Functional Coexpression of the Mitochondrial Alternative Oxidase and Uncoupling Protein Underlies Thermoregulation in the Thermogenic Florets of Skunk Cabbage

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°C to 23.1°C or from 8.3°C to 24.9°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.Thermogenesis is a phenomenon in which the temperature of a specific floral tissue increases due to endogenous heat production. To date, thermogenesis has been reported in several species of the arum lily family (Philodendron spp.; Nagy et al., 1972;Gottsberger and Amaral, 1984; Symplocarpus spp.; Seymour and Blaylock, 1999;Ito et al., 2004;Seymour, 2004; Helicodiceros muscivorus;Seymour et al., 2003), and also in palms (Bactris gasipaes;Schroeder, 1978; Carludovica palmata;Gottsberger, 1990; Bactris spp.;Listabarth, 1996), lotus (Nelumbo spp.; Miyake, 1898; Schultze-Motel, 1996, 1998;Watling et al., 2006), cycads (Ceratozamia miqueliana and Zamia furfuracea; Skubatz et al., 1993; Macrozamia machinii;Seymour et al., 2004), and water lilies (Victria spp.; Prance and Arias, 1975;Skubatz et al., 1990).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 AOX of plant mitochondria accepts electrons from the ubiquinone pool and uses them to reduce oxygen to water, with no conservation of energy through pro...

Effects of floral thermogenesis on pollen function in Asian skunk cabbage Symplocarpus renifolius

“…Moreover, because pollination is practically impossible below 88C, thermogenesis is essential for successful reproduction at natural environmental temperatures. S. renifolius commonly proceeds through the stigma stage when average ambient temperatures are below 58C (Onda et al 2008). Without thermogenesis or the ability of pollen to adapt to lower temperatures, skunk cabbage could not successfully flower as early in the season as it does.…”

“…S. foetidus can maintain spadix temperature within a 3.58C range (22.7-26.28C) over an ambient air temperature range of 37.48C (K10.3 to 27.18C) (Seymour 2004). Precise temperature regulation also occurs in Symplocarpus renifolius from Japan (Onda et al 2008). The inflorescence of both species consists of a fleshy, reddish-brown-coloured spathe around a spadix having approximately 50-100 bisexual florets (Uemura et al 1993;Seymour & Blaylock 1999).…”

In the heat of the night – alternative pathway respiration drives thermogenesis in Philodendron bipinnatifidum

“…For example, FM florets have a higher surface area to volume ratio and thinner cuticle than SM florets (Grant, 2010). The strong relationship between AOX flux and heating in FM florets, and the substantial proportions of total flux via AOX in both FM and SM florets, suggest there is little room for contribution by pUCPs, except alongside AOX to totally uncouple respiration via concurrent operation of pUCPs and AOX (Onda et al, 2008;Wagner et al, 2008). If pUCPs alone were responsible for heat generation in P. bipinnatifidum, then we would expect an increase in flux through the cytochrome pathway during thermogenesis; however, we detected no change in COX flux during heating by FM florets across all thermogenic stages, and comparatively low proportions of total flux via COX in peak heating SM florets when measured under increased O 2 supply.…”

“…Araceae; Lance, 1974). The Araceae contains more thermogenic species than any other family (Meeuse, 1975;Meeuse & Raskin, 1988;Gibernau et al, 2005), and has attracted much attention from researchers aiming to understand heating mechanisms (Wagner et al, 1998(Wagner et al, , 2008Ito et al, 2003a,b;Crichton et al, 2005;Ito & Seymour, 2005;Onda et al, 2008), or to characterize the ecological significance of thermogenesis in plant-pollinator interactions (Gottsberger, 1999;Gibernau & Barabé, 2002). Among thermogenic arums, the capacity for heat generation differs markedly, from approx.…”

Photosynthetic Performance and Fertility Are Repressed in GmAOX2b Antisense Soybean

“…Thermogenesis was proposed to facilitate the evaporation of scent compounds to attract pollinators (Meeuse and Raskin, 1988) and/or to provide an optimum temperature for floral development (Seymour and SchultzeMotel, 1998). Recently, thermogenesis was proposed to prevent low-temperature damage to developing pollen (Onda et al, 2008) and to optimize pollen germination and pollen tube growth in skunk cabbage (Symplocarpus renifolius), ensuring successful fertilization in early spring (Seymour et al, 2009).…”