Linum usitatissimum ABI3 enhances the accumulation of seed storage reserves and tolerance to environmental stresses during seed germination and seedling establishment in Arabidopsis thaliana

Flax (Linum usitatissimum) is an important oil crop in arid and semi-arid regions of North and Northwest China, and its seeds are rich in nutritious storage reserves, such as polyunsaturated fatty acids (FAs) and proteins. However, the regulatory networks that control the accumulation of seed storage reserves in flax are still largely unknown. In this study, we found that LuABI3-1 and LuABI3-2 homologs from the flax cultivar 'Longya 10' play important roles in regulating the accumulation of seed storage reserves in Arabidopsis thaliana. The results of subcellular localization and transcriptional activity assays showed that both LuABI3-1 and LuABI3-2 function as transcription factors. Overexpression of either LuABI3-1 or LuABI3-2 resulted in the significant increase in the contents of total seed FAs and storage proteins, but did not alter other key agronomic traits in A. thaliana. Accordingly, the expression of key genes involved in the biosynthesis of FAs and storage proteins was also greatly up-regulated in the developing seeds of LuABI3-1-overexpression lines. Additionally, both LuABI3-1 and LuABI3-2 enhanced the tolerance to the high salt and mannitol stresses during seed germination and seedling establishment in A. thaliana. These results increase our understanding of the LuABI3 regulatory functions and provide promising targets for genetic manipulation of L. usitatissimum to innovate the germplasm resources and cultivate high yield and quality varieties.