Submitted to: Functional Plant Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/15/2002
Publication Date: 2/5/2003
Citation: Kornyeyer, D., Logan, B., Payton, P.R., Allen, R., Holaday, S. 2003. Elevated chloroplastic glutathione reductase activities decrease chilling-induced photoinhibition by increasing rates of photochemistry, but not thermal energy dissipation, in transgenic cotton. Functional Plant Biology. 30(1): 101-110. Interpretive Summary: Chilling temperatures can result in decreased photosynthesis in chilling-sensitive plants. In crop plants, such as cotton, exposure to moderate chilling can result in cellular damage and ultimately decreased yield. One effect of chilling temperatures in high light conditions is an increase in cellular damage from reactive oxygen species (oxidative stress). Plants possess protective mechanisms that minimize damage from reactive oxygen species, but these systems can be overwhelmed during exposure to chilling in the light, especially in chilling-sensitive species. In an effort to protect cotton plants from oxidative damage, we have engineered plants to over-express a key enzyme in reactive oxygen scavenging, glutathione reductase (GR). Previously, we have shown that increasing the activity of this enzyme in cotton chloroplasts, a major source of reactive oxygen, resulted in a decrease in the photoinhibition of photosynthesis during chilling in the light. The aim of this study was to examine the possible mechanisms underlying this enhanced chilling tolerance. The conclusions drawn here are that increased GR activity and subsequent consumption of reducing power partly explains the maintenance of electron flow through the photosynthetic machinery, minimizing the reduction of oxygen, the production of reactive oxygen species, and resultant oxidative damage. Thermal energy dissipation, another protective mechanism against oxidative damage, was unaffected by elevated GR activities.
Technical Abstract: Abstract. The effect of the overproduction of glutathione reductase (GR+) in cotton (Gossypium hirsutum L. cv. Coker 312) chloroplasts on the response of photosynthetic parameters to chilling in the light was examined. After 180 min at 10ºC and 500 mol photons m-2 s-1 in the chamber of an oxygen electrode, leaf discs of GR+ plants exhibited lower levels of sustained PSII photoinhibition than leaf discs of wildtype plants. No genotypic differences in thermal energy dissipation, leaf pigment composition, or the dynamics of xanthophyll cycle de-epoxidation were observed. The rate of induction and steady-state levels of photochemistry were greater for GR+ in comparison to wildtype plants. Enhanced photochemistry in GR+ plants could not be attributed to higher rates of CO2 assimilation at 10ºC. Although GR overproduction afforded some increased protection against PSI photoinactivation, suggesting improved scavenging of reactive oxygen species, higher PSI activities could not completely explain the greater rates of photochemistry. Pools of glutathione and ascorbate were significantly more reduced in GR+ plants. Increased demand for reducing power to maintain these constituents in the reduced state may contribute to the higher rates of photochemistry observed in GR+ plants.