Identification of SUT genes and their expression models during the development of seed in pomegranate

Abstract：【Objective】Photosynthetically produced sugar, principally sucrose, is moved from source leaves to support growth of, and carbon storage by, heterotrophic sink organs. Membrane proteins play pivotal roles in mediating sucrose transport within plants. Pomegranate (Punica granatum L.), native to central Asia, is an ancient medicinal fruit crop grown worldwide, with considerable economic value. Pomegranate is well famed as a highly valuable fruit with high nutritive and medical attributes. The pomegranate’s flower, fruit peel, aril (juice), and seeds, are useful for the prevention and treatment of a wide range of diseases. Their functional advantages have dramatically stimulated the market demand, which has opened the avenue for the breeding programs in pomegranate. Mapping genes related to horticulturally important traits is helpful to the molecular marker-assisted (MAS) breeding. To data, SUTs activities had been proved to have a major impact upon regulating many plant developmental processes, such as pollen germination, flowering, tuberization, restraining plant growth, fruit size reduction, seed development, biomass partitioning, plant growth rates, crop yields and ethylene biosynthesis. Here, the objective of this study was to identify the genomic SUT genes, and analyze their gene structure，promoter，phylogenetic relationship and expression models. It will provide the reference for further research on the function and molecular regulation of SUT family in pomegranate.【Methods】Blastp and HMMERmodel were performed to identify the SUT genes; The phylogenetic relationships of SUTs and their promoters were analyzed by adjacent method and gene structure analysis.【Results】There were nine sucrose transporter genes described in Arabidopsis, whereas the rice genome contained five SUT genes. In the study, a total of 10 SUT genes were detected on the Chr1, Chr3, Chr4, Chr5, Chr6 and Chr7 in pomegranate, whose number was more than that of Arabidopsis and rice. The average sequence length of SUT genes was 4 406.7 bp. The average content of A, C, G and T for SUT genes was 28.50%, 21.57%, 22.77% and 27.14%, respectively. The average content of GC for SUT genes was 44.35%, which was comparable to that of‘Tunisia’genome. Additionally, the mean length of SUT genes was significantly related to that of GC content with the relationship coefficient being 0.87 at p < 0.01 level. The length of SUT proteins varied from 92 aa to 1 251 aa, and the molecular weight of SUT proteins changed from 10 260.37 to 142 472.24 D. Relationship analysis indicated that the length of SUT proteins was significantly negatively related to the content of His, but positively related to the content of Met at p < 0.05, and the relationship coefficients were -0.67 and 0.65, respectively. Increasing availability of molecular information could provide new opportunities to detect physiological roles for, and regulation of, sucrose transporters. In the study, the phylogenetic analysis indicated that these SUT genes were divided into three groups, named group1, group2 and group3. The group1, group2 and group3 contained four, one and five SUT genes, respectively. The GC contents of SUT genes and their promoters in pomegranate were significantly lower than those of Arabidopsis thaliana and Oryza sativa. The GC contents of SUT genes were significantly higher than those of their promoters in pomegranate. There was a significant relationship between the GC contents of SUT genes and their length. We also detected ten MYB elements in the promoter domain for the SUT genes. The MYB elements mainly contained MYB2AT, MYB2CONSENSUSAT, MYBCORE, MYBCOREATCYCB1, MYBGAHV, MYBST1, MYBPLANT, MYBPZM, MYB1AT and MYB1LEPR. PgL0145810.1, PgL0237030.1 and PgL0181920.1 all contained more than 20 MYB elements, while PgL0099690.1 only contained seven elements. In addition, four SUT genes containing PgL0328370.1, PgL0099690.1, PgL0145810.1 and PgL0145770.1 were detected to differentially express during the development of the seed by previous transcriptomic sequencing in pomegranate. PgL0099690.1 was down- regulated after 120 flowering days in‘Sanbai’seeds compared to after 60 flowering days. Both PgL0145810.1 and PgL0145770.1 were down- regulated in ‘Sanbai’ seeds after 120 flowering days compared to after 60 flowering days. Conversely, PgL0328370.1 was up-regulated after 120 flowering days in‘Sanbai’or‘Tunisia’seeds compared to after 60 flowering days. For the different cultivars, PgL0145770.1 was down-regulated after 60 flowering days in‘Tunisia’seeds compared to that in‘Sanbai’.【Conclusion】The SUTs were encoded by a multi-gene family. The SUT family has been characterized in many plant species. The molecular characteristics, structures, and phylogeny of plant SUTs have been well studied and reviewed. Our results indicated that SUT genes were relatively conservative in their sequences and gene structures during evolution in pomegranate. However, their promoter sequences changed greatly in evolutionary process; Developing plant embryos depended on nutrition from maternal tissues via the seed coat and endosperm. Sucrose, the major transport form of carbohydrate in plants, was delivered via the phloem to the maternal seed coat and then secreted from the seed coat to feed the embryo. Thus, the expression variation of SUT genes regulated the development of the seed in pomegranate. Our findings will provide a foundation for further functional studies on SUTs in pomegranate, and contribute to elucidating SUT roles in seed development.