Regulation of root respiration and sugar-mediated gene expression in plants

The respiration rate of plant tissues is related to both substrate supply a nd demand for respiratory energy. At the biochemical level, it is considere d to be regulated by the supply of adenosine diphosphate. In the whole plan ts, respiration rate is correlated with carbohydrate content suggesting con trol by substrates. There is evidence to support both the hypotheses, and t hey can be reconciled if the capacity of tissues for respiration can alter within a few hours and sucrose can control that capacity. Supplying exogeno us sugars to the starved tissue not only increases the rate of respiration of that tissue but also leads to induction of enzymes which are involved in reversing the carbohydrate starvation-induced effects in plants. Expressio n of proteins and metabolic systems may be under the direct or indirect con trol of sugar. Sucrose or some product of its metabolism can control gene e xpression of these proteins, probably by increasing transcription.