Abstract
Two distinct mitochondrial energy dissipating systems, alternative oxidase (AOX) and uncoupling protein (UCP), have been implicated as crucial components of thermogenesis in plants and animals, respectively. To further clarify the physiological roles of AOX and UCP during homeothermic heat production in the thermogenic skunk cabbage (Symplocarpus renifolius), we identified the thermogenic cells and performed expression and functional analyses of these genes in this organism. Thermographic analysis combined with in situ hybridization revealed that the putative thermogenic cells surround the stamens in the florets of skunk cabbage and coexpress transcripts for SrAOX, encoding Symplocarpus AOX, and SrUCPb, encoding a novel UCP that lacks a fifth transmembrane segment. Mitochondria isolated from the thermogenic florets exhibited substantial linoleic acid (LA)-inducible uncoupling activities. Moreover, our results demonstrate that LA is capable of inhibiting the mitochondrial AOX pathway, whereas the proportion of pyruvate-stimulated AOX capacity was not significantly affected by LA. Intriguingly, the protein expression levels for SrAOX and SrUCPb were unaffected even when the ambient air temperatures increased from 10.3 degrees C to 23.1 degrees C or from 8.3 degrees C to 24.9 degrees C. Thus, our results suggest that functional coexpression of AOX and UCP underlies the molecular basis of heat production, and that posttranslational modifications of these proteins play a crucial role in regulating homeothermic heat production under conditions of natural ambient temperature fluctuations in skunk cabbage.
Highlights
Two distinct mitochondrial energy dissipating systems, alternative oxidase (AOX) and uncoupling protein (UCP), have been implicated as crucial components of thermogenesis in plants and animals, respectively
Our results suggest that both AOX and UCP molecules are involved in tissue-specific thermoregulation in this plant
Because the interior of an intact spadix cannot be cooler than the florets in the steady state, our results strongly suggest that the florets are the thermogenic tissues in skunk cabbage
Summary
Two distinct mitochondrial energy dissipating systems, alternative oxidase (AOX) and uncoupling protein (UCP), have been implicated as crucial components of thermogenesis in plants and animals, respectively. Heat production in thermogenic plants is thought to be associated with a large increase in the activity of the cyanide-resistant, nonphosphorylating electron transport pathway in mitochondria This pathway is mediated by alternative oxidase (AOX) and is shared among plants, fungi, and nematodes (Berthold and Siedow, 1993; McIntosh, 1994; Day and Wiskich, 1995; Wagner and Krab, 1995; Moore et al, 2002; McDonald and Vanlerberghe, 2004; Clifton et al, 2006; McDonald and Vanlerberghe, 2006; Onda et al, 2007). The functions of UCPs in nonthermogenic plants have been extensively studied (Vercesi et al, 2006), the roles of these UCPs in thermogenic plants remain poorly understood
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