ZEA3 - A PLEIOTROPIC MUTATION AFFECTING COTYLEDON DEVELOPMENT, CYTOKININ RESISTANCE AND CARBON-NITROGEN METABOLISM

When photomorphogenesis takes place during early plant development, th e cotyledons undergo a metabolic transition from heterotrophic sink me tabolism to autotrophic source metabolism. A mutant screen was devised for seedlings affected in the regulation of nitrate assimilation duri ng this early sink-source transition in Nicotiana plumbaginifolia. A m utant (EMS 203.6) was isolated for its inability to grow on low nitrat e concentration. In contrast to wild-type (WT) plants, the mutant coty ledons remained tightly attached to each other throughout seedling dev elopment. It was found that a low carbon/nitrogen ratio (C/N ratio) in the medium was required for mutant growth. The higher the ratio was, the more the growth was inhibited. Mutant EMS 203.6 accumulated all am ino acids in permissive conditions (low C/N ratio), and all amino acid s and sugars also in selective (high C/N ratio) conditions. In additio n, sucrose in the medium repressed light-regulated genes involved in n itrate assimilation and in photosynthesis in the mutant but not in the WT plants. The mutation was mapped to the Zea3 complementation group which confers resistance to zeatin. This zeatin resistance was associa ted with a hypertrophy of mutant cotyledons in response to cytokinin. Both cytokinin resistance and sensitivity to a high C/N ratio were not observed in etiolated mutant seedlings and were restricted to the joi nted-cotyledon developmental stage. Previous physiological studies sho wed evidence for a role of cytokinins in the expression of nitrate red uctase. Here, the first genetic evidence for a link between carbohydra te/nitrogen metabolism and cytokinin action during early development i s provided.