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 F2 generations were obtained from selfing F1. 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.
Highlights
Watermelon (Citrullus lanatus (Thunb.) Matsum & Nakai) is an annual herb that belongs to the gourd family and is an important horticultural crop worldwide
The flesh hardness of each individual in the three F2 population was between the tested parents, showing a continuous distribution; the overall trend characterized by obvious partial separation of the distribution variation range was relatively large, indicating that there were major genes controlling the firmness of watermelon center flesh
The main manifestation is the separation of the flesh cells by the degradation of intercellular pectin substances (Goulao and Oliveira, 2008)
Summary
Watermelon (Citrullus lanatus (Thunb.) Matsum & Nakai) is an annual herb that belongs to the gourd family and is an important horticultural crop worldwide. A change in fruit hardness is a common physiological phenomenon in nature and is mainly caused by changes in flesh cell structure and material, the degradation of cell wall material in the flesh and the degradation of pectin, cellulose, and hemicellulose structures, which initiates fruit softening. In the study of peach fruit hardness, the hardness of peach fruit was positively correlated with the contents of propectin and cellulose and negatively correlated with the content of soluble pectin (Hu et al, 2007). The degradation of the cell wall-plasma membrane leads to the softening of fruit, which is related to a significant decrease in the glycoprotein and pectin contents and the remodeling of their permutations (Agata et al, 2018)
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