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Dongfeng Liu, Junbei Ni, Ruiyuan Wu and Yuanwen Teng

Sorbitol is the main photosynthetic product and primary translocated carbohydrate in the Rosaceae and plays fundamental roles in plant growth, fruit quality, and osmotic stress adaptation. To investigate the effect of frequent high temperature during advanced fruit development on fruit quality of chinese sand pear [Pyrus pyrifolia (Burm. f.) Nakai], we analyzed sorbitol metabolism in mature leaves and fruit flesh of potted ‘Wonhwang’ pear trees. In mature leaves, sorbitol synthesis catalyzed by NADP+-dependent sorbitol-6-phosphate dehydrogenase (S6PDH) was repressed, while sorbitol utilization mainly catalyzed by NAD+-dependent sorbitol dehydrogenase (NAD+-SDH) and NADP+-dependent sorbitol dehydrogenase (NADP+-SDH) was higher than that before high-temperature treatment, which resulted in decreased sorbitol accumulation. In contrast, sucrose accumulation in mature leaves was significantly enhanced in response to high temperatures. In fruit flesh, accumulation of sorbitol and sucrose was increased at the time of harvest under high temperatures. Among sorbitol metabolic enzymes, only NAD+-SDH was sensitive to high temperature in fruit flesh, and significant decrease of NAD+-SDH activity indicated that the fruit sorbitol-uptake capacity was undermined under high temperatures. Transcription analysis revealed tissue-specific responses of NAD+-SDH genes (PpSDH1, PpSDH2, and PpSDH3) to high-temperature treatment. The NAD+-SDH activity and regulation of PpSDH1 and PpSDH3 were positively correlated in mature leaves. However, the downregulation of PpSDH1 and PpSDH2 was consistent with decreased enzyme activity in the fruit flesh. With regard to sorbitol transport, two sorbitol transporter genes (PpSOT1 and PpSOT2) were isolated, and downregulation of PpSOT2 expression in mature leaves indicated that the sorbitol-loading capability decreased under high-temperature conditions because of the limited sorbitol supply. These findings suggested that sorbitol metabolism responded differently in mature leaves and fruit flesh under high temperature, and that these dissimilar responses influenced fruit quality and may play important roles in adaptation to high temperatures.

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Yu Zong, Ping Sun, Xiaoyan Yue, Qingfeng Niu and Yuanwen Teng

Pyrus betulaefolia is one of the most popular pear (Pyrus) rootstocks in China and other east Asian countries because of its good adaptability to versatile environments. However, the number of wild P. betulaefolia populations is decreasing because of habitat destruction and fragmentation. An urgent evaluation of P. betulaefolia genetic diversity and population structure is necessary to develop a conservation strategy for this important wild species. Thirteen simple sequence repeat loci were detected to infer the genetic composition of 18 P. betulaefolia populations in northern China. The average number of different alleles for each locus was 7.1. The number of effective alleles among loci ranged from 1.77 to 5.94. The overall mean values of expected and observed heterozygosity were 0.702 and 0.687, respectively. The Taihang Mountains, which run from northeast to southwest, acted as natural boundary in shaping the genetic diversity of P. betulaefolia in northern China. The distinct pattern, which was also observed in the distribution of chloroplast DNA (cpDNA) variation, appeared to be obscured by pollen-mediated gene flow in the distribution of nuclear microsatellite variation. Large populations with high allelic richness (e.g., populations BT, ZN, and QS) are considered suitable for in situ conservation because of the potential for adaptation to future environmental change. The smaller populations had mixed gene pools (e.g., populations GQ and XF) and should therefore also be considered for ex situ conservation. Preserving genetic diversity in seeds was proposed when field collections are fully characterized.

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Yuanwen Teng, Kenji Tanabe, Fumio Tamura and Akihiro Itai

A total of 118 Pyrus sp. (pear) and cultivars native mainly to east Asia were subjected to randomly amplified polymorphic DNA (RAPD) analysis to evaluate genetic variation and relationships among the accessions. Two hundred fifty RAPD markers were scored from 20 decamer primers. RAPD markers specific to species were identified. Clustering analysis revealed two divisions: one comprising cultivars of P. communis L., and the other including all accessions of Pyrus native to east Asia. The grouping of the species and cultivars by RAPD data largely agrees with morphological pear taxonomy. However, some noted incongruence existed between two classification methods. Pyrus calleryana Dcne. clustered together with P. koehnei Schneid., P. fauriei Schneid. and P. dimorphophylla Makino. Pyrus betulaefolia Bge. clustered with P. ×hopeiensis Yu and P. ×phaeocarpa Rehd. A noncultivated clone of P. aromatica Kikuchi et Nakai grouped with P. aromatica cultivars. Pyrus hondoensis Nakai et Kikuchi and cultivars of P. ussuriensis Max. formed a single group. Some accessions from Korea (named Korean pear) had species-specific RAPD markers and comprised an independent group. Most of the Chinese white pears clustered together with most of the Chinese sand pears. Based on the present results, the new nomenclature P. pyrifolia var. sinensis (Lindley) Teng et Tanabe for Chinese white pear was suggested. Most accessions of Japanese pears fell into one main group, whereas pear cultivars from Kochi Prefecture of Japan subclustered with some Chinese sand pears and one accession from Korea. Our results infer that some local Japanese pear cultivar populations may have been derived from cultivars native to Kochi Prefecture in Shikoku region, and that the latter may have been introduced from ancient China and/or Korea.

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Shuang Jiang, Haishan An, Xiaoqing Wang, Chunhui Shi, Jun Luo and Yuanwen Teng

Simple sequence repeats (SSRs) are widely used in cultivar identification, genetic relationship analysis, and quantitative trait locus mapping. Currently, the selection of hybrid progeny plants in molecular marker-assisted breeding mostly relies on SSR markers because of their ease of operation. In Pyrus, a large number of SSR markers have been developed previously. The method to identify polymorphic SSRs quickly is still lacking in cultivated as well as wild pear species. We present a large number of polymorphic SSRs identified using a quick in silico approach applied across 30 cultivated and wild accessions from Pyrus species. A total of 49,147 SSR loci were identified in Pyrus, and their genotypes were evaluated by whole-genome resequencing data of 30 Pyrus accessions. The results show that most SSR loci were dinucleotide repeat motifs located in intergenic regions. The genotypes of all SSR loci were revealed in all accessions. A total of 23,209 loci were detected, with more than one genotype in all Pyrus accessions. We selected 702 highly polymorphic SSR loci to characterize the pear accessions with an average polymorphism information content value of 0.67, suggesting that these SSR loci were highly polymorphic. The genetic relationship of Pyrus species in the neighbor-joining (NJ) tree and population structure showed a clear division between the oriental and occidental accessions. The population structure split all oriental pears into two groups: cultivars and wild accessions. These new findings of the polymorphic SSR loci in this study are valuable for selecting appropriate markers in molecular marker-assisted breeding in Pyrus.