Transcript-protein decoupling of chloroplast targeted enzymes in a stress inducible GOC photorespiratory bypass in rice

Abstract Background Photorespiration is a major source of carbon loss in C 3 plants, and introducing photorespiratory bypasses into chloroplasts has been pursued as a strategy to improve photosynthetic efficiency. Constitutive expression of the GOC (glycolate oxidase, oxalate oxidase, catalase) bypass has been shown to enhance biomass in rice and potato. Here, we constructed a drought-inducible version of the GOC bypass in rice, aiming to increase the CO 2 concentration around Rubisco and thereby improve the stress tolerance of rice. Results Under drought stress, transcripts of all three transgenes were strongly induced to levels tens of times higher than the endogenous control. However, neither the corresponding proteins nor their enzymatic activities increased proportionally; under ABA or osmotic treatment, they still remained at wild type levels or even decreased. Consistent with the absence of functional enzyme accumulation, transgenic lines showed no improved tolerance to drought or osmotic stress, and exhibited even greater membrane damage than wild type plants under osmotic stress. These results reveal a striking disconnect between transcription and functional protein accumulation for chloroplast targeted enzymes under stress. We propose that stress-activated blockade of chloroplast protein import (possibly via a CHLORAD-like mechanism), together with insufficient translational capacity for multiple transgenes, may account for this bottleneck. Conclusion The transcript-protein decoupling of chloroplast targeted enzymes in inducible GOC bypass provides a cautionary empirical note for future efforts to engineer stress-inducible metabolic pathways in chloroplasts: transcript induction alone does not guarantee protein synthesis and/or delivery, and protein-level verification under the target stress conditions is indispensable.