Surveying the diversity of Rubisco activase genes in Australian wild crop relatives

Rubisco activase (RCA) is the regulatory partner of Rubisco – the enzyme that fixes CO2 into sugars. RCA removes inhibitory compounds from the active sites of Rubisco. Thus, RCA can reactivate Rubisco, increasing the efficiency of biomass accumulation in plants. Under heat stress the association between RCA and Rubisco breaks down and plant productivity declines. However,wild crop relatives that occur in hot environments have divergent RCA isoforms that can function during heat stress. Because average daily temperatures are increasing, RCA has garnered interest for transgenic studies. This study aimed to characterise the heat response of photosynthesis and RCA abundance, and the sequence diversity of RCA in domestic and wild species of cotton (Gossypium) and tobacco (Nicotiana) and three populations of kangaroo grass (Themeda triandra). I found that wild species’ photosynthetic rates demonstrate a range of responses to increased temperature. I identified four isoforms of RCA in both Gossypium and Nicotiana species, and one isoform in Themeda as well as three RCA mutant isoforms in Gossypium and one in Nicotiana. This study showed that while the total RCA content of each of these species did not change in response to heat, thermotolerant species of both genera demonstrated unique patterns of RCA accumulation, indicating the importance of RCA stoichiometry in thermotolerance. Finally, I report the occurrence of polymorphisms in RCA from Gossypium, Nicotiana, and T. triandra which may contribute to changes in RCA rigidity but which do not fit with current ideas about RCA thermostability.