Different species of edible seed watermelons ( Citrullus spp.) are cultivated in Asia and Africa for their colorful nutritious seeds. Consumer preference varies for watermelon seed coat color. Therefore, it is an important consideration for watermelon breeders. In 1940s, a genetic model of four genes, R , T , W and D , was proposed to elucidate the inheritance of seed coat color in watermelon. In this study, we developed three segregating F 2 populations: Sugar Baby (dotted black seed, RRTTWW ) × plant introduction (PI) 482379 (green seed, rrTTWW ), Charleston Gray (dotted black seed, RRTTWW ) × PI 189225 (red seed, rrttWW ), and Charleston Gray (dotted black seed, RRTTWWdd ) × UGA147 (clump seed, RRTTwwDD ) to re-examine the four-gene model and to map the four genes. In the dotted black × green population, the dotted black seed coat color ( R_ ) is dominant to green seed coat color ( rr ). In the dotted black × red population, the dominant dotted black seed coat color and the recessive red seed coat color segregate for the R and T genes, where the R gene is dominantly epistatic to the T gene. However, the inheritance of the T locus did not fit the four-gene model, thus we named it T 1 . In the dotted black × clump population, the clump seed coat color and the dotted black seed coat color segregate for W and D , where D is recessively epistatic to W . The R , T 1 , W , and D loci were mapped on chromosomes 3, 5, 6, and 8, respectively, using QTL-seq and genotyping-by-sequencing (GBS). Kompetitive Allele Specific PCR (KASP™) assays and SNP markers linked to the four loci were developed to facilitate maker-assisted selection (MAS) for watermelon seed coat color.

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Multipartite viral vectors provide a simple, inexpensive and effective biotechnological tool to transiently manipulate (i.e. reduce or increase) gene expression in planta and characterise the function of genetic traits. The development of virus-induced gene regulation (VIGR) systems usually involve the targeted silencing or overexpression of genes involved in pigment biosynthesis or degradation in plastids, thereby providing rapid visual assessment of success in establishing RNA- or DNA-based VIGR systems in planta. Carotenoids pigments provide plant tissues with an array of yellow, orange, and pinkish-red colours. VIGR-induced transient manipulation of carotenoid-related gene expression has advanced our understanding of carotenoid biosynthesis, regulation, accumulation and degradation, as well as plastid signalling processes. In this review, we describe mechanisms of VIGR, the importance of carotenoids as visual markers of technology development, and knowledge gained through manipulating carotenogenesis in model plants as well as horticultural crops not always amenable to transgenic approaches. We outline how VIGR can be utilised in plants to fast-track the characterisation of gene function(s), accelerate fruit tree breeding programs, edit genomes, and biofortify plant products enriched in carotenoid micronutrients for horticultural innovation.