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
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.