Soybean Phosphomethylpyrimidine Synthase GmTHIC Contributes to Thiamine Biosynthesis and Salt Tolerance

Soybean is a major oil and grain crop. Thiamine plays an essential role in both human and plant health; improving plant thiamine contents is one of the key goals for soybean scientists and breeders; however, the genes and mechanisms underlying the control of soybean thiamine remain elusive. In this study, we identify a phosphomethylpyrimidine synthase, GmTHIC, in soybean, which enhances thiamine biosynthesis and plant salt resistance. Cell-free assay shows that the E3 ligase GmPUB21 mediates the degradation of GmTHIC, which is particularly induced by pretreatment with ABA or NaCl. Interestingly, the degradation is not attenuated by the protease inhibitor MG132, indicating that GmPUB21 ubiquitinates GmTHIC possibly via a 26S proteasome-independent degradation system. In addition, exogenous application of thiamine effectively alleviates salt stress symptoms in soybean plants, reduces reactive oxygen species accumulation, and thereby enhances salt tolerance. These results reveal that GmTHIC is required for thiamine biosynthesis and salt tolerance in soybean.