Multiple paths of sugar‐sensing and a sugar/oxygen overlap for genes of sucrose and ethanol metabolism

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Abstract
The two‐fold purpose of this work is, first, to review current hypotheses for multiple paths of sugar‐sensing in an oxygen‐responsive context, and second, to present evidence for the extent of sugar/oxygen overlap regulating genes for sucrose and ethanol metabolism. Current data indicate that sugar signals in plants may be initiated by (a) hexokinases, (b) membrane sensors, (c) acetate and/or respiratory metabolites, and (d) other signals and/or crosstalk. Responses may also involve concurrent input along transduction paths by effectors such as energy charge, P status, and phytohormones. Prime candidates for initiation and/or integration of such signal integration include SNF1‐ and SCF‐like, multi‐enzyme complexes. In addition, different paths of sugar signal transduction may be linked to contrasting roles of responsive genes during feast, famine or pathogen attack. Oxygen can potentially alter sugar signals at several points, so its influence on feast and famine responses was initially tested with genes for sucrose metabolism in maize root tips. The Sus1and Sh1sucrose synthases in maize (typically up‐regulated by carbohydrate abundance and deprivation, respectively) showed parallel responses to hypoxia (3% O2 [0.03l l−1 O2]) and anoxia (0% O2 [0l l−1 O2]) that were consistent with involvement of similar signals. In contrast, the differential sugar‐responses of the Ivr1and Ivr2invertases were not evident under low oxygen, and both genes were rapidly repressed. A third response was evident in the marked, sugar‐regulation of an oxygen‐responsive Adh1gene for alcohol dehydrogenase, which was sensitive to sugar availability from deficit to abundance, regardless of oxygen status (anaerobic to fully aerobic [40% O2 (0.04l l−1 O2)]. A clear interface is thus evident between sugar and oxygen signals, but this varies markedly with the genes involved and probable differences in respective transduction paths.