Transcriptome analysis of flower bud identified genes associated with pistil abortions between long branches and spur twigs in apricots (Prunus armeniaca L.)

Abstract

Pistil abortions of flower buds occur frequently in many apricot cultivars, especially in long branches. However, the molecular mechanism underlying pistil abortion in apricots remains unclear. To better understand the molecular mechanism of pistil abortions between long branches and spur twigs, paraffin sections and high-throughput sequencing technology were employed to analyze the expression patterns of genes associated with pistil abortions during later flower bud development stage in 'Shajinhong' apricot. The result of stage III (separation of bud scales) was the critical stage of pistil abortion in apricots. A total of 163 differentially expressed genes were identified as candidate genes related to pistil abortion in long branches. These genes are implicated in programmed cell death, hormone signaling, cell wall degeneration, and the carbohydrate metabolism pathway. The results showed that the up-regulation of gene expression of Xyloglucan endotransglucosylase/hydrolase and β-glucosidase in flower buds might be the direct cause of cell wall breakdown and pistil necrosis in long branches. We hypothesize that there is a molecular relationship between pistil abortion before blooming and cellulose degradation, and then carbohydrate transport in the case of carbon deficiency in long branches. Our work provides new insights into cellulose degradation in abortion pistils and valuable information on flower development in apricots, and also provides a useful reference for cultivation regulation in apricot or other fruit crops.

Fig 1. Morphology of flower buds of ‘Shajinhong’ apricots.

(A) The features of flower bud of spur twigs (up) and long branches (down) at stage I. (B) The features at stage II. (C) The features at stage III. (D) The features of normal flowers. (E) The features of aborted flowers. (F) and (G), the vertical section of normal flower bud. (H) and (I), the vertical section of aborted flower bud.

Fig 2. Annotation of all expressed genes of ‘Shajinhong’ apricots.

(A) KOG function classification of all genes. (B) Gene ontology classification of all genes.

Fig 3

Principal component analysis (A) and cluster analysis (B) of transcriptome data. Three biological replicates per sample were analyzed for each development stage. The percentages on the axes indicate the values explained by each PCA; and the dots of the same color represent the transcriptomes of the samples obtained from the same shoot type at the same time.

Fig 4. Twelve groups of gene co-expression trend of differentially expressed genes (DEGs) among three stages of flower bud development.

Fig 5

Comparison (a) and Venn diagram (b) of DEGs between long branches and spur twigs. A, SC2vsSC3; B, SD3vsSC3; C, SD2vsSD3; D, SD2vsSC2.

Fig 6. Enrich center plot of DEGs between long branches and spur twigs at Stage III.

(A) KEGG. (B) Biological process component of GO classify. (C) Cellular component of GO classify. (D) Molecular function of GO classify. Red indicates high expression; blue indicates low expression; the color of lines indicates different pathways, and the color of gene nodes indicates fold of difference. The larger the nodes represents the more genes are enriched into this pathway.

Fig 7. Expression analyses of genes related to hormones or transcription factors.

The x-axis indicates the different stages. The y-axis shows the expression values of the FPKMs. All values were repeated three times and the data are presented as the mean±SD. Different letters indicate significant differences at P<0.01. ACC: CU_locus32671; ACO: CU_locus22526; CYP71A1: CU_locus30850; CML19: CU_locus33375; CML38:CU_locus24324; bHLH35: CU_locus1151; bHLH92: CU_locus476; WRKY22: CU_locus16052; WRKY33: CU_locus40517.

Fig 8. Expression analyses of genes related to cellulose degradation.

The x-axis indicates the different stages. The y-axis shows the expression values of the FPKMs. All values were repeated three times and the data are presented as the mean±SD. Different letters indicate significant differences at P<0.01. XTH1: CU_locus1414; XTH2: CU_locus1411; EBG: CU_locus7296; BGS1: CU_locus37299; BGS2: CU_locus24729; HK2: CU_locus39380.

Fig 9. Expression analyses of genes related to starch breakdown, sucrose synthesis and transportation.

The x-axis indicates the different stages. The y-axis shows the expression values of the FPKMs. All values were repeated three times and the data are presented as the mean±SD. Different letters indicate significant differences at P<0.01. GPHl: CU_locus25159; GPHh: CU_locus35718; UDPase: CU_locus18418; CAS3: CU_locus34862; CAS5: CU_locus29673; BGA1: CU_locus24690; BGA3: CU_locus7299; BGA6: CU_locus43; SUS: CU_locus46184; BFS: Glucosucrase, CU_locus32011; GPT1: CU_locus27685; GPT2: CU_locus24728.

Fig 10. Verification of the selected genes related to the pistil abortion by qRT-PCR.

The x-axis indicates the different stages. The y-axis shows the qRT-PCR expression values. The qRT-PCR values were determined by the actin (ACT) gene as an internal reference using the 2-ΔΔCT values. All values were repeated three times and the data are presented as the mean±SD. Different letters indicate significant differences at P<0.01. ACO: CU_locus22526; CYP71A1: CU_locus30850; WRKY33: CU_locus40517; bHLH92: CU_locus476; XTH1: CU_locus1414; EBG: CU_locus7296; HK2: CU_locus39380; GPHh: CU_locus35718; CAS3: CU_locus34862; BGA1: CU_locus24690; SUS: CU_locus46184; GPT1: CU_locus27685.

Fig 11. Schematic diagram of the mechanism of pistil abortion in apricots.