Mechanisms of the decline in grain protein concentration in wheat under elevated atmospheric CO2 concentration

Mechanisms of the decline in grain protein concentration in wheat under elevated atmospheric CO2 concentration

Wheat is one of the most produced crops worldwide and is an important source of protein for many people. Climate conditions predicted for the future will have a major impact on the growth and nutritional quality of wheat, due to the increasing atmospheric carbon dioxide concentration ([CO2]). By the end of the century, the [CO2] is predicted to increase to at least 700 μmol mol-1, rising from the current concentration of approximately 400 μmol mol-1. Many studies show that wheat grown under these conditions assimilates more carbon and, therefore, increases in biomass and yield. On the other hand, studies have also demonstrated that wheat grown under future [CO2] declines in grain protein concentration (GPC). Currently, there is a lack of information on what causes this decline in wheat under elevated [CO2]. Current explanations for the decline in GPC consist of biomass dilution, whereby an increase in grain biomass dilutes the grain protein, and inhibition of nitrate assimilation, where nitrogen remains in the form of nitrate in leaves and is unable to be assimilated and remobilised to the grain. However, these mechanisms do not completely explain the decline in GPC. Due to the lack of understanding in this area, this thesis aimed to investigate three unexplored aspects of GPC decline: i) Identifying whether difference exists in the GPC of three wheat types (tetraploid, hexaploid and synthetic hexaploid) in response to e[CO2]; ii) Determining the traits with the greatest contribution to wheat GPC under e[CO2]; iii) Investigating potential sugar sensing pathways in roots of wheat which control expression of nitrogen uptake and assimilation related genes under e[CO2].

Wheat belonging to three types (tetraploid, hexaploid and synthetic hexaploid) were grown under ambient and elevated [CO2] (e[CO2]) to identify whether the response of GPC was different between the three wheat types. In addition, biomass measurements were taken to explore the extent of biomass dilution in explaining GPC decline. The response of GPC to e[CO2] was found to be genotype dependent, rather than wheat type dependent and biomass dilution could not completely explain the change in GPC. Understanding the extent that e[CO2] affects other traits that contribute to the plant’s GPC is important in order to identify any specific mechanisms controlling GPC response. Traits associated with nitrogen uptake and remobilisation were measured in addition to plant biomass and rate of photosynthesis. Elevated [CO2] did not have a consistent effect on each of the traits studied. The decline in GPC appeared to be caused by a combination of traits, rather than a single trait, although GPC typically declined due to both an increase in grain biomass and decline in nitrogen uptake. Studies have shown that photosynthesis is downregulated under e[CO2] due to sugar sensing. As such, this study aimed to identify whether a change in sugar was associated with a decline in nitrogen uptake and assimilation in roots of wheat seedlings through gene expression and proteomics analysis. While increased sugar was associated with an increase in expression of an ammonium transporter and glutamine synthetase, there was not sufficient evidence to indicate regulation of nitrogen uptake and assimilation related gene expression by e[CO2] through sugar sensing pathways. Overall, this thesis further increases the knowledge available on the mechanisms affecting GPC in response to e[CO2].