Role of PROTEIN PHOSPHATASE 7-LIKE and the EUKARYOTIC TRANSLATION ELONGATION FACTOR COMPLEX 1B in plant development and stress responses in Arabidopsis thaliana

Abstract

Growth and development of plants, as sessile organisms, are dependent on many internal signaling factors and on external environmental cues. Coordinated root growth is required for an optimal supply with nutrients and water. Undifferentiated stem cells within the root apical meristem regulate cell division and thereby coordinate continuous replenishment of root cells. DNA integrity is of utmost importance in stem cells to hinder damaged DNA from being passed onto the next generation of cells. A heteroprotein complex consisting of (at least) three proteins plays an important role for genome stability and primary root growth. MAINTENANCE OF MERISTEMS (MAIN), MAINTENANCE OF MERISTEMS-LIKE1 (MAIL1) and PROTEIN PHOSPHATASE 7-LIKE (PP7L) interact with each other and are acting within the same signaling pathway. T-DNA insertional of each of these three proteins show similar phenotypes with drastically reduced primary root growth, accumulation of dead cells within the root apical meristem and the release of transposable elements from silencing. Currently, it is unknown in which signaling pathway the MAIN-MAIL1-PP7L complex is active. In the first part of this thesis, different approaches were utilized to characterize the MAIN-MAIL1-PP7L complex. On the one hand, a suppressor screen was performed on pp7l mutants to identify suppressor mutations that reverse the short root phenotype. Potential suppressor lines were isolated. Identification of the affected genes will give insight about the potential signaling pathway. On the other hand, protein interaction partners of PP7L were identified using co-immunoprecipitation coupled with mass spectroscopy. Environmental cues strongly influence plant development. Climate change including increasing temperatures challenge plants to constant adaption. Protein biosynthesis and a balanced protein homeostasis are essential for plant growth. Both processes have a high energy consumption and are therefore tightly regulated. Upon stress conditions, global protein biosynthesis is inhibited and solely translation of stress-specific proteins is performed. Translation rates are mostly regulated at the translation initiation step, but recently a regulation at the translation elongation step has been considered. The second part of this thesis focused on the characterization of the translation elongation factor complex eEF1B. eEF1B has a conserved GDP/GTP exchange function on the translation elongation factor complex eEF1A. eEF1A transports aminoacyl-tRNAs to the translating ribosomes at the mRNA and translocates the aminoacyl-tRNAs onto the growing polypeptide chain under GTP hydrolysis. The Arabidopsis eEF1B complex consists of three subunits, alpha, beta and gamma, whose function have scarcely been analyzed. Here, a function of the Arabidopsis eEF1Bgamma subunit for efficient translation and for plant development has been identified. Mutants with reduced eEF1Bgamma protein levels show a delayed plant development with defects in cell division. The global protein biosynthesis rates are similar to WT, but an increased number of total ribosomes indicate a compensation of reduced translation elongation rates in an eef1bgamma double mutant. A direct function of eEF1Bgamma in plant heat stress response has not been identified. Nonetheless, all three eEF1B subunits were found to localize to heat-induced stress granules. Stress granules are stress-induced accumulations of mRNA and proteins in the cytoplasm, which are formed via liquid-liquid phase separation. The eEF1Bbeta subunit shows an increased accumulation within stress granules and is able to enhance the accumulation of the other two eEF1B subunits to stress granules upon heat stress. The accumulation of eEF1B subunits might be a mechanism to finetune the regulation of protein biosynthesis during stress conditions.