Loss of PROTEIN TARGETING TO STARCH 2 has variable effects on starch synthesis across organs and species

Loss of PTST2, a protein involved in starch granule initiation, had highly variable effects between three grass species on carbohydrate metabolism and physiology of leaves and granule morphology of seeds. Recent work has identified several proteins involved in starch granule initiation, the first step of starch synthesis. However, the degree of conservation in the granule initiation process remains poorly understood, especially among grass species differing in patterns of carbohydrate turnover in leaves, and granule morphology in the endosperm. We therefore compared mutant phenotypes of Hordeum vulgare (barley), Triticum turgidum (durum wheat), and Brachypodium distachyon defective in PROTEIN TARGETING TO STARCH 2 (PTST2), a key granule initiation protein. We report striking differences across species and organs. Loss of PTST2 from leaves resulted in fewer, larger starch granules per chloroplast and normal starch content in wheat, fewer granules per chloroplast and lower starch content in barley, and almost complete loss of starch in Brachypodium. The loss of starch in Brachypodium leaves was accompanied by high levels of ADP-glucose and detrimental effects on growth and physiology. Additionally, we found that loss of PTST2 increased granule initiation in Brachypodium amyloplasts, resulting in abnormal compound granule formation throughout the seed. These findings suggest that the importance of PTST2 varies greatly with the genetic and developmental background and inform the extent to which the gene can be targeted to improve starch in crops.

Automated summary

Loss of PTST2 protein has variable effects on carbohydrate metabolism, physiology, and granule morphology in different grass species. Loss of PTST2 results in fewer and larger starch granules in wheat, fewer granules and lower starch content in barley, and almost complete loss of starch in Brachypodium. The loss of starch in Brachypodium leaves is accompanied by high levels of ADP-glucose and detrimental effects on growth and physiology. Loss of PTST2 also leads to abnormal granule formation in Brachypodium seeds.