Arabidopsis MDA1, a Nuclear-Encoded Protein, Functions in Chloroplast Development and Abiotic Stress Responses

<div><p>Most chloroplast and mitochondrial proteins are encoded by nuclear genes, whose functions remain largely unknown because mutant alleles are lacking. A reverse genetics screen for mutations affecting the mitochondrial transcription termination factor (mTERF) family in <em>Arabidopsis thaliana</em> allowed us to identify 75 lines carrying T-DNA insertions. Two of them were homozygous for insertions in the At4g14605 gene, which we dubbed <em>MDA1</em> (<em><u>M</u>TERF <u>D</u>EFECTIVE IN <u>A</u>rabidopsis1</em>). The <em>mda1</em> mutants exhibited altered chloroplast morphology and plant growth, and reduced pigmentation of cotyledons, leaves, stems and sepals. The <em>mda1</em> mutations enhanced salt and osmotic stress tolerance and altered sugar responses during seedling establishment, possibly as a result of reduced ABA sensitivity. Loss of <em>MDA1</em> function caused up-regulation of the <em>RpoTp/SCA3</em> nuclear gene encoding a plastid RNA polymerase and modified the steady-state levels of chloroplast gene transcripts. Double mutant analyses indicated that <em>MDA1</em> and the previously described <em>mTERF</em> genes <em>SOLDAT10</em> and <em>RUG2</em> act in different pathways. Our findings reveal a new role for mTERF proteins in the response to abiotic stress, probably through perturbed ABA retrograde signalling resulting from a disruption in chloroplast homeostasis.</p>