Fig. 6.

Possible role of pyruvate orthophosphate dikinase (PPDK) in rice coleoptiles in adaptation to anoxia. PP_i could be produced or consumed by PPDK or pyrophosphate:fructose 6-phosphate 1-phosphotransferase (PFP). Two scenarios are possible; either PP_i produced during polymer synthesis exceeded PP_i required, or vice versa. Possible roles of PPDK and PFP during these two scenarios are shown. Scenario 1. PP_i produced > PP_i required. PP_i produced during cell wall synthesis and protein synthesis allows a large glycolytic flux and economizes on energy spent during cell wall synthesis and amino acid activation. Scenario 2. PP_i produced < PP_i required, then the first possibility, scenario 2·1, is that both PPDK and PFP synthesize PP_i for (a) sucrose conversion starting with sucrose synthase (SS) via UDP glucose pyrophosphorylase and for (b) tonoplast energization (via H⁺-PP_iase). However, phosphofructokinase (PFK) would then have to catalyse the combination of the glycolytic flux and the flux of the substrate cycle flowing from PFK via F-1,6-bisP back to PFK, and in rice coleoptiles this may be well be above the capacity of PFK (see text and Noguchi, 2002). Hence we favor scenario 2·2: PP_i would be synthesized by PPDK in a substrate cycle with pyruvate kinase (PK), while some of the PP_i produced is used by PFP, allowing a large glycolytic flux to produce additional ATP for PP_i synthesis in the PPDK substrate cycle.