Search

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]

1 Loss of VST1 restricts sugar loading into the intercellular space for protection against aphid infestation in watermelon. a Leaf curl phenotypes of WT, vst1-1 and vst1-2 lines upon aphid infestation. Remarkably, the OE plants showed severe leaf curl phenotype than WT plants after aphid infestation (Figure S1a), suggesting that the sugar transporter VST1 plays a negative role in aphid tolerance. [Extracted from the article]

Key message: CRISPR/Cas9 system can precisely edit genomic sequence and effectively create knockout mutations in T0 generation watermelon plants. Abstract: Genome editing offers great advantage to reveal gene function and generate agronomically important mutations to crops. Recently, RNA-guided genome editing system using the type II clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) has been applied to several plant species, achieving successful targeted mutagenesis. Here, we report the genome of watermelon, an important fruit crop, can also be precisely edited by CRISPR/Cas9 system. ClPDS, phytoene desaturase in watermelon, was selected as the target gene because its mutant bears evident albino phenotype. CRISPR/Cas9 system performed genome editing, such as insertions or deletions at the expected position, in transfected watermelon protoplast cells. More importantly, all transgenic watermelon plants harbored ClPDS mutations and showed clear or mosaic albino phenotype, indicating that CRISPR/Cas9 system has technically 100% of genome editing efficiency in transgenic watermelon lines. Furthermore, there were very likely no off-target mutations, indicated by examining regions that were highly homologous to sgRNA sequences. Our results show that CRISPR/Cas9 system is a powerful tool to effectively create knockout mutations in watermelon. [ABSTRACT FROM AUTHOR]

Watermelon, Citrullus lanatus Thunb. Matsum. & Nakai is an important vegetable crop worldwide. Due to its narrow genetic base, detection and utilization of the genetic variations, cultivar identification and increasing genetic diversity are some important tasks for watermelon breeders. Molecular markers, especially microsatellites or simple sequence repeats (SSRs) are playing increasingly important roles for these purposes. In the present study, a core set of 23 highly informative SSR markers was developed for watermelon genetic diversity analysis. Based on whole genome sequencing of 17 watermelon inbred lines, we identified 3.9 million single nucleotide polymorphisms (SNPs) which were used to construct a SNP-based dendrogram for the 17 lines. Meanwhile, from the sequenced genome, 13,744 SSRs were developed, of which 704 were placed on a high-resolution watermelon linkage map. To develop the core set SSR markers, 78 of the 704 mapped SSRs were selected as the candidate markers. Using the SNP-based dendrogram as calibration, 23 SSR markers evenly distributed across the genome were identified as the core marker set for watermelon genetic diversity analysis. Each marker was able to detect 2-7 alleles with polymorphism information content values ranging from 0.45 to 0.82. The dendrograms of 17 watermelon lines based on SNPs, the base set of 78 SSRs and the core set of 23 SSRs were highly consistent. The utility of this core set SSRs was demonstrated in 100 commercial watermelon cultivars and elite lines, which could be placed into six clusters that were largely consistent with previous classification based on morphology and parentage data. This core set of SSR markers should be very useful for genotyping and genetic variation analysis in watermelon. [ABSTRACT FROM AUTHOR]