Previous experiments have shown that carbohydrate partitioning in leaves of potato (Solanum tuberosum L.) plants can be modified by antisense repression of the triose phosphate translocator (TPT), favoring starch accumulation during the light period, or by leafspecific antisense repression of ADP-glucose pyrophosphorylase (ACPase), reducing leaf starch content. These experiments showed that starch and sucrose synthesis can partially replace each other.To determine how leaf metabolism acclimates to an inhibition of both pathways, transgenic potato (S. tuberosum 1. cv Désirée) plants, with a 30% reduction of the TPT achieved by antisense repression, were transformed with an antisense cDNA of the small subunit of ACPase, driven by the leaf-specific ST-LS1 promoter.These double-transformed plants were analyzed with respect to their carbohydrate metabolism, and starch accumulation was reduced in all lines of these plants. In one line with a 50% reduction of AGPase activity, the rate of CO, assimilation was unaltered. In these plants the stromal level of triose phosphate was increased, enabling a high rate of triose phosphate export in spite of the reduction of the TPT protein by antisense repression. In a second line with a 95% reduction of ACPase activity, the amount of chlorophyll was significantly reduced as a consequence of the lowered triose phosphate utilization capacity.

When spinach (Spinacia oleracea L.) leaf disks were incubated in 10% polyethylene glycol to induce water stress, the ratio of glucose-1-phosphate to glucose-6-phosphate increased. This increase indicated an imbalance in the phosphoglucomutase (EC 2.7.5.1) reaction, which was earlier observed to be close to equilibrium, and was accompanied by higher fructose-1,6-bisphosphate and ribulose-1,5-bisphosphate concentrations. Because starch degradation was assumed to be the source of the glucose-1-phosphate accumulation, the kinetic properties of plastidic phosphoglucomutase were analysed. It was found that physiological concentrations of both sugar bisphosphates inhibited phosphoglucomutase by about 50%. From this observation it was concluded that under conditions in which fructose-1,6-bisphosphate and ribulose-1,5-bisphosphate accumulated, an inhibition of phosphoglucomutase activity restricted the carbon exchange between the Calvin cycle and starch turnover.

A survey is given of possible solutions and open-ended questions in the biotransformation of steroids (without side chain degradation of sterols) by using immobilized cells. The data of literature between 1975 and 1990 and results of preliminary experiments from the microbial, biochemical, biophysical, physiological, as well as the biotechnological point of view are summarized and discussed.

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