The mitochondrial respiratory chain is important as an electron sink to avoid over-reduction of chloroplasts in Arabidopsis thaliana leaves under high-light stress

The respiratory chain in plant mitochondria has two pathways: the ATP-coupling cytochrome pathway (CP) and ATP-uncoupling alternative oxidase (AOX). Under high-light (HL) conditions, AOX has been considered to have important roles in avoiding the over-reduction of chloroplasts. The inhibition of the respiratory chain under HL conditions accelerates photosystem II (PSII) damage and represses PSII repair in Arabidopsis thaliana leaves. However, it is not known precisely how the respiratory chain contributes to maintenance of photosynthetic electron transport activity. In this study, we investigated photosynthetic electron transport activity, protein amounts of the photosynthetic electron transport, and electron pool size in the intersystem photosynthetic electron transport and the stroma, before and after 2 h-HL treatment in the leaves of wild-type (WT) and AOX1a-knockout (aox1a) A. thaliana, in which the CP was partially inhibited by a complex-III inhibitor. The electron transport rate around PSII was decreased by CP inhibition in aox1a. After 2 h-HL treatment, CP inhibition barely changed the protein amounts of photosynthetic electron transport, but the protein amount of AOX was increased in WT. The lack of AOX further led to over-reduction of chloroplasts under CP inhibition. Under HL conditions, AOX is important in preventing over-reduction of chloroplasts as an electron sink.