Expression of sporophytic storage proteins in the corm of the quillwort (Isoetes echinospora Dur.)

Parenchyma cells from the corm tissue of the aquatic lycopod Isoetes echinospora Dur. were shown by electron microscopy to be packed with amyloplasts, lipid bodies, and protein bodies. The protein bodies are morphologically similar to those identified in seeds and certain vegetative tissues of higher plants. Globoid-containing protein bodies (1-10 micrometers) isolated in a sucrose gradient possessed a buoyant density of 1.28 g/mL and contained globulin (salt-soluble) proteins. Sucrose gradient centrifugation of crude globulins revealed only two components with mean sedimentation coefficients of approximately 2S and 11S. The 2S component, designated VSP-IsA, was composed of a 15.7-kD polypeptide. The 11S component, designated VSP-IsB, had a molecular mass of 215 kD as estimated by gel filtration and was composed of 39- to 42-kD polypeptides. Two-dimensional gel electrophoresis showed constituent polypeptides distinguished by differences in net charge and molecular mass. Affinity-purified antibodies against VSP-IsA and VSP-IsB prepared and used as probes on immunoblots crossreact only with their specific antigens, suggesting that the proteins are not immunologically related. Indirect immunolocalization studies confirmed that VSP-IsB is deposited in protein bodies. These globulin proteins, like those from some seeds, form the principal storage reserves of the corm tissue