Rice MtN3/saliva/SWEET gene family: Evolution, expression profiling, and sugar transport

The rice MtN3/saliva/SWEET gene family consists of 21 paralogs. However, their functions in physiological processes are largely unknown, although at least three of the 21 paralogs are used by pathogenic bacteria to infect rice. Here, we report the evolutionary features, transcriptional characteristics, and putative functions in sugar transport of this gene family. The wild rice accessions in this study included those with AA, BB, CC, BBCC, CCDD, EE, and GG genomes, which appeared approximately 0.58-14.6 million years ago. The structures, chromosomal locations, phylogenetic relationships, and homologous distribution among the accessions suggest that the number of rice MtN3/saliva/SWEET paralogs gradually increased as the Oryza genus evolved, and one third of the paralogs may have originated recently. These paralogs are differentially expressed in vegetative and reproductive tissues, in the leaf senescence process, and in signaling dependent on gibberellic acid, cytokinin, or 1-naphthalene acetic acid (an analog of auxin), suggesting that they may be associated with multiple physiological processes. Four paralogs could transport galactose in yeast, which suggests that they may have a similar function in rice. These results will help to elucidate their roles and biochemical functions in rice development, adaptation to environment, host-pathogen interaction, and so forth.