Zhihang Feng, Hiroshi Nagao, Baohai Li, Naoyuki Sotta, Yusuke Shikanai, Katsushi Yamaguchi, Shuji Shigenobu, Takehiro Kamiya*, Toru Fujiwara*


Mg2+ is among the most abundant divalent cations in living cells. In plants, investigations on Mg homeostasis are restricted to the functional characterization of Mg2+ transporters. Here, we demonstrate that the splicing factors SUPPRESSORS OF MEC-8 AND UNC-52 1 (SMU1) and 2 (SMU2) mediate Mg homeostasis in Arabidopsis thaliana. A low-Mg sensitive A. thaliana mutant was isolated and the causal gene was identified as SMU1. Disruption of SMU2, a protein that can form a complex with SMU1, resulted in a similar low-Mg sensitive phenotype. In both mutants, a Mg2+ transporter gene, Mitochondrial RNA Splicing 2 (MRS2-7), showed altered splicing patterns. Genetic evidence indicated that MRS2-7 functions in the same pathway as SMU1 and SMU2 for low-Mg adaptation. In contrast to previous results showing that the SMU1-SMU2 complex is the active form in RNA splicing, MRS2-7 splicing was promoted in the smu2 mutant overexpressing SMU1, indicating that complex formation is not a prerequisite for the splicing. We found here that formation of the SMU1-SMU2 complex is an essential step for their compartmentation in the nuclear speckles, a type of nuclear body enriched with proteins that participate in various aspects of RNA metabolism. Taken together, our study reveals the involvement of the SMU splicing factors in plant Mg homeostasis and provides evidence that complex formation is required for their intranuclear compartmentation.

Paper Information

: Plant Physiology