Distinct effects of PTST2b and MRC on starch granule morphogenesis in potato tubers

The molecular mechanisms underpinning the formation of the large, ellipsoidal starch granules of potato tuber are poorly understood. Here, we demonstrate the distinct effects of PROTEIN TARGETING TO STARCH2b (PTST2b) and MYOSIN RESEMBLING CHLOROPLAST PROTEIN (MRC) on tuber starch granule morphology. A gene duplication event in the Solanaceae resulted in two PTST2 paralogs (PTST2a and PTST2b). PTST2b is expressed in potato tubers, and unlike PTST2a, it had no detectable interaction with STARCH SYNTHASE 4. MRC expression was detectable in leaves, but not in tubers. Using transgenic potato lines in the variety Clearwater Russet, we demonstrate that MRC overexpression leads to the formation of granules with aberrant shapes, many of which arise from multiple initiation points. Silencing PTST2b led to the production of striking near-spherical granules, each arising from a single, central initiation point. Contrary to all reported PTST2 mutants in other species, we observed no change in the number of granules per cell in these lines, suggesting PTST2b is specifically involved in the control of starch granule shape. Starch content and tuber yield per plant were not affected by PTST2b silencing, but MRC overexpression led to strong decreases in both parameters. Notably, the spherical granules in PTST2b silencing lines had a distinctively altered pasting profile, with higher peak and final viscosity than the wild type. Thus, PTST2b and MRC are promising target genes for altering starch granule size and shape in potato tubers, and can be used to create novel starches with altered physicochemical and/or functional properties.