VISUALIZATION OF STARCH GRAINS: A RAPID OBSERVATION METHOD TO ISOLATE MUTANTS WITH DEFECTS IN STARCH GRAIN MORPHOLOGY

Abstract

Starch is the most important carbohydrate for the human energy source contained in staple cereals and tubers and is also used for food additives such as thickeners and stabilizer. Starch consists of a large number of glucose units joined by glycosidic bonds  and synthesized to form starch grains (SGs) inside plant cells. Despite the simple glucose polymer composition of starch, SGs exhibit various morphologies depending on plant species. The morphological variation of SGs supports the wide range of applications of starch, however the underlying molecular mechanisms have not yet been determined. We developed an effective method for preparing thin sections of seed endosperms without chemical fixation and conventional resin embedding that clearly visualize subcellular starch grains. We applied this method to genetic screening to  isolate rice mutants in which starch grains were morphologically altered. In five mutants named ssg (substandard starch grain), increased numbers of small starch grains (ssg1  to  ssg3), enlarged starch grains (ssg4), and abnormal interior structures of starch grains (ssg5) were observed. The observation method is also applicable to other cereal grains, such as barley, wheat and maize. This method will also serve as a useful technique to  study the molecular diversity of starch grain morphologies and to monitor the cereal grain qualities.