The effects of reduced amounts of lipid unsaturation on chloroplast ultrastructure and photosynthesis in a mutant of Arabidopsis

Inter-Organellar Effects of Defective ER-Localized Linolenic Acid Formation on Thylakoid Lipid Composition, Non-Photochemical Quenching of Chlorophyll Fluorescence and Xanthophyll Cycle Activity in the Arabidopsis fad3 Mutant

Monogalactosyldiacylglycerol (MGDG) is the main lipid constituent of thylakoids and a structural component of photosystems and photosynthesis-related proteo-lipid complexes in green tissues. Previously reported changes in MGDG abundance upon stress treatments are hypothesized to reflect mobilization of MGDG-based polyunsaturated lipid intermediates to maintain extraplastidial membrane integrity. While exchange of lipid intermediates between compartmental membranes is well documented, physiological consequences of mobilizing an essential thylakoid lipid, such as MGDG, for an alternative purpose are not well understood. Arabidopsis seedlings exposed to mild (50 mM) salt treatment displayed significantly increased abundance of both MGDG and the extraplastidial lipid, phosphatidylcholine (PC). Interestingly, similar increases in MGDG and PC were observed in Arabidopsis fad3 mutant seedlings defective in endoplasmic reticulum (ER)-localized linolenic acid formation, in which compensatory plastid-to-ER-directed mobilization of linolenic acid-containing intermediates takes place. The postulated (salt) or evident (fad3) plastid-ER exchange of intermediates concurred with altered thylakoid function according to parameters of photosynthetic performance. While salt treatment of wild-type seedlings inhibited photosynthetic parameters in a dose-dependent manner, interestingly, untreated fad3 mutants did not show overall reduced photosynthetic quantum yield. By contrast, we observed a reduction specifically of non-photochemical quenching (NPQ) under high light, representing only part of observed salt effects. The decreased NPQ in the fad3 mutant was accompanied by reduced activity of the xanthophyll cycle, leading to a reduced concentration of the NPQ-effective pigment zeaxanthin. The findings suggest that altered ER-located fatty acid unsaturation and ensuing inter-organellar compensation impacts on the function of specific thylakoid enzymes, rather than globally affecting thylakoid function.

Loss of Function of Fatty Acid Desaturase 7 in Tomato Enhances Photosynthetic Carbon Fixation Efficiency

Fatty Acid Desaturase 7 (FAD7) generates polyunsaturated fatty acids, promoting the desaturation of chloroplast membranes; it also provides an essential precursor for the synthesis of jasmonic acid (JA), a phytohormone that can influence plant growth, development, and primary metabolism. This study examined the effects of spr2, a null mutation in SlFAD7, on the growth, morphology, and photosynthetic traits of tomato, Solanum lycopersicum. Although the spr2 mutant had a lower density of stomata than wild type plants, the two genotypes had comparable stomatal conductance, transpiration rates, and intracellular CO2 levels; in addition, spr2 had significantly thinner leaf blades, which may help maintain normal levels of CO2 diffusion despite the lower number of stomata. Surprisingly, spr2 also had significantly higher carbon assimilation (A) and maximum quantum efficiency of PSII (Fv/Fm) than wild type plants at both of the light intensities tested here (220 or 440 µmol m-2 s-1), despite having lower levels of chlorophyll than wild type plants under low light (220 µmol m-2 s-1). Furthermore, CO2 response curves indicated higher in vivo Rubisco activity (Vcmax) in spr2 compared to wild type plants, as well as an enhanced maximum rate of electron transport used in the regeneration of ribulose-1,5-bisphosphate (Jmax). These data indicate that loss of function of FAD7 can enhance the efficiency of both light-dependent and light-independent reactions in photosynthesis. Consistent with this, the spr2 mutant also displayed enhanced growth, with significantly more leaves and a more compact growth habit. In contrast to spr2, another tomato mutant impaired in JA synthesis (acx1) showed no enhancements in growth or photosynthetic efficiency, suggesting that the enhancements observed in spr2 are independent of the effects of this mutation on JA synthesis. These data demonstrate that loss of function of FAD7 can enhance photosynthesis and growth, potentially through its impacts on the chloroplast membranes.

Long-Chain Polyunsaturated Fatty Acids in the Green Microalga Lobosphaera incisa Contribute to Tolerance to Abiotic Stresses

Lobosphaera incisa is a green microalga that accumulates high levels of the valuable omega-6 long-chain polyunsaturated fatty acids (LC-PUFA) arachidonic acid (ARA, 20:4n-6) in triacylglycerols (TAG) under nitrogen (N) starvation. LC-PUFA accumulation is a rare trait in photosynthetic microalgae with insufficiently understood physiological significance. In this study, RNAi was attempted, for the first time in L. incisa, to produce knockdown lines for the Δ5 desaturase gene. Two lines, termed modified lines, which were isolated during screening for transgenic events, demonstrated alterations in their LC-PUFA profile, ARA-biosynthesis gene expression and lipid class distribution. In line M5-78, which appeared to carry a mutation in the Δ6 elongase gene, LC-PUFA were substituted by 18:3n-6 in all glycerolipids. Line M2-35, for which the exact genetic background has not been established, displayed a dramatic reduction in 20:4n-6, concomitant with an augmented proportion of 18:1n-9, in particular in the extraplastidial membrane lipids and TAG. The physiological responses of the modified lines to stressful conditions were compared with the wild type and the Δ5 desaturase mutant. In the N-replete cells of modified lines, the frequency of lipid droplets was reduced, while a number of starch grains increased, suggesting altered partitioning of assimilated carbon into reserve products. Furthermore, both lines exhibited reduced ability to accumulate TAG under N deprivation and recover from N starvation. Both lines demonstrated lower photosynthetic pigment contents, impairments in photosynthesis under a range of stressful conditions, and less efficient functioning of photoprotection under optimal conditions. Possible implications of fatty acids modifications in the stress response of L. incisa are addressed.

Medium-Chain Polyprenols Influence Chloroplast Membrane Dynamics in Solanum lycopersicum

The widespread occurrence of polyprenols throughout the plant kingdom is well documented, yet their functional role is poorly understood. These lipophilic compounds are known to be assembled from isoprenoid precursors by a class of enzymes designated as cis-prenyltransferases (CPTs), which are encoded by small CPT gene families in plants. In this study, we report that RNA interference (RNAi)-mediated knockdown of one member of the tomato CPT family (SlCPT5) reduced polyprenols in leaves by about 70%. Assays with recombinant SlCPT5 produced in Escherichia coli determined that the enzyme synthesizes polyprenols of approximately 50-55 carbons (Pren-10, Pren-11) in length and accommodates a variety of trans-prenyldiphosphate precursors as substrates. Introduction of SlCPT5 into the polyprenol-deficient yeast Δrer2 mutant resulted in the accumulation of Pren-11 in yeast cells, restored proper protein N-glycosylation and rescued the temperature-sensitive growth phenotype that is associated with its polyprenol deficiency. Subcellular fractionation studies together with in vivo localization of SlCPT5 fluorescent protein fusions demonstrated that SlCPT5 resides in the chloroplast stroma and that its enzymatic products accumulate in both thylakoid and envelope membranes. Transmission electron microscopy images of polyprenol-deficient leaves revealed alterations in chloroplast ultrastructure, and anisotropy measurements revealed a more disordered state of their envelope membranes. In polyprenol-deficient leaves, CO2 assimilation was hindered and their thylakoid membranes exhibited lower phase transition temperatures and calorimetric enthalpies, which coincided with a decreased photosynthetic electron transport rate. Taken together, these results uncover a role for polyprenols in governing chloroplast membrane dynamics.

Polyprenols Are Synthesized by a Plastidial cis-Prenyltransferase and Influence Photosynthetic Performance

Plants accumulate a family of hydrophobic polymers known as polyprenols, yet how they are synthesized, where they reside in the cell, and what role they serve is largely unknown. Using Arabidopsis thaliana as a model, we present evidence for the involvement of a plastidial cis-prenyltransferase (AtCPT7) in polyprenol synthesis. Gene inactivation and RNAi-mediated knockdown of AtCPT7 eliminated leaf polyprenols, while its overexpression increased their content. Complementation tests in the polyprenol-deficient yeast ∆rer2 mutant and enzyme assays with recombinant AtCPT7 confirmed that the enzyme synthesizes polyprenols of ∼55 carbons in length using geranylgeranyl diphosphate (GGPP) and isopentenyl diphosphate as substrates. Immunodetection and in vivo localization of AtCPT7 fluorescent protein fusions showed that AtCPT7 resides in the stroma of mesophyll chloroplasts. The enzymatic products of AtCPT7 accumulate in thylakoid membranes, and in their absence, thylakoids adopt an increasingly "fluid membrane" state. Chlorophyll fluorescence measurements from the leaves of polyprenol-deficient plants revealed impaired photosystem II operating efficiency, and their thylakoids exhibited a decreased rate of electron transport. These results establish that (1) plastidial AtCPT7 extends the length of GGPP to ∼55 carbons, which then accumulate in thylakoid membranes; and (2) these polyprenols influence photosynthetic performance through their modulation of thylakoid membrane dynamics.

Glycerolipids in photosynthesis: composition, synthesis and trafficking

Glycerolipids constituting the matrix of photosynthetic membranes, from cyanobacteria to chloroplasts of eukaryotic cells, comprise monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. This review covers our current knowledge on the structural and functional features of these lipids in various cellular models, from prokaryotes to eukaryotes. Their relative proportions in thylakoid membranes result from highly regulated and compartmentalized metabolic pathways, with a cooperation, in the case of eukaryotes, of non-plastidic compartments. This review also focuses on the role of each of these thylakoid glycerolipids in stabilizing protein complexes of the photosynthetic machinery, which might be one of the reasons for their fascinating conservation in the course of evolution. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

Plant lipids: metabolism, mutants, and membranes

The mechanisms that regulate plant lipid metabolism determine the dietary and industrial value of storage oils found in economically important species and may control the ability of many plants to survive exposure to temperature extremes. Many of the problems researchers have in defining the pathways, enzymes, and genes involved in plant lipid metabolism appear to be amenable to analysis by genetic approaches. Mutants with alterations in membrane lipid composition have also been used to study the structural and adaptive roles of lipids. The application of genetic engineering methods affords opportunities for researchers to apply knowledge gained about plant lipid metabolism toward enhanced use of plant oils as abundant and renewable sources of reduced carbon.

Variable amounts of DNA related to the size of chloroplasts III. Biochemical determinations of DNA amounts per organelle

Plastid genomes (plastomes) are part of the integrated compartmentalised genetic system of photoautotrophic eukaryotes. They are highly redundant and generally dispersed in several regions (nucleoids) within organelles. DNA quantities and number of DNA-containing regions per plastid vary and are developmentally regulated in a way not yet understood. Reliable quantitative data describing these patterns are scarce. We present a protocol to isolate fractions of pure plastids with varying average sizes from leaflets (8 microm average diameter, corresponding from approximately a dozen to 330 genome equivalents per organelle and on average four to seven copies per nucleoid. The ratio of plastid/nuclear DNA changed continuously during leaf development from as little as 0.4% to about 20% in fully developed leaves. On the other hand, mesophyll cells of mature leaves differing in ploidy (di-, tri- and tetraploid) appeared to maintain a relatively constant nuclear genome/plastome ratio, equivalent to about 1,700 copies per C-value.

Tocotrienols, the unsaturated forms of vitamin E, can function as antioxidants and lipid protectors in tobacco leaves

Vitamin E is a generic term for a group of lipid-soluble antioxidant compounds, the tocopherols and tocotrienols. While tocotrienols are considered as important vitamin E components in humans, with functions in health and disease, the protective functions of tocotrienols have never been investigated in plants, contrary to tocopherols. We took advantage of the strong accumulation of tocotrienols in leaves of double transgenic tobacco (Nicotiana tabacum) plants that coexpressed the yeast (Saccharomyces cerevisiae) prephenate dehydrogenase gene (PDH) and the Arabidopsis (Arabidopsis thaliana) hydroxyphenylpyruvate dioxygenase gene (HPPD) to study the antioxidant function of those compounds in vivo. In young leaves of wild-type and transgenic tobacco plants, the majority of vitamin E was stored in thylakoid membranes, while plastoglobules contained mainly delta-tocopherol, a very minor component of vitamin E in tobacco. However, the vitamin E composition of plastoglobules was observed to change substantially during leaf aging, with alpha-tocopherol becoming the major form. Tocotrienol accumulation in young transgenic HPPD-PDH leaves occurred without any significant perturbation of photosynthetic electron transport. Tocotrienols noticeably reinforced the tolerance of HPPD-PDH leaves to high light stress at chilling temperature, with photosystem II photoinhibition and lipid peroxidation being maintained at low levels relative to wild-type leaves. Very young leaves of wild-type tobacco plants turned yellow during chilling stress, because of the strongly reduced levels of chlorophylls and carotenoids, and this phenomenon was attenuated in transgenic HPPD-PDH plants. While sugars accumulated similarly in young wild-type and HPPD-PDH leaves exposed to chilling stress in high light, a substantial decrease in tocotrienols was observed in the transgenic leaves only, suggesting vitamin E consumption during oxygen radical scavenging. Our results demonstrate that tocotrienols can function in vivo as efficient antioxidants protecting membrane lipids from peroxidation.

Regulation of membrane fatty acid composition by temperature in mutants of Arabidopsis with alterations in membrane lipid composition

BACKGROUND

A wide range of cellular responses occur when plants are exposed to elevated temperature, including adjustments in the unsaturation level of membrane fatty acids. Although membrane bound desaturase enzymes mediate these adjustments, it is unknown how they are regulated to achieve these specific membrane compositions. Furthermore, the precise roles that different membrane fatty acid compositions play in photosynthesis are only beginning to be understood. To explore the regulation of the membrane composition and photosynthetic function in response to temperature, we examined the effect of temperature in a collection of mutants with altered membrane lipid fatty acid composition.

RESULTS

In agreement with previous studies in other species, the level of unsaturation of membrane fatty acids in Arabidopsis was inversely correlated with growth temperature. The time required for the membrane fatty acids to attain the composition observed at elevated temperature was consistent with the timing required for the synthesis of new fatty acids. Comparisons of temperature-induced fatty acid alterations in membranes were made among several Arabidopsis lines including wild-type Columbia, and the compositional mutants, fad5, fad6, act1 and double mutants, fad7 fad8 and act1 fad6. The results revealed key changes that occur in response to elevated temperature regardless of the specific mutations in the glycerolipid pathway, including marked decreases in trienoic fatty acids and consistent increases in unsaturated 16:0 and in dienoic 18:2 levels. Fluorescence measurements of various mutants indicated that photosynthetic stability as well as whole plant growth at elevated temperature is influenced by certain membrane fatty acid compositions.

CONCLUSIONS

The results of this study support the premise that defined proportions of saturated and unsaturated fatty acids in membrane lipids are required for photosynthetic thermostability and acclimation to elevated temperature. The results also suggest that changes in the membrane fatty acid composition brought about in response to temperature are regulated in such a way so as to achieve highly similar unsaturation levels despite mutations that alter the membrane composition prior to a high-temperature exposure. The results from examination of the mutant lines also suggest that interorganellar transfer of fatty acids are involved in mediating temperature-induced membrane alterations, and reveal steps in the fatty acid unsaturation pathway that appear to have key roles in the acclimatization of membranes to high temperature.

Mutants of Arabidopsis reveal many roles for membrane lipids

Polyunsaturated acyl lipids constitute approximately 50% of the hydrophobic membrane barriers that delineate the compartments of cells. The composition of these lipids is critically important for many membrane functions and, thus, for proper growth and development of all living organisms. In the model plant Arabidopsis, the isolation of mutants with altered lipid compositions has facilitated biochemical and molecular approaches to understanding lipid metabolism and membrane biogenesis. Just as importantly, the availability of a series of plant lines with specific changes in membrane lipids have provided a new resource to study the structural and adaptive roles of lipids. Now, the sequencing of the Arabidopsis genome, and the development of reverse-genetics approaches provide the tools needed to make additional discoveries about the relationships between lipid structure and membrane function in plant cells.

Preparation and functional characterization of thylakoids from Arabidopsis thaliana

A protocol for the isolation of functional thylakoids from Arabidopsis thaliana leaves was developed. The critical factor in obtaining active, coupled and stable preparation is the inclusion of EDTA and EGTA in the grinding buffer. Preparations were characterized with respect to the whole or partial electron transport chain, ATP/NADPH, ATP/O(2) and PS II/chlorophyll ratios. Sensitivity to a light-chill photoinhibitory treatment was also determined by evaluating the decrease in both maximal photochemical efficiency (Fv/Fm) and in electron transport rate.

Trienoic fatty acids are required to maintain chloroplast function at low temperatures

The chloroplast membranes of all higher plants contain very high proportions of trienoic fatty acids. To investigate how these lipid structures are important in photosynthesis, we have generated a triple mutant line of Arabidopsis that contains negligible levels of trienoic fatty acids. For mutant plants grown at 22 degrees C, photosynthetic fluorescence parameters were indistinguishable from wild type at 25 degrees C. Lowering the measurement temperature led to a small decrease in photosynthetic quantum yield, Phi(II), in the mutant relative to wild-type controls. These and other results indicate that low temperature has only a small effect on photosynthesis in the short term. However, long-term growth of plants at 4 degrees C resulted in decreases in fluorescence parameters, chlorophyll content, and thylakoid membrane content in triple-mutant plants relative to wild type. Comparisons among different mutant lines indicated that these detrimental effects of growth at 4 degrees C are strongly correlated with trienoic fatty acid content with levels of 16:3 + 18:3, approximately one-third of wild type being sufficient to sustain normal photosynthetic function. In total, our results indicate that trienoic fatty acids are important to ensure the correct biogenesis and maintenance of chloroplasts during growth of plants at low temperatures.

Identification and characterization of an animal delta(12) fatty acid desaturase gene by heterologous expression in Saccharomyces cerevisiae

We have cloned a Caenorhabditis elegans cDNA encoding a Delta12 fatty acid desaturase and demonstrated its activity by heterologous expression in Saccharomyces cerevisiae. The predicted protein is highly homologous both to the cloned plant genes with similar function and to the published sequence of the C. elegans omega-3 fatty acid desaturase. In addition, it conforms to the structural constraints expected of a membrane-bound fatty acid desaturase including the canonical histidine-rich regions. This is the first report of a cloned animal Delta(12) desaturase gene. Expression of this cDNA in yeast resulted in the accumulation of 16:2 and 18:2 (linoleic) acids. The increase of membrane fluidity brought about by this change in unsaturation was measured. The production of polyunsaturated fatty acids in yeast cells and the concomitant increase in membrane fluidity was correlated with a modest increase in growth rate at low temperature and with increased resistance to ethanol and oxidative stress.

Characterization of transgenic tobacco with an increased alpha-linolenic acid level

Microsomal omega-3 fatty acid desaturase catalyzes the conversion of 18:2 (linoleic acid) to 18:3 (alpha-linolenic acid) in phospholipids, which are the main constituents of extrachloroplast membranes. Transgenic tobacco (Nicotiana tabacum) plants with increased 18:3 contents (designated SIIn plants) were produced through the introduction of a construct with the tobacco microsomal omega-3 fatty acid desaturase gene under the control of the highly efficient promoter containing the E12Omega sequence. 18:3 contents in the SIIn plants were increased by about 40% in roots and by about 10% in leaves compared with the control plants. With regard to growth at 15 degreesC and 25 degreesC and the ability to tolerate chilling at 1 degreesC and 5 degreesC, there were no discernible differences between the SIIn and the control plants. Freezing tolerance in leaves and roots, which was assessed by electrolyte leakage, was almost the same between the SIIn and the control plants. The fluidity of plasma membrane from the SIIn plants was almost the same as that of the control plants. These results indicate that an increase in the 18:3 level in phospholipids is not directly involved in compensation for the diminishment in growth or membrane properties observed under low temperatures.

Low-temperature damage and subsequent recovery of fab1 mutant Arabidopsis exposed to 2 degrees C

The fab1 mutant of Arabidopsis thaliana, which contains increased levels of saturated fatty acids, was indistinguishable from the wild type when it was grown at 22 or 12 degrees C. During the first 7 to 10 d after transfer to 2 degrees C, the growth and photosynthetic characteristics of the fab1 plants remained indistinguishable from the wild type, with values for the potential quantum efficiency of photosystem II decreasing from 0.8 to 0.7 in plants of both lines. Whereas wild-type plants maintained quantum efficiency of photosystem II at approximately 0.7 for at least 35 d at 2 degrees C, this parameter declined rapidly in the mutant after 7 d and reached a value of less than 0.1 after 28 d at 2 degrees C. This decline in photosynthetic capacity was accompanied by reductions in chlorophyll content and the amount of chloroplast glycerolipids per gram of leaf. Electron microscopic examination of leaf samples revealed a rapid and extensive disruption of the thylakoid and chloroplast structure in the mutant, which is interpreted here as a form of selective autophagy. Despite the almost complete loss of photosynthetic function and the destruction of photosynthetic machinery, fab1 plants retained a substantial capacity for recovery following transfer to 22 degrees C. These results provide a further demonstration of the importance of chloroplast membrane unsaturation to the proper growth and development of plants at low temperature.

Tissue-specific and light-responsive regulation of the promoter region of the Arabidopsis thaliana chloroplast omega-3 fatty acid desaturase gene (FAD7)

The Arabidopsis FAD7 gene encodes a chloroplast omega-3 fatty acid desaturase that catalyzes the desaturation of lipid-linked dienoic fatty acids (18:2 and 16:2). An 825 bp FAD7 promoter fragment upstream from the transcriptional start point contained several short sequences which were homologous to the cis-elements (box II, G-box, etc.) conserved in many light-responsive genes. We introduced the FAD7 promoter fused to the beta-glucuronidase (GUS) or the luciferase (LUC) reporter gene into tobacco plants. The -825 promoter sequence conferred tissue-specific and light-responsive expression to both these reporter genes in transgenic tobacco, indicating that these expressions of the FAD7 gene were regulated mainly at the transcriptional level. Histochemical GUS staining showed that the activity of the FAD7 promoter is restricted to the tissues with chloroplast-containing cells although the staining was noticeably absent in the chloroplast-containing cells associated with vascular systems. The 5' deletion experiments of the promoter revealed that the -362/-166 region, containing two putative box II sequences, was responsible for the tissue-specific and light-responsive expression of the FAD7 gene.

Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against low-temperature photoinhibition in transgenic tobacco plants

Using tobacco plants that had been transformed with the cDNA for glycerol-3-phosphate acyltransferase, we have demonstrated that chilling tolerance is affected by the levels of unsaturated membrane lipids. In the present study, we examined the effects of the transformation of tobacco plants with cDNA for glycerol-3-phosphate acyltransferase from squash on the unsaturation of fatty acids in thylakoid membrane lipids and the response of photosynthesis to various temperatures. Of the four major lipid classes isolated from the thylakoid membranes, phosphatidylglycerol showed the most conspicuous decrease in the level of unsaturation in the transformed plants. The isolated thylakoid membranes from wild-type and transgenic plants did not significantly differ from each other in terms of the sensitivity of photosystem II to high and low temperatures and also to photoinhibition. However, leaves of the transformed plants were more sensitive to photoinhibition than those of wild-type plants. Moreover, the recovery of photosynthesis from photoinhibition in leaves of wild-type plants was faster than that in leaves of the transgenic tobacco plants. These results suggest that unsaturation of fatty acids of phosphatidylglycerol in thylakoid membranes stabilizes the photosynthetic machinery against low-temperature photoinhibition by accelerating the recovery of the photosystem II protein complex.

Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against low-temperature photoinhibition in transgenic tobacco plants

Using tobacco plants that had been transformed with the cDNA for glycerol-3-phosphate acyltransferase, we have demonstrated that chilling tolerance is affected by the levels of unsaturated membrane lipids. In the present study, we examined the effects of the transformation of tobacco plants with cDNA for glycerol-3-phosphate acyltransferase from squash on the unsaturation of fatty acids in thylakoid membrane lipids and the response of photosynthesis to various temperatures. Of the four major lipid classes isolated from the thylakoid membranes, phosphatidylglycerol showed the most conspicuous decrease in the level of unsaturation in the transformed plants. The isolated thylakoid membranes from wild-type and transgenic plants did not significantly differ from each other in terms of the sensitivity of photosystem II to high and low temperatures and also to photoinhibition. However, leaves of the transformed plants were more sensitive to photoinhibition than those of wild-type plants. Moreover, the recovery of photosynthesis from photoinhibition in leaves of wild-type plants was faster than that in leaves of the transgenic tobacco plants. These results suggest that unsaturation of fatty acids of phosphatidylglycerol in thylakoid membranes stabilizes the photosynthetic machinery against low-temperature photoinhibition by accelerating the recovery of the photosystem II protein complex.

Isolation and characterization of mutants affected in lipid metabolism of Chlamydomonas reinhardtii

Two mutants affected in lipid metabolism of Chlamydomonas reinhardtii were isolated by treating cells with ultraviolet light. Both mutants showed high chlorophyll fluorescent yields, as compared with parents, and were designated as hf-2 and hf-9 (for high fluorescence). hf-2 was shown to be defective in the synthesis of a chloroplast-specific lipid, sulfoquinovosyl diacylglycerol. hf-9 was shown to be defective in desaturation at the omega 6 position of fatty acids of monogalactosyl diacylglycerol, digalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol and phosphatidylglycerol. The mutants exhibited alterations in photosynthetic activity and chloroplast ultrastructure.

Tansley Review No. 75 Arabidopsis- its use in the genetic and molecular analysis of plant morphogenesis

In the last decade, the weed Arabidopsis thaliana has come to prominence as a major new model system for investigating genetic and molecular aspects of developmental plant morphology. Extensive genetic and molecular information about the Arabidopsis genome, facilitated by international collaborations and the production of novel mutagenic systems, has enabled a vast array of mutants to be identified, most of which reveal nuclear genes that control different aspects of plant developmental processes. An ever increasing number of these newly identified genes have been isolated and within the next few years an overall view of the molecular control of plant development is likely to emerge. Particularly prevalent amongst these Arabidopsis mutants are those which alter morphogenic processes cither by changes in differentiation patterns of specific cell types, homeotic conversion of entire structures or abnormal patterns of cell division. Mutants in the control of morphogenesis of most parts of the Arabidopsis plant have been identified and characterized. The most abundant classes are in embryogenesis including seedling pattern formation, root morphogenesis, floral morphology (including pollen and anther formation) and mutunts affecting shoot apical inenstern morphology. The first genes to be isolated from morphological mutants have been identified as transcription factors capable of controlling expression of other gene classes as part of a hierarchy of gene control. The relative ease with which many interesting and potentially important genes in morphogenesis have been revealed by identificatic.n of mutants makes it highly likely that with the aid of Arabidopsis thatiana, an understanding of the extremely complex molecular basis of plant morphogenesis may at last be Hirbin reach. Contents Summary 19 I. Introduction 19 II. Why Arabidopsis? 20 III. The developing seed 22 IV. The root 25 V. The shoot apex 27 VI. The Leaf 28 VII. Whole plant morphology 30 VIII. Floral development 31 IX. Sub-cellular tnorphology 32 X. Discussion and future possihihties 32 XI. Acknowledgements 32 XII. References 34.

Structural characteristics of thylakoid membranes of Arabidopsis mutants deficient in lipid fatty acid desaturation

The ultrastructure of thylakoid membranes from Arabidopsis thaliana wild-type, JB67 and LK3 fatty acid desaturation deficient mutants was studied by thin-section and freeze-fracture electron microscopy. There was a decrease in the amount of the appressed and non-appressed membranes in JB67 and LK3 Arbidopsis mutants when compared to the wild type, resulting in a reduction in the length of photosynthetic membrane per plastid. The results from freeze-fracture showed a decrease in size and a marked increase in packing density of membrane-associated particles on the exo- and endoplasmic fracture faces of the mutants. In addition, areas of the appressed membranes of the mutants contained particles in regular arrays under conditions where no such arrays were observed in wild-type thylakoid membranes. These observations suggest, that the decreased level of lipid fatty acid unsaturation affects the ability of the lipid matrix to mediate the assembly of chloroplast membrane components. The role of polyunsaturated membrane lipids is considered in terms of their ability to promote functional oligomeric assemblies of components of the photosynthetic apparatus.

Contribution of membrane lipids to the ability of the photosynthetic machinery to tolerate temperature stress

The contribution of the unsaturation of membrane lipids to the ability of the photosynthetic machinery to tolerate temperature stress was studied in a transgenic cyanobacterium. Anacystis nidulans R2-SPc was transformed with the desA gene, which encodes the Delta12-desaturase that desaturates the fatty acids of membrane lipids in Synechocystis PCC6803. The transformant acquired the ability to introduce a second double bond into palmitoleic and oleic acids. The transformation enhanced the tolerance of the photosynthetic machinery to chilling stress but it had no detectable effect on the ability to tolerate heat stress. The transformation itself did not have any effect on photosynthetic activity. These results imply that an increase in the unsaturation of membrane lipids enhances the tolerance of the photosynthetic machinery toward chilling stress but not toward heat stress and that such an increase does not affect photosynthesis within the range of physiological temperatures.

Unsaturation of fatty acids in membrane lipids enhances tolerance of the cyanobacterium Synechocystis PCC6803 to low-temperature photoinhibition

Effect of the unsaturation of fatty acids in the glycerolipids of thylakoid membranes on low-temperature photoinhibition of photosynthesis was studied by mutation and transformation of the cyanobacterium Synechocystis PCC6803. When grown at 34 degrees C, the wild type contained mono-, di-, and triunsaturated lipids; a mutant, designated Fad6, contained mono- and diunsaturated lipids; and a transformant of Fad6, with a disrupted gene for desaturation and designated Fad6/desA::Kmr, contained only monounsaturated lipids. Fad6/desA::Kmr was the most susceptible among these strains to low-temperature photoinhibition of photosynthesis, whereas Fad6 and the wild type were apparently indistinguishable in terms of sensitivity to photoinhibition. This result suggests that the presence of diunsaturated fatty acids is important in protecting against low-temperature photoinhibition. The photoinhibition at room temperature, although much less significant than that at low temperature, was also affected by the unsaturation of fatty acids. By contrast, the photosynthetic transport of electrons, measured at various temperatures, was not affected by changes in extent of fatty acid unsaturation.

A chilling sensitive mutant of Arabidopsis with altered steryl-ester metabolism

A chilling-sensitive mutant of Arabidopsis thaliana was isolated and subjected to genetic, physiological, and biochemical analysis. The chilling-sensitive nature of the mutant line is due to a single recessive nuclear mutation at a locus designated chs1. In contrast to wild-type plants, which are not adversely affected by low temperatures, the chs1 mutant is killed by several days of exposure to temperatures below 18 degrees C. Following exposure to chilling temperatures, the mutant displays two common symptoms of chilling injury-leaf chlorosis and electrolyte leakage. In these respects, the physiological response of the mutant to low temperatures mimics the response observed in some naturally occurring chilling sensitive species. The biochemical basis of chilling sensitivity was explored by examining the pattern of incorporation of (14)CO(2) into soluble metabolites and lipids in wild-type and mutant plants. The only difference observed between the mutant and wild type was that following low temperature treatment, the mutant accumulated 10-fold more radioactivity in a specific class of neutral lipids which were identified by a variety of criteria to be steryl-esters. The accumulation of radioactivity in the steryl-ester fraction occurs 24 hours before there is any visible evidence of chilling injury. These results suggest one of two possible explanations: either the mutation directly affects sterol metabolism, which in turn leads to chilling sensitivity, or the mutation affects another unidentified function and the accumulation of radioactivity in steryl-esters is a secondary consequence of chilling injury.

Enhanced thermal tolerance in a mutant of Arabidopsis deficient in palmitic Acid unsaturation

A mutant of Arabidopsis thaliana, deficient in the activity of a chloroplast omega9 fatty acid desaturase, accumulates high amounts of palmitic acid (16:0), and exhibits an overall reduction in the level of unsaturation of chloroplast lipids. Under standard conditions the altered membrane lipid composition had only minor effects on growth rate of the mutant, net photosynthetic CO(2) fixation, photosynthetic electron transport, or chloroplast ultrastructure. Similarly, fluorescence polarization measurements indicated that the fluidity of the membranes was not significantly different in the mutant and the wild type. However, at temperatures above 28 degrees C, the mutant grew more rapidly than the wild type suggesting that the altered fatty acid composition enhanced the thermal tolerance of the mutant. Similarly, the chloroplast membranes of the mutant were more resistant than wild type to thermal inactivation of photosynthetic electron transport. These observations lend support to previous suggestions that chloroplast membrane lipid composition may be an important component of the thermal acclimation response observed in many plant species which are photosynthetically active during periods of seasonally variable temperature extremes.

Different Roles for Phytochrome in Etiolated and Green Plants Deduced from Characterization of Arabidopsis thaliana Mutants

We have isolated a new complementation group of Arabidopsis thaliana long hypocotyl mutant (hy6) and have characterized a variety of light-regulated phenomena in hy6 and other previously isolated A. thaliana hy mutants. Among six complementation groups that define the HY phenotype in A. thaliana, three (hy1, hy2, and hy6) had significantly lowered levels of photoreversibly detectable phytochrome, although near wild-type levels of the phytochrome apoprotein were present in all three mutants. When photoregulation of chlorophyll a/b binding protein (cab) gene expression was examined, results obtained depended dramatically on the light regime employed. Using the red/far-red photoreversibility assay on etiolated plants, the accumulation of cab mRNAs was considerably less in the phytochrome-deficient mutants than in wild-type A. thaliana seedlings. When grown in high-fluence rate white light, however, the mutants accumulated wild-type levels of cab mRNAs and other mRNAs thought to be regulated by phytochrome. An examination of the light-grown phenotypes of the phytochrome-deficient mutants, using biochemical, molecular, and morphological techniques, revealed that the mutants displayed incomplete chloroplast and leaf development under conditions where wild-type chloroplasts developed normally. Thus, although phytochrome may play a role in gene expression in etiolated plants, a primary role for phytochrome in green plants is likely to be in modulating the amount of chloroplast development, rather than triggering the initiation of events (e.g., gene expression) associated with chloroplast development.

Manipulating membrane Fatty Acid compositions of whole plants with tween-Fatty Acid esters

This paper describes a method for manipulating plant membrane fatty acid compositions without altering growth temperature or other conditions. Tween-fatty acid esters carrying specific fatty acids were synthesized and applied to various organs of plants growing axenically in glass jars. Treated plants incorporated large amounts of exogenous fatty acids into all acylated membrane lipids detected. Fatty acids were taken up by both roots and leaves. Fatty acids applied to roots were found in leaves, while fatty acids applied to leaves appeared in both leaves higher on the plant and in roots, indicating translocation (probably in the phloem). Foliar application was most effective; up to 20% of membrane fatty acids of leaves above the treated leaf and up to 40% of root membrane fatty acids were exogenously derived. Plants which took up exogenous fatty acids changed their patterns of fatty acid synthesis such that ratios of saturated to unsaturated fatty acids remained essentially unaltered. Fatty acid uptake was most extensively studied in soybean (Glycine max [L.] Merr.), but was also observed in other species, including maize (Zea mays L.), mung beans (Vigna radiata L.), peas (Pisum sativum L.), petunia (Petunia hybrida L.) and tomato (Lycopersicon esculentum Mill.). Potential applications of this system include studying internal transport of fatty acids, regulation of fatty acid and membrane synthesis, and influences of membrane fatty acid composition on plant physiology.

Different Roles for Phytochrome in Etiolated and Green Plants Deduced from Characterization of Arabidopsis thaliana Mutants

We have isolated a new complementation group of Arabidopsis thaliana long hypocotyl mutant (hy6) and have characterized a variety of light-regulated phenomena in hy6 and other previously isolated A. thaliana hy mutants. Among six complementation groups that define the HY phenotype in A. thaliana, three (hy1, hy2, and hy6) had significantly lowered levels of photoreversibly detectable phytochrome, although near wild-type levels of the phytochrome apoprotein were present in all three mutants. When photoregulation of chlorophyll a/b binding protein (cab) gene expression was examined, results obtained depended dramatically on the light regime employed. Using the red/far-red photoreversibility assay on etiolated plants, the accumulation of cab mRNAs was considerably less in the phytochrome-deficient mutants than in wild-type A. thaliana seedlings. When grown in high-fluence rate white light, however, the mutants accumulated wild-type levels of cab mRNAs and other mRNAs thought to be regulated by phytochrome. An examination of the light-grown phenotypes of the phytochrome-deficient mutants, using biochemical, molecular, and morphological techniques, revealed that the mutants displayed incomplete chloroplast and leaf development under conditions where wild-type chloroplasts developed normally. Thus, although phytochrome may play a role in gene expression in etiolated plants, a primary role for phytochrome in green plants is likely to be in modulating the amount of chloroplast development, rather than triggering the initiation of events (e.g., gene expression) associated with chloroplast development.

Altered chloroplast structure and function in a mutant of Arabidopsis deficient in plastid glycerol-3-phosphate acyltransferase activity

Mutants of Arabidopsis thaliana deficient in plastid glycerol-3-phosphate acyltransferase activity have altered chloroplast membrane lipid composition. This caused an increase in the number of regions of appressed membrane per chloroplast and a decrease in the average number of thylakoid membranes in the appressed regions. The net effect was a significant decrease in the ratio of appressed to nonappressed membranes. A comparison of 77 K fluorescence emission spectra of thylakoid membranes from the mutant and wild type indicated that the ultrastructural changes were associated with an altered distribution of excitation energy transfer from antenna chlorophyll to photosystem II and photosystem I in the mutant. The changes in leaf lipid composition did not significantly affect growth or development of the mutant under standard conditions. However, at temperatures above 28 degrees C the mutant grew slightly more rapidly than the wild type, and measurements of temperature-induced fluorescence yield enhancement suggested an increased thermal stability of the photosynthetic apparatus of the mutant. These effects are consistent with other evidence suggesting that membrane lipid composition is an important determinant of chloroplast structure but has relatively minor direct effects on the function of the membrane proteins associated with photosynthetic electron transport.

Enhanced thermal tolerance of photosynthesis and altered chloroplast ultrastructure in a mutant of Arabidopsis deficient in lipid desaturation

A mutant of Arabidopsis thaliana, deficient in activity of the chloroplast n-6 desaturase, accumulated high levels of C(16:1) and C(18:1) lipids and had correspondingly reduced levels of polyunsaturated lipids. The altered lipid composition of the mutant had pronounced effects on chloroplast ultrastructure, thylakoid membrane protein and chlorophyll content, electron transport rates, and the thermal stability of the photosynthetic membranes. The change in chloroplast ultrastructure was due to a 48% decrease in the amount of appressed membranes that was not compensated for by an increased amount of nonappressed membrane. This resulted in a net loss of 36% of the thylakoid membrane per chloroplast and a corresponding reduction in chlorophyll and protein content. Electrophoretic analysis of the chlorophyll-protein complexes further revealed a small decrease in the amount of light-harvesting complex. Relative levels of whole chain and protosystem II electron transport rates were also reduced in the mutant. In addition, the mutation resulted in enhanced thermal stability of photosynthetic electron transport. These observations suggest a central role of polyunsaturated lipids in determining chloroplast structure and maintaining normal photosynthetic function and demonstrate that lipid unsaturation directly affects the thermal stability of photosynthetic membranes.

A mutant of Arabidopsis deficient in the chloroplast 16:1/18:1 desaturase

Leaf tissue of a mutant of Arabidopsis thaliana contains reduced levels of both 18-carbon and 16-carbon polyunsaturated fatty acids and increased levels of the 18:1 and cis-16:1 precursors due to a single nuclear mutation at a locus designated fadC. Analysis of the fatty acid compositions of individual lipids and the kinetics of lipid labeling with [(14)C]acetate in vivo indicate that the mutant lacks activity of the chloroplast glycerolipid omega-6 desaturase. As a result, lipids synthesized by the prokaryotic pathway are not desaturated further than 18:1 and 16:1. Lipids derived from the eukaryotic pathway are desaturated-presumably by the endoplasmic reticulum 18:1 phosphatidylcholine desaturase. However, an increase in the level of 18:1 on all the phospholipids derived from the eukaryotic pathway in leaves of the mutant suggests that the mutation does exert an effect on the composition of extrachloroplast membranes. Synthesis of monogalactosyldiacylglycerol (MGD) by the prokaryotic pathway is reduced 30 to 35% in the mutant and there is a corresponding increase in MGD synthesis by the eukaryotic pathway. This shift in metabolism which results in a more unsaturated MGD pool, may reflect the existence of a regulatory mechanism which apportions lipid synthesis between the two pathways in response to alterations in the physical properties of the chloroplast membranes.

A Mutant of Arabidopsis thaliana that Exhibits Chlorosis in Air but Not in Atmospheres Enriched in CO(2)

A mutant of Arabidopsis thaliana (L.) Heynh. which requires a high concentration (2% by volume) of atmospheric CO(2) for growth has been isolated. Unlike previous mutants of this type, this line does not have any apparent defect in photosynthetic CO(2)-fixation, photorespiration, or photosynthetic electron transport. The mutant is abnormally susceptible to pigment bleaching in air but not in 2% CO(2). The presence of normal or above-normal levels of antioxidants, carotenoids, and enzymes involved in reactive oxygen detoxification suggests that the mutant is equipped to detoxify activated oxygen species. Although it was not possible to establish a biochemical basis for the lesion, the properties of the mutant suggest the existence of a previously unidentified role for CO(2).