Gustaf E. Degen, Dawn Worrall, and Elizabete Carmo-Silva

Rubisco activase (Rca) is a key regulator of carbon fixation. It uses energy from ATP hydrolysis
to remove naturally occurring inhibitory sugar phosphate derivatives from the active sites of the
CO 2 -fixing enzyme Rubisco. However, Rca is a thermolabile enzyme and constrains
photosynthesis at moderately high temperatures. Thus, the regulation of Rubisco by Rca
constitutes a bottleneck under future climate scenarios, which are predicted to increase
temperatures above the optimum for production of important food crops, such as wheat. Here
we present the temperature response of the native Rca isoforms from bread wheat (Triticum
aestivum) and show that Rca 1β exhibits higher temperature optimum of ATP hydrolysis and
Rubisco activation than Rca 2β and 2α. The rate of Rubisco activation by Rca 1β still occurred
at 42.5 °C, while Rca 2β and 2α were no longer active, but at 20-35 °C Rca 1β was
considerably slow at activating Rubisco compared to the less thermostable Rca 2β and 2α. Site-
directed mutagenesis of Rca 2β was used to replace a methionine by an isoleucine in the AAA+
domain, which increased the relative Rca activity at temperatures between 35-42.5 °C. The
mutant Rca 2β M160I improved thermostability by 4.6 °C and increased Rubisco activation at
35 and 40 °C by 30% compared to Rca2β. M160I appears to act as a thermal switch and can be
explored to improve Rubisco activation as temperatures become warmer and more volatile,
increasing wheat resilience to climate change.