Induced Expression of AtLEC1 and AtLEC2 Differentially Promotes Somatic Embryogenesis in Transgenic Tobacco Plants

Arabidopsis LEAFY COTYLEDON (LEC) genes, AtLEC1 and AtLEC2, are important embryonic regulators that play key roles in morphogenesis and maturation phases during embryo development. Ectopic expression of AtLEC1 and AtLEC2 in tobacco caused abnormality in transgenic seedling. When transgenic seeds germinated on medium containing 30 µM DEX, LEC1 transgenic seedlings were ivory and fleshy, with unexpanded cotyledons, stubby hypocotyls, short roots and no obvious callus formation at the shoot meristem position. While LEC2 transgenic seedlings formed embryonic callus on the shoot apical meristem and somatic embryo-like structures emerged from the surface of the callus. When callus were transferred to hormone free MS0 medium more shoots were regenerated from each callus. However, shoot formation was not observed in LEC1 overexpressors. To investigate the mechanisms of LEC2 in somatic embryogenesis, we studied global gene expression by digital gene expression profiling analysis. The results indicated that ectopic expression of LEC2 genes induced accumulation of embryo-specific proteins such as seed storage proteins, late embryogenesis abundant (LEA) proteins, fatty acid biosynthetic enzymes, products of steroid biosynthesis related genes and key regulatory genes of the embryo development. Genes of plant-specific transcription factors such as NAC domain protein, AP2 and GRAS family, resistance-related as well as salicylic acid signaling related genes were up-regulated in LEC2 transgenic seedlings. Ectopi c expression of LEC2 induced large number of somatic embryo formation and shoot regeneration but 20 d DEX induction of LEC1 is not sufficient to induce somatic embryogenesis and shoot formation. Our data provide new information to understand the mechanisms on LEC2 gene's induction of somatic embryogenesis.