A spontaneous mutation in a key C4 pathway gene significantly alters leaf δ13C uncoupling its relationship with WUE and photosynthetic performance in Zea mays

Increases in global temperature and drought are negatively impacting the yields of major crops. Therefore, targeted improvements to intrinsic water use efficiency (WUEi) are needed to reduce the water required for agricultural production. While it is very time-consuming to directly measure WUEi, stable carbon isotope ratios ({delta}13C) are a reliable high throughput proxy trait for quantifying WUEi in C3 species. While genetic studies have improved our understanding of the relationship between WUEi and {delta}13C in C4 species, the knowledge needed to implement {delta}13C in breeding schemes is incomplete. Using a Zea mays line with an extremely negative {delta}13C value, a quantitative genetics approach was used to identify a large deletion in carbonic anhydrase1 (cah1). Carbonic anhydrase is the first enzymatic step of the C4 photosynthetic pathway and is known to affect {delta}13C. Surprisingly, the line with the mutant allele has significantly higher carbonic anhydrase activity with a concurrent reduction in {delta}13C, opposite of what would be expected based on C4 carbon isotope fractionation theory. These observed decouple {delta}13C and WUEi, which calls for further investigation into carbon isotope discrimination in C4 species.