Overexpression of alfalfa γ-tocopherol methyltransferase (γ-TMT) gene increases salt susceptibility of transgenic Arabidopsis in seed germination

As antioxidants, tocopherols deactivate reactive oxygen species and prevent lipids from oxidation in response to abiotic stresses. gamma-Tocopherol methyltransferase (gamma-TMT) catalyzes the conversion of gamma-tocopherol into alpha-tocopherol which has the highest biological activity. To investigate roles of gamma-TMT in seed germination under salinity stress, we heterologously overexpressed an alfalfa MsTMT gene in Arabidopsis. MsTMT transgenic seeds germinated much slower than that of Arabidopsis wild-type (WT) seeds under salt stress or exogenous abscisic acid (ABA) treatment, indicating enhanced osmotic and ABA sensitivity in transgenic seeds. Under salinity stress, expression levels of ABA biosynthesis genes (NCED4 and NCED9) and signaling genes (ABI3 and ABI5) were increased in transgenic seeds. Meanwhile, the expression of GA biosynthesis genes (GA3OX1 and GA3OX2) were repressed and that of GA signal suppressor genes RGL2 was enhanced. Moreover, overexpression of MsTMT promoted the release of seed mucilage and contributed to the redistribution of pectins. Interestingly, removal of seed mucilage eliminated the difference in the initiation of seed germination between WT and transgenic lines. Taken together, MsTMT had a strong influence on the response to salinity stress in transgenic Arabidopsis during seed germination. Our results reveal a novel role of MsTMT in mediating the regulations of ABA and GA signaling in seed germination, which is also associated with mucilage release and structure. This study provides new insights into the regulatory network controlled by tocopherol biosynthesis gene in response to abiotic stress in plants.