Succinate dehydrogenase assembly factor 2 is needed for assembly and activity of mitochondrial complex II and for normal root elongation in <scp>A</scp>rabidopsis

Summary Mitochondria complex II (succinate dehydrogenase, SDH ) plays a central role in respiratory metabolism as a component of both the electron transport chain and the tricarboxylic acid cycle. We report the identification of an SDH assembly factor by analysis of T ‐ DNA insertions in A t5g51040, a protein with unknown function that was identified by mass spectrometry analysis as a low abundance mitochondrial protein. This gene is co‐expressed with a number of genes encoding mitochondrial proteins, including SDH 1‐1, and has low partial sequence similarity to human SDHAF 2, a protein required for flavin‐adenine dinucleotide (FAD) insertion into SDH . In contrast to observations of other SDH deficient lines in Arabidopsis, the sdhaf2 line did not affect photosynthetic rate or stomatal conductance, but instead showed inhibition of primary root elongation with early lateral root emergence, presumably due to the low SDH activity caused by the reduced abundance of SDHAF 2. Both roots and leaves showed succinate accumulation but different responses in the abundance of other organic acids and amino acids assayed. Isolated mitochondria showed lowered SDH 1 protein abundance, lowered maximal SDH activity and less protein‐bound flavin‐adenine dinucleotide (FAD) at the molecular mass of SDH 1 in the gel separation. The short root phenotype and SDH function of sdhaf2 was fully complemented by transformation with SDHAF 2 . Application of the SDH inhibitor, malonate, phenocopied the sdhaf2 root architecture in WT . Whole root respiratory assays showed no difference between WT and sdhaf2 , but micro‐respirometry of the tips of roots clearly showed low oxygen consumption in sdhaf2 which could explain a metabolic deficit responsible for root tip growth.