Molecular cloning, characterization and expression profile of the sucrose synthase gene family in Litchi chinensis

Abstract

Sucrose synthase (SUS, EC 2.4.1.13) is widely considered as a key enzyme involved in plant sucrose metabolism, and the gene family encoding different SUS isozymes has been identified and characterized in several plant species. However, to date scant information about the SUS genes is available in Litchi chinensis Sonn. Here, we identified five SUS genes in litchi. These LcSUSs shared high levels of similarity in both nucleotide and amino acid sequences. Their gene structure, phylogenetic relationships, and expression profiles were characterized. Gene structure analysis indicated that the LcSUSs have similar exon-intron structures. Phylogenetic analysis revealed that the five members could be classified into three groups (LcSUS1 and LcSUS2 in SUS II, LcSUS4 and LcSUS5 in SUS III, and LcSUS3 in SUS I), demonstrating evolutionary conservation in the SUS family across litchi and other plant species. The expression levels of LcSUSs were investigated via real-time PCR in various tissues and different developmental stages of aril. For tissues and organs, LcSUSs exhibited distinct but partially redundant expression profiles in litchi, being predominantly expressed in young leaves (sink). During aril development, the expression pattern of LcSUS1 was consistent with the trend of sugar accumulation, indicating it may play important roles in determination of sink strength in aril. Moreover, transcript levels of LcSUS2, LcSUS4, and LcSUS5 varied between cultivars with different hexose/sucrose ratios, which may regulate the sugar composition in aril. Our results provide insights into physiological functions of SUS genes in litchi, especially roles in regulating sugar accumulation in aril.