A fresh starch: Creating new starch granule morphologies in potato tuber

Starch is the major storage carbohydrate in plants and is organised into semicrystalline granules. The size, shape, and composition of these granules greatly affects how they are digested, and which industrial applications they are suitable for. Therefore, the enzymes controlling starch synthesis are incredibly valuable targets for breeders, despite being poorly understood outside of the model organism Arabidopsis. Hochmuth et al.  investigated the role of two of these proteins in potato tubers: PROTEIN TARGETING TO STARCH2b (PTST2b) and MYOSIN RESEMBLING CHLOROPLAST PROTEIN (MRC). Tubers deficient in the newly-identified PTST2b paralogue due to silencing (referred to as siPTST2b plants) had granules that were the same size as the control, but were starkly spherical, rather than the wild-type ellipsoid shape. Notably, PTST2b did not interact with STARCH SYNTHASE 4, which is key to its function in Arabidopsis, suggesting these proteins may play different roles in tubers. MRC expression was high in the leaves but undetectable in tubers, and therefore its role was investigated using overexpression (MRC OE) lines. While potato starch granules typically are built from a single initiation point, the MRC OE lines had multiple initiation points per granule, and a smaller granule size than wild type tubers. Together, these potato mutants demonstrate the importance of PTST2b and MRC in starch granule morphology, and highlight the mechanistic differences in starch synthesis across different species and tissues. This work also generated two unique starch phenotypes which may be useful in the food industry. (Summary by Ciara O’Brien) Plant Biotechnol. J. 10.1111/pbi.14505