A major challenge to the papaya industry is inconsistency in fruit quality and, in particular, flavour, which is a complex trait that comprises taste perception in the mouth (sweetness, acidity, or bitterness) and aroma produced by several volatile compounds. Current commercial varieties vary greatly in their taste, likely due to historical prioritised selection for fruit appearance as well as large environmental effects. Therefore, it is important to better understand the genetic and biochemical mechanisms and biosynthesis pathways underpinning preferable flavour in order to select and breed for better tasting new commercial papaya varieties. As an initial step, objectively measurable standards of the compound profiles that provide papaya’s taste and aroma, together with ‘mouth feel’, are required. This review presents an overview of the approaches to characterise the flavour profiles of papaya through sugar component determination, volatile compound detection, sensory panel testing, as well as genomics-based studies to identify the papaya flavour.
BackgroundThe identification and characterisation of quantitative trait loci (QTL) is an important step towards identifying functional sequences underpinning important crop traits and for developing accurate markers for selective breeding strategies. In this study, a genotyping-by-sequencing (GBS) approach detected QTL conditioning desirable fruit quality traits in papaya.ResultsFor this, a linkage map was constructed comprising 219 single nucleotide polymorphism (SNP) loci across 10 linkage groups and covering 509 centiMorgan (cM). In total, 21 QTLs were identified for seven key fruit quality traits, including flesh sweetness, fruit weight, fruit length, fruit width skin freckle, flesh thickness and fruit firmness. Several QTL for flesh sweetness, fruit weight, length, width and firmness were stable across harvest years and individually explained up to 19.8% of the phenotypic variance of a particular trait. Where possible, candidate genes were proposed and explored further for their application to marker-assisted breeding.ConclusionsThis study has extended knowledge on the inheritance and genetic control for key papaya physiological and fruit quality traits. Candidate genes together with associated SNP markers represent a valuable resource for the future of strategic selective breeding of elite Australian papaya cultivars.
Inconsistency in flavour is one of the major challenges to the Australian papaya industry. However, objectively measurable standards of the compound profiles that provide preferable taste and aroma, together with consumer acceptability, have not been set. In this study, three red-flesh papayas (i.e., ‘RB1’, ‘RB4’, and ‘Skybury’) and two yellow-flesh papayas (i.e., ‘1B’ and ‘H13’) were presented to a trained sensory panel and a consumer panel to assess sensory profiles and liking. The papaya samples were also examined for sugar components, total soluble solids, and 14 selected volatile compounds. Additionally, the expression patterns of 10 genes related to sweetness and volatile metabolism were assessed. In general, red papaya varieties had higher sugar content and tasted sweeter than yellow varieties, while yellow varieties had higher concentrations of citrus floral aroma volatiles and higher aroma intensity. Higher concentrations of glucose, linalool oxide, and terpinolene were significantly associated with decreased consumer liking. Significant differences were observed in the expression profiles of all the genes assessed among the selected papaya varieties. Of these, cpGPT2 and cpBGLU31 were positively correlated to glucose production and were expressed significantly higher in ‘1B’ than in ‘RB1’ or ‘Skybury’. These findings will assist in the strategic selective breeding for papaya to better match consumer and, hence, market demand.
High sugar content is a valued papaya fruit quality trait that is difficult to select due to a lack of knowledge regarding the major contributing genetic components and the potential for large environmental effects. The initial step towards better understanding this complex trait is to identify the functional genes involved in the sugar synthesis and accumulation pathways. This was achieved in the current study through differential expression analyses of a suit of sweetness-related genes among two phenotypically contrasting papaya cultivars. The major sugar detected in leaf and ripening fruit tissues of two genotypes (120 days after anthesis to colour break 100%) was sucrose, which accounted for 40-60% of the total sugars detected. In general, genotype 'Sunrise Solo' had higher sugar content than genotype 'RB2' in all tissue types assessed. Subsequently, differential expression of 11 genes potentially associated with the main sugar synthesis and accumulation pathways were investigated. Of these, sucrose phosphate synthase (cpSPS1, cpSPS2, cpSPS3 and cpSPS4) and invertase (cpCWINV1and cpAVIN2) gene family members were expressed significantly differently. In all tissue types, cpSPS2 was between 0.5 to 3 times more highly expressed in 'Sunrise Solo' than 'RB2'. The maximum expression of cpSPS2 in 'Sunrise Solo' was observed in mature fruit at 120 days after anthesis, while its expression remained constant in 'RB2'. Similarly, genes cpCWINV1 and cpAVIN2 were significantly more expressed in 'Sunrise Solo' compared to 'RB2' during ripening stages. Further validation and investigation of the expression of these gene sequences for association with the sweetness trait at specific fruit ripening stages and in different environments will aid in their identification as candidate sweetness markers for future selective breeding strategies.
The application of biotechnology to plant breeding includes the development of tools and knowledge to increase accuracy and efficiency for pre-breeding and selection for crop improvement that have multiple advantages over traditional technologies. This chapter provides information on the in vitro culture, genetic transformation, genome project and sequencing technology, gene identification and characterization, genetic linkage maps and identification of quantitative trait loci (QTL), and molecular markers for crop improvement of pawpaws (Carica papaya).
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