Heat tolerance in a wild Oryza species is attributed to maintenance of Rubisco activation by a thermally stable Rubisco activase ortholog.

The mechanistic basis of tolerance to heat stress was investigated in Oryza sativa and two wild rice species, Oryza meridionalis and Oryza australiensis. The wild relatives are endemic to the hot, arid Australian savannah. Leaf elongation rates and gas exchange were measured during short periods of supra-optimal heat, revealing species differences. The Rubisco activase (RCA) gene from each species was sequenced. Using expressed recombinant RCA and leaf-extracted RCA, the kinetic properties of the two isoforms were studied under high temperatures. Leaf elongation was undiminished at 45 degrees C in O.australiensis. The net photosynthetic rate was almost 50% slower in O.sativa at 45 degrees C than at 28 degrees C, while in O.australiensis it was unaffected. Oryza meridionalis exhibited intermediate heat tolerance. Based on previous reports that RCA is heat-labile, the Rubisco activation state was measured. It correlated positively with leaf elongation rates across all three species and four periods of exposure to 45 degrees C. Sequence analysis revealed numerous polymorphisms in the RCA amino acid sequence from O.australiensis. The O. australiensis RCA enzyme was thermally stable up to 42 degrees C, contrasting with RCA from O.sativa, which was inhibited at 36 degrees C. We attribute heat tolerance in the wild species to thermal stability of RCA, enabling Rubisco to remain active.