Peanut cotyledonary cells were observed with transmission and scanning electron microscopy during starch granule formation. Developing starch granules were primarily electron translucent, and both simple and compound granules were observed. Fully developed starch granules were observed during both early and late stages of seed development. Major findings include observation of electron dense materials within the stroma of developing starch granules that may be ultrastructural evidence of the location of enzymes involved in the synthesis of starch.
The role of starch granules and their development in peanut cotyledonary cells are poorly understood.
Ten randomly selected peanut plants (cv. NC 7) were harvested from the Central Crops Research Station, Clayton, North Carolina. Peanut pods were opened, and twenty seeds were classified into early and late developmental stages according to criteria modified from
Testa was carefully removed from seeds with a razor blade. The individual seeds were separated into the two cotyledons that comprise each seed. Tissue blocks (1 mm3) of mid-region cotyledon were cut with a razor blade and fixed in Karnovsky's fixative (
Dehydrated tissue for scanning electron microscopy was critical point dried in a Tousimis unit (Ladd Research, Williston, VT) using liquid carbon dioxide. Dried sections were mounted on aluminium specimen stubs with double-sided tape and silver conducting paint. Tissue blocks on stubs were coated with 30 nm of gold-palladium alloy with a Hummer V sputter-coater fitted with a Technics digital thickness monitor. Specimens were viewed with a Phillips 505T SEM at a working distance of 15 mm and an accelerating voltage of 15 kV.
Dehydrated tissue for transmission electron microscopy was also embedded in Spurr's resin for long pot-life (
Peanut cotyledonary cell starch granules were observed to originate and grow in amyloplasts (
Due to the similar spherical nature of protein bodies and simple starch granules, SEM was capable of distinguishing only the compound starch granules (
The way in which the structure of starch granules is determined remains one of the least understood aspects of starch synthesis in storage organs such as peanut seed. It is generally accepted that granule growth occurs in a zone at the surface of the granule and that the synthesis in this zone of two different kinds of polymers, namely amylose and amylopectin, is a function of the existence of multiple isoforms of starch synthetase and starch-branching enzyme with different properties and spatial locations (
Electron dense material within the stroma of the developing starch granules in peanut cotyledonary cells was observed and the spatial location of this electron dense material may be ultrastructural evidence of the location of the enzymes involved in the synthesis of starch within the peanut cotyledonary cell amyloplast. Future prospects for progress for understanding the regulation of starch synthesis in peanut cotyledonary cells should include not only the study of enzymes of starch synthesis but also these enzymes' relationship to the chemistry of starch polymers and to the ultrastructure of starch granules.
We are grateful to Ms. Valerie M. Knowlton of the North Carolina State University Electron Microscopy Center for assistance with transmission electron microscopy. The research reported in this publication was a cooperative effort of the Agric. Res. Ser. of the U.S. Dept. of Agric. and the NC Agric. Res. Ser., Raleigh, NC 27695-7643. The use of trade names in this publication does not imply endorsement by the United States Department of Agriculture or the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned.
Professor (deceased), Dept. of Food Science, Box 7624; Res. Chemist, U.S. Dept. of Agric. Res. Serv., Dept. of Botany, Box 7625, Microscopist, Dept. of Food Science, Box 7624, Supv. Plant Physiologist, U.S. Dept. of Agric. Res. Serv., Dept of Food Science, Box 7624, North Carolina State Univ., Raleigh, NC 27695