Growth at moderately elevated temperature alters the physiological response of the photosynthetic apparatus to heat stress in pea (Pisum sativum L.) leaves

The impact of heat stress on the functioning of the photosynthetic apparatus was examined in pea (Pisum sativum L.) plants grown at control (25 °C; 25 °C-plants) or moderately elevated temperature (35 °C; 35 °C-plants). In both types of plants net photosynthesis (P_n) decreased with increasing leaf temperature (LT) and was more than 80% reduced at 45 °C as compared to 25 °C. In the 25 °C-plants, LTs higher than 40 °C could result in a complete suppression of P_n. Short-term acclimation to heat stress did not alter the temperature response of P_n. Chlorophyll a fluorescence measurements revealed that photosynthetic electron transport (PET) started to decrease when LT increased above 35 °C and that growth at 35 °C improved the thermal stability of the thylakoid membranes. In the 25 °C-plants, but not in the 35 °C-plants, the maximum quantum yield of the photosystem II primary photochemistry, as judged by measuring the F_v/F_m ratio, decreased significantly at LTs higher than 38 °C. A post-illumination heat-induced reduction of the plastoquinone pool was observed in the 25 °C-plants, but not in the 35 °C-plants. Inhibition of P_n by heat stress correlated with a reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Western-blot analysis of Rubisco activase showed that heat stress resulted in a redistribution of activase polypeptides from the soluble to the insoluble fraction of extracts. Heat-dependent inhibition of P_n and PET could be reduced by increasing the intercellular CO₂ concentration, but much more effectively so in the 35 °C-plants than in the 25 °C-plants. The 35 °C-plants recovered more efficiently from heat-dependent inhibition of P_n than the 25 °C-plants. The results show that growth at moderately high temperature hardly diminished inhibition of P_n by heat stress that originated from a reversible heat-dependent reduction of the Rubisco activation state. However, by improving the thermal stability of the thylakoid membranes it allowed the photosynthetic apparatus to preserve its functional potential at high LTs, thus minimizing the after-effects of heat stress.     

Mapping photoautotrophic metabolism with isotopically nonstationary (13)C flux analysis

Understanding in vivo regulation of photoautotrophic metabolism is important for identifying strategies to improve photosynthetic efficiency or re-route carbon fluxes to desirable end products. We have developed an approach to reconstruct comprehensive flux maps of photoautotrophic metabolism by computational analysis of dynamic isotope labeling measurements and have applied it to determine metabolic pathway fluxes in the cyanobacterium Synechocystis sp. PCC6803. Comparison to a theoretically predicted flux map revealed inefficiencies in photosynthesis due to oxidative pentose phosphate pathway and malic enzyme activity, despite negligible photorespiration. This approach has potential to fill important gaps in our understanding of how carbon and energy flows are systemically regulated in cyanobacteria, plants, and algae.     

Influence of benomyl on photosynthetic capacity in soybean leaves

This investigation was performed to study the influence of benomyl on photosynthetic pigments and enzymes in soybean leaves. Chlorophyll and pheophytin levels were reduced by benomyl 45 days after greening. These results indicate that chlorophylla andb, and pheophytin must be controlled by benomyl. SDS-PAGE analysis showed that 50 and 14.5 kD polypeptides represented as the large and small subunits of rubisco. In the both of these subunits, the band intensity of the control was significantly higher than that after benomyl treatment, indicating that these two subunits are affected by benomyl. Benomyl strongly inhibited both the activity and content of rubisco as its concentration was gradually increased. However, it remains unclear whether this reduction of rubisco level was due to a reduced level of rubisco activase. Two major polypeptides of 46 and 42 kD were identified as rubisco activase subunits by SDS-PAGE. The intensity of these two bands was shown to be higher in the control than after benomyl treatment. These results indicate that the rubisco decrease resulting from increased benomyl concentrations was caused by rubisco activase. A significant decrease in both the activity and content of rubisco activase by benomyl was also observed. These results suggest that the decrease in rubisco level caused by benomyl is accompanied by a decrease in both the activity and content of rubisco activase.     

小麦Rubisco活化酶基因的克隆和表达特性

Rubisco活化酶是广泛存在于光合生物中调节Rubisco活性的酶,我们利用PCR技术,从小麦(Triticum aestivum)叶片cDNA文库中克隆得到Rubisco活化酶基因cDNA片段,该片段长度为850 bp,编码201个氨基酸.Northern blot表明,小麦叶片在暗诱导衰老的条件下,叶片中活化酶基因表达水平逐渐下降;同时,小麦叶片的光合特性、叶绿素含量和Rubisco活性呈现下降趋势.这些结果表明,衰老时小麦叶片Rubisco活化酶基因表达水平下降与光合速率下降密切相关.     

Changes in protein quantities of phosphoenolpyruvate carboxylase and Rubisco activase in various wheat genotypes

In early seedlings of wheat genotypes two isoforms of Rubisco activase with molecular weights of 42 and 46 kDa are expressed. Amounts of both isoforms significantly increase in early seedlings of the durum wheat genotype Barakatli-95 exposed to salt stress. But at the beginning of the tillering stage, the changes in quantities of both RCA isoforms are different in durum and bread wheat genotypes subjected to a 3-day drought stress. In the leaves of the early seedlings of the studied wheat genotypes exposed to drought stress quantities of PEPC subunits increase compared to the control but they remain relatively stable in early roots and germinating seeds. However, quantities of its subunits decrease sharply in roots and germinating seeds of early seedlings under the influence of 100 mM NaCl. In flag leaves and ear elements of the Barakatli-95 genotype grown under normal water supply conditions protein quantities of PEPC subunits change differently depending on time. Changes in protein quantities of RCA, PEPC and Rubisco enzymes have been studied comparatively in ear elements and flag leaves after the fourth day of anthesis.     

Heat sensitivity of Rubisco,Rubisco activase and Rubisco binding protein in higher plants

During the past few years the investigations concerning Rubisco and the changes of its activity and properties at elevated temperature were reconsidered with special reference to the important role of Rubisco activase and Rubisco binding protein. The major changes in Rubisco, Rubisco activase and Rubisco binding protein reported recently are presented in this review. New information on these proteins, including their changes under heat stress conditions, is discussed together with open questions.     

Ribulose 1,5-bisphosphate carboxylase/oxygenase large subunit translation is regulated in a small subunit-independent manner in the expanded leaves of tobacco

Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is composed of small subunits (SSs) encoded by rbcS on the nuclear genome and large subunits (LSs) encoded by rbcL on the chloroplast genome, and it is localized in the chloroplast stroma. Constitutive knockdown of the rbcS gene reportedly causes a reduction in LS quantity and the level of translation in tobacco and the unicellular green alga Chlamydomonas. Constitutively knockdown of the rbcS gene also causes a reduction in photosynthesis, which influences the expression of photosynthetic genes, including the rbcL gene. Here, to investigate the influence of the knockdown of the rbcS gene on the expression of the rbcL gene under normal photosynthetic conditions, we generated transgenic tobacco plants in which the amount of endogenous rbcS mRNA can be reduced by inducible expression of antisense rbcS mRNA with dexamethasone (DEX) treatment at later stages of growth. In already expanded leaves, after DEX treatment, the level of photosynthesis, RuBisCO quantity and the chloroplast ultrastructure were normal, but the amount of rbcS mRNA was reduced. An in vivo pulse labeling experiment and polysome analysis showed that LSs were translated at the same rate as in wild-type leaves. On the other hand, in newly emerging leaves, the rbcS mRNA quantity, the level of photosynthesis and the quantity of RuBisCO were reduced, and chloroplasts failed to develop. In these leaves, the level of LS translation was inhibited, as previously described. These results suggest that LS translation is regulated in an SS-independent manner in expanded leaves under normal photosynthetic conditions.