Characteristics of photosynthesis in rice plants transformed with an antisense Rubisco activase gene

Transgenic rice plants with an antisense gene inserted via Agrobacterium tumefaciens were used to explore the impact of the reduction of Rubisco activase (RCA) on Rubisco and photosynthesis. In this study, transformants containing 15% to 35% wild type Rubisco activase were selected, which could survive in ambient CO2 concentration but grew slowly compared with wild type controls. Gas exchange measurements indicated that the rate of photosynthesis decreased sig- nificantly, while stomatal conductance and transpiration rate did not change; and that the intercellular CO2 concentration even increased. Rubisco determination showed that these plants had approximately twice as much Rubisco as the wild types, although they showed 70% lower rate of photosynthesis, which was likely an acclimation response to the reduction in Rubsico activase and/or the reduction in carbamylation.

Enhancing crop yield by using Rubisco activase to improve photosynthesis under elevated temperatures

With the rapid growth of world population,it is essential to increase agricultural productivity to feed the growing population.Over the past decades,many methods have been used to increase crop yields.Despite the success in boosting the crop yield through these methods,global food production still needs to be increased to be on par with the increasing population and its dynamic consumption patterns.Additionally,given the prevailing environmental conditions pertaining to the global temperature increase,heat stress will likely be a critical factor that negatively affects plant biomass and crop yield.One of the key elements hindering photosynthesis and plant productivity under heat stress is the thermo-sensitivity of the Rubisco activase(RCA),a molecular chaperone that converts Rubisco back to active form after it becomes inactive.It would be an attractive and practical strategy to maintain photosynthetic activity under elevated temperatures by enhancing the thermo-stability of RCA.In this context,this review discusses the need to improve the thermo-tolerance of RCA under current climatic conditions and to further study RCA structure and regulation,and its limitations at elevated temperatures.This review summarizes successful results and provides a perspective on RCA research and its implication in improving crop yield under elevated temperature conditions in the future.

Diurnal Changes of Rubisco and RCA Activities and Their Cellular Localization in Rice

The cellular localization of Rubisco and Rubisco activase (RCA) in rice (Oryza sativa subsp. indica cv. Zhenong 952) leaf was investigated with immunogold-labeled electron microscope techniques on the basis of determining the diurnal changes of photosynthetic rate (Pn), Rubisco and RCA activities, and quantifying two enzyme contents in the leaf with immuno-diffusion method in order to understand why RCA activity decreased in the midday when its contents was high. The results showed that Rubisco mainly was located in chloroplast, and RCA were found both in chloroplast and mitochondria. The lowering of Rubisco in chloroplast as well as Rubisco activity at noon could be one of good reasons to explain the photosynthetic midday depression in leaf. The density of RCA in chloroplast reached the maximum at 14:00 and a valley at 11:00. The result much coincided with the activity of RCA in leaf. In mitochondria, the density of RCA changed abruptly in one day with the highest at 13:00 and it can well elucidate why the activities of Rubisco declined at noon when its amount was increasing. Therefore the cellular localization and/or distribution of Rubisco and RCA during a day is more important for Pn, Rubisco and RCA activities.

Characteristics of Gas Exchange, Chlorophyll Fluorescence and Expression of Key Enzymes in Photosynthesis During Leaf Senescence in Rice Plants

Abstract: Gas exchange, chlorophyll (Chl) fluorescence, photosynthetic pigments, and the expression of Rubisco and Rubisco activase in flag leaves of rice (Oryza sativa L.) from the heading to mature grain stage were investigated. The results showed that the photosynthetic capacity declined after full expansion of flag leaves. The decline of photosynthetic rate (Pn) in two cultivars during natural senescence was accompanied by a decrease in Chl content, as well as in the Rubisco and Rubisco activase contents. The Rubisco and Rubisco activase contents in Zhenong 966 decreased faster than that in Zhenong 952. The Pn diminished without a decrease in intercellular CO2 concentration during the early senescence of flag leaves, indicating that lower photosynthetic capacity is not caused by stomatal limitation but by reduced carboxylation efficiency. During the senescence of flag leaves, the correlation between the change in photosynthetic capacity and the variation in the abundance of Rubisco and Rubisco activase suggested that the decline in Pn of flag leaves could be attributed to the lower level of rbcS and rca transcripts. The PSII and the electron transport rate appeared in the same rate as Pn. However, excitation pressure (1-qP) showed a different pattern and there was an inverse linear correlation between increased excitation pressure and the reduced Rubisco. Therefore, it is likely that the increased excitation pressure resulted from saturation of the electron transport chain, owing to a limitation of the reductant used by the Calvin cycle; in addition, the change in excitation pressure could further mediate the expression of the rbcS and rca genes, resulting in a fast reduction in Rubisco content.

Regulation of Calvin-Benson cycle enzymes under high temperature stress

The Calvin Benson cycle(CBC)consists of three critical processes,including fixation of CO_(2) by Rubisco,reduction of 3-phosphoglycerate(3PGA)to triose phosphate(triose-P)with NADPH and ATP generated by the light reactions,and regeneration of ribulose 1,5-bisphosphate(RuBP)from triose-P.The activ-ities of photosynthesis-related proteins,mainly from the CBC,were found more significantly affected and regulated in plants challenged with high temperature stress,incuding Rubisco,Rubisco activase(RCA) and the enzymes involved in RuBP regeneration,such as sedoheptulose-1,7-bisphosphatase(SBPase).Over the past years,the regulatory mechanism of CBC,especially for redox-regulation,has attracted major interest,because balancing flux at the various enzymatic reactions and maintaining metabolite levels in a range are of critical importance for the optimal operation of CBC under high temperature stress,providing insights into the genetic manipulation of photosynthesis.Here,we summarize recent progress regarding the identification of various layers of regulation point to the key enzymes of CBC for acclimation to environmental temperature changes along with open questions are also discussed.