Molecular analyses of carotenoid-associated proteins from chromoplasts of cucumissativus corollas

Chromoplasts are carotenoid-accumulating plastids found in the corollas and fruits of many higher plants. In most cases the pigment in these plastids is accumulated within unique structures with the aid of carotenoid-associated proteins. We isolated and characterized the cDNA (CHRC) from Cucumis sativus corollas which encodes the chromoplast-specific carotenoid-associated protein CHRC. Analysis of CHRC transcript level in Cucumis sativus revealed its temporal and tissue-specific regulation: the transcript was detected only in corollas, where its level increased in parallel to flower development, peaking just before anthesis. A CHRC fragment that includes the potential active site of the protein was used as a probe in northern blot analyses of floral and fruit tissues from various plants containing chromoplasts of different types: CHRC homologues of similar sizes were revealed in all cases. To further study chromoplast biogenesis in floral-tissues, a minor (in abundance) protein of ca. 14 kD (CHRD) was isolated from cucumber corolla chromoplasts. Immunological characterization revealed it to be chromoplast-specific, and its steady-state level in corollas increased in parallel to flower development. The protein was not detected in cucumber leaves or fruits. Using an in vitro bud-culture system, the effect of phytohormones on CHRD expression was found to be similar to that on CHRC. GA₃ very rapidly enhanced, whereas ABA and paclobutrazol downregulated the steady-state level of CHRD. Ethylene also downregulated the protein's steady-state level and etiolation increased CHRD accumulation. It is suggested that hormonal control of chromoplastogenesis is tightly regulated at the tissue/organ level and that mainly developmental signals control carotenoid accumulation in non-photosynthetic tissues.