Jean-Luc Poëssel

Induced resistance by Myzus persicae in the peach cultivar ‘Rubira’

The effect of a previous infestation by the green peach aphid Myzus persicae (Sulzer) on the settling behaviour and reproduction of the same aphid species was investigated in the resistant peach cultivar Rubira, and compared with that observed in the susceptible control cultivar GF305. A previous infestation of 48 h triggered induced resistance in Rubira. There were significantly fewer aphids settling on preinfested than on uninfested plants, indicating an increased rejection of Rubira as a host plant. The level of induced resistance in preinfested plants was positively related to the duration of the first infestation. In GF305, previous infestation had no detrimental effect on aphid settlement and even slightly enhanced larviposition by adult females. The aphid probing behaviour after a 48‐h preinfestation was also monitored for 8 h with the electrical peneration graph (EPG) technique. On preinfested GF305, most EPG parameters indicated an enhanced host plant acceptance. On preinfested GF305, aphids produced less sieve element salivation and more continuous sap ingestion than on uninfested GF305, indicating that the previous aphids provoked changes in plant properties beneficial to the test aphids. In Rubira, a major induced factor of resistance was thought to be expressed in the sieve element as phloem sap ingestion was 4‐fold shorter on preinfested than on uninfested plants. The time taken by the aphid stylets to reach a sieve element was also significantly increased on preinfested Rubira, suggesting the induction of resistance factors outside the phloem. The originality of the Rubira/M. persicae interaction is discussed in the perspective of a better understanding of plant induced responses to aphids.

Volatile organic compound emissions induced by the aphid Myzus persicae differ among resistant and susceptible peach cultivars and a wild relative

Little is known on aphid-induced emissions of volatile organic compounds (VOCs) from trees and particularly on their intraspecific variability in association with resistance traits. We compared VOC emissions from five peach cultivars (Prunus persica (L.) Batsch) and a wild relative (Prunus davidiana (Carrière) Franch) that differ in their level (susceptible/resistant) and type (antixenosis, antibiosis) of resistance to the green peach aphid, Myzus persicae (Sulzer). Additionally, the kinetics of VOC induction in response to aphids was compared with that by mechanical wounding. Qualitative and overall quantitative differences among peach genotypes were found in VOC emissions that were mainly composed of methyl-salicylate, farnesenes, (E)-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene. Irrespective of the type of resistance, all resistant genotypes had increased VOC emissions upon aphid attack, while in susceptible genotypes emissions remained low. Emission increases were highest in the genotypes that express increased aphid resistance during second infestations, which had also the highest proportions of methyl-salicylate in their emissions. VOC induction by aphids proceeded slowly with a delay of several hours. Artificial wounding of leaves did not result in emissions of aphid-induced VOCs but caused an immediate burst of green leaf volatiles and benzaldehyde. We conclude that VOC induction in resistant peach cultivars is part of a general defence syndrome that is being avoided or suppressed by M. persicae in the susceptible genotypes. The induction likely involves an aphid-specific elicitor and (methyl)-salicylate in the subsequent signalling and regulation processes that should include gene activation due to the marked delay in the emission response. The results are compared with those of the literature and discussed in view of their ecological and environmental significance.

Marker-assisted selection for the wide-spectrum resistance to root-knot nematodes conferred by the Ma gene from Myrobalan plum (Prunus cerasifera) in interspecific Prunus material

Prunus species express a more or less wide spectrum of resistance to root-knot nematodes (RKN) of the genus Meloidogyne. Among them, sources from Myrobalan plum (P. cerasifera) control all major and minor RKN species tested. In this outbreeding species, the clones P.2175 and P.2980 are heterozygous for the Ma single dominant gene and carry the alleles Ma1 and Ma3, respectively. Each allele confers a high-level resistance to the predominant RKN, M. arenaria, M. incognita and M. javanica and to the Florida isolate of an unknown Meloidogyne sp. which overcomes the resistance from peach and almond sources. The polymorphism of two coupling-phase SCAR markers tightly linked to Ma, SCAL19690 and SCAFLP2202, was evaluated within diverse diploid Prunus accessions. This material belongs to the subgenera Prunophora (Myrobalan and apricot) or Amygdalus (peach, almond and almond-peach) and includes the RKN resistance sources ‘Nemared’, ‘Alnem 1’ and ‘GF.557’. The alleles SCAL19690 and SCAFLP2202 were not present in three apricot cultivars (‘Moniqui’, ‘Luizet’ and ‘Stark Early Orange’) representative of the genetic diversity of this species and they segregated in an interspecific cross between P.2980 and apricot. These results suggest that apricot, reported as resistant to M. arenaria, M. incognita and M. javanica, and the Myrobalan plum might possess two different resistance systems. SCAL19690 and SCAFLP2202 were also absent from all tested Amygdalus material, whatever its resistance to RKN. Eight Myrobalan×Amygdalus segregating progenies including bispecific (P.2175 or P.2980×peach or almond) and trispecific (P.2175 or P.2980×almond-peach) hybrids were tested with the Florida isolate to identify individuals carrying the Ma resistance alleles. Both SCARs were then evaluated for segregation in these progenies to develop marker-assisted selection of Prunus interspecific rootstocks. SCAL19690 and SCAFLP2202 could be clearly detected and their tight linkage to Ma1 and Ma3 was confirmed. Consequently these SCARs appear to be powerful tools to screen for RKN resistance conferred by the Ma gene. They should also facilitate marker-assisted pyramiding of Ma with other resistance genes from the Amygdalus subgenus or from the botanically-related Armeniaca section.

Nitrogen fertilization effects on Myzus persicae aphid dynamics on peach: vegetative growth allocation or chemical defence?

Plant nitrogen (N) fertilization is a common cropping practice that is expected to serve as a pest management tool. Its effects on the dynamics of the aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) were examined on young peach [Prunus persica (L.) Batsch (Rosaceae)] trees grown under five N treatments, ranging from N shortage to supra‐optimal supply for growth. Aphid population increased over time at the three intermediate N levels. It remained stable at the lowest N level and decreased at the highest N level. Four weeks after the start of infestation, the number of aphids displayed a parabolic response to N level. The relationships between N status and parameters of plant vegetative growth (stem diameter) or biomass allocation (lateral‐total leaf area and root‐shoot ratio) were consistent with responses proposed by models of adaptive plasticity in resource allocation patterns. However, the variation in plant growth predicted aphid population dynamics only partially. Whereas aphid number was positively correlated with plant N status and vegetative growth up to the intermediate N level, it was negatively correlated with plant N status above this level, but not with vegetative growth. The concentrations of primary and secondary (plant defence‐related) metabolites in the plant shoots were modified by N treatments: amino acids (main nutritional resource of aphids) and prunasin increased, whereas chlorogenic acid decreased with increasing N availability. Constitutive changes in plant chemistry in response to N fertilization could not directly explain the reduced aphid performance for the highest N level. Nevertheless, the indirect effect of N on the induction of plant defence compounds by aphid feeding warrants further investigation. The study focuses on the feasibility of handling N fertilization to control M. persicae in orchards, but findings may also be relevant for our understanding of the physiological relationships between the host’s nutritional status and the requirements of the insect.

Genetic dissection of resistance to root-knot nematodes Meloidogyne spp. in plum, peach, almond, and apricot from various segregating interspecific Prunus progenies

Sources of resistance in Prunus spp. exhibit different spectra to the root-knot nematodes (RKN) Meloidogyne incognita, Meloidogyne javanica and Meloidogyne floridensis. In this Prunus genus, two dominant genes, Ma with a complete spectrum from the heterozygous Myrobalan plums P.2175 and P.2980 (section Euprunus; subgenus Prunophora) and RMia with a more restricted spectrum from the peaches Nemared and Shalil (subgenus Amygdalus), have been identified. This study characterizes the resistance spectra of interspecific crosses involving (1) previous Myrobalan and peach sources, (2) two Alnem almonds (subgenus Amygdalus) resistant to M. javanica, and (3) the apricot A.3923, representing a species considered RKN-resistant (section Armeniaca; Prunophora). For both latter species, genetic data could be obtained through F1 crosses with genetically characterized Myrobalans that conferred their rooting ability for clonal multiplication of the hybrids and permitted their simultaneous evaluation to the three RKN. Crosses involving either Ma or RMia or both generated the expected resistance spectra. Nemared confirmed the species-specific resistance to M. incognita conferred by RMia. This rootstock, also previously considered resistant to M. javanica, was susceptible to the M. javanica isolate used, what illustrates an isolate-specific resistance to this species. Alnem accessions were shown homozygous resistant to M. javanica. In the progeny P.2980 × A.3923, Ma markers allowed to distinguish resistant individuals carrying that gene from resistant individuals lacking it. Distribution of non-Ma individuals in this cross suggested, in the apricot parent, (1) the absence of a major gene allelic to Ma and (2) the presence of a non RKN specific polygenic resistance.

Development and mapping of peach candidate genes involved in fruit quality and their transferability and potential use in other Rosaceae species

The goal of the present study was to identify candidate genes (CGs) involved in fruit quality in peach that can be transferred to other Rosaceae species. Two cDNA libraries from fruit of the “Fantasia” peach cultivar, constructed at two stages of development, were used to generate a set of expressed sequence tag sequences. A total of 1,730 peach unigenes were obtained after clustering. Sequences and corresponding annotations were stored in a relational database and are available through a web interface. Fifty-nine CGs involved in fruit growth and development or fruit quality at maturity, focusing on sweetness, acidity, and phenolic compound content, were selected according to their annotation. Fifty-five primer pairs, designed from peach CG sequences and giving PCR products in peach, were tested in strawberry and 36 gave amplified products. Eight CGs were mapped in peach, 14 in strawberry, four in both species and confirmed the pattern of synteny already proposed using comparative mapping. In peach, the CGs are located in three linkage groups (3, 5, 7), and in strawberry they are distributed in all seven Fragaria linkage groups. Colocalization between some of these CGs and quantitative trait loci for fruit quality traits were identified and are awaiting confirmation in further analyses.

A Systems Biology Study in Tomato Fruit Reveals Correlations between the Ascorbate Pool and Genes Involved in Ribosome Biogenesis, Translation, and the Heat-Shock Response

Changing the balance between ascorbate, monodehydroascorbate, and dehydroascorbate in plant cells by manipulating the activity of enzymes involved in ascorbate synthesis or recycling of oxidized and reduced forms leads to multiple phenotypes. A systems biology approach including network analysis of the transcriptome, proteome and metabolites of RNAi lines for ascorbate oxidase, monodehydroascorbate reductase and galactonolactone dehydrogenase has been carried out in orange fruit pericarp of tomato (Solanum lycopersicum). The transcriptome of the RNAi ascorbate oxidase lines is inversed compared to the monodehydroascorbate reductase and galactonolactone dehydrogenase lines. Differentially expressed genes are involved in ribosome biogenesis and translation. This transcriptome inversion is also seen in response to different stresses in Arabidopsis. The transcriptome response is not well correlated with the proteome which, with the metabolites, are correlated to the activity of the ascorbate redox enzymes—ascorbate oxidase and monodehydroascorbate reductase. Differentially accumulated proteins include metacaspase, protein disulphide isomerase, chaperone DnaK and carbonic anhydrase and the metabolites chlorogenic acid, dehydroascorbate and alanine. The hub genes identified from the network analysis are involved in signaling, the heat-shock response and ribosome biogenesis. The results from this study therefore reveal one or several putative signals from the ascorbate pool which modify the transcriptional response and elements downstream.