Evidence of the crucial role of sucrose synthase for sink strength using transgenic potato plants (Solanum tuberosum L.)

Abstract

Sink strength of growing potato tubers is believed to be limited by sucrose metabolism and/or starch synthesis. Sucrose synthase (Susy) is most likely responsible for the entire sucrose cleavage in sink tubers, rather than invertases. To investigate the unique role of sucrose synthase with respect to sucrose metabolism and sink strength in growing potato tubers, transgenic potato plants were created expressing Susy antisense RNA corresponding to the T-type sucrose synthase isoform. Although the constitutive 35S CaMV promoter was used to drive the expression of the antisense RNA the inhibition of Susy activity was tuber-specific, indicating that independent Susy isoforms are responsible for Susy activity in different potato organs. The inhibition of Susy leads to no change in sucrose content, a strong accumulation of reducing sugars and an inhibition of starch accumulation in developing potato tubers. The increase in hexoses is paralleled by a 40-fold increase in invertase activities but no considerable changes in hexokinase activities. The reduction in starch accumulation is not due to an inhibition of the major starch biosynthetic enzymes. The changes in carbohydrate accumulation are accompanied by a decrease in total tuber dry weight and a reduction of soluble tuber proteins. The reduced protein accumulation is mainly due to a decrease in the major storage proteins patatin, the 22 kDa proteins and the proteinase inhibitors. The lowered accumulation of storage proteins is not a consequence of the availability of the free amino acid pool in potato tubers. Altogether these data are in agreement with the assumption that sucrose synthase is the major determinant of potato tuber sink strength. Contradictory to the hypothesis that the sink strength of growing potato tubers is inversely correlated with the tuber number per plant, no increase in tuber number per plant was found in Susy antisense plants.

Combined expression of glucokinase and invertase in potato tubers leads to a dramatic reduction in starch accumulation and a stimulation of glycolysis

“…Furthermore, since we have obtained substantive new results about the changes in respiration in these invertase lines, we subsequently confirmed that the key features of these changes also occur in other invertaseexpressing lines (see Experimental Procedures). Our values for hexokinase and acid invertase activity in wild-type tubers were comparable to those measured by other groups Zrenner et al, 1994). In the transgenic lines which expressed the glucokinase we found a doubling of activity in developing tubers and up to a fivefold increase in mature tubers, when compared with the wild-type.…”

“…It is interesting to note that the hexokinase activity was significantly higher in the line U-IN2-30 than in the wildtype (typically 50%). A similar increase was also observed in transgenic potato tubers where the activity of sucrose synthase was inhibited through antisense repression (Zrenner et al, 1994). Thus, it appears that the activity of the endogenous hexokinases can be influenced by dramatic changes in sucrose metabolism.…”

Modification of soybean sucrose synthase by S-thiolation with ENOD40 peptide A

“…The activity of this enzyme is often correlated with the ''sink strength'' of these tissues [22], which is the ability of a sink organ to attract sucrose. In legume root nodules, hydrolysis of sucrose by SuSy is a key step in nitrogen fixation and deficient SuSy activity in nodules renders them incapable of effective nitrogen fixation [3,23].…”

Starch synthesis‐, and tuber storage protein genes are differently expressed in Solanum tuberosum and in Solanum brevidens

“…Transgenic plants with antisense-RNA inhibition of SUT show a dramatic reduction in tuber yield [6]. In the absence of Susy or AGPase sugar-storing tubers with decreased amount of soluble proteins are formed [4,7].…”

“…Transgenic plants with antisense-RNA inhibition of SUT show a dramatic reduction in tuber yield [6]. In the absence of Susy or AGPase sugar-storing tubers with decreased amount of soluble proteins are formed [4,7].An alternative approach toward understanding the molecular mechanism of tuber morphogenesis might be to study the evolution of tuberization. Solanum brevidens is a close relative of potato (S. tuberosum), however, with no tuber-forming ability.…”