, 14, 15 and 17 were stable across the year or the studied material. QTL detected on LG1, 14 and 17 for quercitrin, p‐coumaroylquinic acid, rutin and chlorogenic acid confirmed results of previous studies. However, no significant QTL was obtained on the LG16 where a major locus for flavanols was previously located. With the two previous studies, this study shows the diversity of genomic regions controlling traits of interest in apple

Genome mapping of three major resistance genes to woolly apple aphid (Eriosoma lanigerum Hausm.)

International audienceWoolly apple aphid (WAA; Eriosoma lanigerum Hausm.) can be a major economic problem to apple growers in most parts of the world, and resistance breeding provides a sustainable means to control this pest. We report molecular markers for three genes conferring WAA resistance and placing them on two linkage groups (LG) on the genetic map of apple. The Er1 and Er2 genes derived from 'Northern Spy' and 'Robusta 5',respectively, are the two major genes that breeders have used to date to improve the resistance of apple rootstocks to this pest. The gene Er3, from 'Aotea 1' (an accession classified as Malus sieboldii), is a new major gene for WAA resistance. Genetic markers linked to the Er1 and Er3 genes were identified by screening random amplification of polymorphic deoxyribonucleic acid (DNA; RAPD) markers across DNA bulks from resistant and susceptible plants from populations segregating for these genes. The closest RAPD markers were converted into sequence-characterized amplified region markers and the genome location of these two genes was assigned to LG 08 by aligning the maps around the genes with a reference map of 'Discovery' using microsatellite markers. The Er2 gene was located on LG 17 of 'Robusta 5' using a genetic map developed in a M.9 x 'Robusta 5' progeny. Markers for each of the genes were validated for their usefulness for marker-assisted selection in separate populations. The potential use of the genetic markers for these genes in the breeding of apple cultivars with durable resistance to WAA is discusse