The effect of N6-benzyladenin and gibberellic acid on chromoplast differentiation in sepals and fruit of Physalis alkekengi

The transformation of chloroplasts into chromoplasts is a complex and precisely controlled process which is characterized by degradation of the photosynthetic apparatus, synthesis of carotenoids, and formation of specific carotenoid-bearing structures. However, the specific mechanisms which are involved in the regulation of these processes are poorly investigated. In the present report, we described the role of the plant hormones, N6-benzyladenine (BA) and gibberellic acid (GA3), on the differentiation of chromoplasts in the sepals and fruit of Physalis alkekengi. Treatments were performed by injecting 1mM BA and 0.4 mM GA3 into the lumen of the bladder formed by the sepals. The tips of the sepals were tied up with thread and lifted up in such a fashion that the fruit was in a horizontal position. The lower part (about one-quarter of its volume) of the bladder was filled with the solution of the plant hormones. The treatments started when the bladders were completely green. For ultrastructural analyses, the tissue was fixed with 1% glutaraldehyde in cacodylate buffer (pH 7.2) and postfixed with 1% OsO4 in the same buffer. After dehydration in ethanol, the tissue was embedded in Araldite. Ultrathin sections were stained with uranyl acetate and lead citrate and examined with an FEI Morgagni 268D electron microscope. In the controls, the sepals of the green bladder contained typical chloroplasts with numerous plastoglobules (0.1-1 μ m in diameter) forming large accumulations between the thylakoids. As the fruit ripened, the chloroplasts gradually differentiated into chromoplasts thus changing the color of the bladder from green to orange, and finally to intensely red. During this process, crystalloid inclusions appeared inside the plastoglobules so the fully differentiated chromoplasts in the red bladder were filled with carotene crystals (0.1-1 μ m in length). Tubular structures were also often observed in the chromoplast stroma. At the time when the controls turned red, the bladders containing a solution of 1mM BA, became orange in the upper part, while their lower part was still green. The upper part of these bladders contained chromoplasts with crystalloids extending from the plastoglobules. On the other hand, in their lower part remained chloroplasts with typical grana and numerous plastoglobules without crystalloid inclusions. At the same time, the bladders treated with 0.4 mM GA3 became completely orange. They contained chromoplasts with crystalloids formed inside the plastoglobules. The effect of BA and GA3 on the fruit was similar to those observed in sepals. These results indicate that both plant hormones, BA and GA3, retard chromoplast differentiation in the sepals and fruit of Physalis alkekengi. However, the effect of BA was stronger on the tissue which was in direct contact with the hormone solution and was more pronounced than the influence of GA3.