Plum pox virus (PPV) infects Prunus trees across the globe, causing the serious Sharka disease. Breeding programs in the past 20 years have been successful, generating plum varieties hypersensitive to PPV that show resistance in the field. Recently, a single tree displaying typical PPV symptoms was detected in an orchard of resistant plums. The tree was eradicated, and infected material was propagated under controlled conditions to study the new PPV isolate. Performing overlapping PCR analysis, the viral sequence was reconstructed, cloned and tested for infectivity in different 'Jojo'-based resistant plums. The results confirmed that the isolate, named PPV-D 'Herrenberg' (PPVD-H), was able to infect all these varieties. Analyses of chimeras between PPVD-H and a PPV-D standard isolate (PPVD) revealed that the NIa region of PPD-H, carrying three amino acid changes, was enough to break the resistance of these plums. Experiments with single and double mutants showed that all changes were essential to preserve the escaping phenotype. Additionally, one of the changes at the VPg-NIapro junction suggested the involvement of controlled endopeptidase cleavage in the viral response. Transient expression experiments in Nicotiana benthamiana confirmed that NIa cleavage in PPVD-H was reduced, compared to PPVD, linking the observed behavior to an NIa cleavage modulation.

Abstract Background Birch pollen‐related apple allergy is the most frequent IgE‐mediated food allergy in Central‐Northern Europe with Mal d 1 as major allergen. Its concentration in apples varies with the cultivar and storage time. Year‐round appealing, hypoallergenic cultivars still are needed to satisfy the nutritional needs of affected individuals. We characterized three promising cultivars by multidisciplinary in vitro assays including long‐term storage and by clinical challenges of allergic individuals before and after the birch pollen season. Methods Proteins were extracted from fruits of ‘Santana’, ‘Golden Delicious’ (GD), and three genuine cultivars in November 2018 and April 2019. Mal d 1‐levels were analysed by mass spectrometry, SDS‐PAGE, immunoblotting, competitive ELISA, and basophil activation tests. Twenty‐eight allergic individuals underwent single‐blinded open food challenges and skin testing with the cultivars and birch pollen in November 2018 and May 2019. Allergen‐specific IgE‐levels were determined. Results After storage all cultivars except ‘Santana’ were of appealing appearance and taste. Their Mal d 1 content had increased, also reflected by significantly amplified basophil activation and stronger reactions in clinical challenges. Besides, individuals showed boosted reactivity after pollen exposure indicated by enhanced allergen‐specific IgE‐levels and skin reactions to birch pollen. Still, all cultivars remained significantly less allergenic than GD and comparable to Santana in November 2018 in all assessments except for skin testing. Conclusions Combined expertise in pomology and allergology identified promising new cultivars for allergic consumers. The evaluation of hypoallergenic apples should incorporate long‐term storage and birch pollen exposure. Basophil activation tests may be suitable in the selection of promising cultivars for oral challenges.

The aim of the study was to identify plum genes which are associated with plant hypersensitive response to Sharka caused by Plum pox virus (PPV) and to explain the molecular mechanism of hypersensitive reaction (HR) in response to PPV infection in selected genotypes of plum and search for molecular markers related of hypersensitivity. To identify candidate genes whose expression is modified during PPV infection, cDNA-AFLP technique was used. The analyses were done using pools of RNA isolated from hypersensitive and sensitive plants, both healthy and infected. Products of selective amplification showing differential expression in PPV-infected and non-infected plants were re-amplified and sequenced. Analysis using Basic Local Alignment Search Tool (BLAST) revealed their homology to genes encoding proteins involved in defence and host-pathogen interaction, cellular stress, signal perception and transduction, transmembrane transport, regulation of transcription and translation and various enzymes involved in other metabolic pathways. Especially interesting for their potential function in plant defence reactions are genes which were overexpressed in infected plants, both hypersensitive and susceptible. Some of them are proposed as potential molecular markers of hypersensitive reaction to PPV infection, useful in early selection of hypersensitive genotypes in plum breeding.

Prunus domestica (European plum) and Prunus armeniaca (apricot) are related species which can be hybridized. However, there are only very few reports on true interspecific hybrids between P. domestica and P. armeniaca. These hybrids must not be confused with the so called pluots or plumcots which are hybrids between P. salicina (Japanese plum) and P. armeniaca. In 2005, a program for breeding interspecific hybrids between P. domestica and P. armeniaca was initiated at Technische Universitat Munchen. As female parent, a P. domestica clone with hypersensitivity resistance to Plum pox virus (PPV) was used. As male parent, apricot cultivars with different degrees of susceptibility to PPV were chosen. Several interspecific hybrids were obtained. Leaves, shoots and fruits of the seedlings are intermediate between European plum and apricot. The seedlings were tested for their resistance to PPV. Interspecific hybrids with strong resistance to PPV could be obtained. To the best of our knowledge, this is the first report on the generation of PPV resistant hybrids between European plum and apricot. The hybrids could help to better understand the genes which are involved in hypersensitivity resistance to PPV. Moreover, the interspecific hybrid can be used for transferring the PPV resistance to the apricot genome by repeatedly backcrossing them with apricot. The interspecific hybrids represent a new kind of fruit species as the outer and inner fruit traits are intermediate between the parent species.

ETHYLENE action inhibition by 1-methylcyclopropene (1-MCP) extends postharvest life of many climacteric fruits. This work evaluates the effect of 1-MCP (0.625 ppm) treatment on the physiological behavior and quality during postharvest life of ‘Katinka’ and ‘Haganta’ European plum (

Polyphenols in fruits are of increasing interest for consumers and for plant scientists because of their health beneficial potential and their role in plant physiology and disease resistance. Anthocyanins contribute significantly to the attractive pigmentation of red and blue plums. Mirabelles and several reineclaudes do usually not accumulate anthocyanins in the skin. Is this linked to a general low phenolic level? Both the health aspect and the pigmentation are interesting traits for the breeder. For this purpose, rapid analytical methods are necessary. One time consuming step is the extraction of polyphenols. However, fruit juices are easily produced and are anyhow used for estimation of quality traits such as sugars and acidity. Here we show that HPLC analysis of plum juices represent the phenolic profiles of the whole fruits. We analysed the phenolic patterns of juices from 43 plum varieties with yellow, blue and dark blue fruit skins. In most cases, a weak red pigmentation co-occurs with a low total phenol level. However, there are exceptions that may help the breeder to combine yellow fruit skin with a high level of health beneficial phenolic compounds by using the appropriate donor genotypes. The method described here offers a valuable tool for selection.