Exploiting diversity in the regulation of carbon assimilation to improve wheat productivity

Degen, Gustaf E. and Carmo-Silva, Elizabete and Parry, Martin (2020) Exploiting diversity in the regulation of carbon assimilation to improve wheat productivity. PhD thesis, Lancaster University.

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Abstract

Rubisco activase (Rca) is a key regulator of carbon assimilation. It removes inhibitory sugar phosphate derivatives from the active site of Rubisco by using the energy from ATP hydrolysis. However, Rca is thermosensitive and limits photosynthesis at moderately high temperatures and during photosynthetic induction. A better understanding of the properties of Rca isoforms present in wheat, a staple crop for human nutrition, will unlock the potential to improve the efficiency and resilience of carbon assimilation to meet future food demands in the face of climate change. The three wheat Rca isoforms were found to vary in the capacity to activate Rubisco in vitro. Rca1β had lower Rubisco activation activity than Rca2β and Rca2α. Ratios of Rubisco active sites to Rca above 6-11:1 resulted in maximal Rubisco activation. Furthermore, residues that conferred ADP sensitivity of Rca1β were identified using site-directed mutagenesis of the ADP-insensitive Rca2β isoform. These findings can inform future efforts to optimise Rubisco activation during shade to sun transitions in crops. To investigate the thermotolerance of native wheat Rca isoforms, temperature responses of ATP hydrolysis and Rubisco activation were assessed. Rca1β exhibited increased thermotolerance but reduced Rubisco activation activity, in contrast to Rca2β. An isoleucine residue was shown to confer thermostability in the mutant Rca2β-M159I, whilst maintaining high Rubisco activation rates and efficiency. To expand the understanding of thermotolerance and isoform abundance in planta, the effect of heat stress on wheat plants was investigated. The thermotolerant Rca1β increased to 6% of the total Rca protein pool after 5 days of heat stress, but Rubisco activation state remained reduced in heat stressed plants. Furthermore, activity of CA1Pase, which metabolises Rubisco inhibitors, increased in plants in recovery conditions (4 h after relief from heat stress), suggesting a build-up of inhibitors during heat stress and a wider role of this enzyme in Rubisco regulation. Exploiting diversity of wheat wild relatives have been suggested as a strategy for improving modern wheat cultivars. Rca in wild relatives was highly conserved and highly similar to the donor-species of the three wheat sub-genomes. Despite the small differences, the observed variation in Rca expression and protein levels might be associated with the ability of these species to adapt to high temperatures. Overall the results from this thesis suggest that engineering thermotolerant Rca isoforms into wheat to improve carbon assimilation will be the most promising way to make Rubisco regulation in wheat more resilient to heat stress in changing environments.