Plastidial Phosphorylase (Pho1a) is the dominant glucosyltransferase regulating starch granule initiation in potato tubers

Starch granule initiation in potato tubers is initiated by the extension of maltooligosaccharide primers, primarily through the action of glucosyltransferases. While STARCH SYNTHASE 4 (SS4) is crucial in most plants, the plastidial PHOSPHORYLASE 1 (Pho1) also plays a significant role in certain species like rice and wheat. In Arabidopsis, an enzyme-inactive homolog of SS4, STARCH SYNTHASE 5 (SS5), assists in starch granule initiation.

Researchers utilized CRISPR/Cas9 to create knockout mutants of SS4, SS5, and Pho1a in the Clearwater Russet potato variety to determine their impact on granule initiation. They found that tubers lacking SS4 and SS5 had typical starch granule size and shape, indicating that SS4 and SS5 are not essential for normal granule initiation in potatoes.

However, tubers deficient in Pho1a exhibited compound starch granules resulting from multiple initiations, smaller granules, and irregular shapes. Protein interaction studies suggested a possible collaboration between Pho1a and LIKE EARLY STARVATION (LESV). When expressed independently, Pho1a was found in the chloroplast stroma, but when combined with LESV, both proteins congregated on starch granules.

This co-localization, along with the smaller granule formation observed when LESV was knocked out, implies a potential functional partnership in potato tubers. These results conclude that Pho1a is the primary glucosyltransferase responsible for starch granule initiation in Clearwater Russet tubers, working in tandem with LESV.