Comparative transcriptomes reveal molecular mechanisms of apple blossoms of different tolerance genotypes to chilling injury

Abstract

Apple (Malus domestica, Borkh.) is one of the four largest fruits in the world. Freezing damage during the flowering period of apples is one of the main factors leading to the reduction or even extinction of apple production. Molecular breeding of hardy apples is a good solution to these problems. However, the current screening of cold tolerance genes still needs to be resolved. Therefore, in this article, the transcriptome detection and cold tolerance gene screening during the cold adaptation process of apple were studied in order to obtain potential cold-resistant genes. Herein, two high-quality apple tree species (Malus robusta Rehd and M. domestica) were used for cold adaptation experiments and studied under different low-temperature stress conditions (0, -2 and -4°C). The antioxidant levels of two apple flower tissues were tested, and the transcriptome of the flowers after cold culture was tested by next-generation sequencing technology. Antioxidant test results show that the elimination of peroxides in M. robusta Rehd and the adjustment of the expression of antioxidant enzymes promote the cold resistance of this variety of apples. Functional enrichment found that the expression of enzyme activity, cell wall and cell membrane structure, glucose metabolism/gluconeogenesis, and signal transmission are the main biological processes that affect the differences in the cold resistance characteristics of the two apples. In addition, three potential cold-resistant genes AtERF4, RuBisCO activase 1, and an unknown gene (ID: MD09G1075000) were screened. In this study, three potential cold-resistant genes (AtERF4, RuBisCO activase 1, and an unknown gene [ID: MD09G1075000]) and three cold-repressed differential genes (AtDTX29, XTH1, and TLP) were screened.

Keywords: anti-oxidation; apple; chilling gene; cold stress; transcriptome.

Graphical abstract

Figure 1

Antioxidant test of apple flower cells after low-temperature treatment: (a) MDA enzyme content, (b) ROS content, (c) SOD content, and (d) CAT content. Statistical differences: *, **, *** represent the results with p values less than 0.05, 0.005, and 0.001 (compared to their respective normal groups).

Figure 2

Transcriptome test quantitative analysis results: (a) gene expression distribution, (b) correlation between samples, and (c) principal component analysis (species, temperature, and treatment).

Figure 3

Analysis of the difference in sequencing results between different apple varieties and treatment temperature groups: (a) differential gene statistics (all, the number of up-regulated and down-regulated genes) and (b) differential gene clustering.

Figure 4

GO enrichment analysis: (a) GO enrichment analysis histogram and (b) GO enrichment analysis scatter plot (A4 vs B4).

Figure 5

KEGG signal pathway analysis scatter diagram.

Figure 6

qRT-PCR verification of the first three DEGs: (a–c) the first three genes whose expression is down-regulated and (d–f) the first three genes whose expression is up-regulated. (a) MD16G1102700, (b) MD13G1134600, (c) MD04G1064200, (d) MD03G1231800, (e) MD09G1075000, and (f) MD11G1095300. Statistical differences: *, **, *** represent the results with p values less than 0.05, 0.005, and 0.001 (yellow and blue are compared to their respective normal groups).