Antisense inhibition of the iron-sulphur subunit of succinate dehydrogenase enhances photosynthesis and growth in tomato via an organic acid-mediated effect on stomatal aperture

Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of the Sl SDH2-2 gene encoding the iron sulfur subunit of the succinate dehydrogenase protein complex in the antisense orientation under the control of the 35S promoter exhibit an enhanced rate of photosynthesis. The rate of the tricarboxylic acid (TCA) cycle was reduced in these transformants, and there were changes in the levels of metabolites associated with the TCA cycle. Furthermore, in comparison to wild-type plants, carbon dioxide assimilation was enhanced by up to 25% in the transgenic plants under ambient conditions, and mature plants were characterized by an increased biomass. Analysis of additional photosynthetic parameters revealed that the rate of transpiration and stomatal conductance were markedly elevated in the transgenic plants. The transformants displayed a strongly enhanced assimilation rate under both ambient and suboptimal environmental conditions, as well as an elevated maximal stomatal aperture. By contrast, when the Sl SDH2-2 gene was repressed by antisense RNA in a guard cell-specific manner, changes in neither stomatal aperture nor photosynthesis were observed. The data obtained are discussed in the context of the role of TCA cycle intermediates both generally with respect to photosynthetic metabolism and specifically with respect to their role in the regulation of stomatal aperture.     

The enigmatic contribution of mitochondrial function in photosynthesis

Considerable cumulative evidence has accrued suggesting a vital role for mitochondrial function in optimizing photosynthesis. Both pharmacological approaches using respiratory inhibitors and reverse genetic approaches have recently underscored the high degree of interconnection between photosynthesis and respiration--the major pathways of energy production which are largely confined to the plastid and mitochondria, respectively. Here recent studies into the nature of these interactions are reviewed, with particular focus on (i) the recently described link between the mitochondrial electron transport chain activity, ascorbate biosynthesis, and photosynthesis; and (ii) the contribution of mitochondrial metabolism to the photorespiratory process. Whilst there is increasing evidence of a role for ascorbate in co-ordinating the rates of respiration and photosynthesis, some data are presented here for plants grown under extreme environmental conditions that suggest that this relationship is not absolute. It thus seems likely that interactions between these compartments are perhaps more numerous and complicated than previously thought. This observation suggests that although the elucidation of the genetic bases of both photorespiration and the Wheeler-Smirnoff pathway of ascorbate biosynthesis has recently been completed, much further research is probably necessary in order to understand fully how energy metabolism is co-ordinated in the illuminated leaf.     

The Arabidopsis onset of leaf death5 mutation of quinolinate synthase affects nicotinamide adenine dinucleotide biosynthesis and causes early ageing

Leaf senescence in Arabidopsis thaliana is a strict, genetically controlled nutrient recovery program, which typically progresses in an age-dependent manner. Leaves of the Arabidopsis onset of leaf death5 (old5) mutant exhibit early developmental senescence. Here, we show that OLD5 encodes quinolinate synthase (QS), a key enzyme in the de novo synthesis of NAD. The Arabidopsis QS was previously shown to carry a Cys desulfurase domain that stimulates reconstitution of the oxygen-sensitive Fe-S cluster that is required for QS activity. The old5 lesion in this enzyme does not affect QS activity but it decreases its Cys desulfurase activity and thereby the long-term catalytic competence of the enzyme. The old5 mutation causes increased NAD steady state levels that coincide with increased activity of enzymes in the NAD salvage pathway. NAD plays a key role in cellular redox reactions, including those of the tricarboxylic acid cycle. Broad-range metabolite profiling of the old5 mutant revealed that it contains higher levels of tricarboxylic acid cycle intermediates and nitrogen-containing amino acids. The mutant displays a higher respiration rate concomitant with increased expression of oxidative stress markers. We postulate that the alteration in the oxidative state is integrated into the plant developmental program, causing early ageing of the mutant.     

CO2 enrichment and supporting material impact the primary metabolism and 20-hydroxyecdysone levels in Brazilian ginseng grown under photoautotrophy

Plant cell and organ cultures via the implementation of effective elicitation strategies can offer attractive biotechnological platforms for the enhanced production of phytochemicals of pharmaceutical interest. For the first time, the elicitation of exogenous signal molecules was conducted to enhance the production of pharmacologically active alkaloids and flavonoids in Isatis tinctoria L. hairy root cultures (ITHRCs). ITHRCs III and V correspondingly possessing high alkaloid and flavonoid productivity were adopted for elicitation treatments. The maximum accumulation of alkaloids in ITHRCs III elicited by 142.61 µM salicylic acid for 28.18 h and flavonoids in ITHRCs V elicited by 179.54 µM methyl jasmonate for 41.87 h increased 5.89- and 11.21-folds as compared with controls, respectively. Moreover, expressions of 11 genes involved in alkaloid and flavonoid biosynthetic pathways were significantly up-regulated following elicitation, among which YUCCA, CHI and F3′H genes might play a crucial role in the target phytochemical augmentation. Overall, two effective elicitation protocols were provided here to improve the yields of bioactive alkaloids and flavonoids in ITHRCs, which was useful for the scale-up production of these valuable compounds to meet the demands for natural bioactive ingredients by pharmaceutical industries.

     

Deficiency of mitochondrial fumarase activity in tomato plants impairs photosynthesis via an effect on stomatal function

Transgenic tomato (Solanum lycopersicum) plants expressing a fragment of a fumarate hydratase (fumarase) gene in the antisense orientation and exhibiting considerable reductions in the mitochondrial activity of this enzyme show impaired photosynthesis. The rate of the tricarboxylic acid cycle was reduced in the transformants relative to the other major pathways of carbohydrate oxidation and the plants were characterized by a restricted rate of dark respiration. However, biochemical analyses revealed relatively little alteration in leaf metabolism as a consequence of reducing the fumarase activity. That said, in comparison to wild-type plants, CO(2) assimilation was reduced by up to 50% under atmospheric conditions and plants were characterized by a reduced biomass on a whole plant basis. Analysis of further photosynthetic parameters revealed that there was little difference in pigment content in the transformants but that the rate of transpiration and stomatal conductance was markedly reduced. Analysis of the response of the rate of photosynthesis to variation in the concentration of CO(2) confirmed that this restriction was due to a deficiency in stomatal function.