Formation of Proto-Kranz in C3 Rice Induced by Spike-Stalk Injection Method.

Introduction of C4 photosynthetic traits into C3 crops is an important strategy for improving photosynthetic capacity and productivity. Here, we report the research results of a variant line of sorghum-rice (SR) plant with big panicle and high spikelet density by introducing sorghum genome DNA into rice by spike-stalk injection. The whole-genome resequencing showed that a few sorghum genes could be integrated into the rice genome. Gene expression was confirmed for two C4 photosynthetic enzymes containing pyruvate, orthophosphate dikinase and phosphoenolpyruvate carboxykinase. Exogenous sorghum DNA integration induced a series of key traits associated with the C4 pathway called "proto-Kranz" anatomy, including leaf thickness, bundle sheath number and size, and chloroplast size in bundle sheath cells. Significantly, transgenic plants exhibited enhanced photosynthetic capacity resulting from both photosynthetic CO2-concentrating effect and improved energy balance, which led to an increase in carbohydrate levels and productivity. Furthermore, such rice plant exhibited delayed leaf senescence. In summary, this study provides a proof for the feasibility of inducing the transition from C3 leaf anatomy to proto-Kranz by spike-stalk injection to achieve efficient photosynthesis and increase productivity.

Enhanced photorespiration in transgenic rice over-expressing maize C4 phosphoenolpyruvate carboxylase gene contributes to alleviating low nitrogen stress.

The objective of this study was to reveal the physiological and molecular mechanisms of low-nitrogen (N) tolerance in transgenic plant lines containing C4 phosphoenolpyruvate carboxylase (C4-PEPC) gene. The transgenic rice lines only over-expressing the maize C4-PEPC) (PC) and their untransformed wild type, Kitaake (WT), were used in this study. At different N levels, the dry weight, total N content, carbon and N levels, photorespiration-related enzymatic activities, gene expression levels and photorespiration-related product accumulations were measured, as were the transgenic lines' agronomic traits. The PC line, having lower total N and higher soluble sugar contents, was more tolerant to low-N stress than WT, which was consistent with its higher PEPC and lower N-assimilation-related enzyme activity levels. The photosynthetic parameters, enzymatic activity levels, transcripts and products related to photorespiration in PC were also greater than in WT under low-N conditions. This study showed that increased carbon levels in transgenic rice lines overexpressing C4-PEPC could help regulate the photorespiratory pathway under low-N conditions, conferring low-N tolerance and a higher grain yield per plant.

The existence of C4-bundle-sheath-like photosynthesis in the mid-vein of C3 rice.

BACKGROUND: Recent studies have shown that C4-like photosynthetic pathways partly reside in photosynthetic cells surrounding the vascular system of C3 dicots. However, it is still unclear whether this is the case in C3 monocots, especially at the molecular level. RESULTS: In order to fill this gap, we investigated several characteristics required for C4 photosynthesis, including C4 pathway enzymes, cyclic/non-cyclic photophosphorylation rates, the levels and assembly state of photosynthetic machineries, in the mid-veins of C3 monocots rice with leaf laminae used as controls. The signature of photosystem photochemistry was also recorded via non-invasive chlorophyll a fluorescence and reflectance changes at 820 nm in vivo. Our results showed that rice mid-veins were photosynthetically active with higher levels of three C4 decarboxylases. Meanwhile, the linear electron transport chain was blocked in mid-veins due to the selective loss of dysfunctional photosystem II subunits. However, photosystem I was sufficient to support cyclic electron flow in mid-veins, reminiscent of the bundle sheath in C4 plants. CONCLUSIONS: The photosynthetic attributes required for C4 photosynthesis were identified for the first time in the monocotyledon model crop rice, suggesting that this is likely a general innate characteristic of C3 plants which might be preconditioned for the C4 pathway evolution. Understanding these attributes would provide a base for improved strategies for engineering C4 photosynthetic pathways into rice.

Photosynthesis performance, antioxidant enzymes, and ultrastructural analyses of rice seedlings under chromium stress.

The present study was conducted to examine the effects of increasing concentrations of chromium (Cr(6+)) (0, 25, 50, 100, and 200 µmol) on rice (Oryza sativa L.) morphological traits, photosynthesis performance, and the activities of antioxidative enzymes. In addition, the ultrastructure of chloroplasts in the leaves of hydroponically cultivated rice (O. sativa L.) seedlings was analyzed. Plant fresh and dry weights, height, root length, and photosynthetic pigments were decreased by Cr-induced toxicity (200 µM), and the growth of rice seedlings was starkly inhibited compared with that of the control. In addition, the decreased maximum quantum yield of primary photochemistry (Fv/Fm) might be ascribed to the decreased the number of active photosystem II reaction centers. These results were confirmed by inhibited photophosphorylation, reduced ATP content and its coupling factor Ca(2+)-ATPase, and decreased Mg(2+)-ATPase activities. Furthermore, overtly increased activities of antioxidative enzymes were observed under Cr(6+) toxicity. Malondialdehyde and the generation rates of superoxide (O2̄) also increased with Cr(6+) concentration, while hydrogen peroxide content first increased at a low Cr(6+) concentration of 25 µM and then decreased. Moreover, transmission electron microscopy showed that Cr(6+) exposure resulted in significant chloroplast damage. Taken together, these findings indicate that high Cr(6+)concentrations stimulate the production of toxic reactive oxygen species and promote lipid peroxidation in plants, causing severe damage to cell membranes, degradation of photosynthetic pigments, and inhibition of photosynthesis.

High light acclimation of Oryza sativa L. leaves involves specific photosynthetic-sourced changes of NADPH/NADP⁺ in the midvein.

Previous studies have shown that exposure of Arabidopsis leaves to high light (HL) causes a systemic acquired acclimation (SAA) response in the vasculature. It has been postulated that C4-like photosynthesis in the leaf veins triggers this response via the Mehler reaction. To investigate this proposed connection and extend SAA to other plants, we examined the redox state of NADPH, ascorbate (ASA), and glutathione (GSH) pools; levels and histochemical localization of O2- and H2O2 signals; and activities of antioxidant enzymes in the midvein and leaf lamina of rice, when they were subjected to HL and low light. The results showed that (1) high NADPH/NADP(+) was generated by C4-like photosynthesis under HL in the midvein and (2) SAA was colocally induced by HL, as indicated by the combined signaling network, including the decrease in redox status of ASA and GSH pools, accumulation of H2O2 and O2- signals, and high superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. The high correlations between these occurrences suggest that the enhanced NADPH/NADP(+) in HL-treated midveins might alter redox status of ASA and GSH pools and trigger H2O2 and O2- signals during SAA via the Mehler reaction. These changes in turn upregulate SOD and APX activities in the midvein. In conclusion, SAA may be a common regulatory mechanism for the adaptation of angiosperms to HL. Manipulation of NADPH/NADP(+) levels by C4-like photosynthesis promotes SAA under HL stress in the midvein.

Photosynthetic changes of flag leaves during senescence stage in super high-yield hybrid rice LYPJ grown in field condition.

Photosynthetic activities and thylakoid membrane protein patterns as well as the ultrastructure of chloroplasts in flag leaves were investigated during the senescence processes in high-yield hybrid rice LYPJ under field condition. The earlier decrease of PS I activity than PS II in LYPJ was primarily due to the significant degradation of PS I chlorophyll-protein complex. The degradation rate for each chlorophyll-protein complex was different and the order for the stability of thylakoid membrane complexes during flag leaf senescence in rice LYPJ was: LHCII > OEC > PSII core antenna > PSII core > PSI core > LHCI, which was partly supported by the BN-PAGE gel combined with immunoblot analysis. A decrease in the chlorophyll a/b ratio at the senescence stage was observed to coincide with stability of the LHCII subunits. Ultrastructural investigations revealed that the chloroplasts have large loosen stacking grana without interconnecting stroma thylakoids during the senescence processes. It was hypothesized that the stability of grana thylakoids harboring the major LHCII under high radiation condition in summer might played a key role in the dissipation of excess light energy. This alternative strategy would protect photosynthetic apparatus from photodamage and might be causally related to the high yield of this rice cultivar.

Promotion of photosynthesis in transgenic rice over-expressing of maize C4 phosphoenolpyruvate carboxylase gene by nitric oxide donors.

We determined the effects of exogenous nitric oxide on photosynthesis and gene expression in transgenic rice plants (PC) over-expressing the maize C4pepc gene, which encodes phosphoenolpyruvate carboxylase (PEPC). Seedlings were subjected to treatments with NO donors, an NO scavenger, phospholipase inhibitors, a Ca(2+) chelator, a Ca(2+) channel inhibitor, and a hydrogen peroxide (H2O2) inhibitor, individually and in various combinations. The NO donors significantly increased the net photosynthetic rate (PN) of PC and wild-type (WT), especially that of PC. Treatment with an NO scavenger did inhibit the PN of rice plants. The treatments with phospholipase inhibitors and a Ca(2+) chelator decreased the PN of WT and PC, and photosynthesis was more strongly inhibited in WT than in PC. Further analyses showed that the NO donors increased endogenous levels of NO and PLD activity, but decreased endogenous levels of Ca(2+) both WT and PC. However, there was a greater increase in NO in WT and a greater increase in PLD activity and Ca(2+) level in PC. The NO donors also increased both PEPC activity and pepc gene expression in PC. PEPC activity can be increased by SNP alone. But the expression of its encoding gene in PC might be regulated by SNP, together with PA and Ca(2+).