Expression of plastid and nuclear genes during chromoplast differentiation in bell pepper (Capsicum annuum) and sunflower (Helianthus annuus)

Abstract

The expression of genes involved in plastid differentiation is subject to developmental control. To understand better the molecular basis of this regulation, we have analyzed in parallel the changes in relative levels of four chloroplast and two nuclear transcripts during differentiation of chloroplasts into chromoplasts in ripening bell pepper fruits and in maturing sunflower petals, as well as in the case of chemically-induced chromoplast formation in leaves. Amongst the prominent features, we noted that transcripts of nuclear genes coding for chloroplast proteins, such as the major chlorophyll a/b-binding protein (encoded by cab) and the small subunit of ribuloses-1,5-bisphosphate carboxylase (encoded by rbcS), were detected in chromoplast-containing bell pepper leaves and sunflower petals, whilst they disappeared in bell pepper fruit chromoplasts. Transcripts of the plastid gene coding for the large subunit of ribulose-1,5-bisphosphate carboxylase were detected during the whole differentiation process. The steady-state level of the mRNA corresponding to the 32 kDa herbicide-binding protein increased in fruit and petal chromoplasts. Changes in the relative transcriptional activities of plastid genes and in their relative mRNA levels occurred during bell pepper fruit ripening. However, no significant change in the overall transcriptional activity was found when comparing both types of plastids, whereas dramatic changes in translational activity occur, indicating the prevalence of translational control of plastid gene expression in the chromoplast differentiation process.