Abstract
Sugar content is an important trait of fleshy fruit, and elevating Suc levels is a major goal in horticultural crop breeding. Here, we examined the sugar content in two varieties of the Ussurian pear (Pyrus ussuriensis), 'Nanguo' (NG) and its bud sport (BNG), and we found that Suc content was higher in BNG fruit than in NG fruit. We compared the transcriptomes of the two varieties using RNA sequencing and identified a SWEET (Sugars Will Eventually be Exported Transporter) gene, PuSWEET15, expressed at higher levels in BNG fruit. Heterologous expression of PuSWEET15 in a SUSY7/ura yeast (Saccharomyces cerevisiae) strain showed that PuSWEET15 is an active Suc transporter. Overexpression of PuSWEET15 in NG pear fruit increased Suc content, while silencing of PuSWEET15 in BNG fruit decreased Suc content. The WRKY transcription factor PuWRKY31 was also expressed more highly in BNG fruit than in NG fruit, and we found that PuWRKY31 bound to the PuSWEET15 promoter and induced its transcription. The histone acetylation level of the PuWRKY31 promoter was higher in BNG fruit, suggesting a mechanism by which Suc levels can be elevated.
Highlights
Sugar content is an important trait of fleshy fruit, and elevating Suc levels is a major goal in horticultural crop breeding
Based on measurements from two years (2014 and 2018), we found that the total soluble solid content was higher in bud sport variety of NG (BNG) fruit than in cv NG fruit (Supplemental Fig. S2, A and B)
The expression profile of PuSWEET15 was investigated in NG and BNG fruits during development, and we found that it was expressed at significantly higher levels in BNG fruit from 105 to 134 DAFB (Fig. 2A; Supplemental Fig. S5), consistent with the change in Suc content (Fig. 1B)
Summary
Sugar content is an important trait of fleshy fruit, and elevating Suc levels is a major goal in horticultural crop breeding. We examined the sugar content in two varieties of the Ussurian pear (Pyrus ussuriensis), ‘Nanguo’ (NG) and its bud sport (BNG), and we found that Suc content was higher in BNG fruit than in NG fruit. We compared the transcriptomes of the two varieties using RNA sequencing and identified a SWEET (Sugars Will Eventually be Exported Transporter) gene, PuSWEET15, expressed at higher levels in BNG fruit. The WRKY transcription factor PuWRKY31 was expressed more highly in BNG fruit than in NG fruit, and we found that PuWRKY31 bound to the PuSWEET15 promoter and induced its transcription. We determined that a WRKY transcription factor, PuWRKY31, which was expressed at higher levels in BNG fruit, bound to the PuSWEET15 promoter and upregulated its expression. We show here that increased histone acetylation in the PuWRKY31 promoter is associated with its higher expression in BNG fruit
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