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Abstract
Phosphate esters are responsible for valuable and unique functionalities of starch for industrial applications. Also in the cell phosphate esters play a role in starch metabolism, which so far has not been well characterized in storage starch. Laforin, a human enzyme composed of a carbohydrate-binding module and a dual-specificity phosphatase domain, is involved in the dephosphorylation of glycogen. To modify phosphate content and better understand starch (de)phosphorylation in storage starch, laforin was engineered and introduced into potato (cultivar Kardal). Interestingly, expression of an (engineered) laforin in potato resulted in significantly higher phosphate content of starch, and this result was confirmed in amylose-free potato genetic background (amf). Modified starches exhibited altered granule morphology and size compared to the control. About 20–30% of the transgenic lines of each series showed red-staining granules upon incubation with iodine, and contained higher phosphate content than the blue-stained starch granules. Moreover, low amylose content and altered gelatinization properties were observed in these red-stained starches. Principle component and correlation analysis disclosed a complex correlation between starch composition and starch physico-chemical properties. Ultimately, the expression level of endogenous genes involved in starch metabolism was analysed, revealing a compensatory response to the decrease of phosphate content in potato starch. This study provides a new perspective for engineering starch phosphate content in planta by making use of the compensatory mechanism in the plant itself.
Highlights
Starch is the predominant storage carbohydrate in higher plants and is a semi-crystalline composite substrate consisting of two biopolymers, amylose and amylopectin
Contrary to the expectation, the phosphate content increased substantially in starches from the transgenic lines compared with that of the control plants regardless of the construct and genetic background (Kardal and amf). This finding seems to conflict at first glance with the results of Worby, Gentry [32], who found that laforin dephosphorylates potato amylopectin in vitro
The environment in potato amyloplasts is much more intricate than that in the in vitro system, and one important difference is the presence of other endogenous genes, which may be an obstacle to detecting the individual effect of laforin expression on phosphate content in potato
Summary
Starch is the predominant storage carbohydrate in higher plants and is a semi-crystalline composite substrate consisting of two biopolymers, amylose and amylopectin. Studies in Arabidopsis have indicated that phosphoglucan phosphatase starch excess 4 (SEX4) cooperates with like-SEX4 1 and 2 (LSF1 and LSF2) proteins to remove phosphate groups [12,13,14]. This reversible phosphorylation is essential for starch degradation in leaves. It is not known whether the dephosphorylation occurs in storage starches, and in case it does the mechanism of phosphate removal is unknown
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