Comparative Analysis of SUS Gene Family between Saccharum officinarum and Saccharum spontaneum

Sugarcane is a major sugar-producing crop, which contributed 80% of the world’s sugar in 2010. Saccarhum officinarum is a domestic species with high sugar content, while, Saccarhum spontaneum is a wild species with stress tolerance. The highly complex polyploid genome of modern sugarcane cultivars arose from the interspecific hybridization between S. officinarum and S. spontaneum. Sucrose synthase (SUS) is a key enzyme for sucrose metabolism in plants, where activity is bidirectional: both synthetic and separate. In this study, nine genomic sequences of S. officinarum and eight genomic sequences of S. spontaneum for five SUS genes were identified. Phylogenetic analysis showed that the Saccharum SUS3 and SUS5 genes were generated from ρ duplication, SUS1 and SUS2 were duplicated after the split of dicot and monocot species, and SUS4 was retained from the last common ancestor before the origination of Angiospermae. The gene structure and Ka/Ks analysis suggested the functional constraint of SUS genes in the two Saccharum species. Gene expression based on RNA-seq analysis revealed that SUS1was dominantly expressed in source tissues including the internodes and the basal zone of the leaves, SUS2 was detectable in all tissues examined, and the remaining three SUS genes were expressed at low levels in the examined tissues, indicating SUS1 is the key member involved in sucrose accumulation. In addition, SUS genes were observed to be present at higher expression levels in S. officinarum than in S. spontaneum, while SUS2 presented different expression patterns during the circadian rhythm in S. spontaneum and S. officinarum, suggesting the two SUS genes contribute to the differential sugar levels in these species. Our comprehensive study in Saccharum provides the foundations for further functional studies of the SUS gene family.