Extremely high activities of phosphoenolpyruvate carboxylase in thermogenic tissues of Araceae

Metabolic interplay between cytosolic phosphoenolpyruvate carboxylase and mitochondrial alternative oxidase in thermogenic skunk cabbage, Symplocarpus renifolius

Skunk cabbage (Symplocarpus renifolius) blooms in early spring and its inflorescence, referred to as the spadix, can produce enough heat to melt snow. Here, we investigated glycolytic carbon flow at the PEP branch-point in thermogenic spadices. Our analyses revealed that petals and pistils in thermogenic florets exhibited higher expression of SrPEPC and SrAOX transcripts than those of SrPK, SrPEPCK, and SrPEPtase. Moreover, enzymatic analyses showed high activities of PEPC in the extracts from thermogenic florets. Finally, mitochondria from thermogenic florets showed low respiratory activities when pyruvate was used as a substrate, although a significant malate-mediated cyanide-insensitive respiration was observed. Collectively, these results suggest that PEP metabolism, primarily catabolized by PEPC, plays a critical role in thermogenesis in S. renifolius.

Leaf anatomy, water relations and crassulacean acid metabolism in the chlorenchyma and colourless internal water-storage tissue of Carpobrotus edulis and Senecio ?mandraliscae

Both Carpobrotus edulis and Senecio ?mandraliscae possess leaves with a peripheral chlorenchyma and colourless internal water-storage tissue. Water stress in C. edulis growing under semi-natural conditions resulted in the induction of weak Crassulacean acid metabolism (CAM) whereas well-watered plants of S. ?mandraliscae exhibited a similar degree of CAM. Titratable acidity in the separated water-storage tissue was substantially lower than in the chlorenchyma in both species but, nevertheless, increased during the night and decreased during the day either when sampled from the intact plant or from incubated tissue slices. Indeed, the increase in nocturnal titratable acidity produced by the water-storage tissue in situ accounted for approx. 30% of total acidification on a per-leaf basis. It appears that during the night the water-storage tissue in these species is able to fix CO2 which is subsequently released during the day to enter the photosynthetic carbon-reduction cycle of the chlorenchyma. Diurnal rhythms of water potential (Ψ) and osmotic potential (Ψs) were measured in separated chlorenchyma and water-storage tissue by thermocouple psychrometry. Both parameters increased during the latter part of the daytime and initial nocturnal period and decreased during the rest of the night and into the post-dawn period. The chlorenchyma of water-stressed plants of C. edulis appeared to possess a marked negative turgor pressure (as determined from Ψ-Ψs) but this was caused by a severe underestimation in the measurement of the chlorenchyma Ψ. It is suggested that this artefact arose from release of colloidal polysaccharide mucilage, or possibly tannins, from broken tannin cells producing a lowering of water activity when measured using thermocouple psychrometry.

Role and location of NAD malic enzyme in thermogenic tissues of Araceae

This work was done to discover how those nonphotosynthetic tissues of the Araceae that become thermogenic release, as CO2, carbon recently fixed by phosphoenolpyruvate carboxylase. Extracts of clubs of the spadix of Arum maculatum showed no activity for phosphoenolpyruvate carboxykinase and low activities of NADP malic enzyme. NAD malic enzyme activity in the above extracts and in those of thermogenic tissues of other Araceae was appreciable. Analysis of homogenates of clubs of Typhonium giraldii by differential centrifugation and sucrose gradients showed that NAD malic enzyme was confined to mitochondria. Centrifugation of mitochondria after freezing and thawing left all the NAD malic enzyme in the supernatant. NAD malic enzyme in isolated, intact mitochondria was completely latent, and was completely protected from exogenous trypsin. The responses of this latency and protection to different concentrations of Triton X-100 suggested that none of the NAD malic enzyme was accessible from either the outside or the intermembrane space of the mitochondria. Treatment of excised clubs of A. maculatum with 2-N-butylmalonate largely prevented the development of the rapid respiration responsible for thermogenesis, and severely inhibited dark fixation of 14CO2. The conclusion is that in mature clubs of the Araceae phosphoenolpyruvate is converted to malate in the cytosol by phosphoenolpyruvate carboxylase and NAD malate dehydrogenase, and that this malate then enters the mitochondrial matrix where it is converted to pyruvate by NAD malic enzyme.