Galactolipids rule in seed plants

Chloroplast membranes contain high levels of the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). The isolation of the genes involved in the biosynthesis of MGDG and DGDG, and the identification of galactolipid-deficient Arabidopsis mutants has greatly facilitated the analysis of galactolipid biosynthesis and function. Galactolipids are found in X-ray structures of photosynthetic complexes, suggesting a direct role in photosynthesis. Furthermore, galactolipids can substitute for phospholipids, as suggested by increases in the galactolipid:phospholipid ratio after phosphate deprivation. The ratio of MGDG to DGDG is also crucial for the physical phase of thylakoid membranes and might be regulated.     

Effect of indole-3-acetic acid on surface properties of the wheat plastid lipids

Surface parameters of polar lipids extracted from winter wheat plastids were investigated by the Langmuir and X-ray differentiation scattering methods. Highly purified plastids were isolated from non-embryogenic (NE) and embryogenic (E) calli initiated from inflorescences. NE plastids contained more monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and less phospholipids (PL) fraction than E plastids. Moreover, in E calli, unsaturated fatty acids were detected in a higher proportion than in NE for both MGDG and DGDG. No significant differences in fatty acids saturation of PL between NE and E objects were detected. Aqueous surface monolayers were prepared from separate lipids and from mixtures of glycolipids and PL. In the case of MGDG, isotherms showed specific shoulders, contrary to continuous isotherms obtained for other investigated lipids. On the base of pi-A isotherms, the surface parameters: limiting area (A(lim)) and collapse pressure (pi(coll)) were calculated. Indole-3-acetic acid (IAA) increased the A(lim) of all separated lipids about 4-10 angstrom2/mol. However, for NE lipid mixture, the effect of IAA was much smaller (about 2 angstroms2/mol) than for other objects (usually about 5 angstroms2/mol). X-ray experiments for liposomes, obtained from mixtures of glycolipids and PL of NE and E plastids, showed continuous scattering curves with maxima characteristic for lipid bilayer membranes. Calculations of distance distribution functions indicated that bilayer thickness was 41 and 38 angstroms for NE and E, respectively. IAA influence on membrane structures was detected especially in E liposomes and increased the distance between head groups by about 2 angstroms. It is suggested that changes occur during embryogenesis in specific structure of plastid membranes determined also the formation of domains, similar to that suggested for plasmalemma (Plant Sci. 165 (2003) 265). IAA treatment influenced the membrane structure, especially E plastids increasing distances between polar groups.     

Galactosyl headgroup interactions control the molecular packing of wheat lipids in Langmuir films and in hydrated liquid-crystalline mesophases

The behavior of the two major galactolipids of wheat endosperm, mono- (MGDG) and di-galactosyldiacylglycerol (DGDG) was studied in aqueous dispersion and at the air/liquid interface. The acyl chains of the pure galactolipids and their binary equimolar mixture are in the fluid or liquid expanded phase. SAXS measurements on liquid-crystalline mesophases associated with the electron density reconstructions show that the DGDG adopts a lamellar phase L(alpha) with parallel orientation of the headgroups with respect to the plane of the bilayer, whereas MGDG forms an inverse hexagonal phase H(II) with a specific organization of galactosyl headgroups. The equimolar mixture shows a different behavior from those previously described with formation of an Im3m cubic phase. In comparing monolayers composed of the pure galactolipids and their equimolar mixtures, PM-IRRAS spectra show significant differences in the optical properties and orientation of galactosyl groups with respect to the interface. Furthermore, Raman and FTIR spectroscopies show that the acyl chains of the galactolipid mixture are more ordered compared to those of the pure components. These results suggest strong interactions between MGDG and DGDG galactosyl headgroups and these specific physical properties of galactolipids are discussed in relation to their biological interest in wheat seed.     

The digalactosyldiacylglycerol (DGDG) synthase DGD1 is inserted into the outer envelope membrane of chloroplasts in a manner independent of the general import pathway and does not depend on direct interaction with monogalactosyldiacylglycerol synthase for DGDG biosynthesis.

Galactolipids make up the bulk of chloroplast lipids. Therefore, the genes involved in the synthesis of the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) play a critical role in chloroplast development. In this study, we analyzed the subcellular localization of the Arabidopsis DGDG synthase DGD1, which was recently identified by complementation of the Arabidopsis dgd1 mutant. In vitro import experiments demonstrated that DGD1 was targeted to the chloroplast outer envelope in an ATP-independent manner. DGD1 could not be extracted from the membranes by high salt or alkali, suggesting that it is an integral membrane protein. Uptake experiments with truncated versions of DGD1 indicated that the information for targeting and insertion into the outer envelope resides in the N-terminal half of DGD1, but not in the first 33 amino acids. DGD1 apparently does not contain a cleavable signal peptide. Antibodies to Arabidopsis DGD1 detected a 90-kDa protein localized to the chloroplast envelopes of both pea and Arabidopsis. Transformation of DGD1 constructs into cyanobacteria resulted in the expression of active DGDG synthase and demonstrated that DGDG synthesis depends on MGDG lipid, but does not require direct interaction with the plant MGDG synthase.     

Galactosyl headgroup interactions control the molecular packing of wheat lipids in Langmuir films and in hydrated liquid-crystalline mesophases

The behavior of the two major galactolipids of wheat endosperm, mono- (MGDG) and di-galactosyldiacylglycerol (DGDG) was studied in aqueous dispersion and at the air/liquid interface. The acyl chains of the pure galactolipids and their binary equimolar mixture are in the fluid or liquid expanded phase. SAXS measurements on liquid-crystalline mesophases associated with the electron density reconstructions show that the DGDG adopts a lamellar phase L_α with parallel orientation of the headgroups with respect to the plane of the bilayer, whereas MGDG forms an inverse hexagonal phase H_II with a specific organization of galactosyl headgroups. The equimolar mixture shows a different behavior from those previously described with formation of an Im3m cubic phase. In comparing monolayers composed of the pure galactolipids and their equimolar mixtures, PM-IRRAS spectra show significant differences in the optical properties and orientation of galactosyl groups with respect to the interface. Furthermore, Raman and FTIR spectroscopies show that the acyl chains of the galactolipid mixture are more ordered compared to those of the pure components. These results suggest strong interactions between MGDG and DGDG galactosyl headgroups and these specific physical properties of galactolipids are discussed in relation to their biological interest in wheat seed.     

Physicochemical Aspects of Reaction of Ozone with Galactolipid and Galactolipid–Tocopherol Layers

The impact of reaction of galactolipids with ozone on the physicochemical properties of their monolayers was examined. In Megli and Russo (Biochim Biophys Acta, 1778:143–152, 2008), Cwiklik and Jungwirth (Chem Phys Lett, 486:99–103, 2010), Jurkiewicz et al. (Biochim Biophys Acta, 1818:2388–2402, 2012), Khabiri et al. (Chem Phys Lett, 519:93–99, 2012), and Conte et al. (Biochim Biophys Acta, 1828:510–517, 2013), the properties of layers formed from model mixtures composed of chosen lipids and selected oxidation products were studied, whereas in this work, question was raised as to how the oxidation reactions taking place in situ affect the physical properties of the galactolipid layers. So, set experiment should take into account the effect of all reaction products. The mechanical characteristics of monolayers of monogalactosyldiacyl-glycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were determined by Langmuir trough technique, and the electrical properties of liposomes formed from these lipids by measuring their electrophoretic mobility. Considerable loss of galactolipid molecules forming monolayers was found at ozone concentrations (in aqueous medium) higher than 0.1 ppm with a stronger effect measured for MGDG. That goes along with the greater amounts of MDA found in the extracts of oxidized MGDG films compared with DGDG. Based on this, it was concluded that an additional galactose group present in DGDG molecules acts protectively under oxidative conditions. The surface tension of the solutions (of small volume) contacting the oxidized galactolipids films was significantly reduced, indicating the presence of soluble in polar media, surface active reaction products. The presence of α-tocopherol in mixtures with tested galactolipids at a molar ratio of lipid to tocopherol equal to 1.7:1 caused some inhibition of lipid oxidation, reducing the decrease of amount of lipid particles forming the monolayer. Here, also protective effect of α-tocopherol was greater for the MGDG compared to DGDG.     

Biosynthesis and desaturation of prokaryotic galactolipids in leaves and isolated chloroplasts from spinach

Mono- and digalactosyldiacylglycerol (MGDG and DGDG) were isolated from the leaves of sixteen 16:3 plants. In all of these plant species, the sn-2 position of MGDG was more enriched in C₁₆ fatty acids than sn-2 of DGDG. The molar ratios of prokaryotic MGDG to prokaryotic DGDG ranged from 4 to 10. This suggests that 16:3 plants synthesize more prokaryotic MGDG than prokaryotic DGDG. In the 16:3 plant Spinacia oleracea L. (spinach), the formation of prokaryotic galactolipids was studied both in vivo and in vitro. In intact spinach leaves as well as in chloroplasts isolated from these leaves, radioactivity from [1-¹⁴C]acetate accumulated 10 times faster in MGDG than in DGDG. After 2 hours of incorporation, most labeled galactolipids from leaves and all labeled galactolipids from isolated chloroplasts were in the prokaryotic configuration. Both in vivo and in vitro, the desaturation of labeled palmitate and oleate to trienoic fatty acids was higher in MGDG than in DGDG. In leaves, palmitate at the sn-2 position was desaturated in MGDG but not in DGDG. In isolated chloroplasts, palmitate at sn-2 similarly was desaturated only in MGDG, but palmitate and oleate at the sn-1 position were desaturated in MGDG as well as in DGDG. Apparently, palmitate desaturase reacts with sn-1 palmitate in either galactolipid, but does not react with the sn-2 fatty acid of DGDG. These results demonstrate that isolated spinach chloroplasts can synthesize and desaturate prokaryotic MGDG and DGDG. The finally accumulating molecular species, MGDG(18:3/16:3) and DGDG(18:3/16:0), are made by the chloroplasts in proportions similar to those found in leaves.     

Critical regulation of galactolipid synthesis controls membrane differentiation and remodeling in distinct plant organs and following environmental changes

The plant galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the most abundant lipids in chloroplast membranes, and they constitute the majority of total membrane lipids in plants. MGDG is synthesized by two types of MGDG synthase, type-A (MGD1) and type-B (MGD2, MGD3). These MGDG synthases have distinct roles in Arabidopsis. In photosynthetic organs, Type A MGD is responsible for the bulk of MGDG synthesis, whereas Type B MGD is expressed in non-photosynthetic organs such as roots and flowers and mainly contributes to DGDG accumulation under phosphate deficiency. Similar to MGDG synthesis, DGDG is synthesized by two synthases, DGD1 and DGD2; DGD1 is responsible for the majority of DGDG synthesis, whereas DGD2 makes its main contribution under phosphate deficiency. These galactolipid synthases are regulated by light, plant hormones, redox state, phosphatidic acid levels, and various stress conditions such as drought and nutrient limitation. Maintaining the appropriate ratio of these two galactolipids in chloroplasts is important for stabilizing thylakoid membranes and maximizing the efficiency of photosynthesis. Here we review progress made in the last decade towards a better understanding of the pathways regulating plant galactolipid biosynthesis.     

A simple method for <Emphasis Type="Italic">in vivo</Emphasis> labelling of lipid fractions in developing seeds of <Emphasis Type="Italic">Brassica campestris</Emphasis> L

An in vivo method of labelling lipid fractions in developing seeds of Brassica campestris using [1–¹⁴C] acetate has been developed. The “wick” method for introducing label into the intact plant is quite effective, safe and easy to use. The results obtained were reproducible and comparable to those reported earlier for seeds procured from greenhouse grown plants. The labelling pattern showed that rapid oil deposition began around 20 days after anthesis (DAA) and continued until about 45 DAA. The proportion of label in polar lipids declined and that in non-polar lipids increased during the phase of active oil synthesis. Among phospholipids, the label was incorporated mainly in phosphatidyl choline (PC), which was found to be the major fraction of phospholipids. During development, the two galactolipids i.e. monogalactosyl diglyceride (MGDG) and digalactosyl diglyceride (DGDG) followed patterns exactly opposite to each other. The content of the label in MGDG decreased, while that in DGDG increased, indicating the conversion of MGDG to DGDG during maturation.     

Galactolipid depletion in blast fungus‐infected rice leaves

This research focuses on galactolipid depletion in blast fungus‐infected rice leaves. Two major galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), from rice leaves were isolated and purified. The chemical structure of MGDG was identified as 1,2‐dilinolenyl‐3‐O‐β‐d ‐galactopyranosyl‐sn‐glycerol, and that of DGDG as 1,2‐dilinolenyl‐3‐O‐[α‐d ‐galactopyranosyl‐(1→6)‐O‐β‐d ‐galactopyranosyl]‐sn‐glycerol. Both the MGDG and DGDG content in the incompatible blast fungus race‐infected leaves decreased more than those in the compatible blast fungus race‐infected leaves during the infection process. Active oxygen species had the ability to peroxygenate and de‐esterify MGDG or DGDG in vitro, suggesting that active oxygen species play an important role in galactolipid depletion during the process of rice blast fungus invasion. Other possible functions of rice galactolipids during disease resistance are also discussed.     

Breakdown of Galactolipids in Senescent Barly Leaves

The changes of galactolipids (MGDG and DGDG, largely 18:3/18:3), free fatty acids (FFA), and phosphatidylcholine (PC) taking place during senescence of primary barley leaves were analysed employing HPLC and GLC. Upon induction of senescence MGDG and, with some delay, DGDG began to disappear and were largely broken down at the end of the senescence period. A concomitant appearance of a pool of FFA could not be observed. However, PC accumulated during the main period of galactolipid breakdown. This change was due to the marked increase of the 18:3/18:3 molecular species of PC. An inverse correlation between the changes of galactolipids and PC could be established. A hypothesis featuring the conversion of galactolipids via diacylglycerol to PC is presented as the principal route of galactolipid breakdown.     

Polar lipid composition of a plastid ribosome-deficient barley mutant

Green and white leaves of the barley mutant line `albostrians' were compared for their polar lipid content and fatty acid composition. The mutant plastids of the white leaves have a double-layered envelope, but in contrast with the normal chloroplasts, lack 70 S ribosomes and thylakoids. In the green leaves, the amount of monogalactosyldiacylglycerol (MGDG) consistently exceeds the amount of digalactosyldiacylglycerol (DGDG) and the amount of galactolipids exceeds the amount of phospholipids. In contrast, in white leaves the amount of DGDG exceeds the amount of MGDG and the amount of phospholipids exceeds the amount of galactolipids. In white leaves, the galactolipid composition reflects the plastid envelope composition which is rich in DGDG, whereas in green leaves the galactolipid composition reflects the thylakoid composition which is rich in MGDG. These results demonstrate the likelihood that all the enzymes involved in galactolipid, sulfolipid and fatty acid synthesis are coded by the nuclear genome.     

Differential changes in galactolipid and phospholipid species in soybean leaves and roots under nitrogen deficiency and after nodulation

The availability of nitrogen (N) to plants has a profound impact on carbohydrate and protein metabolism, but little is known about its effect on membrane lipid species. This study examines the changes in galactolipid and phospholipid species in soybean as affected by the availability of N, either supplied to soil or obtained through Bradyrhizobium japonicum nodulation. When N was limited in soil, the content of galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacyglycerol (DGDG), decreased drastically in leaves, while a smaller decrease of DGDG was observed in roots. In both leaves and roots, the overall content of different phospholipid classes was largely unchanged by N limitation, although some individual phospholipid molecular species did display significant changes. Nodulation with Bradyrhizobium of soybean grown in N-deficient soil resulted in a large increase in levels of plastidic lipid classes, MGDG, DGDG, and phosphatidylglycerol, along with smaller increases in non-plastidic phospholipids in leaves. Nodulation also led to higher levels of phospholipids in roots without changes in root levels of MGDG and DGDG. Overall, N availability alters lipid content more in leaves than roots and more in galactolipids than phospholipids. Increased N availability leads to increased galactolipid accumulation in leaves, regardless of whether N is supplied from the soil or symbiotic fixation.     

Mono- and digalactosyldiacylglycerol composition of dinoflagellates. VI. Biochemical and genomic comparison of galactolipid biosynthesis between Chromera velia (Chromerida), a photosynthetic alveolate with red algal plastid ancestry,and the dinoflagellate,Lingulodinium polyedrum

Chromera velia is a recently discovered, photosynthetic, free-living alveolate that is the closest free-living relative to non-photosynthetic apicomplexan parasites. Most plastids, regardless of their origin, have membranes composed chiefly of two galactolipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively). Because of the hypothesized shared red algal origin between the plastids of C. velia and dinoflagellates, our primary objectives were to examine how growth temperature affects MGDG and DGDG composition via positive-ion electrospray/mass spectrometry (ESI/MS) and positive ion/electrospray/mass spectrometry/mass spectrometry (ESI/MS/MS), and to examine galactolipid biosynthetic genes to determine if shared ancestry translates into shared MGDG and DGDG composition. When growing at 20°C, C. velia produces eicosapentaenoic acid-rich 20:5(n-3)/20:5(n-3) (sn-1/sn-2) MGDG and 20:5(n-3)/20:5(n-3) DGDG as its primary galactolipids, with relative percentage compositions of approximately 35 and 60%, respectively. At 30°C these are lessened by approximately 5 and 8%, respectively, by the corresponding production of 20:5/20:4 forms of these lipids. The presence of 20:5 at the sn-1 position is similar to what has been observed previously in a cluster of peridinin-containing dinoflagellates, but the presence of 20:5(n-3) at the sn-2 position is extremely rare. Thus, the forms of MGDG and DGDG in C. velia displayed similarities and differences to what has been observed in peridinin-containing dinoflagellates, such as Lingulodinium polyedrum, which produces 20:5/18:5 and 20:5/18:4 as the major forms of MGDG and DGDG. We develop conceptual models from the galactolipids observed and galactolipid-relevant gene annotations to explain the presence of polyunsaturated fatty acid-containing MGDG and DGDG in both L. polyedrum and C. velia.     

Temperature modulation and the presence of C20 fatty acids in mono‐ and digalactosyldiacylglycerol of Euglena gracilis and Lepocinclis acus: A modern interpretation of euglenid galactolipids using positive‐ion electrospray ionization/mass spectrometry

Mono‐ and digalactosyldiacylglycerol (MGDG and DGDG, respectively) are important galactolipids that comprise photosynthetic membranes in almost all photosynthetic organisms. Intact forms of MGDG and DGDG of Euglena gracilis and Lepocinclis acus, two example euglenids with secondary plastids of green algal origin, were elucidated with fatty acid regiochemistry via positive‐ion electrospray ionization/mass spectrometry at two growth temperatures. At 20°C, E. gracilis and L. acus produced predominantly 18:3/16:4 (sn‐1/sn‐2) MGDG, whereas at 30°C this was supplanted by 18:2/16:2 MGDG. At both temperatures were also observed a variety of other MGDG and DGDG forms, including C₂₀ fatty acid‐containing forms not expected in a green algal‐derived plastid. In addition to providing structural details of MGDG and DGDG not available in past studies, these results suggest a previously unknown relationship between these two organisms and the red algae. This study also illustrates that temperature modulation of galactolipids occurs via modification of unsaturation of both the sn‐1 and sn‐2 fatty acids; this is fundamentally different from previously published studies from our laboratory on other algal classes.     

Functional roles of the major chloroplast lipids in the violaxanthin cycle

Monogalactosyldiacylglyceride (MGDG) and digalactosyldiacylglyceride (DGDG) are the major membrane lipids of chloroplasts. The question of the specialized functions of these unique lipids has received limited attention. One function is to support violaxanthin de-epoxidase (VDE) activity, an enzyme of the violaxanthin cycle. To understand better the properties of this system, the effects of galactolipids and phosphatidylcholines on VDE activity were examined by two independent methods. The results show that the micelle-forming lipid (MGDG) and bilayer forming lipids (DGDG and phosphatidylcholines) support VDE activity differently. MGDG supported rapid and complete de-epoxidation starting at a threshold lipid concentration (10 μM) coincident with complete solubilization of violaxanthin. In contrast, DGDG supported slow but nevertheless complete to nearly complete de-epoxidation at a lower lipid concentration (6.7 μM) that did not completely solubilize violaxanthin. Phosphotidylcholines showed similar effects as DGDG except that de-epoxidation was incomplete. Since VDE requires solubilized violaxanthin, aggregated violaxanthin in DGDG at low concentration must become solubilized as de-epoxidation proceeds. High lipid concentrations had lower activity possibly due to formation of multilayered structures (liposomes) that restrict accessibility of violaxanthin to VDE. MGDG micelles do not present such restrictions. The results indicate VDE operates throughout the lipid phase of the single bilayer thylakoid membrane and is not limited to putative MGDG micelle domains. Additionally, the results also explain the differential partitioning of violaxanthin between the envelope and thylakoid as due to the relative solubilities of violaxanthin and zeaxanthin in MGDG, DGDG and phospholipids. The violaxanthin cycle is hypothesized to be a linked system of the thylakoid and envelope for signal transduction of light stress.     

Galactolipid multibilayers modified with xanthophylls: orientational and diffractometric studies

Oriented multibilayers of chloroplast galactolipids: monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) modified with violaxanthin and zeaxanthin were examined by X-ray diffractometry and linear dichroism. The results obtained suggest that zeaxanthin, in contrast to violaxanthin, has a significant ordering effect on galactolipid bilayers. The best ordered system consists of DGDG and zeaxanthin. In this case, the angle between the long axis of zeaxanthin molecule and the normal to the plane of bilayers amounts to 9 degrees and system has a periodicity of 61.7 A. The analogous angles in systems MGDG + violaxanthin, MGDG + zeaxanthin and DGDG + violaxanthin are clearly wider (35 degrees, 17 degrees and 28 degrees, respectively) but diffractograms show no distinct maxima.     

Effect of tocopherol on surface properties of plastid lipids originating from wheat calli cultivated in cadmium presence

The behaviour of equimolar mixtures of α-tocopherol with monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and phospholipids (PL) isolated from wheat calli cultured on media with and without cadmium was investigated at the air–water interface by surface pressure–area (π–A) measurements established using an automated Langmuir-type film balance. It was found that monolayers of all studied compounds were expanded. The additivity rule was not fulfilled and the collapse pressure of mixtures was different from these recorded for pure components. This can be related with the existence of interactions between molecules in mixed monolayers. Tocopherol diminished the differences between parameters of monolayers formed by lipids extracted from objects cultivated on various media (with and without cadmium).     

Phase equilibria of mixtures of plant galactolipids. The formation of a bicontinuous cubic phase

The chloroplast galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were isolated from wheat leaves. The phase equilibria of galactolipid-water systems with MGDG / DGDG molar ratios equal to 0:1, 1:2, 1.2:1, 2:1 and 1:0 were investigated, using nuclear magnetic resonance (NMR) methods. MGDG and DGDG form reversed hexagonal and lamellar phases, respectively, at temperatures between 10 and 40°C at all water contents studied (up to about 14 mol ²H₂O per mol lipid). The galactolipid mixtures show a complex phase forming reversed hexagonal, lamellar and reversed cubic phases, depending on water content and temperature. It was found that the water hydration is similar for the lamellar and hexagonal phases formed by DGDG and MGDG, respectively. The non-lamellar phase areas increase with increasing content of MGDG. Small-angle X-ray measurements show that the cubic phase belongs to the Ia3d space group. From translational diffusion studies by NMR it is concluded that the structure of this cubic phase is bicontinuous.     

Age dependent changes in phospholipids and galactolipids in primary bean leaves (Phaseolus vulgaris)

Primary leaves of Phaseolus vulgaris show concomitant changes in phospholipid, galactolipid, chlorophyll and fresh weight during leaf development from 3 to 32 days after planting. Phosphatidyl choline, phosphatidyl ethanolamine, and phosphatidyl inositol show only small changes on a mole per cent lipid phosphate basis during leaf development. The chloroplast lipids, phosphatidyl glycerol, monogalactosyl diglyceride (MGDG) and digalactosyl diglyceride (DGDG) all show marked increases and decreases which are coincident with chloroplast development. The decline in the leaf content of chloroplast polar lipids and chlorophyll become evident upon reaching maximal leaf size. The molar ratio of galactolipids (MGDG/DGDG), reaches a maximum value of 2.3 in expanding leaves, but steadily declines during senescence to a minimum value of 1.5 at abscission. The declining ratio is caused by a preferential loss of MGDG in the senescing leaves.