Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 15, 1998
Publication Date: N/A
Interpretive Summary: The germination of cereal seeds and growth of the emerging seedling is governed by the production of certain proteins within the seeds, known as hydrolytic enzymes. These enzymes break down stored molecules within the seed (proteins, starch, cell wall structural components, lipids) into smaller components. This allows their transport to nourish the growing root and shoot. The production of these enzymes is turned on by a hormone known as gibberellin (GA), and it is inhibited by another hormone known as abscisic acid (ABA). Despite years of study, many of the basic processes controlled by these hormones remain poorly understood. However, in the past three years there have been a number of breakthroughs which are beginning to elucidate these processes. This review article summarizes present knowledge of how genes encoding hydrolytic enzymes are inhibited during seed development. It then explores research into the possible mechanisms by which GA and ABA are perceived on the surface of the cell's membrane during germination and how this generates a signal which is sent, via various intracellular intermediates, to genes in the cell's nuclear DNA. Knowledge of the DNA sequences, which are the hormone's ultimate sites of action, of particular genes and the possible proteins which may coordinate the activation of the genes is also summarized. The impact of this article will be in helping other researchers to rapidly learn of the current research in the area of activation of cereal seed enzyme genes, how this research is interrelated with studies in other systems, and emerging theories in this area.
Technical Abstract: The analysis of hydrolytic enzyme gene expression in germinating cereals has progressed from a determination of hormone-responsive promoter elements to the cloning of positive and negative promoter-binding proteins and the direct in vivo demonstration of their effects. Interesting parallels with animal systems have emerged, particularly with regard to the mediation of gibberellin (GA) signal transduction through transcriptional regulators encoded by oncogene homologues, calcium signalling pathways, and (possibly) through the phosphoinositide turnover pathway. New applications of microinjection, fluorescent probes and transient expression plasmids have allowed direct examination of the roles of signal transduction factors and the ultimate targets of their pathways on hormone- and sugar-responsive gene promoter elements. Arabidopsis hormone response mutants have provided cloned candidates for genes encoding trans-acting regulatory proteins. These have rapidly led to counterparts in cereals. The identification of the initial receptor for GA and abscisic acid (ABA) still proves to be an intractable problem, but candidates have emerged from recent studies. The puzzle of how GA and ABA signal transductions lead to opposing regulatory events proves to be a continuing challenge, but recent studies provide intriguing insights into these complex events. These studies are providing testable models as to how hydrolytic enzyme gene expression in developing and germinating cereal grains is regulated. The following review summarizes recent physiological, genetic and molecular studies in these areas and, where appropriate, provides correlative information from past studies.