Genome-wide identification and expression profile analysis of citrus sucrose synthase genes: investigation of possible roles in the regulation of sugar accumulation.

Mohammad Zahidul Islam,Long-Fei Jin,Shu-Ang Peng,Xiao-Mei Hu,Yong-Zhong Liu,Yuepeng Han

doi:10.1371/journal.pone.0113623

Abstract

Sucrose synthase (Sus) (EC 2.4.1.13) is a key enzyme for the sugar accumulation that is critical to form fruit quality. In this study, extensive data-mining and PCR amplification confirmed that there are at least six Sus genes (CitSus1-6) in the citrus genome. Gene structure and phylogeny analysis showed an evolutionary consistency with other plant species. The six Sus genes contain 12–15 exons and 11–14 introns and were evenly distributed into the three plant Sus groups (CitSus1 and CitSus2 in the Sus I group, CitSus3 and CitSus6 in the Sus II group, and CitSus4 and CitSus5 in the Sus III group). Transcripts of these six CitSus genes were subsequently examined. For tissues and organs, CitSus1 and 2 were predominantly expressed in fruit juice sacs (JS) whereas CitSus3 and 4 were predominantly expressed in early leaves (immature leaves), and CitSus5 and 6 were predominantly expressed in fruit JS and in mature leaves. During fruit development, CitSus5 transcript increased significantly and CitSus6 transcript decreased significantly in fruit JS. In the fruit segment membrane (SM), the transcript levels of CitSus2 and 5 were markedly higher and the abundant levels of CitSus3 and 6 gradually decreased. Moreover, transcript levels of CitSus1-4 examined were higher and the CitSus5 transcript level was lower in the fruit SM than in fruit JS, while CitSus6 had a similar transcript level in fruit JS and SM. In addition, transcripts of CitSus1-6 responded differently to dehydration in mature leaves or to mild drought stress in fruit JS and SM. Finally, the possible roles of Sus genes in the regulation of sugar accumulation are discussed; however, further study is required.

Highlights

Sucrose is the principal form of photoassimilate for export from the source to sink organ in some plants, such as citrus [1]
A previous study was carried out to detect Sucrose synthase (Sus) activities in Ponkan fruits and, the results showed that changes in Sus activity varied between fruit segment membrane (SM) and juice sacs (JS) during fruit development and ripening [3]: in fruit SM, Sus activity of the cleavage direction stayed at a relatively high level and showed only a slight change while Sus activity of the synthetic direction was slightly lower but increased continuously; in JS, Sus activity of the cleavage direction was increased, it was slightly lower than Sus activity of the synthetic direction, which kept a relatively constant level during fruit development and ripening
CitSus gene expression profiles in the present study revealed that CitSus1, 2, 5 and 6 were predominantly expressed in fruit JS (Figure 3A, B, E and F) and transcript levels of CitSus1-4 were significantly higher in fruit SM compare to JS (Figure 4A–D)

Summary

Introduction

Sucrose is the principal form of photoassimilate for export from the source to sink organ in some plants, such as citrus [1]. The utilization of sucrose in the plant cell requires its cleavage, which is performed by two key enzymes, sucrose synthase (Sus, EC 2.4.1.13) and invertase (Inv). Sus has shown its potential roles in fruit development and in promoting sugar accumulation in the juice sacs (JS) of grapefruit (Citrus paradisi) [10]. Hockema and Etxeberria [5] suggested that the increase of sugar content in citrus fruit under drought stress is due to the increase in Sus activity that promotes photoassimilate partitioning into fruit JS. Egan 1) showed that the significant increase of sugar accumulation in the juice sacs under soil plastic film mulch was attributed to the enhancement of Sus activity (cleavage direction) in the fruit segment membrane (SM) and Sus synthetic activity in fruit JS [3]

Methods

Results

Discussion

Conclusion