Quantification of Photosynthetic Enzymes in Leaf Extracts by Immunoblotting

Antibody Design for the Quantification of Photosynthetic Proteins and Their Isoforms

Antibodies are a valuable research tool, with uses including detection and quantification of specific proteins. By using peptide fragments to raise antibodies, they can be designed to differentiate between structurally similar proteins, or to bind conserved motifs in divergent proteins. Peptide sequence selection and antibody validation are crucial to ensure reliable results from antibody-based experiments. This chapter describes the steps for the identification of peptide sequences to produce protein- or isoform-specific antibodies using recombinant technologies as well as the subsequent validation of such antibodies. The photosynthetic protein Rubisco activase is used as a case study to explain the various steps involved and key aspects to take into consideration.

Purification of Rubisco from Leaves

Rubisco fixes CO2 through the carboxylation of ribulose 1,5-bisphosphate (RuBP) during photosynthesis, enabling the synthesis of organic compounds. The natural diversity of Rubisco properties represents an opportunity to improve its performance and there is considerable research effort focusing on better understanding the properties and regulation of the enzyme. This chapter describes a method for large-scale purification of Rubisco from leaves. After the extraction of Rubisco from plant leaves, the enzyme is separated from other proteins by fractional precipitation with polyethylene glycol followed by ion-exchange chromatography. This method enables the isolation of Rubisco in large quantities for a wide range of biochemical applications.

Gas exchange measurements in the unsteady state

Leaf level gas exchange is a widely used technique that provides real-time measurement of leaf physiological properties, including CO₂ assimilation (A), stomatal conductance to water vapour (g_sw ) and intercellular CO₂ (C_i ). Modern open-path gas exchange systems offer greater portability than the laboratory-built systems of the past and take advantage of high-precision infrared gas analyzers and optimized system design. However, the basic measurement paradigm has long required steady-state conditions for accurate measurement. For CO₂ response curves, this requirement has meant that each point on the curve needs 1-3 min and a full response curve generally requires 20-35 min to obtain a sufficient number of points to estimate parameters such as the maximum velocity of carboxylation (V_c,max ) and the maximum rate of electron transport (J_max ). For survey measurements, the steady-state requirement has meant that accurate measurement of assimilation has required about 1-2 min. However, steady-state conditions are not a strict prerequisite for accurate gas exchange measurements. Here, we present a new method, termed dynamic assimilation, that is based on first principles and allows for more rapid gas exchange measurements, helping to make the technique more useful for high throughput applications.

Heat-induced changes in the abundance of wheat Rubisco activase isoforms

The Triticum aestivum (wheat) genome encodes three isoforms of Rubisco activase (Rca) differing in thermostability, which could be exploited to improve the resilience of this crop to global warming. We hypothesized that elevated temperatures would cause an increase in the relative abundance of heat-stable Rca1β. Wheat plants were grown at 25° C : 18°C (day : night) and exposed to heat stress (38° C : 22°C) for up to 5 d at pre-anthesis. Carbon (C) assimilation, Rubisco activity, CA1Pase activity, transcripts of Rca1β, Rca2β, and Rca2α, and the quantities of the corresponding protein products were measured during and after heat stress. The transcript of Rca1β increased 40-fold in 4 h at elevated temperatures and returned to the original level after 4 h upon return of plants to control temperatures. Rca1β comprised up to 2% of the total Rca protein in unstressed leaves but increased three-fold in leaves exposed to elevated temperatures for 5 d and remained high at 4 h after heat stress. These results show that elevated temperatures cause rapid changes in Rca gene expression and adaptive changes in Rca isoform abundance. The improved understanding of the regulation of C assimilation under heat stress will inform efforts to improve wheat productivity and climate resilience.

Photosynthesis across African cassava germplasm is limited by Rubisco and mesophyll conductance at steady state, but by stomatal conductance in fluctuating light

Sub-Saharan Africa is projected to see a 55% increase in food demand by 2035, where cassava (Manihot esculenta) is the most widely planted crop and a major calorie source. Yet, cassava yield in this region has not increased significantly for 13 yr. Improvement of genetic yield potential, the basis of the first Green Revolution, could be realized by improving photosynthetic efficiency. First, the factors limiting photosynthesis and their genetic variability within extant germplasm must be understood. Biochemical and diffusive limitations to leaf photosynthetic CO2 uptake under steady state and fluctuating light in 13 farm-preferred and high-yielding African cultivars were analyzed. A cassava leaf metabolic model was developed to quantify the value of overcoming limitations to leaf photosynthesis. At steady state, in vivo Rubisco activity and mesophyll conductance accounted for 84% of the limitation. Under nonsteady-state conditions of shade to sun transition, stomatal conductance was the major limitation, resulting in an estimated 13% and 5% losses in CO2 uptake and water use efficiency, across a diurnal period. Triose phosphate utilization, although sufficient to support observed rates, would limit improvement in leaf photosynthesis to 33%, unless improved itself. The variation of carbon assimilation among cultivars was three times greater under nonsteady state compared to steady state, pinpointing important overlooked breeding targets for improved photosynthetic efficiency in cassava.

Spectrophotometric Determination of RuBisCO Activity and Activation State in Leaf Extracts

RuBisCO plays a central role in photosynthesis and, due to its catalytic inefficiencies, frequently limits CO₂ assimilation in fully illuminated leaves at the top of unstressed crop canopies. The CO₂-fixing enzyme is heavily regulated and not all the enzyme present in the leaf is active at any given moment. In this chapter, a spectrophotometric assay is described for measuring RuBisCO activity and activation state in leaf extracts. Most of the assay components are available commercially and others can be produced by established protocols, making adoption of the assay achievable by most plant biochemistry laboratories. Its relative high-throughput capacity enables large-scale experiments aimed at screening germplasm for improved RuBisCO function.

Survey of Tools for Measuring In Vivo Photosynthesis

Measurements of in vivo photosynthesis are powerful tools that probe the largest fluxes of carbon and energy in an illuminated leaf, but often the specific techniques used are so varied and specialized that it is difficult for researchers outside the field to select and perform the most useful assays for their research questions. The goal of this chapter is to provide a broad overview of the current tools available for the study of in vivo photosynthesis so as to provide a foundation for selecting appropriate techniques, many of which are presented in detail in subsequent chapters. This chapter also organizes current methods into a comparative framework and provides examples of how they have been applied to research questions of broad agronomical, ecological, or biological importance. The chapter closes with an argument that the future of in vivo measurements of photosynthesis lies in the ability to use multiple methods simultaneously and discusses the benefits of this approach to currently open physiological questions. This chapter, combined with the relevant methods chapters, could serve as a laboratory course in methods in photosynthesis research or as part of a more comprehensive laboratory course in general plant physiology methods.

Extraction of RuBisCO to Determine Catalytic Constants

RuBisCO enables net carbon fixation through the carboxylation of RuBP during photosynthesis. Its complex biochemistry and catalytic diversity found among different plants make characterization of RuBisCO properties useful for investigations aimed at improving photosynthetic performance. This chapter reports methods for rapid extraction of soluble proteins to examine RuBisCO catalytic properties, and for large-scale purification of RuBisCO from leaves to measure the specificity of the enzyme toward its gaseous substrates.