Ultrastructure, origin, and composition of the protein bodies in the ligule of Isoetes lacustris L.

The basal cells in the ligule of Isoetes lacustris contain numerous protein bodies, the contents of which can be digested enzymically by pronase and are stained red by treatment with ninhydrin Schiff's reagent. Two types of protein bodies can be distinguished ultrastructurally: spherically-shaped bodies with granular contents and spindle-like bodies with fibrillar contents. Both are ensheathed by single membranes and do not show any solid inclusions within their matrix.The protein bodies probably arise from dilatation of the endoplasmic reticulum (ER) cisternae. This conclusion is based upon three observations: (a) The protein bodies occasionally show membrane continuity with the ER; (b) ribosomes and polysomes are frequently attached to the protein-body membranes; (c) the contents of the protein bodies and of the dilated ER cisternae show similar ultrastructural features.The dilatation of the ER cisternae is assumed to be a result of protein accumulation in the intracisternal space.Based upon the results of polyacrylamide gel electrophoresis, it is likely that the spherically-shaped protein bodies store predominately two proteins with molecular weights of 51300 and 55800 D, while the spindle-like bodies store two proteins with molecular weights of 92000 and 98000 D.The results presented do not permit a definite conclusion regarding the function of the ligule of Isoetes lacustris but it is suggested that it may have a nutritive role. © 1981 Annals of Botany Company.