Unravelling the genomics of sucrose-associated sweetness in papaya
Usana Nantawan is a plant scientist and PhD candidate at Griffith University whose research is focused on molecular strategies for improving papaya fruit quality. Along with flavour, an important papaya fruit quality trait to preserve at point of sale is sweetness, and in particular sweetness that is derived from sucrose. This is directly regulated by a suite of genes involved in sucrose synthesis (ss) and it is unknown how the expression of these genes is affected during fruit maturation, presumably leading to differences in sweetness of the harvested fruit. Therefore, the aims of this study were to assess the differential expression of nine key genes known to be involved in governing the production of sucrose through a time course of fruit maturity and to examine transcription profiles for association with sucrose accumulation. This included assessing three sucrose phosphate synthase (cpSPS), four invertase (cpAVIN, cpCWINV) and two sucrose synthase (cpSUS) genes in two papaya genotypes ‘RB2’ and ‘Sunrise Solo’. The fruit of each cultivar were harvested at five maturity stages; mature green, three colour break stages (25, 50, 75%) and fully ripe. Accordingly, similar patterns in sucrose accumulation were observed among the genotypes over the maturity stages, accounting for 40-60% of the total sugar detected. The majority of ss genes were expressed in higher levels in ‘Sunrise Solo’ than in ‘RB2’, ranging from 0.3- to 12.5-fold higher with cpSPS2 most changed in mature fruit. At all harvest stages, ‘Sunrise Solo’ showed greater expression of cpSPS2 than ‘RB2’ and ranged from 1.5- to 6-fold differences. Other differentially expressed genes were cpCWINV1 and cpAVIN2, which were highly expressed in genotype ‘Sunrise Solo’ during ripening stages. The genomic locations of the differentially expressed genes were identified using an F2 to generate a physical map of ‘RB2’ × ‘Sunrise Solo’ using genotyping-by-sequencing (GBS). The proximity of unique single nucleotide polymorphisms (SNPs) within these target genes to major quantitative trait loci (QTL) for sweetness was determined as further evidence for their direct role in the sweetness trait and potential for validation as candidate sweetness markers in future selective breeding strategies.
Usana Nantawan won an ISHS Young Minds Award for the best oral presentation at the VII International Symposium on Tropical and Subtropical Fruits at IHC2018 in Turkey in August 2018.
Usana Nantawan, School of Environment and Science, Griffith University, Nathan, Australia, and Environmental Futures Research Institute, Griffith University, Gold Coast, Australia, e-mail: email@example.com