Unravelling the response of the alternative pathway of mitochondrial electron transport to abiotic stress in plants

Waterman CD, Sweetman C, Rainbird BM, Day DA, Jenkins CLD and Soole KL

College of Science and Engineering, Flinders University of South Australia, GPO Box 5100, Adelaide, SA, Australia.

Plant mitochondria are comprised of a complex network of respiratory proteins regulating metabolism and driving production of ATP. Among these are the well characterised cytochrome c (CytC) pathway enzymes. Lesser known is the alternative pathway (AP). Unlike the conventional CytC pathway, the AP role remains unclear. Where the CytC pathway contributes to a proton gradient and ATP production, the AP does not. Utilising NAD(P)H, the first enzymes in this pathway, (the type II dehydrogenases) do not pump protons. Analogous to cytochrome c oxidase, the alternative oxidase (AOX) oxidises ubiquinol but again contributes no protons across the membrane. This begs the question, why is a pathway with the potential to be wasteful not only conserved across the plant kingdom, but also conserved across many isoforms. Transgenic knockdown and overexpressing models have demonstrated across multiple organisms that some of these enzymes are crucial to an adequate stress response. In Arabidopsis, an isoform of AOX, AOX1a, has been shown to not only be important to the stress response but improve the plants ability to deal with various abiotic stressors when overexpressed. Recent work in our laboratory has shown that plants over-expressing both AOX and external NADH dehydrogenase NDB2 exhibit enhanced recovery from combined drought and high light stress. Here we have explored transcriptional and metabolic networks in these transgenic lines to investigate the basis to this capacity for enhanced stress tolerance.