Energy and Fermentation Metabolism in Hypoxic Rice Coleoptiles — A Multinuclear NMR Approach

Abstract

In vivo ³¹P NMR was employed to follow changes in intracellular pH, concentrations of phosphate metabolites, and the unidirectional rate of ATP synthesis in rice shoot tips during hypoxia. Under extended hypoxia (16–22 hours), rice shoot tips exhibited a small and transient decrease in cytoplasmic pH, while the ATP concentration and unidirectional rate constant of ATP synthesis were maintained at 90 and 75% of the control level, respectively. This is in contrast to actively growing flood-sensitivet tissues such as maize root tips which showed a two-fold decrease in ATP concentration and a ten-fold decline in the rate of ATP synthesis. In addition, carbon flow through biochemical pathways including fermentation was followed using ¹³C-enriched acetate and bicarbonate precursors, and ¹H NMR, ¹³C NMR and GC-MS techniques. Ethanol, alanine, succinate, γ-aminobutyrate and lactate were major fermentation products that accumulated in hypoxic rice shoot tips; succinate and γ-aminobutyrate (via glutamate) were major carbon sinks for acetate metabolism through the Krebs cycle. Moreover, both the wet weight and length of rice shoot tips increased comparably under control and hypoxic conditions. These results indicated that hypoxic rice shoot tips possessed active fermentation metabolism and continued to operate the Krebs cycle, both of which contributed to maintenance of ATP synthesis, reduced cytoplasmic acidification, and ultimately flood tolerance. However, the lack of unique fermentation products in rice shoot tips suggested that a quantitative instead of a qualitative difference in fermentation metabolism may account for this tolerance. In addition, rice shoot tips may utilise other oxidative pathways such as nitrate respiration to couple with the Krebs cycle to maintain their high ATP turnover under hypoxia.