Impaired reductive activation of stromal bisphosphatases in tomato leaves following low-temperature exposure at high light.

Photosynthesis in domestic tomato (Lycopersicon esculentum L.) is highly sensitive to low temperature, particularly when accompanied by high light. Since previous studies have established that the inhibited plants retain photosynthetic electron transfer and ATP formation competence, we sought to identify specific steps in the photosynthetic carbon reduction pathway that could account for the lost photosynthetic capacity. Measurements of steady-state photosynthetic metabolite pool sizes showed an accumulation of fructose 1,6-bisphosphate and sedohepulose 1,7-bisphosphate following chilling in the light. Measurements of in vivo turnover rates of the metabolite pools accompanied by direct determinations of enzymatic activity showed that the capacity of the stromal bisphosphatases was substantially reduced following chilling in the light and was the cause of the bisphosphate accumulation. The time course of the loss of phosphatase activity closely mimicked that of the inhibition of net photosynthesis, further indicating that impaired phosphatase function is the underlying cause of the sensitivity of photosynthesis in tomato to light and chilling. Fructose 1,6-bisphosphatase extracted from inhibited tomato plants could be fully activated in the presence of dithiothreitol, indicating that chilling in the light disrupts the normal, thioredoxin-dependent, activation pathway of the stromal bisphosphatases. This disruption could involve a change in the redox potential of the functional disulfide on the phosphatases.