关键词: 西瓜, 蜡粉, 表皮结构, 化学成分, 遗传分析, 基因定位

Abstract:

【Objective】Wax powder is the first protective barrier for plants to resist external stress. In order to find out the physiological and biochemical mechanism of this trait and to get candidate genes, the structure, chemical compositions and inheritance of wax powder on watermelon fruit epidermis were investigated in this research, and the candidate genes were predicted. 【Method】Six-generation populations (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) were constructed by crossing the inbred line ‘Meijiaxuanhei’ (in glossy) and ‘FH’ (in waxy). Scanning electron microscopy (SEM) was used to observe the structure of wax powder on mature watermelon fruit epidermis. The composition of wax powder was determined by gas chromatography-mass spectrometry (GC-MS), and the contents of different chemical components were quantitatively calculated based on the peak area. The wax powder trait was preliminary mapped by BSA-seq, and BLAST software was used to annotate the coding genes in the localization interval in many databases. By analysis of detailed gene annotation information and mutation site, candidate genes were quickly screened. 【Result】The color of wax powder on ‘Meijiaxuanhei’ fruit was gray-white, and the structure was compact plate shaped with about 5 μm in length, while ‘FH’ fruit was found smooth epidermis and no similar structure. 24 kinds of aliphatic compounds were detected in the wax powder, which were classified as hydrocarbons, alcohols, esters, acids, phenols and aldehydes. There were 10 kinds of hydrocarbons were found, which accounted for 77.72% of the total effective chemical extracts of epidermal wax powder and chain length ranged from C17 to C36. Among them, most of the saturated n-alkanes were C27, C28, C29, C32, C33, C34 and C36, 5 kinds of alcohols accounted for 12.60%, 5 kinds of esters accounted for 0.43%, 2 kinds of acids accounted for 0.53%, 1 kind of phenol accounted for 0.80%, and 1 kind of aldehydes accounted for 3.99%, respectively. The top five chemical components in watermelon wax powder were n-34, n-29, 1,30-triacontanediol, n-33 and n-octacosane. The fruit of F1 and BC₁P₂ progenies had wax, the segregation of waxy and glossy in F₂ progenies corresponded to the Mendelian ratio of 3:1, and the segregation ratio in BC₁P₁ progenies was 1:1, so it could be concluded that the hereditary mode of waxy was the dominant heredity of single gene, and the waxy type was the dominant character. By SNP and InDel correlation analysis, 3.16-4.84Mb association region of chromosome 1 was obtained, which contained 144 genes, 138 genes in them were functionally annotated, including 10 non-synonymous mutations and 1 frameshift mutation. Combining with the literature reports, there were five non-synonymous mutation genes might be related to the formation of watermelon epidermal wax powder: Cla002367 was an enoyl ACP-reductase (ECR) gene, which was essential for the synthesis of extra-long chain fatty acids. Cla011514, Cla002337 and Cla002342 were cytochrome P450 (CYP) family genes, and some proteins encoded by the CYP family genes could catalyze the formation of aliphatic compounds through alkyl hydroxylation and other reactions; Cla002353 gene annotation information was ABC transporter, and ABC transporters were closely related to the transport of waxy molecules.【Conclusion】Wax powder on watermelon fruit epidermis was a plate structure, which was mainly composed of aliphatic compounds derived from long chain fatty acids. The trait conformed to the single gene inheritance model, and wax powder was dominant. The association region of chromosome 1 in the range of 1.68Mb was obtained by BSA correlation analysis. 5 non-synonymous mutation genes Cla002367, Cla011514, Cla002337, Cla002342 and Cla002353 were predicted as candidate genes for wax powder on watermelon fruit epidermis.

Key words: watermelon, wax powder, epidermis structure, chemical compositions, genetic analysis, gene mapping