Starch Plays a Key Role in Sporosorus Formation by the Powdery Scab Pathogen Spongospora subterranea.

Powdery scab disease, caused by the soilborne protist Spongospora subterranea f. sp. subterranea, poses a major constraint to potato production worldwide. Disease symptoms include damage to the tuber skin and the formation of root galls. This study aimed to investigate the potential mechanism behind the formation of sporosori, which are aggregates of resting spores, within root galls. Scanning electron microscopy analysis revealed that the early stage of gall formation, characterized by a white color, involved the accumulation of starch grains, which later disappeared as the gall matured and turned brown. The mature brown galls were found to contain fully formed sporosori. Light microscopy examination of ultramicrotome sections of the root galls showed that the high-amylopectin starches were surrounded by a plasmodium, a precursor to sporosorus. These findings suggest that starch grains contribute to the formation of a sponge-like structure within the sporosori. A significant reduction in total starch levels in both the root galls and their associated roots was observed compared with healthy roots. These findings indicate starch consumption by sporosori during the maturation of root galls. Interestingly, analysis of the transcript levels of starch-related genes showed downregulation of genes encoding starch degrading enzymes and an amylopectin-debranching enzyme, whereas genes encoding a starch synthase and a protein facilitating starch synthesis were upregulated in the infected roots. Overall, our results demonstrate that starch is consumed during sporosorus formation, and the pathogen likely manipulates starch homeostasis to its advantage for sporosorus development within the root galls.