ISRAEL JOURNAL OF BOTANY, Vol. 34,1985,pp. 377-395

PROTEIN SYNTHESIS AND SECRETION BY THE BARLEY ALEURONE: A PERSPECTIVE

RussELL L. JoNES Department of Botany, University of California, Berkeley, C4 94720, USA

The secretion of extracellular macromolecules is a process common to all living organisms. In animal cells the secretion of proteins, especially by tissues of the digestive system, is quite well understood at the level of cell structure as well as at the biochemical and molecular levels. In plants, on the other hand, most studies have emphasized the transport of polysaccharides to the cell surface for the formation of the cell wall and other extracellular structures (Fahn, 1979). The cereal aleurone layer and digestive glands of insectivorous plants are especially interesting as they are among the few plant tissues studied that secrete acid hydro lases. In the case of insectivorous plants, research on the structure of the digestive gland dates back to the late nineteenth century and has been reviewed recently by Fahn (I 979) and He"slop­ Harrison and Heslop-Harrison (1981). The present review focuses on the synthesis and secretion of hydro lases by the cereal aleurone.

ROLES OF ALEURONE, SCUTELLUM AND EMBRYO IN REGULATING ENZYME PRODUCTION Endosperm modification in cereals has been intensively studied for more than a century (Sachs, 1882; Haberlandt, 1884), but several aspects of this process that were controversial at the end of the nineteenth century (Haberlandt, 1890; Brown & Morris, 1890) remain so today. One controversy concerns the relative contributions of the aleurone layer and scutellum to the production of the hydrolases that mobilize endo­ sperm reserves during germination, while another concerns the role of the embryo in the regulation of hydrolase production. Although it is widely accepted that both the aleurone layer and scutellum of cereal grains produce a-amylase, the relative contributions of each to total starch breakdown are disputed. Brown and Morris (1890) and more recently Briggs (1964), Gibbons (1979, 1980, 1981), Okamoto et al. (1980), Akazawa and Miyata (1982), Palmer (1982) and Gibbons and Nielsen (1983) have argued that in the germinating grain of barley or rice the scutellum is the principal source of a-amylase. These authors' estimates of the contribution of the aleurone to total grain a-amylase production in barley range from a low of little or none (Palmer, 1982) to a high of 50% (Gibbons & Nielsen, 1983).

Received April16,1985 378 R.L. JONES lsr. J. Bot.

With the exception of the experiments of Briggs (1964) and Pahner (1982) the recent data supporting the notion that the scutellum is the principal source of a­ amylase were obtained from cytological observations of either amylolytic activity or antigenicity in sections of germinating cereal grains. A more direct approach to determining whether a-amylase comes from aleurone or scutellum was taken by Ranki and Sopanen (1984) and MacGregor et al. (1984). These authors isolated various parts of the germinating barley grain and measured either the rate of a-amylase synthesis or the amount of a-amylase using specific antibody. In both cases it was concluded that the aleurone layer was the major source of a-amylase. G.B. Fincher and colleagues (pers. commun.) used a different approach to this problem but also concluded that the aleurone layer was the source of most of the a-amylase in the germinating grain. They isolated poly (At-RNA from aleurone layers and germ (scutellum plus embryo) of germinating barley and translated the mRNA in the reticulocyte lysate cell-free translation mix. They showed that mRNAs for a-amylase and (j-glucanase were ahnost exclusively (92-95%) localized in the aleurone layer. These data, and the observation that when the aleurone is dissected from the germinating grain the aleurone synthesizes most of the a-amylase (Ranki & Sopanen, 1984), provide strong support for the early conclusion of Haberlandt (1884) that the aleurone layer plays a key role in starch mobilization in barley. The role of the scutellum and its importance in endosperm modification should not be overlooked, however. In germinating grain of most cereals a-amylase secretion ahnost always begins in the endosperm adjacent to the scutellum (Briggs, 1973; Akazawa & Miyata, 1982). Synthesis of a-amylase by scutellum and aleurone may therefore be temporally separated such that the scutellum synthesizes a-amylase de novo upon imbibition of water (Akazawa & Miyata, 1982), while the aleurone layer requires a chemical signal to begin a-amylase synthesis (Ho, 1979). Thus, although the aleurone layer may contribute more a-amylase for endosperm breakdown during germination, starch digestion may be initiated by enzyme produced by the scutellum, and this tissue may be the sole source of a-amylase during the first 24 h of imbibition. Anatomical differences between species may also explain the varying conclusions which have been drawn about the relative roles of aleurone and scutellum. In barley, for example, the aleurone layer, which can be as many as three to four cell-layers thick (Jones, 1969a), is the principal source of hydrolases, while in corn, oat, rice, rye and wheat, where the aleurone layer is generally one cell-layer thick (Bhatnagar & Sawhney, 1981), it may be a less important source of these enzymes. The role of the embryo in the control of endosperm mobilization in cereal grains also continues to generate controversy. As early as 1890 Haberlandt, Brown and Morris all concluded that starch breakdown in the endosperm of barley was dependent on a non-enzymatic signal from the embryo. The subsequent demonstration by Yomo (1960) and Paleg (1960) that exogenously supplied gibberellic acid (GA 3 ) would promote enzyme release in de-embryonated barley led to the hypothesis that gib­ berellins (GAs) might act as hormonal signals produced by the embryo. That GAs are synthesized by barley embryos was demonstrated by the experiments of Radley (1967) and Cohen and Paleg (1967), and these data led to the general acceptance of a hypo-