RNA Synthesis inPhaseolusChloroplasts

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Abstract
The rate of RNA synthesis in chloroplasts from the primary leaves of Phaseolus vulgaris L. cv. Canadian Wonder was measured in vitro as plant age increased. The rate per leaf began to fall before the leaf was 70% expanded. At full expansion, activity had fallen by 70%. Chloroplast RNA synthesis per unit chlorophyll was falling before the leaf was 25% expanded. When all parts of the plant above the mature primary leaves were removed (detopping) chloroplast RNA synthesis in these leaves rose within 36 h. The rate increased to a maximum 3–4 d after detopping, when it was 5–10 times control values; thereafter it fell again. The chlorophyll content began to increase about 4 d after detopping, eventually rising by 100%. Detopping caused a 3-fold increase in the Triton X-100-soluble DNA content of chloroplast preparations, measured after 3.5 d. At that time the rate of RNA synthesis per unit Triton-soluble DNA was the same in chloroplasts from the primary leaves of intact and detopped plants. Detopping also resulted in an increase in the depth of the leaf palisade layer. The effects of detopping on chloroplasts were prevented by darkness and reduced by shading. Increased chloroplast RNA polymerase activity was also induced in the primary leaves by placing a polythene bag over intact plants, enclosing everything above these leaves. Removal of the roots from detopped plants prevented the rise in the rate of chloroplast RNA synthesis.

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