Repression of Sex4 and Like Sex Four2 Orthologs in Potato Increases Tuber Starch Bound Phosphate With Concomitant Alterations in Starch Physical Properties

Abstract

To examine the roles of starch phosphatases in potatoes, transgenic lines were produced where orthologs of SEX4 and LIKE SEX FOUR2 (LSF2) were repressed using RNAi constructs. Although repression of either SEX4 or LSF2 inhibited leaf starch degradation, it had no effect on cold-induced sweetening in tubers. Starch amounts were unchanged in the tubers, but the amount of phosphate bound to the starch was significantly increased in all the lines, with phosphate bound at the C6 position of the glucosyl units increased in lines repressed in StSEX4 and in the C3 position in lines repressed in StLSF2 expression. This was accompanied by a reduction in starch granule size and an alteration in the constituent glucan chain lengths within the starch molecule, although no obvious alteration in granule morphology was observed. Starch from the transgenic lines contained fewer chains with a degree of polymerization (DP) of less than 17 and more with a DP between 17 and 38. There were also changes in the physical properties of the starches. Rapid viscoanalysis demonstrated that both the holding strength and the final viscosity of the high phosphate starches were increased indicating that the starches have increased swelling power due to an enhanced capacity for hydration.

Modification of Cassava Root Starch Phosphorylation Enhances Starch Functional Properties

“…In addition, this work shows that targeting genes discovered and studied in Arabidopsis leaves it is possible to change the relative abundance of phosphate at the C3 and C6 positions of the glucosyl residues of cassava starch amylopectin. This, together with another very recent study of the role of LSF2 in potato tubers ( Samodien et al, 2018 ), demonstrates a conserved function between species and plant tissues.…”

“…In that system, the increase in C3:C6 ratio in the lsf2 mutant was greater than was observed here, and it was accompanied by an increase in the total starch bound phosphate. In potato tubers, silencing LSF2 yielded similar findings, with both an increase in C3:C6 ratio and a slight increase in total starch-bound phosphate ( Samodien et al, 2018 ). It seem likely that the magnitude of the differences observed in each case can be explained by whether LSF2 expression is repressed (i.e., in cassava and potato) or completely abolished (i.e., in Arabidopsis).…”

“…Nevertheless, the phosphate levels in the storage root starch were slightly increased, although the strength of this effect was somewhat dependent on the year of harvest (Figure 3A ). Repression of SEX4 in potato also led to an increase in the phosphate content of the starch ( Samodien et al, 2018 ). This is interesting because Arabidopsis sex4 mutants do not have increased amount of bound phosphate per gram of leaf starch.…”

“…This altered starch physico-chemical properties such as gelatinisation enthalpy and also changed features such as starch granule morphology and amylose content ( Carciofi et al, 2011 ; Chen et al, 2017 ). Similarly, repression of SEX4 or LSF2 in potato altered amylopectin chain-length distribution and starch granule size in addition to altering phosphate content ( Samodien et al, 2018 ).…”

Mutations in Glucan, Water Dikinase Affect Starch Degradation and Gametophore Development in the Moss Physcomitrella patens

“…Degradation by these enzymes releases soluble phosphorylated malto-oligosaccharides into the stroma. The phosphate from these is removed by two phosphoglucan phosphatases 33–35 , before they can be further degraded to maltose and glucose through the actions of ∝-, β- and isoamylases 29–31 alongside disproportionating enzyme 1 36–38 . Maltose and glucose are exported from the plastid into the cytosol by two transporters 39–41 where the maltose is further mobilized by disproportionating enzyme 2 37,38,42,43 .…”