Identification of genes associated with soluble sugar and organic acid accumulation in ‘Huapi’ kumquat ( Fortunella crassifolia Swingle) via transcriptome analysis

Background The levels and ratios of sugar and acid are important contributors to fruit taste. Kumquat is one of the most economically important citrus crops, but the information on the soluble sugar and organic acid metabolism in kumquat is limited. Here, two kumquat varieties, 'Rongan' (RA) and its mutant 'Huapi' (HP), were used to assess the soluble sugar and organic acid accumulation and the related genes. Results Soluble sugars include sucrose, glucose, and fructose while malate, quinic acid, and citrate are the dominant organic acids in the fruits of both kumquat varieties. 'HP' accumulated more sugars but fewer organic acids than those in 'RA'. Transcriptome analysis revealed 63 and 40 differentially expressed genes involved in soluble sugar and organic acid accumulation, respectively. The genes associated with sugar synthesis and transport including SUS, SPS, TST, STP, and ERD6L were up-regulated, while INVs, FRK and HXK genes related to sugar degradation were down-regulated in 'HP' kumquat. For organic acids, the up-regulation of PEPC and NAD-MDH could accelerate malate accumulation. In contrast, high expression of NAD-IDH and GS resulted in citric acid degradation during 'HP' fruit development. Additionally, the PK, PDH, PEPCK, and FBPase genes responsible for the interconversion of soluble sugars and organic acids were also significantly altered in the early development stages in 'HP'. Conclusion The high sugar accumulation in 'HP' fruit was associated with up-regulation of SUS, SPS, TST, STP and ERD6L genes. The PEPCK, PEPC, NAD-MDH, NADP-IDH, GS, and FBPase genes played important roles in acid synthesis and degradation in 'HP' kumquat. These findings provide further information into understanding the mechanisms underlying sugars and organic acid metabolism in citrus. This article is protected by copyright. All rights reserved.