Transcriptoma de frutos de acerola durante o amadurecimento: uma visão da regulação gênica do metabolismo do ascorbato, etileno, respiração e firmeza

Abstract

The present work aimed to determine the transcriptomic profile of acerola fruits, confronting it with the metabolic dynamics of ascorbate, ethylene, CO2 and firmness during ripening. To that end, total messenger RNAs of fruits in three developmental stages (unripe, intermediate and ripe) were sequenced, and the ascorbate, ethylene, CO2 and firmness profiles were measured in an attempt to identify candidate genes for the regulation of these processes. In addition, leaves and flowers transcriptomes were analyzed to understand the regulation of AsA in other organs. The expression analysis results revealed 4326 differentially expressed transcripts (DETs) during ripening. Notably, the accumulation of reduced AsA in unripe fruit was associated with greater abundance of transcripts encoding phosphomanomutase, GDPmannose pyrophosphorylase 1, 2 and 3, GDP-mannose epimerase 1 and 2, GDP L-galactose transferase 1 and 2 for biosynthesis (L-galactose pathway), several other of the nucleobase– ascorbate transporter family, monodehydroascorbate reductase 2 and dehydroascorbate reductase 1 for the translocation and recycling processes. The sharp depletion in the AsA content during ripening could be explained by the downregulation of transcripts involved in the biosynthesis, translocation and recycling processes, and the upregulation of ascorbate peroxidase 2 and ascorbate oxidase 2 and 3 involved in the degradation. In flowers and leaves, the downregulation of the L-galacotase pathway transcripts suggested a lower biosynthetic rate, while greater number of ascorbate peroxidase and ascorbate oxidase members upregulated at such organs were consistent with the higher amount of dehydroascorbate found in flowers and moderate in leaves. Regarding ethylene and CO2, their contents declined during maturation. The upregulation of ACC synthase 2 transcripts and downregulation of the ethylene receptors 1 and 2 in fruits in the intermediate stage suggested a higher sensitivity to ethylene. While the higher abundance of ACS synthase 1 and 2 transcripts, ACC oxidase 2 and several ethylene response factors in ripe fruit, indicated the occurrence of posttranscriptional/translational regulation of the ethylene biosynthesis. Simultaneously, the higher respiratory rate of unripe fruit corroborated with the upregulation of transcripts coding for some steps of the glycolytic pathway, several subunits of the complexes I, II and IV of the CTE and ATP synthases. However, in ripe fruit, the positive regulation of other members of the glycolytic pathway, pyruvate dehydrogenase complex, phosphate pentoses pathway, external NAD (P) H dehydrogenase, alternative oxidase, and the negative regulation of succinate dehydrogenase and isocitrate dehydrogenase of the Krebs cycle were verified. These results suggested enhanced activity of the nonphosphoryllative pathways as apotentially efficient strategy to flexibilize the energetic and biosynthetic demands after the intermediate stage. The fruit firmness reduced dramatically during ripening and could be associated to the increase in expression of at least 54 transcripts encoding proteins/enzymes responsible for the degradation/biosynthesis of pectin, hemicellulose and cellulose. Overall, this study reflects the identification of several genes with potential applicability in the improvement of acerola quality and provides a vast amount of information for future investigations dedicated to a better understanding of these and other peculiarities of acerola ripening.