ABSTRACT
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 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 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 did 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, whereas 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 insight into understanding the mechanisms underlying metabolism of sugars and organic acids in citrus. © 2021 Society of Chemical Industry.
Subject(s)
Acids/metabolism , Fruit/metabolism , Plant Proteins/genetics , Rutaceae/genetics , Sugars/metabolism , Acids/analysis , Citric Acid/metabolism , Fruit/genetics , Gene Expression Profiling , Gene Expression Regulation, Plant , Malates/metabolism , Plant Proteins/metabolism , Rutaceae/metabolismABSTRACT
Drought severely affects citrus growth and development. In order to explore the mechanism of drought response of citrus, two cultivars (Sanhuhongju and Sanhuhuahong) that differing in drought tolerance were used as materials. The drought and rewatering treatment was conducted in pot experiments, with leaf photosynthesis, chlorophyll fluorescence, and root architecture being measured. The results showed that drought significantly decreased net photosynthetic rate (Pn), stomatal conductance (gs), transportation rate (Tr), and intercellular CO2 concentration (Ci) of both cultivars, but Sanhuhongju generally showed less reduction. After rewatering, photosynthetic parameters were partly recovered but still lower than that in control. The water use efficiency (WUE) of Sanhuhongju was significantly increased after drought stress for 15 d, but the WUE of Sanhuhuahong was decreased except at the 15 day of drought stress. In addition, the maximum photosynthesis efficiency of PS II (Fv/Fm) was increased in both cultivars, but the photochemical quantum yield of PS II [Y(II)] was increased in Sanhuhuahong under drought. Both the apparent electron transport rate (ETR) and photochemical quenching (qP) were inhibited in the treated seedlings. The non-photochemical quenching (NPQ) was decreased in Sanhuhongju while increased in Sanhuhuahong under drought and rewatering conditions. Drought stress resulted in the decrease of root surface area and volume of both cultivars, and it inhibited root elongation of Sanhuhuahong while improved the root length and root tip number of Sanhuhongju. The length of first lateral roots of Sanhuhongju was increased after drought stress 10 d, but did not change at the drought stress prophase of Sanhuhuahong, and then significantly decreased after 20 d. Furthermore, drought stress inhibited all lateral roots development except the tertiary lateral root of Sanhuhongju, and root growth could not be recovered by rewatering except root tip number. In conclusion, Sanhuhongju showed less reduction in leaf photosynthesis than Sanhuhuahong, with higher WUE and light use efficiency under drought stress. The increases of root tip number and lateral root length would help improve water uptake ability in Sanhuhongju.
