Molecular mechanisms of growth responses to elevated atmospheric carbon dioxide in wheat (Triticum aestivum L.)

The carbon dioxide concentration [CO2] in the current atmosphere is increasing at a significant rate and is predicted to reach up to 700 umol mol-1 by the end of this century. Elevated ([CO2]) has the potential to increase the growth and yield of crops. Being the primary substrate for photosynthesis, increased CO2 levels significantly promote the growth of most C3 crops through increased photosynthesis capacity and reduced stomatal conductance. This stimulation of photosynthesis is central to other post-photosynthetic key metabolic processes such as carbon and nitrogen metabolism, cell cycle functions and hormonal regulation, which may lead to changes in whole plant growth. The magnitude of these responses to elevated [CO2] varies even within same species, indicating a significant genetic variation for CO2 responsiveness within plant communities. In addition, the CO2 responsiveness of plants depends on their ontogeny and is found to be more pronounced in the early development stages of the crops. However, there is a limited understanding of underlying molecular mechanisms of plant growth responses to elevated [CO2] which is crucial for developing crop breeding strategies to improve crop productivity in a changing climate. Therefore, this project broadly aimed to dissect the molecular mechanisms of plant growth responses to elevated [CO2] in wheat (Triticum aestivum L.), focusing on three unexplored aspects of this underlying mechanism. This project focused primarily on the early vegetative stage of wheat to study the increased CO2 responsiveness of crops in their early ontogeny. First, putative quantitative trait loci (QTL) for major early growth traits at elevated [CO2] were identified using a doubled haploid population of a cross between RAC875 and Kukri, to identify the genetic regions potentially associated with CO2 responsiveness. In total 24 putative QTL for CO2 responsiveness were identified for different growth traits. Three QTL, worthy for future research, were identified on chromosome 2A, 1B and 4B that showed an increased response for biomass accumulation at elevated [CO2]. Secondly, the role of photosynthesis and post-photosynthetic metabolic processes in moderating growth responses to elevated [CO2] was investigated through developing an understanding of the source and sink interaction of wheat. Transcript abundance of key genes involved in carbon and nitrogen metabolism, and cell cycle functions varied greatly among CO2 levels (400 and 700 μmol mol-1), organ types (last fully expanded leaves, expanding leaves, leaf cell elongation zone and shoot apex region) and genotypes. Finally, the interplay of different regulatory mechanisms involved in plant growth at elevated [CO2] was investigated through a comparative proteomics analysis. Most of the differentially expressed proteins at elevated [CO2] were involved in carbon metabolism, energy pathways, protein synthesis and cell cycle functions. However, the leaf proteome responses to elevated [CO2] were highly genotype dependent. Overall, the results indicated that post-photosynthetic metabolic processes play a significant role in moderating plant growth responses at elevated [CO2]. Molecular level responses of these processes are subject to developmental regulation and thus, are involved in determining the source and sink integration of plants. This study has demonstrated the intraspecific variability of growth responses to elevated [CO2] at the genetic, transcriptomic and proteomic level. These variable responses provide valuable targets for the selection of genotypes that can thrive well in the future CO2 enriched atmosphere