Circadian oscillation and development-dependent expression of glycine-rich RNA binding proteins in tomato fruits.

Glycine-rich RNA-binding proteins (GRPs) are involved in the modulation of the post-transcriptional processing of transcripts and participate as an output signal of the circadian clock. However, neither GRPs nor the circadian rhythmic have been studied in detail in fleshy fruits as yet. In the present work, the GRP1 gene family was analysed in Micro-Tom tomato (Solanum lycopersicum L.) fruit. Three highly homologous LeGRP1 genes (LeGRP1a-c) were identified. For each gene, three products were found, corresponding to the unspliced precursor mRNA (pre-mRNA), the mature mRNA and the alternatively spliced mRNA (preLeGRP1a-c, mLeGRP1a-c and asLeGRP1a-c, respectively). Tomato GRPs (LeGRPs) show the classic RNA recognition motif and glycine-rich region, and were found in the nucleus and in the cytosol of tomato fruit. By using different Escherichia coli mutants, it was found that LeGRP1s contained in vivo RNA-melting abilities and were able to complement the cold-sensitive phenotype of BX04 cells. Particular circadian profiles of expression, dependent on the fruits' developmental stage, were found for each LeGRP1 form. During ripening off the vine of fruits harvested at the mature green stage, the levels of all LeGRP1a-c forms drastically increased; however, incubation at 4°C prevented such increases. Analysis of the expression of all LeGRP1a-c forms suggests a positive regulation of expression in tomato fruit. Overall, the results obtained in this work reveal a complex pattern of expression of GRPs in tomato fruit, suggesting they might be involved in post-transcriptional modulation of circadian processes of this fleshy fruit.

Expression profile of transcripts encoding cell wall remodelling proteins in tomato fruit cv. Micro-Tom subjected to 15°C storage.

To extend fruit market life, tomatoes are harvested before red ripe and kept at temperatures below optimum (20°C). In this work, Micro-Tom tomatoes stored at 20°C (normal ripening) were compared with those stored at 15°C or 4°C (chilling injury inducer) for 7 days. In contrast to 4°C, storage at 15°C delayed ripening with the benefit of not enhancing oxidative metabolism and of enabling ripening upon being transferred to 20°C. The transcriptional expression profile of enzymes related to cell wall metabolism was compared at the three temperatures. Although endo-ß-1,4-glucanase (Cel1), which is associated with fruit decay, was largely increased after removal from 4°C storage, its expression was not modified in fruits stored at 15°C. Enhanced transcriptional expression of xyloglucan endotransgylcosylase/hydrolases (XTHs) XTH1, -2, -10 and -11, and of two ß-xylosidases (Xyl1-2) was detected in fruits stored at 15°C with respect to those at 20°C. Following 2 days at 20°C, these transcripts remained higher in fruits stored at 15°C and XHT3 and -9 also increased. Ethylene evolution was similar in fruits kept at 15°C and 20°C; thus, the changes in the transcript profile and fruit properties between these treatments may be under the control of factors other than ethylene.