Rice MtN3/Saliva/SWEET Family Genes and Their Homologs in Cellular Organisms

The MtN3/saliva/SWEET-type genes, existing either alone or in a family group, are found in diverse organisms, from monocellular protozoa to higher eukaryotes, indicating their importance in cellular organisms. These genes encode polytopic membrane proteins that feature an MtN3/saliva domain, also known as a PQ loop repeat, The rice MtN3/saliva/SWEET gene family consists of 21 members and is among the largest families in sequenced organisms. Accumulating data suggest that these genes are involved in multiple physiological processes, including reproductive development, senescence, environmental adaptation, and host-pathogen interaction, in different species. In rice, some members of the family, including Xa13/Os8N3/OsSWEET11, which is essential for reproductive development, are used by the pathogenic bacterium Xanthomonas oryzae pv. oryzae to invade its host. Emerging data have also revealed that at least some MtN3/saliva/SWEET-type proteins may regulate different physiological processes by facilitating ion transport via interaction with ion transporters or as sugar transporters. The accumulating knowledge about MtN3/saliva/SWEET- type genes will help to elucidate the molecular bases of their function in different organisms.