Nonphotochemical quenching (NPQ) regulates energy conversion in photosystem II and protects plants from photoinhibition. Here we analyze NPQ capacity in a number of rice cultivars. NPQ was strongly induced under medium and high light intensities in rice leaves. Japonica cultivars generally showed higher NPQ capacities than Indica cultivars when we measured a rice core collection. We mapped NPQ regulator and identified a locus (qNPQ1-2) that seems to be responsible for the difference in NPQ capacity between Indica and Japonica. One of the two rice PsbS homologues (OsPsbS1) was found within the qNPQ1-2 region. PsbS protein was not accumulated in the leaf blade of the mutant harboring transferred DNA insertion in OsPsbS1. NPQ capacity increased as OsPsbS1 expression increased in a series of transgenic lines ectopically expressing OsPsbS1 in an Indica cultivar. Indica cultivars lack a 2.7-kb region at the point 0.4 kb upstream of the OsPsbS1 gene, suggesting evolutionary discrimination of this gene.chlorophyll fluorescence | pulse amplitude modulation | quantitative trait loci analysis | rice subclass P lants have the potential to transform absorbed light energy to chemical energy at relatively high efficiency. However, the actual efficiency is dependent on the inherent capacities of photochemistry and carbon assimilation, and environmental factors including light intensity. The sites at which light energy is absorbed are photosystem I and II (PSI and PSII), which drive photochemistry and production of chemical energy (ATP and NADPH). For PSII, energy transformation processes of chlorophyll excitation energy can be measured by the pulse amplitude modulation technique of chlorophyll fluorescence measurement (1-3).Chlorophyll deexcitation processes are divided into three groups: photochemistry (photosynthetic electron transport), basal dissipation/ non-light induced quenching (NO), and thermal dissipation, which results in nonphotochemical quenching (NPQ) of chlorophyll fluorescence. Basal dissipation consists of chlorophyll fluorescence, internal conversion, and intersystem crossing. With increasing light intensity, there is a decrease in efficiency of use of excitons in photochemistry and an increase in NPQ. More than half of absorbed energy can be lost through NPQ under high illumination (4, 5). NPQ consists of several components. These can be distinguished by the rate of induction of NPQ in the light and by the rate of its relaxation in the dark (6). The NPQ component that is rapidly induced by illumination and relaxes rapidly in the dark is called qE. qE is the energy-dependent quenching linked to the proton motive force.Photoinhibition indicates light stress and, under high illumination, excessive energy results in photodamage with inactivation of the PSII machinery. This leads to a decrease in the photochemical rate constant and thermal loss of energy caused by photoinhibition. Photoinhibition results in inactivation of a part of the PSII reaction centers. The degree of photoinhibition and the intrinsic loss of...

Boron (B) is one of the essential nutrients for plant growth and reproduction. Transcriptome analyses have identified genes regulated by B deficiency, but their function mostly remains elusive. To identify the functions of B deficiency-inducible genes, T-DNA insertion mutants of 10 B deficiency-induced genes were obtained, and their growth properties in response to B conditions were examined. Among the lines examined, mutants of the transcription factor WRKY6 showed growth defect compared with the wild-type under B deficiency, but not under normal conditions. This growth defect was commonly observed among three independently isolated wrky6 mutants. There was no significant difference in B concentration between wrky6-3 and the wild-type. Promoter activity of WRKY6 was induced around the root tip under B deficiency. These results established that WRKY6 is a low-B-induced transcription factor gene that is essential for normal root growth under low-B conditions. Transcriptome analysis around the root tip identified WRKY6-regulated genes under B deficiency. Our findings represent the first identification of a transcription factor involved in the response to B deficiency.

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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...