Suppression of Reactive Oxygen Species Accumulation in Chloroplasts Prevents Leaf Damage but Not Growth Arrest in Salt-Stressed Tobacco Plants.

Crop yield reduction due to salinity is a growing agronomical concern in many regions. Increased production of reactive oxygen species (ROS) in plant cells accompanies many abiotic stresses including salinity, acting as toxic and signaling molecules during plant stress responses. While ROS are generated in various cellular compartments, chloroplasts represent a main source in the light, and plastid ROS synthesis and/or elimination have been manipulated to improve stress tolerance.

An in vivo system involving co-expression of cyanobacterial flavodoxin and ferredoxin-NADP(+) reductase confers increased tolerance to oxidative stress in plants.

Oxidative stress in plants causes ferredoxin down-regulation and NADP(+) shortage, over-reduction of the photosynthetic electron transport chain, electron leakage to oxygen and generation of reactive oxygen species (ROS). Expression of cyanobacterial flavodoxin in tobacco chloroplasts compensates for ferredoxin decline and restores electron delivery to productive routes, resulting in enhanced stress tolerance. We have designed an in vivo system to optimize flavodoxin reduction and NADP(+) regeneration under stress using a version of cyanobacterial ferredoxin-NADP(+) reductase without the thylakoid-binding domain.

Cyanobacterial flavodoxin complements ferredoxin deficiency in knocked-down transgenic tobacco plants.

Ferredoxins are the main electron shuttles in chloroplasts, accepting electrons from photosystem I and delivering them to essential oxido-reductive pathways in the stroma. Ferredoxin levels decrease under adverse environmental conditions in both plants and photosynthetic micro-organisms. In cyanobacteria and some algae, this decrease is compensated for by induction of flavodoxin, an isofunctional flavoprotein that can replace ferredoxin in many reactions.

Transgenic tobacco plants overexpressing chloroplastic ferredoxin-NADP(H) reductase display normal rates of photosynthesis and increased tolerance to oxidative stress.

Ferredoxin-NADP(H) reductase (FNR) catalyzes the last step of photosynthetic electron transport in chloroplasts, driving electrons from reduced ferredoxin to NADP+. This reaction is rate limiting for photosynthesis under a wide range of illumination conditions, as revealed by analysis of plants transformed with an antisense version of the FNR gene. To investigate whether accumulation of this flavoprotein over wild-type levels could improve photosynthetic efficiency and growth, we generated transgenic tobacco (Nicotiana tabacum) plants expressing a pea (Pisum sativum) FNR targeted to chloroplasts.

Transgenic tobacco plants expressing antisense ferredoxin-NADP(H) reductase transcripts display increased susceptibility to photo-oxidative damage.

Ferredoxin-NADP(H) reductase (FNR) catalyses the final step of the photosynthetic electron transport in chloroplasts. Using an antisense RNA strategy to reduce expression of this flavoenzyme in transgenic tobacco plants, it has been demonstrated that FNR mediates a rate-limiting step of photosynthesis under both limiting and saturating light conditions. Here, we show that these FNR-deficient plants are abnormally prone to photo-oxidative injury.

The flavoenzyme ferredoxin (flavodoxin)-NADP(H) reductase modulates NADP(H) homeostasis during the soxRS response of Escherichia coli.

Escherichia coli cells from strain fpr, deficient in the soxRS-induced ferredoxin (flavodoxin)-NADP(H) reductase (FPR), display abnormal sensitivity to the bactericidal effects of the superoxide-generating reagent methyl viologen (MV). Neither bacteriostatic effects nor inactivation of oxidant-sensitive hydrolyases could be detected in fpr cells exposed to MV. FPR inactivation did not affect the MV-driven soxRS response, whereas FPR overexpression led to enhanced stimulation of the regulon, with concomitant oxidation of the NADPH pool.