Search

Watermelon (Citrullus lanatus) as non‐climacteric fruit is domesticated from the ancestors with inedible fruits. We previously revealed that the abscisic acid (ABA) signaling pathway gene ClSnRK2.3 might influence watermelon fruit ripening. However, the molecular mechanisms are unclear. Here, we found that the selective variation of ClSnRK2.3 resulted in lower promoter activity and gene expression level in cultivated watermelons than ancestors, which indicated ClSnRK2.3 might be a negative regulator in fruit ripening. Overexpression (OE) of ClSnRK2.3 significantly delayed watermelon fruit ripening and suppressed the accumulation of sucrose, ABA and gibberellin GA4. Furthermore, we determined that the pyrophosphate‐dependent phosphofructokinase (ClPFP1) in sugar metabolism pathway and GA biosynthesis enzyme GA20 oxidase (ClGA20ox) could be phosphorylated by ClSnRK2.3 and thereby resulting in accelerated protein degradation in OE lines and finally led to low levels of sucrose and GA4. Besides that, ClSnRK2.3 phosphorylated homeodomain‐leucine zipper protein (ClHAT1) and protected it from degradation to suppress the expression of the ABA biosynthesis gene 9'‐cis‐epoxycarotenoid dioxygenase 3 (ClNCED3). These results indicated that ClSnRK2.3 negatively regulated watermelon fruit ripening by manipulating the biosynthesis of sucrose, ABA and GA4. Altogether, these findings revealed a novel regulatory mechanism in non‐climacteric fruit development and ripening. [ABSTRACT FROM AUTHOR]

Key message: The mutation of ClZISO identified in EMS-induced watermelon leads to photosensitive flesh in watermelon. Watermelon (Citrullus lanatus) has a colorful flesh that attracts consumers and benefits human health. We developed an ethyl-methanesulfonate mutation library in red-fleshed line '302' to create new flesh color lines and found a yellow-fleshed mutant which accumulated ζ-carotene. The initial yellow color of this mutant can be photobleached within 10 min under intense sunlight. A long-term light-emitting diode (LED) light treatment turned flesh color from yellow to pink. We identified this unique variation as photosensitive flesh mutant ('psf'). Using bulked segregant analysis, we fine-mapped an EMS-induced G-A transversion in 'psf' which leads to a premature stop codon in 15-cis-ζ-carotene isomerase (ClZISO) gene. We detected that wild-type ClZISO is expressed in chromoplasts to catalyze the conversion of 9,15,9'-tri-cis-ζ-carotene to 9,9'-di-cis-ζ-carotene. The truncated ClZISOmu protein in psf lost this catalytic function. Light treatment can partially compensate ClZISOmu isomerase activity via photoisomerization in vitro and in vivo. Transcriptome analysis showed that most carotenoid biosynthesis genes in psf were downregulated. The dramatic increase of ABA content in flesh with fruit development was blocked in psf. This study explores the molecular mechanism of carotenoid biosynthesis in watermelon and provides a theoretical and technical basis for breeding different flesh color lines in watermelon. [ABSTRACT FROM AUTHOR]

Fruit ripening is a highly complicated process, which is modulated by phytohormones, signal regulators and environmental factors playing in an intricate network that regulates ripening-related genes expression. Although transcriptomics is an effective tool to predict protein levels, protein abundances are also extensively affected by post-transcriptional and post-translational regulations. Here, we used RNA sequencing (RNA-seq) and tandem mass tag (TMT)-based quantitative proteomics to study the comprehensive mRNA and protein expression changes during fruit development and ripening in watermelon, a non-climacteric fruit. A total of 6,226 proteins were quantified, and the large number of quantitative proteins is comparable to proteomic studies in model organisms such as Oryza sativa L. and Arabidopsis. Base on our proteome methodology, integrative analysis of the transcriptome and proteome showed that the mRNA and protein levels were poorly correlated, and the correlation coefficients decreased during fruit ripening. Proteomic results showed that proteins involved in alternative splicing and the ubiquitin proteasome pathway were dynamically expressed during ripening. Furthermore, the spliceosome and proteasome were significantly enriched by Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, suggesting that post-transcriptional and post-translational mechanisms might play important roles in regulation of fruit ripening-associated genes expression, which might account for the poor correlation between mRNAs and proteins during fruit ripening. Our comprehensive transcriptomic and proteomic data offer a valuable resource for watermelon research, and provide new insights into the molecular mechanisms underlying the complex regulatory networks of fruit ripening. [ABSTRACT FROM AUTHOR]

Grafting cultivation is implemented worldwide mainly to resist abiotic and biotic stresses and is an effective method to improve watermelon production. However, grafting may affect fruit development and quality. In our experiment, pumpkin-grafted (PG) watermelon fruits developed slower and the ripening period was extended compared to self-grafted (SG) fruits. We found that the concentrations of abscisic acid (ABA) among endogenous phytohormones were dramatically reduced by pumpkin grafting. In order to understand these changes at the gene expression level, we performed a comprehensive analysis of the fruit flesh transcriptomes between PG and SG during fruit development and ripening. A total of 1,675 and 4,102 differentially expressed genes (DEGs) were identified between PG and SG. Further functional enrichment analysis revealed that these DEGs were associated with carbohydrate biosynthesis, phytohormone signaling transmission, and cell wall metabolism categories. ABA centric phytohormone signaling and fruit quality-related genes including ABA receptor, PP2C proteins, AP2-EREBP transcription factors, sucrose transporter, and carotenoid isomerase were co-expressed with fruit ripening. These results provide the valuable resource for understanding the mechanism of pumpkin grafting effect on watermelon fruit ripening and quality development. [ABSTRACT FROM AUTHOR]

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is an important vegetable crop world-wide. Watermelon fruit quality is a complex trait determined by various factors such as sugar content, flesh color and flesh texture. Fruit quality and developmental process of cultivated and wild watermelon are highly different. To systematically understand the molecular basis of these differences, we compared transcriptome profiles of fruit tissues of cultivated watermelon 97103 and wild watermelon PI296341-FR. We identified 2,452, 826 and 322 differentially expressed genes in cultivated flesh, cultivated mesocarp and wild flesh, respectively, during fruit development. Gene ontology enrichment analysis of these genes indicated that biological processes and metabolic pathways related to fruit quality such as sweetness and flavor were significantly changed only in the flesh of 97103 during fruit development, while those related to abiotic stress response were changed mainly in the flesh of PI296341-FR. Our comparative transcriptome profiling analysis identified critical genes potentially involved in controlling fruit quality traits including α-galactosidase, invertase, UDP-galactose/glucose pyrophosphorylase and sugar transporter genes involved in the determination of fruit sugar content, phytoene synthase, β-carotene hydroxylase, 9-cis-epoxycarotenoid dioxygenase and carotenoid cleavage dioxygenase genes involved in carotenoid metabolism, and 4-coumarate:coenzyme A ligase, cellulose synthase, pectinesterase, pectinesterase inhibitor, polygalacturonase inhibitor and α-mannosidase genes involved in the regulation of flesh texture. In addition, we found that genes in the ethylene biosynthesis and signaling pathway including ACC oxidase, ethylene receptor and ethylene responsive factor showed highly ripening-associated expression patterns, indicating a possible role of ethylene in fruit development and ripening of watermelon, a non-climacteric fruit. Our analysis provides novel insights into watermelon fruit quality and ripening biology. Furthermore, the comparative expression profile data we developed provides a valuable resource to accelerate functional studies in watermelon and facilitate watermelon crop improvement. [ABSTRACT FROM AUTHOR]