Minisymposium 4: Oxidative stress

Abs # 14002: Photosynthetic performance of Arabidopsis APX3 knockout mutants under heat stress conditions

Presenter:	Narendra, Savitha , Savitha.Narendra@ttu.edu
Authors	Narendra, Savitha  (A)   Kornyeyev, Dmytro  (A)   Yan, Juqiang  (A)   Holaday, Scott  (A)   Zhang, Hong  (A)
Affiliations:	(A): Texas Tech University

Enzymatic actions of superoxide dismutase, catalase and ascorbate peroxidase (APX) remove reactive oxygen species (ROS) in plant cells and therefore protect plants from the harmful effects of ROS. APX is a H₂O₂-scanvenging enzyme that exists in five isoforms, and we have been studying the peroxisomal membrane-bound isoform (i.e. APX3) in Arabidopsis. To study the function of APX3, we analyzed the photosynthetic performance of APX3 knockout mutants under heat stress conditions. Chlorophyll fluorescence analysis was used to study energy partitioning in Photsystem II (PSII) complexes. We first light acclimated plants at 350 mmol m^-2s^-1 and 25 ^oC, and then the leaf temperature was raised to 40 ^oC. The temperature shift led to short-term increase in effective quantum yield of PSII photochemistry, the portion of light energy that was used for linear electron transport. This increase can be explained simply by thermodynamic properties of electron transport and metabolic reactions utilizing primary products of photosynthesis. The efficient quantum yield of PSII photochemistry declined after about 0.5 hr under heat stress for both wild type and knockout plants. Such an effect is likely to be associated with temperature-induced damage to photosynthetic machinery including PSII complex. No considerable changes in photosynthetic electron transport capacity occurred when the leaf temperature was elevated up to 38 ^oC. An enhanced efficiency of non-photochemical quenching of excitation energy was observed as a result of leaf temperature elevation. It was caused by PSII damage as well as by down regulation of PSII activity. Such changes in the energy partitioning in PSII complexes allowed plants to minimize the portion of the light energy absorbed by PSII antennae that reaches ‘closed’ reaction centers, which correlates with non-reversible inactivation of PSII (Plant Cell Environ 2003 Vol.44. p.318 and p.1064). It was found that the decrease in the effective quantum yield of PSII photochemistry occurred earlier in the heat-stressed (40 ^oC) leaves of APX3 knockout mutants than in those of wild type. Therefore, one may speculate that APX3 is likely involved in scavenging H₂O₂ generated through photorespiration, because loss of APX3 apparently leads to a modified response of photosynthetic electron transport to elevated temperature. However, loss of APX3 does not appear to affect the plant growth and development under normal conditions.