Changes in Ribosome Organization and Messenger RNA Abundance in Ripening Tomato Fruits

Abstract

The involvement of RNA and protein synthesis in fruit ripening was investigated. Mature-green tomato fruits were found to contain about 30% of their ribosomal RNA in polyribosomes. At the 'breaker' or early ripening stage, about 50% of rRNA was in polyribosomes and this distribution of rRNA was maintained until fruits were fully ripe. The continued presence of polyribosomes is consistent with active protein synthesis persisting through and beyond the climacteric period when the wall-hydrolysing enzyme polygalacturonase accumulates and fruits soften, synthesize lycopene and undergo other ripening related changes. Poly(A)-containing RNA purified from polyribosomes extracted from individual fruits was used to prime the synthesis of [35S]methionine-labelled polypeptides by a wheat germ in vitro translation system. The pattern of polypeptides synthesized in response to RNA from mature-green fruits differed from that given by RNA from ripening fruits. The majority of changes were found to occur within approximately 48 h of the increase of ethylene synthesis and were apparent in all fruits with any pink or red colour. Similar results were obtained by translating total cellular RNA and total polyribosomal RNA indicating that the major RNA species shown in this study to change in abundance during ripening are polyadenylated, and hence most probably cytoplasmic, and do not accumulate as 'stored messages' outside of the polyribosomes. The differences between green and ripening fruits in polyribosome profiles were demonstrated in two cultivars of tomato. Differences in mRNA populations between green and ripe fruits were found in three cultivars.