Decreased expression of sucrose phosphate synthase strongly inhibits the water stress-induced synthesis of sucrose in growing potato tubers

Water stress stimulates sucrose synthesis and inhibits starch synthesis in wild-type tubers. Antisense and cosuppression potato transformants with dec reased expression of sucrose-phosphate synthase (SPS) have been used to ana lyse the importance of SPS for the regulation of this water-stress induced change in partitioning. (i) In the absence of water stress, a 70-80% decrea se in SPS activity led to a 30-50% inhibition of sucrose synthesis and a sl ight (10-20%) increase of starch synthesis in tuber discs in short-term lab elling experiments with low concentrations of labelled glucose. Similar cha nges were seen in short-term labelling experiments with intact tubers attac hed to well-watered plants. Provided plants were grown with ample light and water, transformant tubers had a slightly lower water and sucrose content and a similar or even marginally higher starch content than wild-type tuber s. (ii) When wild type tuber slices were incubated with labelled glucose in the presence of mannitol to generate a moderate water deficit (between -0. 12 and -0.72 MPa), there was a marked stimulation of sucrose synthesis and inhibition of starch synthesis. A similar stimulation was seen in labelling experiments with wild-type tubers that were attached to water-stressed wil d-type plants. These changes were almost completely suppressed in transform ants with a 70-80% reduction of SPS activity. (iii) Decreased irrigation le d to an increase in the fraction of the dry-matter allocated to tubers in w ild-type plants. This shift in allocation was prevented in transformants wi th reduced expression of SPS. (iv) The results show that operation of SPS a nd the sucrose cycle in growing potato tubers may lead to a marginal decrea se in starch accumulation in non-stressed plants. However, SPS becomes a cr ucial factor in water-stressed plants because it is required for adaptive c hanges in tuber metabolism and whole plant allocation.