EXPRESSION OF AN ASPARTATE KINASE HOMOSERINE DEHYDROGENASE GENE IS SUBJECT TO SPECIFIC SPATIAL AND TEMPORAL REGULATION IN VEGETATIVE TISSUES, FLOWERS, AND DEVELOPING SEEDS

Although the regulation of amino acid synthesis has been studied exten sively at the biochemical level, it is still not known how genes encod ing amino acid biosynthesis enzymes are regulated during plant develop ment. In the present report, we have used the beta-glucuronidase (GUS) reporter gene to study the regulation of expression of an Arabidopsis thaliana aspartate kinase-homoserine dehydrogenase (AK/HSD) gene in t ransgenic tobacco plants. The polypeptide encoded by the AK/HSD gene c omprises two linked key enzymes in the biosynthesis of aspartate-famil y amino acids. AK/HSD-GUS gene expression was highly stimulated in api cal and lateral meristems, lateral buds, young leaves, trichomes, vasc ular and cortical tissues of growing stems, tapetum and other tissues of anthers, pollen grains, Various parts of the developing gynoecium, developing seeds, and, in some transgenic plants, also in stem and lea f epidermal trichomes. AK/HSD-GUS gene expression gradually diminished upon maturation of leaves, stems, floral tissues, and embryos. GUS ex pression was relatively low in roots. During seed development, express ion of the AK/HSD gene in the embryo was coordinated with the initiati on and onset of storage protein synthesis, whereas in the endosperm it was coordinated with the onset of seed desiccation. Upon germination, AK/HSD-GUS gene expression in the hypocotyl and the cotyledons was si gnificantly affected by light. The expression pattern of the A. thalia na AK/HSD-GUS reporter gene positively correlated with the levels of a spartate-family amino acids and was also very similar to the expressio n pattern of the endogenous tobacco AK/HSD mRNA as determined by in si tu hybridization.