Although active oxygen species are produced at high rates in both the chloroplasts and peroxisomes of the leaves of C3 plants, most attention has focused on the potentially damaging consequences of enhanced chloroplastic production in stress conditions such as drought. This article attempts to provide quantitative estimates of the relative contributions of the chloroplast electron transport chain and the glycolate oxidase reaction to the oxidative load placed on the photosynthetic leaf cell. Rates of photorespiratory H2O2 production were obtained from photosynthetic and photorespiratory flux rates, derived from steady-state leaf gas exchange measurements at varying irradiance and ambient CO2. Assuming a 10% allocation of photosynthetic electron flow to the Mehler reaction, photorespiratory H2O2 production would account for about 70% of total H2O2 formed at all irradiances measured. When chloroplastic CO2 concentration rates are decreased, photorespiration becomes even more predominant in H2O2 generation. At the increased flux through photorespiration observed at lower ambient CO2, the Mehler reaction would have to account for more than 35% of the total photosynthetic electron flow in order to match the rate of peroxisomal H2O2 production. The potential signalling role of H2O2 produced in the peroxisomes is emphasized, and it is demonstrated that photorespiratory H2O2 can perturb the redox states of leaf antioxidant pools. We discuss the interactions between oxidants, antioxidants and redox changes leading to modified gene expression, particularly in relation to drought, and call attention to the potential significance of photorespiratory H2O2 in signalling and acclimation.

The importance of the mitochondrial electron transport chain in photosynthesis was studied using the tobacco (Nicotiana sylvestris) mutant CMSII, which lacks functional complex I. Rubisco activities and oxygen evolution at saturating CO 2 showed that photosynthetic capacity in the mutant was at least as high as in wild-type (WT) leaves. Despite this, steady-state photosynthesis in the mutant was reduced by 20% to 30% at atmospheric CO 2 levels. The inhibition of photosynthesis was alleviated by high CO 2 or low O 2 . The mutant showed a prolonged induction of photosynthesis, which was exacerbated in conditions favoring photorespiration and which was accompanied by increased extractable NADP-malate dehydrogenase activity. Feeding experiments with leaf discs demonstrated that CMSII had a lower capacity than the WT for glycine (Gly) oxidation in the dark. Analysis of the postillumination burst in CO 2 evolution showed that this was not because of insufficient Gly decarboxylase capacity. Despite the lower rate of Gly metabolism in CMSII leaves in the dark, the Gly to Ser ratio in the light displayed a similar dependence on photosynthesis to the WT. It is concluded that: (a) Mitochondrial complex I is required for optimal photosynthetic performance, despite the operation of alternative dehydrogenases in CMSII; and (b) complex I is necessary to avoid redox disruption of photosynthesis in conditions where leaf mitochondria must oxidize both respiratory and photorespiratory substrates simultaneously.Plant biomass production is ultimately determined by the ratio between photosynthesis and respiratory CO 2 release, and 30% to 70% of fixed carbon can be rereleased within each 24-h cycle (Lambers, 1997). Therefore, manipulation of respiration is a potential approach to improving agricultural yields (Lewis et al., 2000). One current difficulty is that, quite apart from the roles played by respiration in heterotrophic tissues, the importance of leaf respiration for the process of carbon fixation itself is not yet fully established. Leaf mitochondria may have several functions during photosynthesis (for review, see Krö mer, 1995; Hoefnagel et al., 1998; Gardeströ m et al., 2002). These include oxidation of malate generated by the photosynthetic electron transport chain (Backhausen et al., 1998), production of organic acids for chloroplastic ammonia assimilation (Foyer et al., 2001), and generation of ATP to support UDP-Glc formation for Suc synthesis (Krö mer et al., 1988(Krö mer et al., , 1993. A key function of leaf mitochondria during photosynthesis, particularly in C 3 species, is the oxidation of Gly produced in the photorespiratory pathway (Douce and Neuburger, 1989) through reactions catalyzed by Gly decarboxylase (GDC) and Ser hydroxymethyl transferase, enzymes found in abundance in the mitochondrial matrix (Oliver et al., 1990). Although the importance of these enzymes in photorespiratory carbon recycling is well established (Somerville and Ogren, 1981; Blackwell et al., 1990; Heineke et al., 2001), and although is...

The aim of this work was to examine whether carhohydrates are involved in signalling N deficiency through sourcersink imbalance. Photosynthetic metaholism in tohacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated hy shading leaves on some of the plants. In N-sufficient plants over this timescale, there was a small decline in photosynthetic rate, Ruhisco protein and amino acid content, with a larger decrease in carhohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO2), which was caused hy a large decrease in the amount of Rubiseo protein (62% after 8 d) and Ruhisco activity. Higher amounts of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the sourcersink ratio hy shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.

Cellular redox homeostasis is a hub for signal integration. Interactions between redox metabolism and the ABSCISIC ACID-INSENSITIVE-4 (ABI4) transcription factor were characterized in the Arabidopsis thaliana vitamin c defective1 (vtc1) and vtc2 mutants, which are defective in ascorbic acid synthesis and show a slow growth phenotype together with enhanced abscisic acid (ABA) levels relative to the wild type (Columbia-0). The 75% decrease in the leaf ascorbate pool in the vtc2 mutants was not sufficient to adversely affect GA metabolism. The transcriptome signatures of the abi4, vtc1, and vtc2 mutants showed significant overlap, with a large number of transcription factors or signaling components similarly repressed or induced. Moreover, lincomycin-dependent changes in LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN 1.1 expression were comparable in these mutants, suggesting overlapping participation in chloroplast to nucleus signaling. The slow growth phenotype of vtc2 was absent in the abi4 vtc2 double mutant, as was the sugar-insensitive phenotype of the abi4 mutant. Octadecanoid derivative-responsive AP2/ERF-domain transcription factor 47 (ORA47) and AP3 (an ABI5 binding factor) transcripts were enhanced in vtc2 but repressed in abi4 vtc2, suggesting that ABI4 and ascorbate modulate growth and defense gene expression through jasmonate signaling. We conclude that low ascorbate triggers ABA-and jasmonate-dependent signaling pathways that together regulate growth through ABI4. Moreover, cellular redox homeostasis exerts a strong influence on sugar-dependent growth regulation.

Plants co-ordinate information derived from many diverse external and internal signals to ensure appropriate control of gene expression under optimal and stress conditions. In this work, the relationships between catalase (CAT) and H2O2 during drought in wheat (Triticum aestivum L.) are studied. Drought-induced H2O2 accumulation correlated with decreases in soil water content and CO2 assimilation. Leaf H2O2 content increased even though total CAT activity doubled under severe drought conditions. Diurnal regulation of CAT1 and CAT2 mRNA abundance was apparent in all conditions and day/night CAT1 and CAT2 expression patterns were modified by mild and severe drought. The abundance of CAT1 transcripts was regulated by circadian controls that persisted in continuous darkness, while CAT2 was modulated by light. Drought decreased abundance, and modified the pattern, of CAT1 and CAT2 mRNAs. It was concluded that the complex regulation of CAT mRNA, particularly at the level of translation, allows precise control of leaf H2O2 accumulation.