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Plant and Cell Physiology, 1993, Vol. 34, No. 5 697-704
© 1993

Translatable mRNAs for Chloroplast-Targeted Proteins in Detached Radish Cotyledons during Senescence in Darkness

Naoto Kawakami1 and Akira Watanabe

Research Institute for Biochemical Regulation, School of Agricultural Sciences, Nagoya University Chikusa-ku, Nagoya, 464 Japan

A protein-import system prepared with isolated chloroplasts was used to monitor changes in levels of mRNAs for chloroplast-targeted proteins during dark-induced leaf senescence. Biologically active chloroplasts were isolated from young (9-day-old) and aged (14-day-old) radish cotyledons. Poly(A)+-RNA was prepared from radish cotyledons that had been detached from seedlings and placed in darkness to accelerate senescence. The RNA was translated in a wheat germ system, and the products were added to an import system prepared with chloroplasts from young cotyledons. Electrophoretic analysis of the imported proteins suggested that most chloroplast- targeted proteins decreased in abundance during dark treatment of cotyledons. However, the relative abundance of 38 stromal and three thylakoid proteins increased transiently or continuously among the products of translation of RNA isolated during the course of senescence. The efficiency of the uptake of precursor proteins by chloroplasts isolated from aged cotyledons was lower than that by chloroplasts from young tissue. The chloroplasts from aged cotyledons more efficiently imported at least one stromal protein and one thylakoid protein than chloroplasts from the young tissue. The relative abundance of these two proteins increased among the products of translation of RNA from senescing cotyledons when tested in the uptake system with chloroplasts from young cotyledons. These results suggest that some nuclear genes for chloroplast-targeted proteins are expressed in senescing cotyledons more efficiently than in young tissue, and that the machinery for import of proteins into chloroplasts changes during aging of the tissue to allow more efficient import of certain proteins that may be responsible for the senescence of the chloroplasts.

1Present address: Kihara Institute for Biological Research, Yokohama City University, Mutsukawa 3-122-20, Minami-ku, Yokohama, 232 Japan

(Received December 16, 1992; Accepted May 6, 1993)
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