Transgenic overexpression of rubisco subunits and the assembly factor RAF1 are beneficial to recovery from drought stress in maize

Abstract

Finding ways to increase Rubisco activity in crops to improve photosynthesis has long been a key objective. We have developed maize plants with transgenic overexpression of Rubisco subunits and Rubisco assembly factor 1 (RAF1) that resulted in higher Rubisco content, photosynthetic capacity and dry biomass under non-stressed growth conditions. The question remains, however, whether the transgenes conferring these effects could confer a growth penalty under stress conditions, given the nitrogen sink that Rubisco represents. In the present study, control and transgenic maize lines either overexpressing RAF1, both large and small subunits of Rubisco (LSSS), or all three (RAF1-LSSS), were subjected to mild, and severe drought conditions, and then re-watered. The results demonstrate that RAF1-LSSS, which has increased Rubisco abundance relative to the control, was slightly more affected than wild-type Rubisco level plants in terms of gas exchange and chlorophyll content after 14 days of stress. After returning plants to well-watered conditions, however, RAF1-LSSS displayed a more robust recovery compared to other genotypes. We attribute this to its high Rubisco content and the low malondialdehyde content noted prior to recovery, which indicates reduced oxidative degradation of lipids. Strong recovery was also observed in lines overexpressing RAF1 alone, in contrast with its lack of improved performance under non-stressed conditions. In contrast, LSSS displayed improved growth under non-stressed conditions, but showed a clear growth penalty during stress along with poor recovery. Taken together, our results suggest that while the high-Rubisco trait is neutral during water stress, recovery is more rapid once the stress is removed.