No abstract

Plastids are semiautonomous organelles with a wide structural and functional diversity and unique biochemical pathways. As such, they are able to transcribe and translate the information present in their own genome but are strongly dependent on imported proteins that are encoded in the nuclear genome and translated in the cytoplasm. Plastids are present in every plant cell, with very few exceptions (such as the highly specialized male sexual cells), and their structural and functional diversity reflects their role in different cell types. According to their developmental stage, we distinguish them as juvenile (proplastids), differentiating, mature, and senescent. Meristematic cells contain proplastids, which ensure the continuity of plastids from generation to generation and are capable of considerable structural and metabolic plasticity to develop into various types of plastids that remain interconvertible. When leaves are grown in darkness, proplastids differentiate into etioplasts, which can be converted into chloroplasts under illumination.The metabolism of these various types of plastids is linked to the function of the tissue in which they are found. For instance, whereas the chief function of illuminated leaves is the assimilation of CO 2 by chloroplasts, root plastids are mainly involved in the assimilation of inorganic nitrogen. Amyloplasts, which contain large starch grains, behave as storage reservoirs in stems, roots, and tubers. Chromoplasts synthesize large amounts of carotenoids and are present in petals, fruits, and even roots. The interconversions between these different plastids are accompanied by dramatic changes, including the development or regression of internal membrane systems (e.g. thylakoids and prolamellar bodies) and the acquisition of specific enzymatic equipment reflecting specialized metabolism. However, at all stages of these transformations, the two limiting envelope membranes remain apparently unchanged.Located at the interface between plastids and the surrounding cytosol, the envelope is a key structure for the integration of plastid metabolism within the cell. Because plastids are semiautonomous organelles, a tight coordination between plastidial development and cell differentiation is required. Envelope membranes are an essential checkpoint between the expression of plastidial and nuclear genomes, for example, as the site for the specific recognition and transport of the precursor plastid proteins synthesized on cytosolic ribosomes. Plastid membranes contain an astonishing variety of specific lipids, including polar lipids (e.g. galactolipids, phospholipids, and SLs), pigments (e.g. carotenoids and chlorophylls), and prenylquinones (e.g. plastoquinone and tocopherols). This diversity requires complex metabolic pathways that are closely associated with envelope membranes.A unique biochemical machinery (Fig. 1) is present in envelope membranes and reflects the stage of development of the plastid and the specific metabolic requirements of the various tissues. PLASTID ENVELOPE ME...

The degradation of storage compounds just after germination is essential to plant development, providing energy and molecules necessary for the building of a photosynthetic apparatus and allowing autotrophic growth. We identified à bout de souffle ( bou ), a new Arabidopsis mutation. Mutant plants stopped developing after germination and degraded storage lipids, but they did not proceed to autotrophic growth. Neither leaves nor roots developed in the mutant. However, externally added sugar or germination in the dark could bypass this developmental block and allowed mutant plants to develop. The mutated gene was cloned using the transposon Dissociation as a molecular tag. The gene coding sequence showed similarity to those of the mitochondrial carnitine acyl carriers (CACs) or CAC-like proteins. In animals and yeast, these transmembrane proteins are involved in the transport of lipid-derived molecules across mitochondrial membranes for energy and carbon supply. The data presented here suggest that BOU identifies a novel mitochondrial pathway that is necessary to seedling development in the light. The BOU pathway would be an alternative to the well-known glyoxylate pathway.

No abstract

No abstract

Isolated intact spinach chloroplasts were incubated with phospholipase C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3) under mild experimental conditions in which only the phosphatidylcholine localized in the cytosolic leaflet of the outer envelope membrane can be hydrolyzed. Thylakoids, which were protected from phospholipase C degradation, were subsequently prepared from the phospholipase C-treated chloroplasts and found to be devoid of phosphatidylcholine. Previously reported occurrences of phosphatidylcholine in thylakoid preparations probably reflect contamination of the thylakoids by envelope membranes. In the present work, contamination of thylakoids by envelope membranes was determined by measuring the 1,2-diacylglycerol 3-fi-galactosyltransferase [monogalactosyldiacylglycerol (MGDG) synthase; UDPgalactose:1,2-diacylglycerol 3-.t-D-galactosyltransferase, EC 2.4.1.46] in the different chloroplast subfractions. We conclude that phosphatidylcholine is not present in highly purified thylakoids. Phosphatidylcholine is also absent from prokaryotic cyanobacterial membranes, and our results are in agreement with the endosymbiotic origin of higher plant chloroplasts.According to the endosymbiotic theory, plastids in eukaryotic cells could have originated as free-living organisms that found shelter within protoeukaryotes and then became symbiotic elements within them (1, 2). Comparison of the membrane constituents in chloroplasts and cyanobacteria reveal numerous similarities-i.e., the presence of galactolipids, sulfolipid, and phosphatidylglycerol having the same structure-and a few striking differences, such as the absence of phosphatidylcholine, a typical eukaryotic lipid, in cyanobacteria (3). Analyses of the lipid components in envelope membranes from pea (4) and spinach (5) chloroplasts have shown that large amounts of phosphatidylcholine

The envelope from spinach chloroplasts contains an alkaline phosphatidic acid phosphatase which was found to be located on the inner envelope membrane. The diacylglycerol formed by this enzyme from endogenous phosphatidic acid is then used as a substrate for galactolipid synthesis on the inner envelope membrane.

Most seeds are anhydrobiotes, relying on an array of protective and repair mechanisms, and seed mitochondria have previously been shown to harbor stress proteins probably involved in desiccation tolerance. Since temperature stress is a major issue for germinating seeds, the temperature response of pea (Pisum sativum) seed mitochondria was examined in comparison with that of mitochondria from etiolated epicotyl, a desiccation-sensitive tissue. The functional analysis illustrated the remarkable temperature tolerance of seed mitochondria in response to both cold and heat stress. The mitochondria maintained a well-coupled respiration between 23.5°C and 40°C, while epicotyl mitochondria were not efficient below 0°C and collapsed above 30°C. Both mitochondria exhibited a similar Arrhenius break temperature at 7°C, although they differed in phospholipid composition. Seed mitochondria had a lower phosphatidylethanolamine-to-phosphatidylcholine ratio, fewer unsaturated fatty acids, and appeared less susceptible to lipid peroxidation. They also accumulated large amounts of heat shock protein HSP22 and late-embryogenesis abundant protein PsLEAm. The combination of membrane composition and stress protein accumulation required for desiccation tolerance is expected to lead to an unusually wide temperature tolerance, contributing to the fitness of germinating seeds in adverse conditions. The unique oxidation of external NADH at low temperatures found with several types of mitochondria may play a central role in maintaining energy homeostasis during cold shock, a situation often encountered by sessile and ectothermic higher plants.Many organisms need to cope with extreme temperatures, but few are adapted to live and reproduce in such conditions. While extremophilic microorganisms can metabolically adapt, more complex organisms avoid temperature stress by controlling body temperature or by moving to more favorable habitats. As land plants are ectothermic and unable to move, they cannot escape dramatic changes in temperature. Most live in environments where frequent temperature changes of 10°C to 20°C are common, and some, such as alpine plants, may experience fluctuations of more than 40°C in a single day. While much work has been carried out on the acclimation of plants to either low or high temperature, little is known about the mechanisms allowing them to cope with sudden temperature fluctuations that may exist for extended periods. In analyzing this situation, we obtained evidence from seeds that mitochondria play a central role in allowing plants to adapt to extreme temperatures.In the life cycle of higher plants, seeds must complete the crucial task of protecting the embryo and driving it toward the establishment of a new generation. The majority of higher plant seeds are desiccation tolerant, a complex trait that has contributed to the evolutionary success of angiosperms. Desiccationtolerant seeds are in fact anhydrobiotes and certainly represent the most stress-tolerant stage of plants. They are endowed with an impressiv...