Alternative oxidase (AOX) 1a and 1d limit proline-induced oxidative stress and aid salinity recovery in Arabidopsis

Proline (Pro) catabolism and reactive oxygen species production have been linked in mammals and Caenorhabditis elegans, while increases in leaf respiration rate follow Pro exposure in plants. Here, we investigated how alternative oxidases (AOXs) of the mitochondrial electron transport chain accommodate the large, atypical flux resulting from Pro catabolism and limit oxidative stress during Pro breakdown in mature Arabidopsis (Arabidopsis thaliana) leaves. Following Pro treatment, AOX1a and AOX1d accumulate at transcript and protein levels, with AOX1d approaching the level of the typically dominant AOX1a isoform. We therefore sought to determine the function of both AOX isoforms under Pro respiring conditions. Oxygen consumption rate measurements in aox1a and aox1d leaves suggested these AOXs can functionally compensate for each other to establish enhanced AOX catalytic capacity in response to Pro. Generation of aox1a.aox1d lines showed complete loss of AOX proteins and activity upon Pro treatment, yet full respiratory induction in response to Pro remained possible via the cytochrome pathway. However, aox1a.aox1d leaves displayed symptoms of elevated oxidative stress and suffered increased oxidative damage during Pro metabolism compared to the wild-type (WT) or the single mutants. During recovery from salt stress, when relatively high rates of Pro catabolism occur naturally, photosynthetic rates in aox1a.aox1d recovered slower than in the WT or the single aox lines, showing that both AOX1a and AOX1d are beneficial for cellular metabolism during Pro drawdown following osmotic stress. This work provides physiological evidence of a beneficial role for AOX1a but also the less studied AOX1d isoform in allowing safe catabolism of alternative respiratory substrates like Pro. The alternative oxidase of plant mitochondria contributes to proline catabolism by preventing oxidative stress in plant tissues and this aids recovery of photosynthetic rates following salinity stress.

Automated summary

This study reveals the significant role of alternative oxidases (AOXs) in the breakdown of proline in Arabidopsis leaves, allowing for regulation of large, atypical flux and limitation of oxidative stress. The two AOX isoforms can functionally compensate for each other, enhancing AOX catalytic capacity under proline-respiring conditions. AOX1a and AOX1d are beneficial for cellular metabolism and aid in the recovery of photosynthetic rates following salt stress.