A molecular genetic analysis of the requirement of TOR kinase signalling for plant growth : a thesis presented in partial fulfilment of the requirements for the degree Doctor of Philosophy in Plant Biology at Massey University, Palmerston North, New Zealand

Abstract

Eukaryotes have developed a highly complex mechanism to incorporate signals from nutrient, energy, stress, developmental, and environmental cues to modulate their growth. To promote this growth, eukaryotes have to coordinate the expansion in cellular mass and size through macromolecular synthesis with the increase in cell number through division. This demands a complex orchestration of a plethora of cellular processes such as transcription, protein synthesis, metabolism and cell wall synthesis. The TARGET OF RAPAMYCIN (TOR) pathway was identified as a central integrator of this growth-regulating mechanism. Components of this pathway, including the TOR kinase and its interaction partners REGULATORY-ASSOCIATED PROTEIN OF TOR (RAPTOR) and LETHAL WITH SEC 13 PROTEIN 8 (LST8), are highly conserved among eukaryotes. This includes plants, for which the adaptation to changing environmental conditions is particularly important given their sessile lifestyle and highly plastic development. This work sought to further expand the knowledge of how TOR function was adapted to suit the requirements of plants. Therefore, I analysed genetic knock-out mutants of raptor in Arabidopsis thaliana, which resulted in a severe reduction of growth but did not cause an early developmental arrest as reported by previous studies. Detailed analysis of these mutants further revealed defects in the development of trichomes, gametophytes, and the polar extension of root hairs and pollen tubes. Potential causes for these defects were indicated by lower DNA content and limited ROS accumulation in raptor mutants. High similarities between raptor and lst8 mutants indicated that the formation of TOR complexes as found in other eukaryotes might not be functionally conserved in plants. Further, I adapted a CRE/lox system for the induction of mosaic deletions of RAPTOR, which indicated no tissue-specific requirement for RAPTOR functions within the root of A. thaliana, but demonstrated a role in the regulation of meristem size. To conclude, this data presents further evidence for an altered requirement of RAPTOR and LST8 function for TOR signalling in plants compared to fungi and animals. This thesis revealed novel functions of TOR in plant development, ROS homeostasis and endoreduplication. It further draws attention to the connection with other signalling pathways to regulate growth and development in plants.