Current Chemical Biology - Volume 6, Issue 3, 2012

During evolution, different heterotrophic organisms have acquired photosynthetic prokaryotes to transform them into their “tiny green slaves”, the chloroplasts. During endosymbiogenesis plastids have partially lost their independence, and tight co-regulation between the host nucleus and the organelle has been developed. In this work, the general features and the special characteristics (such as plastid morphology, number of plastid bounding membranes, periplastidial space and nucleomorph, thylakoid arrangement, plastid genome, plastid-located storage materials, carboxysomes and pyrenoids, plastoglobuli and eyespots, and other plastid inclusions) of the chloroplasts of the most important algal groups are reviewed in details. Several unicellular algae possess only (one or a few) chloroplast(s). In more complex organisms, such as for instance brown algal thalli and land plants, plastid form and function have been diversified and plastids got gradually specialized in parallel with the evolution of cells with different specific functions. Cells that have lost their photosynthetic activity developed special plastid types with less or no chlorophyllous pigments and thylakoids, but with specific functions such as storage (leucoplasts) or carotenoid synthesis (chromoplasts). In other cells (e.g. in dividing regions) proplastids can be found with poorly developed thylakoid system. In algae, plastid diversification and the interconversion of the different plastid types into each other are discussed as a consequence of (i) changes in the trophic mode of the algae, (ii) increasing complexity of the vegetative body, (iii) increasing complexity of the life cycle, (iv) senescence processes and (v) changes induced in plastid structure by environmental stimuli.

In the course of endosymbiogenesis, the photosynthetic prokaryotes engulfed and retained by different heterotrophic organisms have partially lost their independence during evolution and became semi-autonomous organelles, the chloroplasts. The chloroplast represents the most ancestral form of plastids that has parallelly evolved in several algal groups (reviewed in [1]) and in land plants. After briefly discussing plastid morphology, we review the most important ultrastructural features of the plastids of land plants. Then we discuss how plastids got gradually specialized in parallel with the increasing developmental and/or organizational complexity of the plant body. The plastids of non-photosynthetic tissues and cells do not need to produce and maintain a photosynthetic apparatus, but have adjusted their metabolism to the major function of the host cell (and tissue). This way, different plastid forms specialized for other functions such as storage (e.g. starch storing leucoplasts called amyloplast) or carotenoid synthesis (chromoplasts) have developed. However, the classical ultrastructural characterization and classification of plastids is often problematic. First of all, the term plastid refers to the extremely high plasticity of this organelle, and its capacity to be readily transformed from one type into another one upon different environmental or developmental stimuli. Therefore, transitional (or if persistent, intermediate) plastids with morphological features characteristic for two different plastid types can be often observed. Sometimes plastids with similar ultrastructure can have different specific functions and basically different metabolism, and should be therefore treated separately. After having recalled the different plastid types of land plants we present a dynamic model about their interconversions.

The polar acyl lipids which provide the structural basis of all plant cell membranes all contain fatty acids synthesized in the plastid stroma. Fatty acid synthesis in the plastid stroma is also the source of acyl groups used for the synthesis of storage lipids in seeds and other tissues. While the basic biochemistry of fatty acid and glycerolipid synthesis is relatively well understood, the mechanisms for the transport of lipids between organelles and regulation of these pathways are just beginning to be unraveled. The plastid membranes themselves are composed of lipids synthesized in the plastid envelope. Notably, plastid membranes contain the non-phosphorous sugar-containing acyl lipids mono- (MGDG) and digalactosyl diacylglycerol (DGDG), and the sulfolipid, sulfoquinovosyl diacylglycerol (SQDG). These lipids are assembled in the plastid envelope from diacylglycerol backbones, synthesized by two different pathways: one contained entirely in the plastid envelope and one in the endoplasmic reticulum (ER). Phosphate limitation causes a dramatic remodeling of membrane lipids in the plant cell. In the thylakoid membrane, the anionic phospholipid phosphatidylglycerol (PG) is replaced by SQDG. In extraplastidial membranes (i.e. mitchondria, plasma membrane and the tonoplast) plastid envelopederived DGDG replaces several phospholipid classes. Plastidial lipids are also remodeled under other types of stress and in some cases storage lipids will accumulate inside plastids.

There are many cellular processes occurring in plastids. One of them concerns nitrogen assimilation (nitrate and ammonium). Photosynthesis taking place in the same organelle plays a key role in direct or indirect assimilation of nitrogen. This is involved in the building of the fundamental life molecules: nucleotides, proteins, chlorophyll and numerous other metabolites and cellular components. This review enlightens the mechanisms involved in the transport, reduction and assimilation of nitrate implying specific transporters in the plastid envelopes, specific enzymes as glutamine synthetase/ glutamate synthase pathway in the plastid stroma from plants and algae studied in the lab as models. EPSP synthase, AHA synthase and GS participating to amino acid biosynthesis have been used as targets for widely distributed herbicides. This paper will review the molecular basis of their inhibiting action and the various approaches to obtain herbicide- tolerant crops.

Plastids represent a family of organelles, which are ubiquitous in all plant cells. They have an outer and an inner envelope membrane, specialized in transport between the cytosol and the stroma. Green plastids (chloroplasts) have an additional internal membrane, named thylakoid membrane, specialized in photosynthetic light-dependent reactions. The envelope and thylakoid membrane of chloroplasts display a selective permeability since they allow substances of certain types and sizes to cross in certain amounts and at certain times. This contribution will review the current knowledge about mechanisms in transporting proteins, solutes and metabolites across envelope and thylakoid membranes in chloroplasts and about their importance for plant cell metabolism.

Water supply is crucial for the development and growth of all living organisms. This is especially true for oxygenic photosynthetic organisms (cyanobacteria, algae and terrestrial plants), which use water as a substrate to produce molecular oxygen through the activity of the water-oxidizing photosystem II complex. The precise site of water oxidation is on the lumenal side of the thylakoid membrane, harboring this complex. How water molecules reach the thylakoid lumen to sustain oxygen production is a crucial question. To date, the mechanism of water transport across the thylakoid membrane is unknown. Within the cell, the most common mechanisms for water transport are free diffusion and facilitated diffusion, the latter being mediated by specialized channel proteins named aquaporins. In this review, the following questions are addressed: 1) Could free diffusion through the thylakoid membrane provide sufficient amounts of water for effective photosynthetic reaction? or 2) Are aquaporins involved in water transport across the thylakoid membrane? Biophysical studies and theoretical calculations support the second possibility. Moreover, several aquaporins have been found using mass spectrometry-based proteomics in plant chloroplast membranes. Validation of their chloroplast location and investigation of a potential role in photosynthesis should be the focus of future studies.

The exposure of plants to high light in excess of photosynthetic needs causes a reduction in photosynthetic capacity, which is known as photoinhibition. During photoinhibition reactive oxygen species are produced to an extent that leads to the destruction of carotenoids, chlorophyll, protein and to an increase in membrane lipid peroxidation. Plants have developed several strategies to sustain chloroplast functioning under high light conditions. In this review we summarize the latest knowledge about mechanisms for photoinhibition and photoprotective strategies such as: 1) chloroplast antioxidant systems (i.e. tocochromanols, water-water cycle); 2) the quenching of the triplet chlorophyll and reactive oxygen species by carotenoids.; 3) the reversible conversion of violxanthin to zeaxanthin in the light-harvesting complex (LHC) (xanthophyll cycles).

Crops are often exposed to various environmental stresses, such as high and low temperature, drought, increase of field salinity, and metal toxicity. At cellular level, these stresses disturb redox homeostasis and trigger formation of reactive oxygen species (ROS). The improvement of the antioxidant defence system to attenuate the effects of ROS and to maintain the redox balance even under stress conditions may result in an enhanced stress tolerance of plants. Biotechnology efforts including direct gene transfer techniques or cell and tissue cultures have been applied for improving the abiotic stress tolerance of plants via the enhancement of their antioxidant capacity. In this review, we demonstrate that the doubled haploid (DH) technology, especially through androgenesis, is also a valuable tool for producing plants with enhanced antioxidant capacity tolerant to most kinds of abiotic stresses. The monitoring of changes in photosynthetic processes induced by environmental stresses can be efficiently applied for the characterization of the abiotic stress tolerance of selected DH plants. In addition, we demonstrate that the in vitro selection of microspores can provide a wide range of genetic plant materials with different combinations of elevated activities of antioxidant enzymes, which can contribute to the studying of stress tolerance mechanisms in relation also to the redox regulation.