POTATO PLANTS CONTAIN MULTIPLE FORMS OF SUCROSE-PHOSPHATE SYNTHASE, WHICH DIFFER IN THEIR TISSUE DISTRIBUTIONS, THEIR LEVELS DURING DEVELOPMENT, AND THEIR RESPONSES TO LOW-TEMPERATURE

Antibodies raised against a peptide fragment (residues 60-456) of pota to sucrose phosphate synthase (SPS) were used to investigate whether p otato plants contain multiple forms of SPS. When a partially purified preparation of SPS from cold-stored potato tubers was separated on 5% polyacrylamide gel electrophoresis (PAGE), four immunopositive bands w ere found with estimated molecular weights of 125, 127, 135 and 145 kD a. These bands were also found in rapidly prepared extracts and were t ermed SPS-1a, SPS-1b, SPS-2 and SPS-3, respectively. Direct evidence t hat SPS-1a and SPS-1b represent active SPS was provided by the finding that both are greatly reduced in plants expressing an antisense seque nce derived from the potato leaf SPS gene. SPS-2 was not decreased in the antisense plants, indicating that it has a significantly different sequence. Evidence that SPS-2 represents active SPS was obtained by s howing that the amount of SPS-1a and SPS-1b protein remaining in the l eaves and tubers of antisense potato plants was too low to account for the remaining SPS activity. The four immunopositive SPS forms had dif ferent tissue distributions. SPS-1a was the major form in all tissues except petals, sepals and stamens. SPS-1b and SPS-2 were absent in ver y young growing tissues but were present as minor forms in source leav es and sprouting tubers. The SPS-1b level was especially high in petal s and sepals, and the SPS-2 level was especially high in the stamens. SPS-3 was only detected in very young tissues. The four forms also sho wed different responses to low temperature. Transfer of tubers to 4 de grees C led to a specific and reversible increase of SPS-1b during the next 4 d. The appearance of SPS-1b correlated with a change in the ki netic properties of SPS that has recently been shown (Hill et al. 1996 ) to play a key role in triggering the accumulation of sugars in cold- stored tubers. The appearance of SPS-1b protein at low temperature was accompanied by an increase of SPS transcript. Incubation of tuber sli ces with calyculin A and okadaic acid to alter the phosphorylation sta te of SPS did not lead to appearance or disappearance of SPS-1b. It is concluded that potato plants contain several forms of SPS that have d ifferent functions in growing and mature tissues, in flower parts, and in acclimation to low temperature.