Decreased Content of Leaf Ferredoxin Changes Electron Distribution and Limits Photosynthesis in Transgenic Potato Plants

Abstract

A complete ferredoxin (Fd) cDNA clone was isolated from potato (Solanum tuberosum L. cv Desiree) leaves. By molecular and immunoblot analysis, the gene was identified as the leaf-specific Fd isoform I. Transgenic potato plants were constructed by introducing the homologous potato fed 1 cDNA clone as an antisense construct under the control of the constitutive cauliflower mosaic virus 35S promoter. Stable antisense lines with Fd contents between 40% and 80% of the wild-type level were selected by northern-and western-blot analysis. In short-term experiments, the distribution of electrons toward their stromal acceptors was altered in the mutant plants. Cyclic electron transport, as determined by the quantum yields of photosystems I and II, was enhanced. The CO 2 assimilation rate was decreased, but depending on the remaining Fd content, some lines showed photoinhibition. The leaf protein content remained largely constant, but the antisense plants had a lower total chlorophyll content per unit leaf area and an increased chlorophyll a/b ratio. In the antisense plants, the redox state of the quinone acceptor A in photosystem II (Q A ) was more reduced than that of the wild-type plants under all experimental conditions. Because the plants with lower Fd amounts reacted as if they were grown under a higher light intensity, the possibility that the altered chloroplast redox state affects light acclimation is discussed.Ferredoxins (Fds) are small, iron-and sulfurcontaining proteins that act as low-potential oneelectron carriers. In chloroplasts, Fd distributes the electrons from PSI onto the various electronconsuming reactions in the chloroplast stroma (Arnon, 1988). During nitrogen (N) and sulfur (S) assimilation, reduced Fd is directly used by nitrite reductase, Gln synthase, sulfite reductase (Knaff and Hirasawa, 1991), and by enzymes of secondary metabolism, such as choline monooxygenase (Brouquisse et al., 1989). Moreover, Fd supplies electrons via Ferredoxin-NADP ϩ -Reductase for NADP reduction (Knaff and Hirasawa, 1991). The generated NADPH serves as a soluble reductant in the chloroplast stroma, mainly for the reduction of 1,3bisphos-phoglycerate by NAD(P)-dependent glyceraldehyde 3-phosphate dehydrogenase during CO 2 assimilation (Leegood, 1996). Fd is involved in the regulation of redox-modulated chloroplast enzymes via electron flow toward ferrodoxin-thioredoxin-reductase (FTR) and thioredoxins (Knaff and Hirasawa, 1991). Thioredoxins reduce their target enzymes and, thus, in interplay with specific metabolic effectors, adjust the enzyme activation states to the actual demand (Scheibe, 1991). Moreover, the redox state of thioredoxins also acts on chloroplast gene expression by regulating transcription and translation of several chloroplast proteins (Kim and Mayfield, 1997;Link, 2001).Fd is furthermore involved in nonassimilatory electron fluxes that act to adjust the stromal ATP/2e Ϫ ratio. Electrons are often generated in excess to the amount required for CO 2 fixation or photorespiration (Backhausen et al., 1...

Transgenic Tobacco Plants Overexpressing Chloroplastic Ferredoxin-NADP(H) Reductase Display Normal Rates of Photosynthesis and Increased Tolerance to Oxidative Stress

“…Transgenic plants in which the content of ferredoxin (Fd; Holtgrefe et al, 2003) or the Rieske iron-sulfur (FeS) protein of the cytochrome b 6 f complex (Price et al, 1998) were diminished by expression of antisense RNA exhibited lower CO 2 assimilation rates, which correlated fairly well with the levels of remaining protein. Using a similar strategy, we restrained the expression of chloroplast Fd-NADP(H) reductase (FNR), a flavin adenine dinucleotide-containing enzyme that catalyzes the final step of the PETC, namely, electron transfer from reduced Fd to NADP 1 (Hajirezaei et al, 2002).…”

“…Antisense potato plants with decreased levels of Fd also displayed growth penalties, but attenuated with respect to those caused by depletion of FNR or the Rieske FeS protein (Holtgrefe et al, 2003). The milder phenotype is probably related to the fact that Fd is engaged in electron distribution to other chloroplast processes besides NADP 1 reduction and therefore accumulates above the levels of other PETC components, reaching a concentration of about 80 mM (YonekuraSakakibara et al, 2000).…”

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…Part of the Fd reduced by photosystem I delivers electrons to Fd-NADP ϩ reductase for NADP ϩ photoreduction, but a substantial fraction is used as electron donor to other plastidic enzymes such as nitrite reductase, sulfite reductase, Glu-oxoglutarate aminotransferase, fatty acid desaturase, and the Fd-Trx reductase FTR (30). Then, the function and regulation of metabolisms so essential to plant life as carbon fixation and allocation, nitrogen and sulfur assimilation, amino acid synthesis, and fatty acid desaturation depend on steady provision of reduced Fd (31). As could be anticipated by this multiplicity of functions, transgenic plants in which Fd contents were diminished by expression of an antisense RNA exhibited a pleiotropic phenotype including lower CO 2 assimilation rates, stunted growth, and a combination of other handicaps (31).…”

“…Then, the function and regulation of metabolisms so essential to plant life as carbon fixation and allocation, nitrogen and sulfur assimilation, amino acid synthesis, and fatty acid desaturation depend on steady provision of reduced Fd (31). As could be anticipated by this multiplicity of functions, transgenic plants in which Fd contents were diminished by expression of an antisense RNA exhibited a pleiotropic phenotype including lower CO 2 assimilation rates, stunted growth, and a combination of other handicaps (31).…”

Functional Replacement of Ferredoxin by a Cyanobacterial Flavodoxin in Tobacco Confers Broad-Range Stress Tolerance

“…Such an effect could deeply affect central metabolic pathways of the chloroplast and the cell and contribute to the growth penalties exhibited by plants exposed to adverse conditions. Transgenic potato (Solanum tuberosum) plants in which the contents of Fd were diminished by the expression of antisense RNA displayed lower CO 2 assimilation rates, growth arrest, and altered distribution of reducing equivalents to oxidoreductive pathways of the stroma (Holtgrefe et al, 2003).…”