A comparison of endosperm and embryo proteins of the palm Washingtonia filifera

Comparative studies on the protein composition of the endosperm and embryo of Washingtonia filifera (Lindl.) Wendl. were conducted. On a fresh weight basis, the endosperm contained lower amounts of protein than did the embryo. This correlated well with lower volumes of protein bodies and cytoplasm in the endosperm than in the embryo. Endosperm proteins were derived mainly from the protein bodies because the volume of organelle-free cytoplasm was negligible, whereas the source of the proteins in the embryo may be the protein bodies, organelles, and the organelle-free cytoplasm. Proteins of the two tissues were compared using SDS-polyacrylamide, isoelectric focusing and two-dimensional gel electrophoresis. In the firstdimension analysis, proteins of the two tissues were heterogeneous in molecular mass and charge. In the firstand second-dimension gels, the majority of the protein bands were similar in the two tissues, but a few bands were specific to each tissue type. The similarities in the proteins suggest that the storage proteins are probably the same in the two tissues. The embryospecific proteins are probably derived from cytoplasm and organelles. IN PALM SEEDS stored reserves are present in the cotyledon of the small embryo and in the massive, hard endosperm. During germination, the distal part of the cotyledon enlarges to form an haustorium. The development of the haustorium occurs concomitant with the digestion of the endosperm. Structural, biochemical, and physiological relationships between the endosperm, embryo, and haustorium have been investigated in detail for the date palm Phoenix dactylifera (Sachs, 1862; Meier, 1958; Keusch, 1968; DeMason, 1985; DeMason, Sexton, and Reid, 1983; DeMason et al., 1985; Chandra Sekhar and DeMason, 1988), but little is known about these aspects of other palm seeds. In previous studies on Washingtonia, the structure and ultrastructure of the endosperm (DeMason, 1986) and embryo (DeMason, 1988) were reported. In these studies it was found that both tissues store reserves in the form of protein bodies and lipid bodies. In addition, the endosperm stores carbohydrate in the form of thickened cell walls. Stereological analyses show that the reserves are present in different amounts in the two tissues (Table 1). The ultrastructural appearance and stainability of the protein bodies in the two tissues are different. The endosperm protein bodies are uniformly densely staining I Received for publication 2 December 1986; revision accepted 28 May 1987. This study was supported by funds from NSF Grant PCM 8408574 to DAD. We thank Dr. W. W. Thomson for helpful suggestions on an early draft of the manscript. and contain large phytin crystals and protein crystalloids (DeMason, 1986), whereas those in the cotyledon parenchyma are lightly, but variably staining and have very small phytin crystals (DeMason, 1988). In addition, a third structural class of protein bodies occurs in the epithelial layer of the cotyledon. These protein bodies are very small, darkly staining, and contain few, if any, phytin crystals. Also, palm seed proteins have not yet been characterized. Seed protein electrophoretic characteristics have only been described for one other palm, Phoenix dactylifera (Chandra Sekhar and DeMason, 1988). In this study, we compare the extractable proteins of the endosperm with those of the embryo and address the following questions: 1) How similar are the extractable proteins in the two storage tissues of the Washingtonia seed? 2) Are the ultrastructural differences between the protein bodies of the endosperm and embryo correlated with differences in protein composition? and, 3) Are the Washingtonia seed proteins similar to those of Phoenix seeds? TABLE 1. Tissue composition (% volume) in Washingtonia seeda Cytoplasm and InterorganProtein Lipid Cell cellular Tissue Nuclei elles bodies bodies wall space Endosperm 1 6 16 28 49 0 Embryo 2 19 43 25 7 4 a Data from DeMason (1986; 1988).