Starch is the plant equivalent of glycogen. It consist of long branched chains of glucose residues. Starch is often stored in roots and tubers but it is also deposited in seeds where it can be mobilized following germination.
Mendel didn't know it at the time but he was dealing with the genetics of starch synthesis while crossing peas in his garden.
One of the genetic traits that Gregor Mendel studied was round ( R) vs. wrinkled ( r) peas. The wrinkled pea phenotype is caused by a defect in the gene for starch branching enzyme. Starch synthesis is very similar to glycogen synthesis. The main polymerization enzyme is starch synthase and it works just like glycogen synthase [Glycogen Synthesis]. Both starch and glycogen synthesis require an additonal enzyme to create new branches. In plants this enzyme is starch branching enzyme.
In the absence of this enzyme, starch synthesis is partially blocked and the developing peas have a higher concentration of sucrose. (In plants, glucose is transported as the disacharide sucrose.) This causes the mutant peas to absorb more water than normal peas and they swell to a larger size. When the seeds begin to dry out the peas with the defective enzyme lose more water and their outer surface takes on a wrinkled appearance. One copy of the active gene for starch branching enzyme is sufficient so the presence of one defective allele has no observable phenotype. When two mutant alleles are present the wrinkled phenotype is expressed because there's no active starch branching enzyme. The mutant allele is recessive to the wild type allele.
For some strange reason Mendel is more famous for discovering this simple genetic rule than for his contributions to understanding starch metabolism.
Bhattacharyya, M. K., Smith, A. M., Ellis, T. H., Hedley, C., and Martin, C. (1990) The wrinkled-seed character of a pea described by Mendel is caused by a transposon-like insertion in a gene encoding starch-branching enzyme. Cell 60:115-122.
[©Laurence A. Moran. Some of the text is from Principles of Biochemistry 4th ed. ©Pearson/Prentice Hall]