High photosynthetic capacity of Sahelian C3 and C4 plants

Photosynthesis Research - The semi-arid ecosystems of the African Sahel play an important role in the global carbon cycle and are among the most sensitive ecosystems to future environmental...     

Spatial regulation of photosynthetic development in C4 plants

Leaf development in C4 plants requires the morphological and functional differentiation of two photosynthetic cell types (bundle sheath and mesophyll). Photosynthetic reactions are split between bundle sheath and mesophyll cells, with each cell type accumulating a specific complement of photosynthetic enzymes. Current evidence suggests that in order to activate this cell-specific expression of photosynthetic genes, bundle sheath and mesophyll cells must interpret positional information distributed locally around each vein.     

Evolution of C4 photosynthetic genes and overexpression of maize C4 genes in rice

C3 plants including many agronomically important crops exhibit a lower photosynthetic efficiency due to inhibition of photosynthesis by O₂ and the associated photorespiration. C4 plants had evolved the C4 pathway to overcome low CO₂ and photorespiration. This review first focuses on the generation of a system for high level expression of the C4-specific gene for pyruvate, orthophosphate dikinase (Pdk), one of the key enzyme in C4 photosynthesis. Based on the results with transgenic rice plants, we have demonstrated that the regulatory system controlling thePdk expression in maize is not unique to C4 plants but rice (C3 plant) posses a similar system. Second, we discussed the possibility of the high level expression of maize C4-specific genes in transgenic rice plants. Introduction of the maize intact phosphoenolpyruvate carboxylase gene (Ppc) caused 30–100 fold higher PEPC activities than non-transgenic rice. These results demonstrated that intact C4-type genes are available for high level expression of C4 enzymes in rice plants. The extended abstract of a paper presented at the 13th International Symposium in Conjugation with Award of the International Prize for Biology “Frontier of Plant Biology”     

Overproduction of C4 photosynthetic enzymes in transgenic rice plants: an approach to introduce the C4-like photosynthetic pathway into rice

Four enzymes, namely, the maize C(4)-specific phosphoenolpyruvate carboxylase (PEPC), the maize C(4)-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C(3)-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO(2) assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO(2) assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO(2) assimilation, however, was more marked at higher CO(2) concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO(2) inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C(4)-like photosynthetic pathway into C(3) plants, are discussed.     

Molecular evolution and genetic engineering of C4 photosynthetic enzymes

The majority of terrestrial plants, including many important crops such as rice, wheat, soybean, and potato, are classified as C(3) plants that assimilate atmospheric CO(2) directly through the C(3) photosynthetic pathway. C(4) plants, such as maize and sugarcane, evolved from C(3) plants, acquiring the C(4) photosynthetic pathway in addition to the C(3) pathway to achieve high photosynthetic performance and high water- and nitrogen-use efficiencies. Consequently, the transfer of C(4) traits to C(3) plants is one strategy being adopted for improving the photosynthetic performance of C(3) plants. The recent application of recombinant DNA technology has made considerable progress in the molecular engineering of photosynthetic genes in the past ten years. It has deepened understanding of the evolutionary scenario of the C(4) photosynthetic genes. The strategy, based on the evolutionary scenario, has enabled enzymes involved in the C(4) pathway to be expressed at high levels and in desired locations in the leaves of C(3) plants. Although overproduction of a single C(4) enzyme can alter the carbon metabolism of C(3) plants, it does not show any positive effects on photosynthesis. Transgenic C(3) plants overproducing multiple enzymes are now being produced for improving the photosynthetic performance of C(3) plants.     

Glycolate synthesis in a C3 C4 and intermediate photosynthetic plant type

Excised leaves of a C₃-photosynthetic type, Hordeum vulgare,a C₄-type, Panicum miliaceum, and an intermediate-type, Panicummilioides, were allowed to take up through their cut ends a1 mM solution of butyl hydroxybutynoate (BHB), an irreversibleinactivator of glycolate oxidase. After 30 to 60 min in BHB,extractable glycolate oxidase activity could not be detectedin the distal quarter of the leaf blades. Following this pretreatment,recovery of ¹⁴C-glycolate from ¹⁴CO₂ incorporated in a 10 minperiod was nearly maximal for each of the three plant types.Labeled glycolate was 51% of the total ¹⁴CO₂ incorporated forthe C₃-species, 36% for the intermediate-species, and 27% forthe C₄-species Increased labeling of glycolate was compensatedfor primarily by decreased labeling of the neutral and basicfractions for the C₃ and intermediate-type species. In the C₄-type,label decreased primarily in the neutral and insoluble fractions,but increased in the basic fraction. A lower rate of glycolatesynthesis is indicative of a lower rate of photorespirationand consistent with a lower O₂/CO₂ ratio present in the bundle-sheathcells of C₄-plants. We conclude that both decreased glycolatesynthesis and the refixation of photorespiratory-released CO₂are important in maintaining a lower rate of photorespirationin C₄-plants compared to C₃ plants. Intermediate glycolate synthesisin Panicum milioldes is consistent with its intermediate levelof O₂ inhibition of photosynthesis and intermediate rate ofphotorespiration. (Received May 6, 1978; )     

C4 photosynthetic isotope exchange in NAD-ME- and NADP-ME-type grasses

Monitoring photosynthetic isotope exchange is an important tool for predicting the influence of plant communities on the global carbon cycle in response to climate change. C(4) grasses play an important role in the global carbon cycle, but their contribution to the isotopic composition of atmospheric CO(2) is not well understood. Instantaneous measurements of (13)CO(2) (Delta(13)C) and C(18)OO (Delta(18)O) isotope exchange in five NAD-ME and seven NADP-ME C(4) grasses have been conducted to investigate the difference in photosynthetic CO(2) isotopic fractionation in these subgroups. As previously reported, the isotope composition of the leaf material (delta(13)C) was depleted in (13)C in the NAD-ME compared with the NADP-ME grasses. However, Delta(13)C was not different between subtypes at high light, and, although Delta(13)C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf delta(13)C between the C(4) subtypes are not caused by photosynthetic isotope fractionation and leaf delta(13)C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C(4) grasses may influences Delta(13)C and should be considered when estimating the contribution of C(4) grasses to the global isotopic signature of atmospheric CO(2). It was found that measured Delta(18)O values were lower than those predicted from leaf CA activities and Delta(18)O was similar in all species measured. The Delta(18)O in these C(4) grasses is similar to low Delta(18)O previously measured in C(4) dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of Delta(18)O in C(4) plants.     

Engineering regulatory RNAs

RNA has long been a favoured medium for in vitro evolution and engineering. Functional RNAs produced in vitro can bind small molecules (aptamers), possess catalytic activity (ribozymes) or do both (aptazymes). A plethora of recent work has shown similar strategies used naturally for gene regulation in bacteria. Interest in these natural systems has inspired an effort to engineer and evolve this activity in vivo. A recent paper by Isaacs et al. describes the engineering and in vivo activity of a small RNA that removes translation inhibition by binding the 5' untranslated region of its target mRNA and making the ribosome-binding site accessible.     

重建基因调控网络

分子生物学的主要挑战是如何更好的理解基因间的调控机理。重建基因网络有助于探索生命系统的本质问题。这里对研究基因调控网络的起源、发展动向、目的和方法及目前所面临的挑战进行了综述。     

Prostate cancer regulatory networks

Although the timing with which common epithelial malignancies arise and become established remains a matter of debate, it is clear that by the time they are detected these tumors harbor hundreds of deregulated, aberrantly expressed or mutated genes. This enormous complexity poses formidable challenges to identify gene pathways that are drivers of tumorigenesis, potentially suitable for therapeutic intervention. An alternative approach is to consider cancer pathways as interconnected networks, and search for potential nodal proteins capable of connecting multiple signaling networks of tumor maintenance. We have modeled this approach in advanced prostate cancer, a condition with current limited therapeutic options. We propose that the integration of three signaling networks, including chaperone‐mediated mitochondrial homeostasis, integrin‐dependent cell signaling, and Runx2‐regulated gene expression in the metastatic bone microenvironment plays a critical role in prostate cancer maintenance, and offers novel options for molecular therapy. J. Cell. Biochem. 107: 845–852, 2009. © 2009 Wiley‐Liss, Inc.     

Engineering photosynthetic production of L-lysine

L-lysine and other amino acids are commonly produced through fermentation using strains of heterotrophic bacteria such as Corynebacterium glutamicum. Given the large amount of sugar this process consumes, direct photosynthetic production is intriguing alternative. In this study, we report the development of a cyanobacterium, Synechococcus sp. strain PCC 7002, capable of producing L-lysine with CO₂ as the sole carbon-source. We found that heterologous expression of a lysine transporter was required to excrete lysine and avoid intracellular accumulation that correlated with poor fitness. Simultaneous expression of a feedback inhibition resistant aspartate kinase and lysine transporter were sufficient for high productivities, but this was also met with a decreased chlorophyll content and reduced growth rates. Increasing the reductant supply by using NH₄⁺, a more reduced nitrogen source relative to NO₃^-, resulted in a two-fold increase in productivity directing 18% of fixed carbon to lysine. Given this advantage, we demonstrated lysine production from media formulated with a municipal wastewater treatment sidestream as a nutrient source for increased economic and environmental sustainability. Based on our results, we project that Synechococcus sp. strain PCC 7002 could produce lysine at areal productivities approaching that of sugar cane to lysine via fermentation using non-agricultural lands and low-cost feedstocks.     

Comparative genomic analysis of C4 photosynthetic pathway evolution in grasses

Background  

Sorghum is the first C4 plant and the second grass with a full genome sequence available. This makes it possible to perform a whole-genome-level exploration of C4 pathway evolution by comparing key photosynthetic enzyme genes in sorghum, maize (C4) and rice (C3), and to investigate a long-standing hypothesis that a reservoir of duplicated genes is a prerequisite for the evolution of C4 photosynthesis from a C3 progenitor.     

Leaf anatomy and C4 photosynthetic enzymes in three reed ecotypes

Differences in leaf interveinal distances, chloroplasts distribution in bundle sheath cells (BSC) and activities of C₄ photosynthetic enzymes in the leaves of three ecotypes of Phragmites communis Trinius, namely swamp reed (SR), heavy salt meadow reed (HSMR) and dune reed (DR), occurring in the desert region of northwest China were investigated. The two terrestrial ecotypes, DR and HSMR, had denser vascular system, more and longer BSC chloroplasts and higher capacity of CO₂ concentrating mechanism of NAD-ME subtype as compared with the SR ecotype. The enhanced NADP-ME pathway in the HSMR might contribute to its adaptation to the salinity habitat.     

Effect of varying environments on photosynthetic parameters of C3, C3-C4 and C4 species of Panicum

Summary CO₂ exchange characteristics and the activity of the carboxylating enzymes phosphoenolpyruvate carboxylase (PEP-C, E.C. 4.1.1.31) and ribulose 1,5-bisphosphate carboxylase (RuBP-C, E.C. 4.1.1.39) during one year in the greenhouse and at two levels of light and temperature in growth chambers were determined in the C₃-C₄ intermediate species P. milioides Nees ex. Trin. These results were compared with those of P. bisulcatum Thumb. (C₃) and P. maximum Jacq. (C₄). Under all tested conditions, and even when the influence of leaf surface temperature on photosynthetic rates and CO₂ compensation points were measured, the biochemical and physiological behaviour of the C₃-C₄ intermediate was more similar to that of the C₃ plant than the C₄ species. The C₄ plant P. maximum, however, responded positively, mainly in terms of PEP-C activity and photosynthetic rate, to the regime of high light and temperature. The results presented indicate that in the C₃-C₄ Panicum grown in high light and temperature no direct relationships between a low CO₂ compesation point and superior growth are evident. It has still to be clarified why in nature a photosynthetic-photorespiratory pathway leading to an intermediate CO₂ compensation value has evolved in P. milioides.