Mitochondrial events during development of thermogenesis inSauromatum guttatum (Schott)

Abstract

Changes in the mitochondrial electrontransport chain were followed in the thermogenic inflorescence ofSauromatum guttatum Schott from 5d before thermogenesis to 3d thereafter. The capacities of the alternative and cytochrome pathways of mitochondrial electron transport were found to be developmentally coordinated to contribute to the thermogenic events in the appendix and the sterile floral regions. Electron flow through the alternative pathway, is believed primarily responsible for heat production, and this pathway was expressed to the highest degree in both tissues during thermogenesis. In the appendix, the cytochrome chain was shut down considerably during thermogenesis, forcing electron flow through the alternative pathway and thus yielding maximum heat production. The shut-down of the cytochrome chain does not occur in the sterile floral region which may explain why this region is not as thermogenic as the appendix. Cytochrome-oxidase difference spectra indicated that the cytochrome oxidase of appendix mitochondria was not capable of accepting electrons on the day of thermogenesis, and that this capacity was partially restored by the following day even though the tissue was senescing at this time point. Relative levels of messenger RNAs for cytochrome-oxidase subunits I and II were found to decrease the day before thermogenesis, which could result in lower levels of these proteins in appendix mitochondria on the day of thermogenesis.

The capacity for overall mitochondrial protein synthesis was also investigated and was found to drop continuously from 5d before thermogenesis to 3d thereafter, even though the capacities of the electron-transport chain were changing dramatically. The levels of mitochondrial ribosomal RNA levels decreased during development, which could explain the overall drop in mitochondrial translational efficiency. Experiments concerning the synthesis of the alternative-oxidase proteins indicated that they were most likely nuclearly encoded, and that their expression could be induced by salicylic acid.