Linkage Mapping and Comparative Transcriptome Analysis of Firmness in Watermelon (Citrullus lanatus)

Abstract

Watermelon fruit texture and quality are determined by flesh firmness. As a quality trait, flesh firmness is controlled by multigenes. Defining the key regulatory factors of watermelon flesh firmness is of great significance for watermelon genetic breeding. In this study, the hard-flesh egusi seed watermelon PI186490 was used as the male parent, the soft-flesh cultivated watermelon W1-1 was used as the female parent, and 175 F 2 generations were obtained from selfing F 1 . Primary mapping of the major genes controlling center flesh firmness was achieved by bulked-segregant analysis (BSA)-Seq analysis and molecular marker technology. Finally, major genes were delimited in the physical interval between 6,210,787 and 7,742,559 bp on chromosome 2 and between 207,553 and 403,137 bp on chromosome 8. The content of each cell wall component and hormone was measured, and comparative transcriptome analysis was performed during fruit development in watermelon. The protopectin, cellulose, hemicellulose, indole-3-acetic acid (IAA) and abscisic acid (ABA) contents were measured, and paraffin sections were made during the three fruit developmental stages. The results revealed that protopectin, celluloses, and hemicelluloses exhibited similar trends for flesh firmness, while the IAA and ABA concentrations continued to decrease with fruit ripening. Paraffin sections showed that PI186490 cells were more numerous, were more tightly packed, had clearer cell wall edges and had thicker cell walls than W1-1 cells at every developmental stage. Comparative transcriptome analysis was conducted on RNA samples of flesh during fruit development and ripening in W1-1 and PI186490. The results from the localization interval transcriptome analysis showed that Cla016033 (DUF579 family member), which may influence the cell wall component contents to adjust the flesh firmness in watermelon fruit, was different in W1-1 and PI186490 and that Cla012507 (MADS-box transcription factor) may be involved in the regulation of fruit ripening and affect the hardness of watermelon fruit.

Identification of Key Gene Networks Associated With Cell Wall Components Leading to Flesh Firmness in Watermelon

“…The opposite trend of water-soluble pectin with flesh firmness during key developmental stages showed that the firmness change of watermelon flesh has a strong correlation with water-soluble pectin content. Many related studies have shown that during the development and storage of fruits, the decrease in the flesh firmness is often accompanied by the presence of water-soluble pectin (Fils-Lycaon and Buret, 1990;Wei et al, 2010;Gwanpua et al, 2014;Sun et al, 2020), our results are in agreement with previous studies. Ionic-bound pectin is another major type of pectin.…”

Functional Characterization and in Silico Analysis of Phytoene Synthase Family Genes Responsible for Carotenoid Biosynthesis in Watermelon (Citrullus lanatus L.)

“…Fruit growth and ripening are also controlled by multiple phytohormones. For instance, ABA regulates fruit ripening in the watermelon 97103, W1-1 and Pi186490 cultivar [60,72]. We identified ABA and MeJA-responsive elements in the ClPSY1 promoter of LSW-177, W1-1 and 97103 cultivars, suggesting that multiple factors involved in hormone synthesis and signaling regulate ClPSY1 expression during fruit ripening.…”

“…The nucleotide start site of the ClPSY1 gene was determined using transcription start site (TSS) program of the Softberry software and the cis-regulatory elements were predicted with plantCARE [57] and PLACE [58,59]. The RNA-seq data of watermelon at different ripening stages are available in the NCBI SRA database under the numbers SRX2037189 for cultivar LSW-177 [49] and SUB4001164 for cultivar W1-1 [60]. The expression levels of different transcription factors and ClPSY1 were analyzed at the different stages of fruit ripening.…”

Metabolome and Transcriptome Integration Reveals Insights Into Flavor Formation of ‘Crimson’ Watermelon Flesh During Fruit Development

“…Transcriptome has made progress in studies on watermelon flesh hardness, color, soluble solids, etc. ( Guo et al, 2015 ; Zhu et al, 2017 ; Gao et al, 2018b ; Sun et al, 2020 ). The transcriptional profiling analysis by K-means clustering has been applied to tomato and other crops earlier ( Srivastava et al, 2010 ).…”