Transcriptome analysis of pre-harvest fruit shrinkage in loquat

【Objective】The objective of the experiment is to understand the molecular mechanism of fruit shrinkage of Zaozhong 6 loquat when it was encountering high temperature before harvest, so as to lay a theoretical foundation for cultivating anti-shrinkage loquat varieties. Pre-harvest fruit shrinkage is affected by environmental factors, varieties and hormone levels in the tree. There are few studies on the molecular mechanism of pre-harvest fruit shrinkage and the screening and cultivation of anti-shrinkage varieties. A pre- harvest non- wrinkling loquat variety Sihedaguo was screened out from the resource nursery by our research group. It was used as an anti-wrinkling variety that was compared with Zaozhong 6 to preliminarily analyze the molecular mechanism of the difference in anti- wrinkling ability among the varieties.【Methods】Zaozhong 6 (easy to wrinkle) and Sihedaguo (anti- winkling) loquat fruits were selected as the materials. The physiological [malonaldehyde (MDA) and proline] and biochemical [superoxide dismutase (SOD) and peroxidase (POD)] indexes of pre- havest Zaozhong 6 lo-quat fruits with different shrinkage degrees (normal fruit labeled as ZZS1, slightly shrunken fruit as ZZS2 and shrunken fruit as ZZS3) were analyzed. Transcriptome sequencing of Sihedaguo (DG) and Zaozhong 6 loquat fruits with different shrinkage degrees was carried out. The differentially expressed genes among different degrees of shrunken fruits of Zaozhong 6, and differentially expressed genes between Sihedaguo and Zaozhong 6 normal fruits were analyzed, so as to screen the genes related to fruit shrinkage in loquat.【Results】Compared with the normal fruit (ZZS1), the contents of MDA and proline, and the activity of SOD and POD in the shrunken fruits (ZZS2, ZZS3) of Zaozhong 6 significantly increased. The content of proline and the activity of POD in ZZS3 increased by 81.9% and 94.5%, respectively compared with ZZS1. Four groups of samples (DG, ZZS1, ZZS2 and ZZS3) were subjected to transcriptome sequencing analysis. A total of 76.26 Gb Clean Data was obtained, and the percentage of Q30 bases in each sample was greater than 90%. Compared with ZZS1, there were 4245 differentially expressed genes in ZZS3, of which 2265 were up-regulated and 1980 were down-regulated. In order to screen the key genes affecting the shrinkage of Zaozhong 6 loquat, Venn diagram and differential expression gene analysis were carried out with the comparison of ZZS1 vs ZZS2, ZZS1 vs ZZS3 and ZZS2 vs ZZS3, and 27 common differentially expressed genes were found, of which 20 were up-regulated and 4 were down-regulated in all the three groups. According to the KEGG annotation, these genes are mainly involved in the brassinolide signal transduction pathway, terpenoid biosynthesis pathway, starch and sucrose metabolism pathway, fat metabolism pathway and plant hormone signal transduction pathway. It was found that NAC transcription factor was up- regulated in the three groups, indicating that it may be involved in the regulation of loquat fruit shrinkage-related gene expression. There were 1647 differentially expressed genes between ZZS1 and DG, of which 1028 were up-regulated and 619 were down- regulated. These genes were all differentially expressed genes between the two varieties, and some of them may not be related to the response of fruit shrinkage to high temperature. In order to further screen the genes related to the anti-shrinkage ability of loquat fruit, Venn diagram analysis was carried out in the three groups of DG vs ZZS1, ZZS1 vs ZZS2 and ZZS1 vs ZZS3, and 481 common differentially expressed genes were found in the three groups. According to COG functional classification, these genes are mainly involved in carbohydrate transport and metabolism, lipid transport and metabolism, cell wall/ membrane biosynthesis, secondary metabolite synthesis, transport and decomposition, signal transduction and defense mechanisms. According to KEGG classification, these genes are mainly involved in starch and sucrose metabolism, plant hormone signal transduction, phenylpropanoid biosynthesis (mainly involved in lignin synthesis), glycolysis and other pathways. Among the 12 genes involved in phenylpropanoid biosynthesis, 9 were related to lignin synthesis and were up-regulated in the group of DG vs ZZS1. Among the 13 genes involved in plant hormone signal transduction pathway, 6 (2 GH3 and 4 SAUR) were involved in auxin signal transduction. Two genes (EVM0023097 and EVM0032997) were involved in the brassinolide biosynthesis pathway, and the expression levels were up-regulated in the three groups of DG vs ZZS1, ZZS1 vs ZZS2 and ZZS1 vs ZZS3. The results showed that auxin and brassinolide may be involved in the pre-harvest fruit shrinkage of loquat. Genes involved in lignin synthesis may be related to the ability of different loquat cultivars to resist high temperature and fruit shrink.【Conclusion】The results of transcriptome sequencing showed that the genes that may be related to the pre- harvest fruit shrinkage of loquat were mainly involved in plant hormone signal transduction pathway, phenylpropanoid biosynthesis pathway, starch and sucrose metabolism pathway, brassinolide biosynthesis and signal transduction pathway. NAC transcription factor may be involved in the regulation of gene expression related to loquat fruit shrinkage. Lignin may be related to the abilityof different loquat varieties to cope with high temperature stress, and auxin and brassinolide may play a role in the process of loquat fruit to resist high temperature stress.