Oxalate oxidase transgene expression in American chestnut leaves has little effect on photosynthetic or respiratory physiology

The American chestnut (Castanea dentata) was a widespread foundation species before the spread of the chestnut blight (caused by Cryphonectria parasitica). Resistance to blight has been achieved by the genetic insertion of a single copy of an oxalate oxidase gene into the chestnut genome. Here, we assess potential transgene impacts on American chestnut physiology, with a focus on photosynthesis and respiration. We collected measurements of leaf respiration and photosynthetic capacity for transgenic (T) and non-transgenic (NT) sibling trees in two distinct experiments. Multiple measurements of photosynthesis (light and CO₂ response curves) and foliar traits (leaf mass per unit area, foliar N concentration) were indicative of equally high rates of photosynthetic capacity across T and NT plants, with no significant differences between groups. Photosynthetic rates were equivalent between T and NT plants across two studies in two locations. We observed a modest stimulation of foliar dark respiration in T vs. NT plants (~ 5–15%) across a range of temperatures, but no change in foliar respiration in the light. The modest stimulation of dark respiration did not seem to be associated with an alteration in growth rate, as stem diameter and length were equivalent between T and NT types. Our findings suggest that there may be a minor impact of transgene expression on the respiratory physiology in some situations, but this effect is not likely to strongly impact the physiological ecology of this historically important species.