SET DOMAIN GROUP 721 protein functions in saline–alkaline stress tolerance in the model rice variety Kitaake

To isolate the genetic locus responsible for saline–alkaline stress tolerance, we developed a high‐throughput activation tagging‐based T‐DNA insertion mutagenesis method using the model rice (Oryza sativa L.) variety Kitaake. One of the activation‐tagged insertion lines, activation tagging 7 (AC7), showed increased tolerance to saline–alkaline stress. This phenotype resulted from the overexpression of a gene that encodes a SET DOMAIN GROUP 721 protein with H3K4 methyltransferase activity. Transgenic plants overexpressing OsSDG721 showed saline–alkaline stress‐tolerant phenotypes, along with increased leaf angle, advanced heading and ripening dates. By contrast, ossdg721 loss‐of‐function mutants showed increased sensitivity to saline–alkaline stress characterized by decreased survival rates and reduction in plant height, grain size, grain weight and leaf angle. RNA sequencing (RNA‐seq) analysis of wild‐type Kitaake and ossdg721 mutants indicated that OsSDG721 positively regulates the expression level of HIGH‐AFFINITY POTASSIUM (K(+) ) TRANSPORTER1;5 (OsHKT1;5), which encodes a Na(+)‐selective transporter that maintains K(+)/Na(+) homeostasis under salt stress. Furthermore, we showed that OsSDG721 binds to and deposits the H3K4me3 mark in the promoter and coding region of OsHKT1;5, thereby upregulating OsHKT1;5 expression under saline–alkaline stress. Overall, by generating Kitaake activation‐tagging pools, we established that the H3K4 methyltransferase OsSDG721 enhances saline–alkaline stress tolerance in rice.