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Background Bud dormancy is a critical developmental process that allows perennial plants to survive unfavorable environmental conditions. Pear is one of the most important deciduous fruit trees in the world, but the mechanisms regulating bud dormancy in this species are unknown. Because genomic information for pear is currently unavailable, transcriptome and digital gene expression data for this species would be valuable resources to better understand the molecular and biological mechanisms regulating its bud dormancy. Results We performed de novo transcriptome assembly and digital gene expression (DGE) profiling analyses of ‘Suli’ pear ( Pyrus pyrifolia white pear group) using the Illumina RNA-seq system. RNA-Seq generated approximately 100 M high-quality reads that were assembled into 69,393 unigenes (mean length = 853 bp), including 14,531 clusters and 34,194 singletons. A total of 51,448 (74.1%) unigenes were annotated using public protein databases with a cut-off E-value above 10 -5 . We mainly compared gene expression levels at four time-points during bud dormancy. Between Nov. 15 and Dec. 15, Dec. 15 and Jan. 15, and Jan. 15 and Feb. 15, 1,978, 1,024, and 3,468 genes were differentially expressed, respectively. Hierarchical clustering analysis arranged 190 significantly differentially-expressed genes into seven groups. Seven genes were randomly selected to confirm their expression levels using quantitative real-time PCR. Conclusions The new transcriptomes offer comprehensive sequence and DGE profiling data for a dynamic view of transcriptomic variation during bud dormancy in pear. These data provided a basis for future studies of metabolism during bud dormancy in non-model but economically-important perennial species.

Developed buds of pears ( Pyrus pyrifolia ) remain in a dormant state and do not break dormancy until the following spring. However, climatic factors and leaf diseases trigger defoliation in late summer and early autumn, which typically results in the bud paradormancy release and subsequent sprouting. This not only depletes tree nutrients but also diminishes the quantity of flowers and the yield of fruit in the following summer. In this study, metabolic changes in ‘Cuiguan’ pear buds were investigated following premature leaf fall. A total of 1,533 metabolites were annotated, with the majority being downregulated. Sugar levels decreased during the release of paradormancy, likely to provide energy for subsequent growth. Concurrently, most amino acids were consumed post-defoliation, with only a few showing increased trends. Furthermore, the observed low levels of phenylpropanoid-related metabolites in flower buds may contribute to premature senescence. This metabolic profile contributes to our understanding of the biological mechanism of paradormancy release from defoliation, and clarifies the metabolic changes during this process.

Exocarp color of sand pear is an important trait for the fruit production and has caused our concern for a long time. Our previous study explored the different expression genes between the two genotypes contrasting for exocarp color, which indicated the different suberin, cutin, wax and lignin biosynthesis between the russet- and green-exocarp. In this study, we carried out microscopic observation and Fourier transform infrared spectroscopy analysis to detect the differences of tissue structure and biochemical composition between the russet- and green-exocarp of sand pear. The green exocarp was covered with epidermis and cuticle which was replaced by a cork layer on the surface of russet exocarp, and the chemicals of the russet exocarp were characterized by lignin, cellulose and hemicellulose. We explored differential gene expression between the russet exocarp of ‘Niitaka’ and its green exocarp mutant cv. ‘Suisho’ using Illumina RNA-sequencing. A total of 559 unigenes showed different expression between the two types of exocarp, and 123 of them were common to the previous study. The quantitative real time-PCR analysis supports the RNA-seq-derived gene with different expression between the two types of exocarp and revealed the preferential expression of these genes in exocarp than in mesocarp and fruit core. Gene ontology enrichment analysis revealed divorced expression of lipid metabolic process genes, transport genes, stress responsive genes and other biological process genes in the two types of exocarp. Expression changes in lignin metabolism-related genes were consistent with the different pigmentation of russet and green exocarp. Increased transcripts of putative genes involved the suberin, cutin and wax biosynthesis in ‘Suisho’ exocarp could facilitate deposition of the chemicals and take a role in the mutant trait responsible for the green exocarp. In addition, the divorced expression of ATP-binding cassette transporters involved in the trans-membrane transport of lignin, cutin, and suberin precursors suggests that the transport process could also affect the composition of exocarp and take a role in the regulation of exocarp pigmentation. Results from this study provide a base for the analysis of the molecular mechanism underlying sand pear russet/green exocarp mutation, and presents a comprehensive list of candidate genes that could be used to further investigate the trait mutation at the molecular level.

‘Wangkumbae’ pear (Pyrus pyrifolia Nakai) was used as material and genes encoding enzymes involved in lignin biosynthesis were cloned: two of 4-coumarate:coenzyme A ligase (4CL) genes Pp4CL1 and Pp4CL2, another two cinnamyl alcohol dehydrogenase (CAD) genes PpCAD1 and PpCAD2, and four peroxidase (POD) genes PpPOD1, PpPOD2, PpPOD3 and PpPOD4. The result of real-time PCR analysis showed a similar expression pattern of PpPAL1 and PpPAL2 with higher accumulation of transcripts at fruit developing early stage and low accumulation at late stage. The expression of PpPAL1 and PpPAL2 revealed down-regulation in gibberellin (GA)-treated fruit. The accumulations of Pp4CL1 and Pp4CL2 transcripts were higher in fruit developing early stage compared with those in late stage, and Pp4CL2 expression level was higher than Pp4CL1. Pp4CL1 and Pp4CL2 expression levels in GA-treated fruit were lower compared with in the control fruit. The relative expression level of PpCAD2 showed higher than PpCAD1 and reached a peak at 53 day after blooming. The expressions of PpCAD2 in GA-treated fruit was significantly lower compared with those in the control fruit in early developmental stage, which was consistent with developmental dynamics of sclereid and dynamic changes in lignin contents during the development. The expression of PpPOD4 gene was significantly higher than PpPOD1, PpPOD2 and PpPOD3 during developmental stages. The expressions of PpPOD1, PpPOD2, and PpPOD4 increased firstly and then decreased during the development of the control fruit. PpPOD1 expression in GA-treated fruit was lower than that in the control fruit, which was consistent with the changes in lignin contents.

The study of pear dormancy mechanism is currently a hot topic in pear production and has significant economic significance for agricultural production. Pyrus pyrifolia. 'Mixue' are known to have high nutritional and commercial value. To better understand the molecular mechanisms of bud dormancy release, we selected the southern sand pear cultivar Pyrus pyrifolia. 'Mixue' as the test material. We explored the respiratory metabolism rate and changes in endogenous hormones of pear flower buds during the dormancy process. Meanwhile, RNA sequencing was used to obtain the gene expression profile of Pyrus pyrifolia. 'Mixue' flower buds, exploring the molecular mechanism of pear flower bud dormancy. The results showed that there were 697 differentially expressed genes (DEGs) in E01vs E02, 665 DEGs in E01 vs E03, 738 DEGs in E01 vs E04, 581 DEGs in E03 vs E02, 664 DEGs in E03 vs E04, 668 genes were differentially expressed in E04 vs E02. KEGG pathway analysis showed that these DEGs were predominantly mapped to Plant-pathogen interaction pathway and Circadian rhythm-plant pathways in the process of dormancy. Furthermore, the genes related to asparagine synthetase (encoding the formation of ADP), glutamate synthetase 1 (encoding NADH, the precursor of starch formation), glutamate dehydrogenase glutamine synthetase leaf isozyme, CCA1 gene enriched in the Circadian rhythm-plant pathway and the four genes of the CML (calmodulin like) family (CML38, CML27, CML25, CML23) are significantly different in the process of pear dormancy, which may play an important role in the regulation of pear bud dormancy. In a word, these results will also be of great help in the regulation of endodormancy to cope with climate change and improve fruit production.

The study of pear dormancy mechanisms is currently a major research area in pear production and has high economic significance for agricultural production. We selected the southern sand pear cultivar Pyrus pyrifolia ‘Huanghua’ as the test material to study the pear dormancy process through microscopic observation of pear flower buds. The endodormancy period is abbreviated as D_bud. Similarly, the endodormancy release initiation period and the ecodormancy period are referred to as DB_bud and G_bud, respectively. Meanwhile, RNA sequencing was used to obtain the gene expression profile of Pyrus pyrifolia ‘Huanghua’ flower buds. The RNA sequencing results indicated that there were 224 differentially expressed genes (DEGs) in endodormancy (D) vs. endodormancy release initiation period (DB), while 975 were identified between endodormancy and ecodormancy (G). Finally, a total of 868 DEGs were found in the DB vs. G comparison. The expression levels of the genes Pbr009498.1 (LAX1-1), Pbr012348.1 (LAX1-2), Pbr021158.1 (GH35), and Pbr031621.1 (LAX2) encoding IAA were significantly higher during the DB_bud than during the D_bud. The expression level of Pbr025864.2 (IAA13) during the D_bud was significantly higher than the DB_bud and G_bud. The Pbr041942.1 (GID1B) gene also showed a significant increase during ecodormancy. Taken together, these results suggest that these genes, annotated as LAX1, GH35, LAX2, IAA13, and GID1C, are involved in endodormancy maintenance and in the transition from endodormancy to ecodormancy in Pyrus pyrifolia ‘Huanghua’.

Pyrus pyrifolia is considered one of the most important cultivated Pyrus species. Hundreds of landraces and bred cultivars have been developed through the natural and artificial hybridizations necessary due to self-incompatibility. In this study, the genetic diversity of 478 Pyrus accessions, including Chinese landraces, bred cultivars, and wild samples, as well as introduced pear cultivars from Japan and Korea, was investigated with a set of 17 simple sequence repeat (SSR) markers distributed across all 17 linkage groups of the pear genome. A total of 121 alleles were detected, including 4 rare alleles with a frequency lower than 5%. Diversity statistics indicated a high level of genetic variation as quantified by the average values of the observed heterozygosity, the expected heterozygosity, and Wright’s fixation index, at 0.76, 0.78, and 0.02, respectively. Population structure and discriminant analysis of principal component analysis implied extensive genetic communication between sand pears in China and revealed four contiguous geographical clusters with overlapping geographical regions. The diversity of the four clusters and approximate Bayesian computation (ABC) indicated that sand pear spread from west to east along the Pearl River and Yangtze River valleys. High diversity and polyphyletic genetic components of cultivars in southwestern China further support southwestern China as the probable center of divergence for Pyrus species. A core collection of 80 out of 470 cultivars was selected, accounting for about 17% of accessions, and capturing 91% of all alleles, including all rare alleles. Our research provides a comprehensive understanding of sand pear germplasm in East Asia and constructs a preliminary core collection, which will be useful for association genetics studies, germplasm conservation, and breeding programs.

The economic value of fruit is reduced by having a short shelf life. Whangkeumbae is a type of sand pear (Pyrus pyrifolia) considered a climacteric fruit. The pear is famous for its smooth surface and good flavor. However, its shelf life is very short because of senescence and disease after harvest and a burst of ethylene (ET) production prompting the onset of fruit ripening. In plants, ETHYLENE INSENSITIVE3 (EIN3) and EIN3like (EIL), located in the nucleus, are important components of the ET signaling pathway and act as transcription factors. EIN3s and EILs belong to a small family involved in regulating the expression of ethylene response factor gene (ERF), whose encoding protein is the final component in the ET signaling pathway. The mutation of these components will cause defects in the ethylene pathway. In this study, one gene encoding an EIN3 was cloned and identified from Whangkeumbae and designated PpEIN3b. The deduced PpEIN3b contained a conserved EIN3 domain, a bipartite nuclear localization signal profile (NLS_BP), and an N-6 adenine-specific DNA methylase signature (N6_MTASE). PpEIN3b belongs to the EIN3 super-family by phylogenetic analysis. Quantitative RT-PCR (qRT-PCR) analysis revealed that PpEIN3b was preferentially expressed in fruit. Additionally, its expression was developmentally regulated during fruit ripening and senescence. Furthermore, PpEIN3b transcripts were obviously repressed by salicylic acid (SA) and glucose treatment in pear fruit and in diseased fruit, while it was significantly induced by 1-aminocyclopropane-1-carboxylic acid (ACC) treatment. Taken together, our results reveal the expression and regulation profiles of PpEIN3b and suggest that PpEIN3b might integrate SA, glucose, and ACC signaling to regulate fruit ripening and senescence in pear, which would provide a candidate gene for this regulation to obtain fruit with a long shelf life and improved economic value.

MicroRNAs (miRNAs) are a class of small non-coding RNA molecules that are endogenous regulators of gene expression. miRNAs play a crucial role in cells via degradation of target mRNAs or by inhibition of target protein translation. In the present study, 186 new potentially conserved pear miRNAs belonging to 37 families were identified. The length of mature miRNAs ranged from 19 to 24 nt, and most of the miRNAs (154 out of 186) were 21 nt in length. The length of pre-miRNAs in pear was also found to vary from 62 to 282 nt with an average of 105 ± 43 nt. The potential miRNAs belonged to 29 clusters involving 20 different miRNA families. Using these potential miRNAs, we further scoured of the pear genome and found 326 potential target genes, which included transcription factors, stress responsive genes, and the genes involved in transmembrane transport and signal transduction. Gene ontology analysis of these potential targets suggested that 47 biological processes were potentially regulated by miRNAs, including oxidation–reduction, stress response, transport, etc. KEGG pathway analysis showed that the identified miRNAs were found in 15 metabolism networks which were related to starch and sucrose metabolism, and ascorbate and aldarate metabolism, among others. Our study will help in the further understanding of the essential role of miRNAs in growth and development and stress response of pear.

The color of fruit skin is an important quality parameter, and in many plants, it is the result of coordinative regulation of the anthocyanin pathway. To characterize the mechanism involved in fruit peel coloration of Yunnan red pear ( Pyrus pyrifolia ), we constructed a subtractive cDNA library using the suppression subtractive hybridization (SSH) technology. cDNA of red peel exposed to sunlight (for 2, 4, 6, and 8 days) was subtracted from that of white skin unexposed to sunlight. Over 100 differentially expressed ESTs were obtained, putatively involved in primary and secondary metabolism, stress, and defense response. Expression analysis using semiquantitative reverse transcription polymerase chain reaction (RT-PCR) for 13 genes was performed with two pear cultivars, light-skinned ‘Zaobaimi’ and red-skinned ‘Yunhong-1’, which had been bagged and then exposed to sunlight for 0, 1, 2, 3, 5, and 7 days before harvest. This analysis showed that genes encoding for a metallothionein-like protein and a NADP-malic acid enzyme were constitutively expressed, whereas other selected genes were either down- or up-regulated. Semiquantitative RT-PCR analysis for 7 anthocyanin biosynthetic pathway genes and 3 putative regulatory genes was also performed. Results showed that an R2R3 MYB transcription factor PyMYB10 was up-regulated in both the less-colored pear ‘Zaobaimi’ and well-colored red pear Yunhong-1 after the bag was removed, but that kinetics differed between cultivars. Other anthocyanin-related genes appeared to be coordinately regulated by the MYB–bHLH–WD40 complex. DFR and ANS genes seemed to be limiting factors for the peel coloration of less-colored pear ‘Zaobaimi’, while all biosynthetic steps are up-regulated by 7 days after bag removal in red fruit. This study suggests the regulation of red pear coloring is via differential effects of the MYB–bHLH–WD40 complex on the pear anthocyanin pathway genes.