Functional characterization of the Target of Rapamycin (TOR) gene using Setaria as the model plant: dissecting the role of TOR pathway regulating growth and metabolism in C4 plants

Abstract

Biomass derived from energy crops is a promising renewable source for the production of biofuels and bio-based materials. Plant biomass production is tightly related to growth and is controlled by a complex regulatory network that integrates external and internal cues. One of the key regulators of growth in all eukaryotes is the Target of Rapamycin (TOR) protein kinase. In the model plant Arabidopsis, functional studies with conditional disruption of TOR kinase revealed a positive correlation between TOR expression and growth and remobilization of carbon flux towards reserves compounds like starch and triacylglicerides (TAGs). Furthermore, it has been found that plant TOR responds to glucose, energy status, hormones and stress and some downstream effectors of the plant TOR pathway been identified. Recently, a study has also revealed that TOR signaling pathway orchestrates unprecedented transcriptional networks to control diverse cellular functions in plants. These recent findings offer new perspectives to explore the role of TOR pathway in the control of growth and biomass production in C4 plant species, which are the main dedicated bioenergy crops due to their intrinsic efficiency to convert solar energy to biomass. Despite the rapid progress made to uncover several biological functions of the TOR pathway in Arabidopsis, most of the biological outputs, downstream effectors and primary target genes and proteins of the plant TOR signaling pathway are still largely unknown especially in photosynthetic organisms. The goal of this project is to dissect TOR signaling network involved in growth and metabolism in C4 plants using Setaria as the model specie. Firstly, we will assess the performance of two TOR inhibitors in Setaria seedlings: rapamycin, a traditional drug used to inhibit TOR pathway in mammals and yeast, and AZD-8055, a drug recently discovered as a potent ATP-competitive inhibitor of TOR in plants. The next step will be dissect the gene regulatory network affected by the chemical inhibition of the TOR pathway through a time-series gene expression experiment using Illumina RNA-seq technology. To further gain insights into the TOR functions in Setaria, we propose to generate and characterize transgenic lines with overexpression and conditional silencing of the TOR gene. Our approach is expected to provide insights into the gene regulatory network coordinated by TOR in C4 species and if possible, reveal novel targets not covered by the network orchestrated by TOR in C3 species. These findings will provide a molecular framework for future studies of the TOR pathway regulating metabolism, growth and biomass accumulation in other grasses like sugarcane, sorghum or maize, which are important crops for both first and second generation biofuel production. (AU)