Mechanisms of wheat grain protein reduction under elevated carbon dioxide concentrations

© 2018 Dr. Pramesha Madurangi Singappuli ArachchigeCarbon dioxide concentration [CO2] in the atmosphere has been progressively rising from approximately 280 µmol mol-1 during the pre-industrial era to a current average of approximately 407 µmol mol-1 and is predicted to increase up to 500 -1000 µmol mol-1 by the year 2100. Among the results of the large volume of research to investigate the impact of rising atmospheric [CO2] on the growth and productivity of agricultural crops is the well-established finding that elevated [CO2] (e[CO2]) decreases wheat grain protein concentration and alters protein composition, both of which are undesirable impacts on bread quality and human nutrition. However, the exact physiological and molecular mechanisms responsible for these responses under e[CO2] are not fully understood. The thesis contains a collection of studies to examine the mechanisms of grain protein reduction in wheat (Triticum aestivum L.) at e[CO2]. The experiments were carried out using the Australian Grains free-air CO2 enrichment (AGFACE) facility located in Horsham, Victoria, Australia and environmentally controlled glasshouse facility located in the Creswick Campus, University of Melbourne, Victoria, Australia. Wheat grain protein and proteome analysis carried out using genotypes with contrasting physiological traits showed that grain protein and proteome responses to e[CO2] were genotype dependent (Chapter 2). The reduction of grain protein concentration at e[CO2] was independent of the grain yield stimulation (Chapter 2). Grain protein concentration and the proteome responses to e[CO2] were developmentally regulated (Chapter 3). The major shift in plant nitrogen (N) dynamics occurred at the latter phase of the grain development and this process is mainly responsible for the repression of grain protein synthesis at e[CO2]. A study that investigated the association between tiller appearance and grain protein response to e[CO2] is reported in Chapter 4. Results showed that the grain protein concentration lowering effect at e[CO2] was independent of the order of tiller appearance. In the study described in Chapter 5, a decrease in leaf N concentration, a factor contributing to photosynthetic acclimation to e[CO2], was identified as an important factor that contributes to the reduction in grain protein concentration, particularly in high protein genotypes, even under sufficient soil N supply. Lower pre-anthesis N uptake also contributes to the reduction in grain protein concentration at e[CO2]. The effect of e[CO2] on passive uptake of NH4+ and NO3- and their allocation into different plant organs was the focus of the studies presented in chapter 6. Decreased stomatal conductance at e[CO2] suppress the NH4+ and NO3- uptake via xylem. However, such reduction had no significant impact on grain protein concentration at e[CO2]. Supplementation of NH4+ as a source of N was identified as an effective way to mitigate grain protein reduction at e[CO2]. Grain protein concentration was less affected when NH4NO3 was supplied as the sole source of N. This was mainly due to a large amount of NH4 being transported directly into the grain, and very effectively assimilated to the proteins in the growing wheat grain. In Chapter 7, the effect of e[CO2] on expression of genes involved in N uptake and assimilation and whether any variation could be related to the changes in grain protein concentration were studied. Results revealed that e[CO2] had no significant impact on expression of N uptake and assimilation related genes. Greater genetic potential to uptake more N during post-anthesis, N uptake as NH4+ and higher N remobilization capacity irrespective of the growth [CO2] are likely to facilitate more N transfer into the grains of those genotypes that are less affected at e[CO2] (Chapter 7). In summation, results of these studies indicate the existence of considerable genetic variation in N uptake and allocation into different plant organs. The least reduction in grain protein concentration occurred in genotypes with higher N uptake capacity, and hence, capable of fulfilling the physiological N demand of the plant, and thus, maintain the grain protein concentration at e[CO2]. Grain protein and proteome wide changes that occurred at e[CO2] were genotype and developmental stage specific. Such changes in grain protein concentration and proteome deteriorate the quality of wheat raising concerns about the industrial and nutritive value of the resulting flour. However, CO2 insensitive genotypes can be identified, and their use may yield better results in wheat breeding programs aimed at meeting the climate change challenge. Additionally, NH4+ supplementation can be adapted to maintain grain protein concentration at e[CO2]. Therefore, understanding of the NH4+ assimilation and strategy used in the allocation of NH4+ into different plant organs at e[CO2] deserves further attention. These findings also suggest that NH4+ transporter gene, AMT1.2 is likely to play a key role in N uptake in CO2 insensitive genotypes irrespective of the growth CO2 condition, thus, AMT1.2 can be used as a potential candidate gene for screening genotypes to identify those capable of retaining the grain protein concentration undisturbed at e[CO2]