Elise Nagel Ebstrup:
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During establishment of a proper root system, plants are extremely dependent upon hormonal crosstalk and activation of correct developmental programs. The phytohormone auxin plays a central role in root development by activating the transcription factor Auxin Response Factor (ARF) family. Regulation of these ARFs is crucial for the progressive changes during lateral root formation. Here we report two different aspects of regulation of specifically ARF7 and ARF19 during lateral root development and pluripotency acquisition. Our investigation shows that during pluripotency acquisition, brassinosteroid transcription factor, BZR1, directly binds and upregulates ARF7 and ARF19. Additionally, dominant active mutant, bzr1-D, exhibits increased ectopic callus formation dependent upon ARF7 and ARF19.
Moreover, an important part of regulation is turnover of no longer needed proteins. We report autophagy to partake in the turnover of ARF7 and ARF19 by removing ARF condensates from low auxin responsive tissue. Through NBR1 dependent selective autophagy, ARF7 localizes with autophagosomes and gets targeted for degradation. Thus, autophagy deficient mutants exhibit an increase in ARF7 protein level, triggering a disruption in the oscillation of ARF7 abundance, ultimately affecting the lateral root formation. This thesis comprises a review and two manuscripts, which together adds to the complex regulatory mechanisms of ARFs during lateral root formation in Arabidopsis thaliana, by identifying the importance of BZR1 transcriptional regulation and autophagic turnover of ARF7 and ARF19.