Poster: Ecophysiology

Abs # 12: Hexokinase Activity in Echinochloa phyllopogon: Anaerobic Induction and Subcellular Localization

Presenter:	Fox, Theodore C, tfox@uwf.edu
Authors	Fox, Theodore C (A)
Affiliations:	(A): Department of Biology, University Of West Florida

Unlike most crop plants, Echinochloa phyllopogon is highly adapted for germination in flooded environments and does so even under anaerobic conditions. At the molecular level, E. phyllopogon, synthesizes a normal complement of proteins under low oxygen conditions with only a subset of proteins being up-regulated in response to the stress, the so-called anaerobic stress proteins (ASPs). Hexokinase activity is stimulated 10 to 12 fold by anoxia and at least two isoforms are induced. Since hexokinase is the initial step in glucose utilization via glycolysis, we hypothesized that its induction may be an essential feature of anaerobic metabolism in E. phyllopogon. However, mannoheptulose, an inhibitor of hexokinase, failed to affect the germination rate under either aerobic or anaerobic conditions. A hexokinase fraction that associates with mitochondria was identified and, unlike the soluble cytoplasmic isoforms, was insensitive to mannoheptulose. Although no difference in the K_m for glucose was observed between the two hexokinase fractions, the mitochondria-associated hexokinases were insensitive to ADP concentrations between 0 and 1 mM whereas the cytoplasmic isoforms were inhibited by 80% in the presence of 1 mM ADP. One of the earliest physiological responses of plant cells to low oxygen is the reduction of cytoplasmic pH from approximately 7.4 to 6.8. The mitochondria-associated hexokinases were more responsive to changes in pH between 6.0 and 8.0 than the cytosolic hexokinases and the mitochondria-associated isoforms exhibited higher specific activities at all pHs tested. It is postulated that the mitochondria-associated hexokinase is an important feature of the anaerobic response of E. phyllopogon.