# The Impact of Mitochondrial DNA Depletion on Mitochondrial Ultrastructure, Photosynthesis, and the mTERF Gene Family in Chlamydomonas reinhardtii

## TL;DR

This study shows how losing mitochondrial DNA in Chlamydomonas reinhardtii affects mitochondrial structure, photosynthesis, and the mTERF gene family, suggesting a role in organellar communication.

## Contribution

The first comprehensive analysis of the mTERF gene family in Chlamydomonas and its connection to mitochondrial DNA depletion and organelle crosstalk.

## Key findings

- Mitochondrial DNA depletion causes structural changes like elongation and cristae collapse.
- Loss of mtDNA impairs chloroplast function, damaging photosystem II and increasing excitation pressure.
- Eight mTERF genes were identified, with potential roles in transcription and RNA regulation, and some targeted by miRNAs.

## Abstract

Mitochondrial biogenesis requires coordinated expression from both nuclear and mitochondrial genomes. To understand the consequences of mitochondrial genome loss, we generated a mitochondrial DNA-depleted line (crm−) in Chlamydomonas reinhardtii via long-term ethidium bromide treatment. We then examined how mtDNA disruption affects mitochondrial ultrastructure, chloroplast function, and the mitochondrial transcription termination factor (mTERF) gene family. Our results reveal that mitochondrial dysfunction is associated with severe organelle remodeling, including mitochondrial elongation, matrix condensation, and cristae collapse. Consequently, mitochondria reduce the electron sink capacity which appears to over-reduce the chloroplast electron transport chain, correlating with causing damage to photosystem II (PSII), as indicated by higher plastoquinone PQ redox state and PSII excitation pressure and lower non-photochemical quantum yield [Y(NPQ)]. Furthermore, we identified and characterized eight nuclear-encoded mTERF genes in C. reinhardtii (CrmTERFs). Phylogenetic analysis grouped them into three clades with potential functional conservation. Collinearity analysis suggested potential evolutionary relationships between mTERF genes in Chlamydomonas and Marchantia polymorpha. Gene ontology annotation linked CrmTERFs to transcription termination and RNA biosynthesis regulation. Additionally, in silico prediction identified twelve putative miRNAs targeting seven of the eight CrmTERFs, with CrmTERF3 as the only exception, providing candidates for future experimental validation. This study provides the first comprehensive analysis of the nuclear encoded mTERF gene family in Chlamydomonas and demonstrates that mtDNA loss is correlated with mTERF genes expression, as well as mitochondrial structure and chloroplast photoprotective impairments. These findings suggest a potential role for CrmTERFs in mitochondrial retrograde signaling and organellar crosstalk, though functional validation is required to establish causality.