Metabolome and Photochemical Analysis of Rice Plants Overexpressing Arabidopsis NAD Kinase Gene    

Abstract

The chloroplastic NAD kinase (NADK2) is reported to stimulate carbon and nitrogen assimilation in Arabidopsis (Arabidopsis thaliana), which is vulnerable to high light. Since rice (Oryza sativa) is a monocotyledonous plant that can adapt to high light, we studied the effects of NADK2 expression in rice by developing transgenic rice plants that constitutively expressed the Arabidopsis chloroplastic NADK gene (NK2 lines). NK2 lines showed enhanced activity of NADK and accumulation of the NADP(H) pool, while intermediates of NAD derivatives were unchanged. Comprehensive analysis of the primary metabolites in leaves using capillary electrophoresis mass spectrometry revealed elevated levels of amino acids and several sugar phosphates including ribose-1,5-bisphosphate, but no significant change in the levels of the other metabolites. Studies of chlorophyll fluorescence and gas change analyses demonstrated greater electron transport and CO 2 assimilation rates in NK2 lines, compared to those in the control. Analysis of oxidative stress response indicated enhanced tolerance to oxidative stress in these transformants. The results suggest that NADP content plays a critical role in determining the photosynthetic electron transport rate in rice and that its enhancement leads to stimulation of photosynthesis metabolism and tolerance of oxidative damages.NADP is a ubiquitous coenzyme, required in various metabolic processes, since these metabolites carry electrons through the reversible conversion between oxidized (NAD + , NADP + ) and reduced (NADH, NADPH) forms in all organisms. NAD is highly oxidized and is involved primarily in intracellular catabolic reactions, whereas NADP is predominantly found in its reduced form and participates in anabolic reactions and defense against oxidative stress (Ziegler, 2000;Noctor et al., 2006;Pollak et al., 2007a). Since NAD(H) and NADP(H) play a variety of distinct physiological roles, the regulation of the NAD(H)/ NADP(H) balance is essential for cell survival (Kawai and Murata, 2008;Hashida et al., 2009).One of the key enzymes that regulates NAD(H)/ NADP(H) balance is NAD kinase (NADK; EC 2.7.1.23), which catalyzes NAD phosphorylation in the presence of ATP. The genes encoding NADK were cloned recently from all organisms investigated to date, except for Chlamydia trachomatis (Kawai and Murata, 2008). Only a single gene encoding NADK has been found in some bacteria and mammals (Kawai and Murata, 2008). In contrast, NADK activity was detected in not only the cytosol but also organelles in yeast and plant (Jarrett et al., 1982;Simon et al., 1982;Dieter and Marme, 1984;, and three genes including cytosol-type and organelle-type NADK have been cloned in yeast (Kawai et al., 2001;Outten and Culotta, 2003) and plants (Turner et al., 2004(Turner et al., , 2005.In Arabidopsis (Arabidopsis thaliana), one of the NADK isoforms is localized in the chloroplast (NADK2; Chai et al., 2005) Analysis of Arabidopsis mutants revealed low chlorophyll (chl) content, low photosynthetic activity, growth inhibi...

SALT-RESPONSIVE ERF1 Regulates Reactive Oxygen Species-Dependent Signaling during the Initial Response to Salt Stress in Rice

“…In both wild-type and serf1 roots, a decrease in NADP + and NADPH levels was observed upon salt stress. NADP(H) levels determine the sizes and redox states of GSH and ascorbate pools (Takahara et al, 2010). Total GSH levels in the wild type and serf1 were similar under control conditions (see Supplemental Figures 4E and 4H online).…”

Essential Role of Tissue-Specific Proline Synthesis and Catabolism in Growth and Redox Balance at Low Water Potential

“…oxidases, activity of many NADPH-dependent enzymes, and nitric oxide production (Zhang et al, 2009;Takahara et al, 2010;Dietz and Pfannschmidt, 2011). Therefore, control of NADP/NADPH ratio by Pro metabolism could impact other metabolic and signaling pathways.…”

Molecular distinction in genetic regulation of nonphotochemical quenching in rice

“…Furthermore, overexpression of the same gene in rice showed increased photosynthesis by activated photochemistry (10). In this report, we investigate the genetic regulation of qE capacity in rice.…”

“…Even recognizing that rice leaves reduce light intensity per unit area by maintaining relatively vertical leaf angles, qE would always be strongly induced in many parts of the leaf in daytime. There is loss of efficiency of energy use in photochemistry with increasing light, and photosynthesis in rice is saturated well below full sunlight (10,12). Thus, the ability of qE to safely dissipate excess energy in rice will continue to be important for sustainable growth.…”