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Ke Cao, Lirong Wang, Gengrui Zhu, Weichao Fang, Chenwen Chen and Pei Zhao

The root-knot nematode (RKN) Meloidogyne incognita can cause severe crop loss in economically important Prunus species like peach (P. persica), almond (P. communis), plum (P. salicina), and apricot (P. armeniaca). Some peach rootstock, including Nemaguard (P. persica), Nemared (P. persica), and Myrobalan plum (P. cerasifera), display significant resistance to RKN. We present a genetic linkage map constructed by using simple sequence repeat (SSR) and sequence-related amplified polymorphism (SRAP) for a peach backcross population (190 individuals) of RKN-resistant ‘Honggengansutao’ (P. kansuensis) and susceptible ‘Bailey’ (P. persica). Degenerate primers designed from conserved motifs of known plant resistance gene (R) products were used to amplify genomic DNA sequences. Twenty-two resistance gene analog (RGA) sequences were selected from 48 RGAs with open-reading frames to design sequence-tagged site markers. The linkage map of ‘Honggengansutao’ is composed of 138 loci (30 SSRs, 102 SRAPs, five RGAs, and one morphological marker for RKN resistance) assigned to eight linkage groups. The map covers 616 cM of the peach genome with an average marker spacing of 4.9 cM. The five RGAs were mapped to Groups 2, 7, and 8. One gene (designated PkMi) involved in resistance to RKN was mapped to Group 2 (which also includes the known RKN-resistance RMia gene). BLASTN analysis mapped all RGAs to the peach genome sequence. The map constructed in the study will aid future rootstock breeding with marker-assisted selection to identify additional candidate RGA sequences.

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Huihui Liu, Ke Cao, Gengrui Zhu, Weichao Fang, Changwen Chen, Xinwei Wang and Lirong Wang

Anthocyanins are important molecules that are responsible for fruit color formation and are also beneficial to human health. To date, numerous structural and regulatory genes associated with anthocyanin biosynthesis in peach (Prunus persica) have been reported based on linkage analysis. In this study, we sought to identify further genes associated with anthocyanin content in peach by conducting a genome-wide association analysis of 129 peach accessions to detect markers associated with the trait. Significant association signals were detected when anthocyanin content was considered a qualitative character but not when it was considered a quantitative trait. We detected an association region located between 11.7 and 13.1 Mb in chromosome 1, a region in which only 133 of 146 genes have previously been functionally annotated. Gene ontology annotation of the genes in this region showed that membrane-associated genes (including one gene encoding a chloride channel protein and 17 sugar transport/carrier-associated genes) were significantly enriched, and we focused on these in subsequent analyses. Based on in vitro induction of anthocyanins in fruit flesh using different exogenously applied sugars and subsequent culture, we found that the expression level of 3 of the 18 membrane-associated genes, Prupe.1G156300, Prupe.1G156900, and Prupe.1G157000, increased during induction treatment. Furthermore, during the fruit development period of a white-fleshed and a red-fleshed peach cultivar, the expression of one gene encoding a transmembrane sugar transport protein was observed to be positively correlated with anthocyanin biosynthesis. These results will facilitate understanding of the molecular mechanism of anthocyanin biosynthesis in peach.

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Fengge Hao, Lirong Wang, Ke Cao, Xinwei Wang, Weichao Fang, Gengrui Zhu and Changwen Chen

Crown gall disease caused by Agrobacterium tumefaciens affects a wide range of horticultural plants, and has no effective treatment. During the evaluation of crown gall resistance of peach germplasm resources, we observed enhanced resistance to subsequent invasion that was activated by virulence of A. tumefaciens in two peach cultivars. To further verify the phenotype observed in field experiments, systemic acquired resistance (SAR)-related salicylic acid (SA) and PR1 genes were investigated. The levels of SA were elevated in two cultivars, and these high levels were maintained for 35 days postinoculation. Compared with mock-inoculated controls, eight of the 22 candidate PpPR1 genes in A. tumefaciens-inoculated samples were significantly upregulated and three were downregulated in response to inoculation with A. tumefaciens. These data suggested that SA-induced SAR was activated in two peach cultivars by virulent A. tumefaciens infection. In addition, the eight induced PpPR1 genes can be used as molecular markers in defense studies in peach.