PARENT SESSION

Symposium S6C Photosynthesis, respiration alternative electron sinks
Thursday September 2nd, 2004 10:20 AM-12:20 PM Room 510B
Chair: Greg Vanlerberghe
Co-Chair: Allan Rasmusson

Light regulation of the Arabidopsis respiratory chain: a molecular analysis. Matthew Escobar^*,1, Keara Franklin², Staffan Svensson³, Michael Salter², Garry Whitelam², Allan Rasmusson¹, ¹ Lund University, Lund, Sweden² University of Leicester, Leicester, United Kingdom³ The Royal Veterinary and Agricultural University, Frederiksberg C, Denmark

ABSTRACT- The sequential oxidation reactions catalyzed by the large, proton-pumping complexes of the respiratory chain (complex I, complex III, and complex IV) generate an electrochemical gradient across the mitochondrial inner membrane that is harnessed for ATP production. However, in plants alternative respiratory pathways catalyzed by the alternative oxidases (AOX), uncoupling proteins (UCP), and type II NAD(P)H dehydrogenases (NDA/B/C) allow the maintenance of substrate oxidation while minimizing the production of ATP. We have investigated the role of light in the regulation of these "energy dissipating" respiratory pathways by transcriptional profiling of the aox, ucp, and nda/b/c gene families in etiolated Arabidopsis thaliana seedlings. The type II NAD(P)H dehydrogenase genes nda1 and ndc1 were found to be rapidly and substantially induced by a broad range of light intensities and qualities. Light regulation of both genes appears to be a direct (photoreceptor-mediated) transcriptional effect that is independent of carbon status. Mutant analyses have demonstrated the involvement of two (or more) separate photoreceptor families in nda1 and ndc1 light regulation: the phytochromes (phyA and phyB), and an undetermined blue light photoreceptor (cryptochromes and/or phototropins). The multiple photoreceptors controlling nda1 light regulation display distinct kinetic profiles of activity which are integrated in the response to natural (white) light conditions. Primary transcriptional control of light response was localized to a 99 bp fragment of the nda1 promoter that contains an I-box flanked by two GT-1 sites, a conserved modular arrangement of cis elements that is prevalent in the promoters of the light-regulated rubisco small subunit genes (rbcS). nda1 gene expression was localized to cotyledons/leaves, which, in combination with its strong photocontrol, suggests a potential connection with photosynthetic metabolism. Our current hypothesis is that nda1 (and ndc1) mediate rapid adjustment of respiratory oxidative capacity to support the photorespiratory oxidative decarboxylation of glycine in the mitochondria.

KEY WORDS: type II NAD(P)H dehydrogenase, photoregulation, respiration, light