Arabidopsis NAC Transcription Factors: Roles in Natural and Stress-Induced Senescence
Plant senescence is an important biological phenomenon that involves systematic degradation of plant cells and tissue structures. Whereas, this process is important for the efficient remobilization of nutrients from old tissues to newly developing sinks, induction of premature senescence in response to abiotic and biotic stresses is responsible for immense yield losses in crop plants worldwide. Investigating this process, therefore, holds a great promise in improving yield potential and nutritional quality under optimal and stress conditions. Plants respond to internal and external stimuli through the modulation of gene expression. Several members of the NAC [for NAM (no apical meristem), ATAF, and CUC (cup-shaped cotyledon)] transcription factor family are differentially expressed during developmental as well as during environmentally-induced senescence in Arabidopsis thaliana. The main objective of this thesis was to investigate the roles of three such senescence-induced members in Arabidopsis thaliana, ANAC032, ANAC046 and ANAC087, during natural senescence and under different stress conditions. The study involved the use of different transgenic approaches combined with molecular and biochemical analyses to determine the physiological functions of these transcription factors. The results showed that ANAC032 positively regulates natural and dark-induced leaf senescence. Detailed analyses of ANAC032 transgenic plants further showed that ANAC032 has a senescence promoting role in response to oxidative stress-inducing conditions as well as during osmotic and salinity stresses. ANAC046 and ANA087 were not found to be important during natural leaf senescence. The detailed characterization of ANAC046 and ANAC087 transgenic lines in response to different oxidative stress-inducing conditions showed that ANAC046 positively regulates senescence in response to oxidative stresses imposed by 3-aminotriazole (3-AT) and paraquat treatment. In contrast, ANAC087 was only found to negatively regulate responses to fumonisin B1 (FB1), a fungal toxin which inhibits sphingolipid biosynthesis in plants upon fungal infection and also induces oxidative stress, suggesting that ANAC087 may be important in the regulation of plant defense responses. Furthermore, both ANAC046 and ANAC087 were found to positively regulate seed germination under abiotic stress conditions. This work illustrates the importance of these three NAC transcription factors during natural and stress-induced senescence in Arabidopsis thaliana.