Development of Arabidopsis thaliana leaves at low temperatures releases the suppression of photosynthesis and photosynthetic gene expression despite the accumulation of soluble carbohydrates.

Arabidopsis thaliana plants were grown at 23 degrees C and changes in carbohydrate metabolism, photosynthesis and photosynthetic gene expression were studied after the plants were shifted to 5 degrees C. The responses of leaves shifted to 5 degrees C after development at 23 degrees C are compared to leaves that developed at 5 degrees C. Shifting warm developed leaves to 5 degrees C lead to a severe suppression of photosynthesis that correlated with a rapid and sustained accumulation of hexose phosphates and soluble sugars. Associated with the suppression of photosynthesis and the accumulation of soluble sugars was a reduction in the amount of transcript for genes encoding photosynthetic proteins (cab and rbcS). In contrast, leaves that developed at 5 degrees C showed an increase in photosynthesis and control levels of photosynthetic gene expression. This recovery occurred even though leaves that developed at 5 degrees C maintained large pools of soluble sugars. Leaves that developed at 5 degrees C also showed a strong upregulation of the cytosolic pathway for soluble sugar synthesis but not of the chloroplastic pathway for starch synthesis. This was shown at the level of both enzyme activity and the amount of transcript. Thus, development of Arabidopsis leaves at 5 degrees C resulted in metabolic changes that enabled them to produce and accumulate large soluble sugar pools without any associated suppression of photosynthesis or photosynthetic gene expression. These changes were also associated with enhanced freezing tolerance. We suggest that this reprogramming of carbohydrate metabolism associated with development at low temperature is essential to the development of full freezing tolerance and for winter survival of over-wintering herbaceous annuals.