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.

Somatic and zygotic embryos of Daucus carota L. display different protein patterns until conversion to plants.

Total protein patterns of different developmental stages of carrot zygotic and somatic embryos revealed by one- and two-dimensional gel electrophoresis were compared using statistical dissimilarity index matrix, and some major polypeptides were partially sequenced. In spite of similar morphology, the protein patterns of somatic embryos at the torpedo stage were clearly different from those of zygotic embryos. In particular, none of the proteins specific of zygotic embryos required for maturation, previously identified, were accumulated in somatic embryos, namely the daucin (a globulin-type storage protein), the RAB25 protein (a late embryogenesis abundant protein) (Dodeman et al. 1998), as well as a novel globulin of M(r) 30,000, that we proposed to name apiacin. Somatic plantlets and seedlings also showed different patterns. This discrepancy likely reflects culture conditions, since somatic embryos recover a protein pattern close to that of seedlings after conversion to plant and growth on a carbon-free medium.

Isozyme patterns in zygotic and somatic embryogenesis of carrot.

Isozyme patterns of carrot (Daucus carota L.) zygotic embryos between the torpedo stage up to 5-day-old seedlings have been compared with those of the similar stages from the embryogenic cell suspension culture to the late somatic plantlet. Somatic embryos blocked at the torpedo stage by ß-cyclodextrine have also been analyzed. All these stages have been analyzed with respect to seven different enzyme systems: arylesterase, glucosephosphate isomerase, phosphogluconate dehydrogenase, alcohol dehydrogenase, isocitrate dehydrogenase, aspartate aminotransferase and phosphoglucomutase (EC 2.7.5.1, PGM). The relationships between the different stages of both types of embryogenesis have been visualized using an unrooted tree. Generally, profiles of somatic embryos were different from those of zygotic embryos. Interestingly however, a typical zygotic embryo pattern was found in the cyclodextrine-blocked somatic embryos. Only aspartate aminotransferase patterns revealed a similarity between zygotic and somatic torpedo embryos. Both plantlet types showed close patterns with common isozymes. Moreover, similarities were evident between somatic plantlets and cell suspensions. A few isozymes appeared to be stage specific markers: esterase 10-11 were specific to achenes and early germination, phosphogluconate dehydrogenase 8 was specific to 4-5 day-old seedlings and phosphoglucomutase 1 and 7 and alcohol dehydrogenase 4 were markers for zygotic embryos. No somatic embryogenesis specific isozyme could be found. We show that patterns can be associated with particular tissue formation: mainly, aspartate aminotransferase 2 and 1, phosphoglucomutase 8 and 9 and phosphogluconate dehydrogenase 7 coincided with apical meristem initiation and phosphoglucomutase 4 and 5, zones "b" and "d" of esterase and zone "b" of phosphogluconate dehydrogenase coincided with vascular bundle formation.