The Role of Mitochondrial Function and Retrograde Signalling for Stress Responses in Arabidopsis thaliana

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Life Sciences, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

In plants, mitochondria are the power houses of the cell and critical organelles for plant energy metabolism, biosynthetic pathways and stress responses. Mitochondria also function as stress sensors and are important for stress tolerance through relaying their functional status to the nucleus via retrograde signalling involving components such as the transcription factor ANAC017 and the protein kinase CDKE1. Flooding is a detrimental abiotic stress which severely affects agricultural production by reducing the availability of oxygen, carbon dioxide and light. The importance of mitochondrial signalling for the acclimation to flooding is not well characterised. Although plants vary in flooding tolerance, knowledge on the contribution of mitochondrial function and signalling for adaptation mechanisms is limited. To investigate the role of ANAC017 in mitochondrial retrograde signalling, plants overexpressing ANAC017 were generated and characterised to identify its role in plant growth (Chapter 2, published in Plant Physiology, Meng et al., 2019). The results from phenotypic characterisation and RNA-seq indicated that the overexpression in Arabidopsis thaliana leads to growth retardation, altered leaf development and early leaf senescence by reprogramming the retrograde signalling pathway. A gene regulatory network revealed the extensive interaction of transcription factors and downstream stress responsive genes. A comparative phenotypical, physiological and transcriptomic characterisation of Arabidopsis mitochondrial signalling mutants (cdke1/rao1 and anac017/rao2) and a set of contrasting accessions was performed after submergence (Chapter 3, in revision with The Plant Journal). The results revealed that acclimation to submergence is dependent on mitochondrial retrograde signalling and underlying transcriptional re-programming is used as an adaptation mechanism. Functional characterisation of two WRKY transcription factors (WRKY40 and WRKY45) confirmed their role in conferring tolerance to submergence. A genome wide association study (GWAS) was performed using 387 Arabidopsis accessions. Significant associations of single nucleotide polymorphisms with quantified flooding related traits were determined and underlying genes are related to processes important for flooding tolerance such as carbohydrate metabolism and nutrient uptake (Chapter 4, draft manuscript in preparation for submission).