The wheat chloroplastic proteome

Highlights

• New insights into wheat chloroplast proteins using high through put mass spectrometry

• To describe the salt-responsive proteins in salt-stressed wheat chloroplasts

• To provide the information about the water stress-responsive proteins in wheat chloroplasts

Abstract

With the availability of plant genome sequencing, analysis of plant proteins with mass spectrometry has become promising and admired. Determining the proteome of a cell is still a challenging assignment, which is convoluted by proteome dynamics and convolution. Chloroplast is fastidious curiosity for plant biologists due to their intricate biochemical pathways for indispensable metabolite functions. In this review, an overview on proteomic studies conducted in wheat with a special focus on subcellular proteomics of chloroplast, salt and water stress. In recent years, we and other groups have attempted to understand the photosynthesis in wheat and abiotic stress under salt imposed and water deficit during vegetative stage. Those studies provide interesting results leading to better understanding of the photosynthesis and identifying the stress-responsive proteins. Indeed, recent studies aimed at resolving the photosynthesis pathway in wheat. Proteomic analysis combining two complementary approaches such as 2-DE and shotgun methods couple to high through put mass spectrometry (LTQ-FTICR and MALDI-TOF/TOF) in order to better understand the responsible proteins in photosynthesis and abiotic stress (salt and water) in wheat chloroplast will be focused.

Biological significance

In this review we discussed the identification of the most abundant protein in wheat chloroplast and stress-responsive under salt and water stress in chloroplast of wheat seedlings, thus providing the proteomic view of the events during the development of this seedling under stress conditions.

Chloroplast is fastidious curiosity for plant biologists due to their intricate biochemical pathways for indispensable metabolite functions. An overview on proteomic studies conducted in wheat with a special focus on subcellular proteomics of chloroplast, salt and water stress. We have attempted to understand the photosynthesis in wheat and abiotic stress under salt imposed and water deficit during seedling stage. Those studies provide interesting results leading to a better understanding of the photosynthesis and identifying the stress-responsive proteins. In reality, our studies aspired at resolving the photosynthesis pathway in wheat. Proteomic analysis united two complementary approaches such as Tricine SDS-PAGE and 2-DE methods couple to high through put mass spectrometry (LTQ-FTICR and MALDI-TOF/TOF) in order to better understand the responsible proteins in photosynthesis and abiotic stress (salt and water) in wheat chloroplast will be highlighted. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Introduction

In plants, algal and cyanobacterial photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is an indispensable process for life. Nearly all life depends on it either directly as a source of energy or indirectly as the ultimate source of the energy in its food [1]. In chloroplasts, a complex and highly integrated set of physical and chemical reactions results in the release of oxygen into the atmosphere and the production of chemical energy in the form of reduced carbon, those are essential for heterotrophic living organisms. Firstly, the light phase of photosynthesis, i.e. the exchange of solar energy into stored chemical energy (ATP and NADPH), takes place in thylakoid membranes, a far-reaching and complex system of internal membranes. Secondly, the dark reaction of photosynthesis, i.e. the reduction of carbon dioxide and its conversion into carbohydrates, take place in the stroma, an amorphous matrix rich in soluble proteins. Thirdly, the metabolic channel of communication between the organelle and the rest of the cell, essential to integrate photosynthesis in the whole plant metabolism, is compactly controlled by proteins of the envelope membranes, a pair of membranes adjoining the chloroplast [2].

Salinity induces structural changes in chloroplasts, and a high Na⁺ concentration negatively affects photosynthetic efficiency, stomatal conductance, and transpiration rates [3]. Reactive oxygen species (ROS) production results when the intracellular osmotic balance and other metabolic processes are disturbed by exposure to high salt [4]. The antioxidant system is up-regulated under such stress, as evidenced by proteomic profiles of salt-responsive proteins generated in the chloroplasts of rice [5], wheat [6], grape [7], potato [8] soybean [9], tobacco [10] and maize [11]. In wheat, several oxidative enzymes or metabolites are involved either promoting cell damage or protecting against such injury, including proline [10], superoxide dismutase [12], hydrogen peroxide [13], and catalase [14]. Proteomic analyses have been applied to various aspects of biological processes, e.g., for protein identification, determinations of protein production during normal plant development and under stress conditions, analysis of post-translational modifications (PTMs), and studies of protein–protein interactions. Recent improvements in analytical methods have made it possible to evaluate and identify many more proteins involved in stress responses. For example, combining a gel-free proteomic system with a two-dimensional gel-based proteomic system provides a large amount of information about protein formation. Moreover, detailed analysis of PTMs via mass spectrometry enables the identification of key signaling molecules. Significant results have been obtained through focused analyses of subcellular proteomes.

Drought conditions inhibit photosynthesis within the first few days that water supply is reduced, causing the CO₂ assimilation rate to decline drastically [15]. On exposure to water stress, plants show a wide range of responses at the cellular and molecular levels [16], and plant metabolism [17]. For example, in wheat, those physiological effects include changes in photosynthesis; transpiration rate [3]; stomatal conductance [18]; and the accumulations of free proline [19]. Leaf photosynthetic rate, stomatal conductance and canopy temperature depression were all associated with yield progress which can be measured simply in the field, suggesting a potential methodology for screening physiologically superior lines [20].

Proteomics is a potent tool for understanding basic processes in plant growth and development, as well as for examining changes in specific proteins in response to environmental fluctuations. Here, proteomic approach was used to investigate the effect of salt and water stress in wheat chloroplasts. By analyzing differentially expressed proteins under drought conditions we sought deeper knowledge, attempted to identify key protein-encoding genes that could be used as candidate marker protein [3]. The application of proteomics namely chloroplast proteomics in crop breeding is usually instigated by detection of stress-responsive proteins by expressional candidate genes through evaluation between stressed and control plants. Sequencing of these stress-responsive proteins will then reveal that some of them have functions clearly consistent with the stress tolerance trait. As a final point, one can utilized these protein encoding genes in marker-assisted breeding or gene transformation programs to improve crop tolerance to stress [21]. However, except Arabidopsis, rice and maize, plant proteomic databases (amino acid sequences) have not been complete due to incomplete genome sequence to plant species. To date, the information of wheat chloroplast proteome has been limited due to under-representation in protein databases.