The molecular mechanisms underlying thermal responses in Arabidopsis thaliana

Increased ambient temperature drastically affects all stages of plant growth and development. However, the regulatory pathways underlying these responses are not fully elaborated. The predicted change in global climate patterns and increased instability of ecosystems pose a serious threat to agriculture and is thought to affect food security on a global scale. Thus, it is of utmost importance to decipher the molecular mechanisms underlying temperature responses and its associated phenotypes in plants. In this thesis, I used the model plant Arabidopsis thaliana to explore two angles of temperature responses that are influenced by small RNAs, which are most commonly divided into micro RNAs (miRNAs) and small interfering RNAs (siRNAs). In the first approach, I analysed the role of miRNAs to increased ambient temperature and addressed their contribution to natural variation within this response. To achieve this, I generated genome-wide small RNA expression profiles of four Arabidopsis accessions that had been exposed to a shift in ambient temperature from 16 ºC to 27 ºC. I determined correlation in expression changes of the miR156/miR172 node and accession specific flowering behaviour, which seems to confirm and further support a role for miRNAs in natural variation of temperature responses. My results further show that increased temperature affects the expression of miRNAs in auxin and abiotic stress pathways. Notably, my results may indicate a previously unexplored cross talk between phosphate starvation and temperature responses. Finally, I show the function of miR408, for the first time, as a temperature-responsive miRNA regulating plant morphology. In the second approach, I analyse and reveal a potential mechanism involving small RNAs that mediates the temperature-dependent growth defect of the Arabidopsis accession Bur-0. This growth defect is caused by a drastically expanded TTC/GAA triplet repeat in the third intron of the ISOPRPYL MALATE ISOMERASE LARGE SUBUNIT 1 (IIL1; AT4G13430) and is characterised by distorted leaf growth. This accession serves as a natural model system to study the effects of trinucleotide expansions and shares several characteristics with neurodegenerative diseases in humans. Notably, I found that expanded TTC/GAA triplet repeats correlate with a drastic increase of 24-nt siRNAs that are associated with IIL1. In addition, I demonstrated that impairment of siRNA biogenesis is sufficient to suppress repeat-associated growth defects. My results suggest a potential link between siRNA-mediated mechanisms and triplet repeat expansion disorders (TREDs).