Plant Physiology Preview
Published on August 4, 2006; 10.1104/pp.106.085886
Received June 28, 2006
Accepted July 27, 2006
IMMUTANS does not Act as a Stress-induced Safety Valve in the Protection of the Photosynthetic Apparatus of Arabidopsis during Steady State Photosynthesis
Dominic Rosso , Alexander G. Ivanov , Aigen Fu , Jane Geisler-Lee , Luke Hendrickson , Matt Geisler , Gregory Stewart , Marianna Krol , Vaughan Hurry , Steven R. Rodermel , Denis P. Maxwell , and Norman P.A. Hüner *
Department of Biology and The Biotron, University of Western Ontario, London, ON, Canada, N6A 5B7
Department of Genetics, Development and Cell Biology, Iowa State University, Ames, Iowa, USA, 50011
Department of Biology and The Biotron, University of Western Ontario, London, ON, Canada, N6A 5B7; Umea Plant Science Centre, Department of Plant Physiology, Umea University, Umea S-901 87, Sweden
Umea Plant Science Centre, Department of Plant Physiology, Umea University, Umea S-901 87, Sweden
* Corresponding author; email: nhuner{at}uwo.ca.
IMMUTANS encodes a thylakoid membrane protein that has been hypothesized to act as a terminal oxidase that couples the reduction of O2 to the oxidation of the plastoquinone pool of the photosynthetic electron transport chain. Because IMMUTANS shares sequence similarity to the stress-induced mitochondrial alternative oxidase, it has been suggested that the protein encoded by IMMUTANS acts as a safety valve during the generation of excess photosynthetically generated electrons. We combined in vivo chlorophyll fluorescence quenching analyses with measurements of the redox state of P700 to assess the capacity of IMMUTANS to compete with photosystem I for intersystem electrons during steady state photosynthesis in Arabidopsis thaliana. Comparisons were made between WT plants, immutans mutant plants, as well as transgenics in which IMMUTANS protein levels had been over-expressed 6 (OE-6X) and 16 (OE-16X) times. Immunoblots indicated that IMMUTANS abundance was the only major variant that we could detect between these genotypes. Over-expression of IMMUTANS did not result in increased capacity to keep the plastoquinol pool oxidized compared to either the WT or immutans grown under control conditions (25°C and 150 µmol photons m-2 s-1). Similar results were observed either after a 3 day cold stress at 5°C or after full leaf expansion at 5°C and 150 µmol photons m-2 s-1. Furthermore, IMMUTANS abundance did not enhance protection of either photosystem II (PSII) or PSI from photoinhibition at either 25° or 5°C. Our in vivo data indicate that modulation of IMMUTANS expression and polypeptide accumulation does not alter the flux of intersystem electrons to P700
+ during steady-state photosynthesis and does not provide any significant photoprotection. In contrast to AOX1a, meta-analyses of published Arabidopsis micro-array data indicated that IMMUTANS expression exhibited minimal modulation in response to a myriad of abiotic stresses which is consistent with our functional data. However, IMMUTANS exhibited significant modulation in response to development in concert with changes in AOX1a expression. Thus, neither our functional analyses of the IMMUTANS knockout and over-expression lines nor meta-analyses of gene expression support the model that IMMUTANS acts as a safety valve to regulate the redox state of the plastoquinone pool during stress and acclimation. Rather, IMMUTANS appears to be strongly regulated by developmental stage of Arabidopsis thaliana.
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