Increasing Ascorbate Increases Phenylpropanoids: An Investigation By Whole Transcriptome Analysis

Abstract

The antioxidant nature of compounds such as ascorbic acid (AsA) and the derivatives of the phenylpropanoid (PP) pathway are of great importance and interest due to the vital functions they play in both plant and human health. In humans, the consumption of these compounds is considered to have positive health benefits such as anti-scorbutic, anti-cancer and anti-inflammatory effects. With the apparent positive correlation between AsA and PP content demonstrated in high AsA tomato fruit, the elucidation of these unknown links between these pathways is of novel interest to due to the significant role they could play in the improvement of crop plants towards higher nutrition value. The aim of this thesis was to identify differentially expressed genes from leaf, flower, mature green and red fruit tissue between high AsA and control tomato plant lines associated with PP metabolism through the investigation of the Solanum lycopersicum transcriptome by RNA-sequencing (RNA-seq). Transient transformation analysis on candidate genes was performed in Nicotiana benthamiana and N. tabacum plants to determine whether AsA content is responsible for the trans-activation of genes belonging to the PP pathway and the subsequent increase in PPs. Overall, a total of 2,621 genes were found to be differentially expressed between the control and high AsA lines of S. lycopersicum. Five genes were found to be differentially expressed across three or more tissues with putative functions related to the electron transport chain, isoleucine biosynthesis, embryo development and zinc finger motifs. Interestingly, there appeared to be no significant differential expression of the regulatory genes of the tightly regulated PP pathway. However, a homebox-leucine zipper with homology to Arabidopsis thaliana anthocyaninless2 gene (ANL2) was found to be the only gene to be highly up-regulated in all tissues. Results here report the first correlation between high AsA, PP and ANL2. The analysis carried out in this study provides a valuable basis for further investigation into the regulatory cross-talk between AsA and PP metabolism, with the future potential to inform breeding programs aiming to increase the nutritional content of fruit and vegetables.