Characterization of storage proteins in Daucus carota L.: two novel proteins display zygotic embryo specificity

Although maturation-related proteins are well known in the endosperm of albuminous seeds, an important question is whether the zygotic embryo possesses its own maturation proteins. We report on the isolation and partial characterization of storage proteins of carrot (Daucus carota L. var Nandor) dry achenes and isolated zygotic embryos, using one- and two-dimensional electrophoresis techniques, HPLC and amino acid sequencing. The presence of a series of abundant polypeptides showing charge heterogeneity, that are rapidly degraded upon germination, was revealed in the endosperm. These proteins consisted of glycoproteins, the most abundant of which displayed a molecular mass (M(r)) of 58,000, albumins of M(r) 42,000 comprising at least one beta-1,3-glucanase, and two globulins of M(r) 90,000 and 50,000-55,000 respectively, the second being an oligomer composed of three subunits of M(r) 13,000, 20,000 and 30,000. None of these storage proteins identified in the endosperm were detected in zygotic embryos. In contrast, two novel proteins were isolated from zygotic embryos, namely a globulin family of M(r) 50,000 and pI 6.3-6.8, which was named "daucin", and a late embryogenesis abundant (LEA) protein family of M(r) 25,000 and pI 6.3-6.6, named "RAB25". Since the latter proteins are apparently absent of the endosperm, these results suggest that the maturation of carrot zygotic embryos requires its own specific set of storage and LEA proteins.