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Difference between thylakoids and lamellae in a chloroplast?

Difference between thylakoids and lamellae in a chloroplast?


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I'm slightly confused as to the difference between thylakoids and lamellae. My understanding was that thylakoids are 'discs' that are stacked into grana and there is a membrane between the grana called the integranal lamellae. However in my text book it says that 'the inner membrane is folded into lamellae, which are stacked up like piles of pennies. Each stack of lamallae is called a granum.' This sounds to me like the description of thylakoids. It later says 'grana are stacks of flattened membrane compartments, called thylakoids'…

Could someone clarify this?

Thank you in advance :)


The answer above goes in depth so I will try go off that. Firstly, lamella is the word used to describe plate-like structures. A thylakoid therefore, being a flattened vesicle, would fit this description, so a thylakoid is a type of lamella. As mentioned earlier, a lamellar system consists of uniform thylakoids, the thylakoids being the individual lamella.

I have the same textbook as you, OCR A2 Biology, and I must admit I had the same confusion. What this textbook fails to mention is the fact that thylakoids are just one type of lamellae.

The textbook also states that the inner membrane of the chloroplast is "folded into lamellae (thin plates)". There are two types of lamellae in the chloroplast, thylakoids and intergranal lamellae, both are stacked, however only thylakoids appear as "piles of pennies" as it states. Intergranal lamellae look nothing like pennies, which is why the textbook is strongly misleading, they are big flattened bits of membrane that connect together different grana (stacks of thylakoids).

Note: my answer comes in 3 years late (I'm currently doing my A2), and the textbook in question will see its last use this year as the current A2 level syllabus is updated, I hope my answer still helps to clear up any confusion should OCR mess up again on this one


I am assuming that you understand this basic structure-

Now, let's start with grana which aresmall discrete dark green bodies embedded in lighter coloured stroma

But, in evolutionarily primitive life forms like algae, there is no differentiation among individual granum… rather they consist of continuous layer or lamellae like the sheets of book parallel to each other… like this…

Now,

this lamellate form of chloroplast is found in all green plants

, but

in higher plants there are a no. of more or less separate piles of sheet or lamellae

( unlike algae, as previously mentioned, where the sheets of lamellae run through the whole chloroplast). Each , of these lamellae is composed of double membranes joined at the ends & each membrane is 100-200 angstrom thick…

following Menke (1960) these double lamellae are known as Thylakoids

Now, unifying all these-

Lamellar system consists of uniform thylakoids , which sometimes are stacked up like pennies , each stack being called a granum ( plu. grana)

& this is as simple explanation as I can give of this utterly confusing Q… please feel free to comment about any portion that needs to be elaborated…


Short Essay on Chloroplast

These are green plastids. They have green pigment the chlorophyll. Chloroplast takes part in photosyn­thesis. They are found in mesophyll cells of leaf and chlorenchyma cells in stem. Many algae have only single Chloroplast in their cells but other advanced groups of plants have 20-40 chloroplasts per cell. Their size and shape varies in different groups of plants. In algae they are stellete, ribbon-like, cup-like, reticulate, spiral, sheet like, disc or tablet like. In mosses, ferns and seed plants chloroplasts are flat, circu­lar, ovate, elliptical.

Image Source: discoveringdna.com/wp-content/uploads/2014/11/chloroplast-labelled-copy.jpg

Their detailed structure has been revealed by electron microscope. Each chloroplast has two unit membranes made up of lipoprotein. Inside the membranes is the proteinaceous ground substance called stroma . Stroma contains a variety of particles, osmiophilic droplets, ribosomes, strands of DNA and RNA, dis­solved salts, enzymes. Embedded in the stroma are many membranes, running paral­lel to each other throughout the length of the chloroplast. These are called lamellae, Each lamella is made up of two unit membranes. At some intervals the lamellae have 10-100 rounded flat sac like structures stacked one above the other. Such rounded flat structure is called thylakoid.

One stack of thylakoids is called granum. Different grana are connected with grana of other lamella with the help of tubular connections called stroma lamellae or frets. Each chloroplast contains 40-60 grana. Membrane of grana lamellae are made up of proteins and phospholipids and contains chlorophyll and carotenoids. On the inner wall of thylakoids are found small granules called quantasome. Park and Pon (1961) and later Park and Biggins (1964) reported quantasome. In the past, quantasome was considered as basic photosynthetic unit, but it is not a functioning photosynthetic unit. Each quantasome has 230 chlorophyll molecules.

Chlorophyll molecule has a complex porphyrin ring (head) to which is attached long hydrophobic phytyl (C28 H39) chain (tail). The porphyrin ring is composed princi­pally of four pyrrole nuclei linked together bearing side chains. The metal constituent of the molecule, magnesium, is incorporated in this ring. The hydrocarbon phytyl group upon hydrolysis gives rise to the alcohol phytol which is an isoprenoid derivative.


The receipt to make an edible plant cell model

Before we start, I want to show you my shopping list for the ingredients to make a plant cell model.

A big watermelon, a box of cherry tomato, a can of corn kernels, a pack of Jell-O, a handful of kale, one avocado, one red, and one yellow onion.

[in this figure] My recipes of an edible plant cell model.

Before I tell you the story, would you like to guess why I need these materials and what kinds of organelles they stand for?


9th Class Biology: Chapter 4 Cells and Tissues Short Questions Answer

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Lamella (cell biology)

A lamella (plural: "lamellae") in biology refers to a thin layer, membrane or plate of tissue. [1] This is a very broad definition, and can refer to many different structures. Any thin layer of organic tissue can be called a lamella and there is a wide array of functions an individual layer can serve. For example, an intercellular lipid lamella is formed when lamellar disks fuse together to form a lamellar sheet. It is believed that these disks are formed from vesicles, giving the lamellar sheet a lipid bilayer that plays a role in water diffusion. [2]

Another instance of cellular lamellae can be seen in chloroplasts. Thylakoid membranes are actually a system of lamellar membranes working together, and are differentiated into different lamellar domains. This lamellar system allows plants to convert light energy into chemical energy. [3] Chloroplasts are characterized by a system of membranes embedded in a hydrophobic proteinaceous matrix, or stroma. The basic unit of the membrane system is a flattened single vesicle called the thylakoid thylakoids stack into grana. All the thylakoids of a granum are connected with each other, and the grana are connected by intergranal lamellae. [4]

It is placed between the two primary cell walls of two plant cells and made up of intracellular matrix. The lamella comprises a mixture of polygalacturons (D-galacturonic acid) and neutral carbohydrates. It is soluble in the pectinase enzyme.

Lamella, in cell biology, is also used to describe the leading edge of a motile cell, of which the lamellipodia is the most forward portion. [5]

The lipid bilayer core of biological membranes is also called lamellar phase. [6] Thus, each bilayer of multilamellar liposomes and wall of a unilamellar liposome is also referred to as a lamella.

  1. ^ Merriam-webster.com. (2017). Definition of LAMELLA. [online] Available at: https://www.merriam-webster.com/dictionary/lamella
  2. ^ Swartzendruber, Donald C Wertz, Philip W Kitko, David J Madison, Kathi C Downing, Donald T (1989). "Molecular models of the Intercellular Lipid Lamellae in Mammalian Stratum Corneum". Journal of Investigative Dermatology. 92 (2): 251–7. doi: 10.1111/1523-1747.ep12276794 . PMID2918233.
  3. ^
  4. Shimoni, E (2005). "Three-Dimensional Organization of Higher-Plant Chloroplast Thylakoid Membranes Revealed by Electron Tomography". The Plant Cell Online. 17 (9): 2580–6. doi:10.1105/tpc.105.035030. JSTOR4130938. PMC1197436 . PMID16055630.
  5. ^Oxford Dictionary of Biochemistry and Molecular Biology, Oxford University Press [page needed]
  6. ^
  7. "The Cytoskeleton and Cell Migration - Lamellipodia".
  8. ^
  9. Gabriel, Jean-Christophe P. Camerel, Franck Lemaire, Bruno J. Desvaux, Hervé Davidson, Patrick Batail, Patrick (2001). "Swollen liquid-crystalline lamellar phase based on extended solid-like sheets" (PDF) . Nature. 413 (6855): 504–508. doi:10.1038/35097046. PMID11586355.
  • Yashroy, R. C (1990). "Lamellar dispersion and phase separation of chloroplast membrane lipids by negative staining electron microscopy". Journal of Biosciences. 15 (2): 93–8. doi:10.1007/BF02703373.

This cell biology article is a stub. You can help Wikipedia by expanding it.


Effect of Drought Stress on Anatomical Structure and Chloroplast Ultrastructure in Leaves of Sugarcane

In order to provide a reference for investigating the mechanism of drought resistance in sugarcane, variations of chlorophyll content and chloroplast ultrastructure in sugarcane leaves were analyzed. The present research was conducted using sugarcane cultivars, strongly drought-resistant F172 and weakly drought-resistant YL6, as plant materials in pot experiment under controlled greenhouse condition. At elongation stage, the plants were provided different degrees of drought stress: (1) mild drought with 65–70 % of the soil water capacity (2) moderate drought with 45–65 % of the soil water content (3) severe drought with 25–45 % of soil water capacity and (4) control with 70 % of soil water capacity. Chlorophyll content in leaves was measured, and variations of green leaves number and chloroplast ultrastructure were observed. It was found that the green leaves of sugarcane and chlorophyll content were significantly reduced in the process of drought stress. Upper and lower cuticle thickness was getting thickened during drought stress, but the thickness of lower epidermal cuticle of YL6 variety was reduced under severe drought condition. Ultrastructure observation showed that, in most cases, the chloroplasts were close to the cell wall and well arranged, and the chloroplast thylakoids were orderly arranged in the chloroplasts. With the ongoing of drought stress, plasmosis occurred, the chloroplasts moved closer to the center of the cell, and turned gradually from long oval to nearly round, and starch grains increased. Under severe drought conditions, F172 still maintained integrity of the chloroplast structure while the chloroplast in YL6 were severely deformed and became blurred in shape.

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Chloroplast ultrastructure

My book says that photosystem 1 occurs maily on the intergranal lamellae and photosystem 2 occurs almost exclusively on the grannal lamellae.
what is the difference between intergranal lamellae and granal lamellae?
Also is the following correct:
thylakoids are stacked membranes. Grana are stacks of thylakoids and lamellae are disk-like structures that link thylakoids in different grana.

Alos does NADP carry hydrogen atoms or hydrogen ions (from photolysis)?

Not what you're looking for? Try&hellip

(Original post by The Illuminati)
My book says that photosystem 1 occurs maily on the intergranal lamellae and photosystem 2 occurs almost exclusively on the grannal lamellae.
what is the difference between intergranal lamellae and granal lamellae?
Also is the following correct:
thylakoids are stacked membranes. Grana are stacks of thylakoids and lamellae are disk-like structures that link thylakoids in different grana.

Alos does NADP carry hydrogen atoms or hydrogen ions (from photolysis)?

What your book says is correct. 'Thylakoids' simply refers to the network of membranes that generate the continuous thylakoid lumen, that are not necessarily stacked. In the granal lamellae, they are stacked, in the stroma lamellae, which are the interconnecting membranes between granum, the membrane is not stacked. Look at any chloroplast diagram to get the idea!

NADP carries a hydride ion, (1 proton and 2 electrons aka. H-). Both NADP and NAD remove 2 electrons and 2 protons from the substrate, however 1 proton diffuses into the surroundings. Usually this is written as 'NADPH + H' to emphasise the removal of 2 protons. NADPH2 is, in my opinion, an incorrect way of saying the same thing.

But what is the difference between the granal lamellae and and intergranal lanellae. Intergranal suggests inbetween grana so where is the granal lamellae located?

I have in my book that NADP carries up to two hydrogen atoms to the stroma in order for the light independent reaction to occur (the stage where GP is reduced to TP). Wouldn't NADPH + H+ mean a hydrogen (1 proton, 1 electron) and a proton are being carried which doesn't make much sense.
Why a hydride ion?
By surroundings do you mean into the thylakoid space to create a proton gradient?

Also I always thought that when talking about reduction in terms of NADPH we were talking in terms of electrons and/or hydrogen. However GP and TP both have 7 hydrogens. Where do the hydrogens from NADPH go then as there is no change in hydrogens. The only difference between GP and TP is that GP has an extra oxygen. Does this mean that when we say GP is reduced it means that oxygen is lost and the oxygen combines the two hydrogen atoms to produce water as a byproduct. I don't understand it any other way.

(Original post by The Illuminati)
But what is the difference between the granal lamellae and and intergranal lanellae. Intergranal suggests inbetween grana so where is the granal lamellae located?

I have in my book that NADP carries up to two hydrogen atoms to the stroma in order for the light independent reaction to occur (the stage where GP is reduced to TP). Wouldn't NADPH + H+ mean a hydrogen (1 proton, 1 electron) and a proton are being carried which doesn't make much sense.
Why a hydride ion?
By surroundings do you mean into the thylakoid space to create a proton gradient?

Also I always thought that when talking about reduction in terms of NADPH we were talking in terms of electrons and/or hydrogen. However GP and TP both have 7 hydrogens. Where do the hydrogens from NADPH go then as there is no change in hydrogens. The only difference between GP and TP is that GP has an extra oxygen. Does this mean that when we say GP is reduced it means that oxygen is lost and the oxygen combines the two hydrogen atoms to produce water as a byproduct. I don't understand it any other way.

It's all the same membrane but arranged differently. As you can see here:

The structures which look like coins on top of each other are the granum, the stromal lamellae are the yellow non-stacked connections that join it all up. I'm not sure how to explain NAD and hydride ions to you but I will try

On the left you can see NAD, here it has a nitrogen atom with a positive charge, N+. When NAD removes electrons and protons from something, 1 proton is added to NAD, at the bottom, to form NADH, this comes with 1 electron. The other electron then is added to N+, so it no longer has the charge. This re-arranges the bonds a bit.

So you can see that NAD takes 1 proton, and 2 electrons, so we say that it takes a hydride ion which is hydrogen with an extra electron thus a negative charge:

By surroundings I just mean the proton remains in the thylakoid lumen, the stroma, the liquid around the reaction.


What is Photosynthesis

Photosynthesis is an anabolic process in which green plants synthesize carbohydrates with carbon dioxide and water in the presence of sunlight and evolve oxygen as a product. Study in photosynthesis originated only about 300 years ago. Some landmark experiments are given (i) Joseph Priestley showed that plants have the ability to take up CO2 from the atmosphere and release O2, (ii) Jan Ingenhousz confirmed Priestley work and discovered that release of O2 by plants was possible only in sunlight and only by the green parts of the plants, (iii) Hill reaction was discovered by Robert Hill in 1939, in which isolated chloroplasts produce oxygen and hydrogen when illuminated in the presence of an oxidizing agent (ferric salts) . Photosynthesis equation:

In photosynthesis, CO2 is fixed or reduced to carbohydrates (glucose C6H12O6) . Water is split in the presence of light (called photolysis of water) to release O2. O2 released comes from the water molecule and not from CO2.


Vacuoles

Vacuoles are membrane-bound sacs that function in storage and transport. The membrane of a vacuole does not fuse with the membranes of other cellular components. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 2b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.

The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. You can rescue wilted celery in your refrigerator using this process. Simply cut the end off the stalks and place them in a cup of water. Soon the celery will be stiff and crunchy again.

Figure 2. These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells. Plant cells do not have lysosomes or centrosomes.


Biogenesis of thylakoids

The sophisticated thylakoid ultrastructure seen in mature chloroplasts is not observed in the proplastids found in dark-grown tissues, but its formation is rapidly initiated upon exposure to light. The process of thylakoid biogenesis requires the coordinated assembly of lipids, proteins, and chlorophylls, which together account for >98% of the mass of the thylakoid membrane ( Murphy, 1986). Strikingly, lipids make up only

30% of the membrane surface ( Kirchhoff et al., 2002). Of these, >50% cannot normally form bilayers under the conditions prevailing in thylakoids ( Webb and Green, 1991). Studies on mutant plants have been most helpful in elucidating the interdependence of component assembly during thylakoid biogenesis.

De-etiolation

The membrane system in proplastids is much simpler than that found in mature chloroplasts. Its simple vesicular structures contain only small amounts of proteins linked to photosynthesis ( Adam et al., 2011). In the absence of light, the proplastid matures into an etioplast, whose inner membrane forms a semi-crystalline network of interconnected tubules called a prolamellar body ( Rosinski and Rosen, 1972). The prothylakoids develop as extensions of the prolamellar body, but contain lower amounts of monogalactosyldiacylglycerol (MGDG), thus allowing them to form a planar bilayer structure ( Selstam and Sandelius, 1984) ( Fig. 6A). The prolamellar body is photosynthetically inactive, but analysis of its proteome has identified 64 proteins that are linked to the photosynthetic light reactions, the Calvin cycle, protein synthesis (including chaperones), and pigment biosynthesis ( Blomqvist et al., 2008). By this point, the ATP synthase is fully assembled ( Plöscher et al., 2011) and the dimeric Cyt b6f complex only lacks mature chlorophyll a ( Reisinger et al., 2008). In contrast, PSII biogenesis is arrested at a pre-complex stage, and PSI assembly most probably occurs later in the greening process ( Müller and Eichacker, 1999). In etioplasts, LHCs are not yet inserted into the membrane ( Kuttkat et al., 1997). The most abundant protein in prolamellar bodies is the NADPH:protochlorophyllide oxidoreductase (POR), and mutants that lack this enzyme accumulate precursors of protochlorophyllide and possess no prolamellar bodies ( Lebedev et al., 1995 Sperling et al., 1998). Further major constituents of the prolamellar body are protochlorophyllide, NADPH, and the non-bilayer-forming lipid MGDG ( Selstam and Sandelius, 1984 Adam et al., 2011). The high content of POR (90% of the protein content) is compatible with the cubic lipid structure adopted by MGDG and thus accounts for the semi-crystalline form of prolamellar bodies ( Selstam and Sandelius, 1984 Ryberg and Sundqvist, 1988). As the initial step in light adaptation, protochlorophyllide is reduced by POR to chlorophyllide and later esterified into chlorophyll ( Von Wettstein et al., 1995). At the same time, the prolamellar body loses its semi-crystalline structure and the extruded lamellae align in parallel throughout the stroma ( Fig. 6B, C). Whether the lipids of the prolamellar body are directly incorporated into the prothylakoids or are transferred via vesicles is unclear ( Rosinski and Rosen, 1972 Adam et al., 2011). The transformation of the semi-crystalline prolamellar body into planar thylakoids ( Fig. 6D) can take from 1h to over a day.

Schematic overview of the light-dependent de-etiolation process and the biogenesis of thylakoid membranes. This simplified scheme focuses on PSII, PSI, and LHC. (A) The etioplast contains prothylakoids and the semi-crystalline prolamellar body. The latter is mainly composed of MGDG, POR, and protochlorophyllide. MGDG and POR together form the cubic lipid structure that makes up the prolamellar body. (B) Etio-chloroplast stage. The de-etiolation process is initiated by exposure to light and the light- and NADPH-dependent reduction of protochlorophyllide by POR. The semi-crystalline prolamellar body disassembles. Whether the lipids of the prolamellar body are incorporated into the maturing prothylakoids directly or via vesicular or tubular intermediates is unknown. Mainly monomeric proteins are incorporated into the developing thylakoids. (C) The lamellar structures align in parallel within the chloroplast. With the formation of protein complexes, the thylakoids enter a photoactive state. (D) The grana stacks characteristic of mature thylakoids form upon incorporation of mega- and supercomplexes. It is assumed that lipids reach thylakoids only via (i) a vesicular pathway, but might also be supplied by (ii) soluble glycerolipid transfer proteins or (iii) via invaginations of, or direct contact sites with, the inner envelope. This schematic representation of the processes involved in the de-etiolation of etioplasts does not do justice to the complexity of the intermediate steps (reviewed in Solymosi and Schoefs, 2010). For example, grana stacks and prolamellar bodies have been observed in the same chloroplast in electron micrographs ( Solymosi and Schoefs, 2010).

Schematic overview of the light-dependent de-etiolation process and the biogenesis of thylakoid membranes. This simplified scheme focuses on PSII, PSI, and LHC. (A) The etioplast contains prothylakoids and the semi-crystalline prolamellar body. The latter is mainly composed of MGDG, POR, and protochlorophyllide. MGDG and POR together form the cubic lipid structure that makes up the prolamellar body. (B) Etio-chloroplast stage. The de-etiolation process is initiated by exposure to light and the light- and NADPH-dependent reduction of protochlorophyllide by POR. The semi-crystalline prolamellar body disassembles. Whether the lipids of the prolamellar body are incorporated into the maturing prothylakoids directly or via vesicular or tubular intermediates is unknown. Mainly monomeric proteins are incorporated into the developing thylakoids. (C) The lamellar structures align in parallel within the chloroplast. With the formation of protein complexes, the thylakoids enter a photoactive state. (D) The grana stacks characteristic of mature thylakoids form upon incorporation of mega- and supercomplexes. It is assumed that lipids reach thylakoids only via (i) a vesicular pathway, but might also be supplied by (ii) soluble glycerolipid transfer proteins or (iii) via invaginations of, or direct contact sites with, the inner envelope. This schematic representation of the processes involved in the de-etiolation of etioplasts does not do justice to the complexity of the intermediate steps (reviewed in Solymosi and Schoefs, 2010). For example, grana stacks and prolamellar bodies have been observed in the same chloroplast in electron micrographs ( Solymosi and Schoefs, 2010).

Lipid incorporation

The thylakoid membrane contains five major lipids. The non-bilayer-forming MGDG accounts for 52% of total lipids by weight ( Kirchhoff et al., 2002). Due to its small headgroup, MGDG forms inverted hexagonal (HII) structures in solution at physical pH and temperature ( Goss and Wilhelm, 2010). Digalactosyldiacylglycerol (DGDG) with 27%, sulphoquinovosyldiacylglycerol (SQDG) with 15%, phosphatidylglycerol (PG) with 3%, and phosphatidylcholine (PC) with 3% account for the rest ( Webb and Green, 1991 Kirchhoff et al., 2002). Because the synthesis of all of these lipids is finalized in the chloroplast envelope, a mechanism for their continuous transport to thylakoids must exist ( Jouhet et al., 2007 Benning, 2008). Whether this transfer of lipids occurs via (i) a vesicular pathway (ii) soluble glycerolipid transfer proteins or (iii) invaginations that directly connect the envelope to thylakoids is not clear ( Fig. 6D) ( Holthuis and Levine, 2005 Jouhet et al., 2007). A mutant with a defective MGDG synthase 1 (mgd1) is unable to produce photosynthetically active membranes, but shows invaginations of the inner envelope ( Kobayashi et al., 2013). However, the emergence in cold-incubated plants of vesicles ( Morré et al., 1991) that resemble the COPII vesicles seen in the cytosol under cold conditions when the energy requirement for fusion with target membranes increases ( Saraste et al., 1986 Morré et al., 1989) points to a role for vesicular traffic in the biogenesis and maintenance of thylakoids ( Vothknecht and Westhoff, 2001).

Mutants without the ‘vesicle-inducing protein in plastids’ (VIPP1) lack the aforementioned cold-induced vesicles and are defective in thylakoid biogenesis ( Kroll et al., 2001). Because it is found both at the inner envelope and at the thylakoids ( Li et al., 1994), a role for VIPP1 in the formation of vesicles that transport lipids and hydrophobic carotenoids to the thylakoids has been proposed ( Kroll et al., 2001). Mutants for the cyanobacterial homologue of VIPP1, PspA (bacterial phage shock protein A), also show a thylakoid-defective phenotype ( Westphal et al., 2001), and a maintenance function for membrane integrity was suggested for it ( Hankamer et al., 2004 Standar et al., 2008). Note that the central α-helical domain, conserved between PspA and Vipp1, is responsible for formation of an oligomeric ring structure ( Aseeva et al., 2004), whereas the N-terminal α-helix mediates lipid binding and assembly of a high molecular weight complex ( Otters et al., 2013). The dynamics of this complex are controlled by the HSP70B–CDJ2–CGE1 chaperones ( Liu et al., 2005 Liu et al., 2007), but HSP90 may promote the disassembly of the multimer and, in its absence, only a few thylakoid membranes are formed ( Feng et al., 2014). Other findings hint that VIPP1 might function like its bacterial homologue, namely acting to maintain the chloroplast envelope instead of inducing vesicles, thus having a protective rather than a driving effect on thylakoid biogenesis ( Zhang et al., 2012). It has been speculated that thylakoid-associated VIPP1 has a similar function, but conclusive proof is still missing ( Vothknecht et al., 2012 Zhang and Sakamoto, 2013). Moreover, VIPP1 was also found to enhance binding of substrates for the cpTat import pathway ( Lo and Theg, 2012). Intriguingly, recent results point to a role for VIPP1 in the assembly of thylakoid core complexes ( Nordhues et al., 2012). Based on these findings, Rütgers and Schroda (2013) have presented a model in which VIPP1 fulfils a structural role within thylakoid centres, which are considered as sites from which thylakoid membranes emerge and at which the biogenesis of PSII at least is thought to occur. Furthermore, VIPP1 could create microdomains in the membrane that facilitate the accumulation of specific lipids that, in turn, aid in the function of translocases ( Lo and Theg, 2012 Rütgers and Schroda, 2013).

VIPP1 apparently does not play a role in vesicular transport in mature chloroplasts, but several other proteins remain as candidates for such factors. Thus, a bioinformatics approach has identified chloroplast-located homologues of the COPII vesicular pathway between the endoplasmic reticulum and Golgi apparatus ( Andersson and Sandelius, 2004). One essential component for assembly of the COPII coat is the GTPase Sar1, whose chloroplast-located homologue cpSar1 also shows GTPase activity in vitro and has been linked to thylakoid biogenesis ( Garcia et al., 2010). Although a direct connection with vesicle coat assembly could not be demonstrated, the protein’s presence at the inner envelope and in the stroma is compatible with a function in vesicle initiation. Furthermore, cpSar1 has been detected around cold-induced vesicles ( Garcia et al., 2010). While cpSar1 knock-out mutants show developmental arrest before greening, cpSar1 RNAi (RNA interference) lines show an interesting intermediate phenotype with respect to thylakoid biogenesis. In these lines, plastids contain vesicles of various sizes that eventually coalesce and form the typical mature grana stacks ( Garcia et al., 2010).

The dynamin family member FZL is also localized at the envelope and thylakoids, and shows GTPase activity, but in FZL knock-out plants disruption of thylakoid ultrastructure is less severe ( Gao et al., 2006) than in cpSar1 knock-outs. Although grana stacks are disorganized and vesicles accumulate, FZL is believed to play a more prominent role later in thylakoid development ( Gao et al., 2006 Adam et al., 2011).

The THF1 (THylakoid Formation1) protein is also assumed to be involved in vesicular trafficking because in thf1 mutants white/yellow patches appear that completely lack grana stacks or any form of thylakoid membrane but accumulate membrane vesicles ( Wang et al., 2004). THF1 is identical to Psb29, which is involved in PSII biogenesis ( Keren et al., 2005). This finding is corroborated by the observation that thf1 mutants retain a PSII–LHCII supercomplex in the dark, which implies an important role for THF1/Psb29 in PSII dynamics ( Huang et al., 2013). Therefore, it cannot be excluded that THF1/Psb29 might be involved in the fusion of PSII-loaded vesicles emerging from the envelope ( Khan et al., 2013), although this seems at variance with the continuous influence of THF1 on leaf development including leave senescence ( Huang et al., 2013).

Given the complexity of COPII vesicular transport, a mechanism dedicated solely to the transport of lipids from the envelope to thylakoids is hard to imagine, especially since the non-bilayer-forming nature of MGDG would complicate such mechanisms. In this context, two observations are of interest: (i) the MDGD:DGDG ratio is three times lower in developing than in mature thylakoids ( Andersson et al., 2001) and (ii) the existence of non-bilayer structures in thylakoids and their ability to exchange lipids with the bilayer phase ( Krumova et al., 2008). Thus, the integration of high concentrations of MGDG into a lipid bilayer relies on the presence of membrane proteins. If the protein to MGDG ratio is lowered, MGDG cannot be kept within the bilayer, but migrates into non-bilayer structures. However, whether bilayer and non-bilayer phases can co-exist in vesicles too remains speculative. Alternatively, it was suggested that a vesicle pathway might also transport non-lipid components ( Westphal et al., 2003 Benning, 2009). This idea is in line with the identification of the plasma membrane as the location of initial photosystem biogenesis in cyanobacteria ( Zak et al., 2001), although this apparently does not hold for A. thaliana ( Che et al., 2013). In an ongoing bioinformatics analysis, the search for components of the vesicular transport mechanism in plastids has been expanded to associated factors ( Khan et al., 2013). In this study, chloroplast-targeted homologues of coat proteins, cargo receptors, tethering factors, and SNAREs were identified. Some 80% of the putative cargo proteins could be linked to functions in thylakoids such as biogenesis, stress responses, and photosynthesis ( Khan et al., 2013).

Despite the lack of conclusive experimental proof, the evidence for a vesicular transport system within the chloroplast cannot be easily dismissed ( Brandizzi, 2011).

The role of protein complexes in thylakoid biogenesis

The vast majority of the thylakoid surface is occupied by protein complexes, which account for >70% of the total thylakoid membrane area ( Kirchhoff et al., 2002). Thus, it seems likely that thylakoid biogenesis is influenced by the insertion of protein complexes into the lipid bilayer matrix. Most of the thylakoid proteins are encoded in the nucleus and synthesized in the cytosol, and must be post-translationally imported into the chloroplast. The pathways mediating this transport, and its evolution and regulation, have been extensively reviewed ( Gutensohn et al., 2006 Strittmatter et al., 2010 Shi and Theg, 2013). Recently, it was suggested that lumenal proteins are also essential for thylakoid biogenesis ( Shipman-Roston et al., 2010 Järvi et al., 2013). Their proper maturation may be a key step in the assembly of thylakoids, as plants mutant for the processing peptidase PLSP1, which is involved in the maturation of lumenal proteins (such as OE33, OE23, and plastocyanin), have been shown to accumulate large amounts of vesicles in the stroma but fail to develop intact thylakoids in adult plants ( Inoue et al., 2005 Shipman and Inoue, 2009 Shipman-Roston et al., 2010). Here, the critical step seems to be the removal of the thylakoid-transfer signal. Without its removal, certain newly imported proteins are not released from the thylakoid membrane ( Frielingsdorf and Klösgen, 2007) and are subsequently degraded ( Midorikawa and Inoue, 2013).

Generally, it is difficult to determine unambiguously the importance of integral membrane proteins for thylakoid biogenesis, since their absence results in significant perturbation of photosynthetic activity. In the following, defects in the assembly of the major thylakoid protein complexes will be reviewed in the context of their effects on thylakoid biogenesis.

Mutants without PSI are incapable of photoautotrophic growth. First identified in a series of high chlorophyll fluorescence (hcf) mutants ( Meurer et al., 1996), hcf101 was depleted of PSI and showed an impaired thylakoid ultrastructure completely devoid of stroma lamellae ( Stöckel and Oelmüller, 2004). HCF101 was found to be involved in the provision of Fe–S clusters required for PSI assembly ( Lezhneva et al., 2004 Schwenkert et al., 2010). Interestingly, other mutants specifically lacking PSI form fragmentary stroma lamellae but still express near wild-type levels of the light-harvesting complexes. These include strains defective for the PSI assembly factor PPD1 (PsbP-domain protein1) ( Liu et al., 2012), PSI-F, a subunit of PSI ( Haldrup et al., 2000), and Pale yellow green7 (Pyg7) ( Stöckel et al., 2006), as well as hcf101, hcf113, and hcf140 ( Amann et al., 2004). Therefore, it can be concluded that the presence of PSI is essential for thylakoid biogenesis, more specifically the formation of the stroma lamellae.

The aforementioned PSII assembly mutant hcf136 forms enlarged and denser grana stacks, while light-harvesting complexes assemble normally ( Meurer et al., 1998). In Low PSII Accumulation1 (LPA1) lines, which retain 20% of the wild-type PSII amount, grana stacks are shorter and thinner, but the overall effect on thylakoid ultrastructure is less severe ( Peng et al., 2006). In the absence of AtCtpA, a protein required for maturation of the PSII reaction centre protein D1, no functional PSII complexes, and few grana stacks, could be assembled ( Che et al., 2013). Conversely, overexpression of maize plastidial transglutaminase in tobacco increased the numbers of PSII centres in the appressed grana, leading to larger grana stacks and reduced stroma lamellae ( Ioannidis et al., 2009). The lack of ATAB2 (Arabidopsis homologue of Chlamydomonas Tab2), which is presumably involved in the biogenesis of both photosystems, results in an intermediate thylakoid phenotype ( Dauvillée et al., 2003 Barneche et al., 2006). In atab2 mutants, PSI complexes are absent, PSII is decreased 5-fold, while the Cyt b6f and ATPase complexes are expressed normally, leading to a significant decrease in stroma lamellae and a general decrease in thylakoid membrane content ( Barneche et al., 2006).

Light-harvesting complex proteins (LHCPs)

Post-translational insertion of the LHCPs into thylakoids is mediated by the signal recognition particle (cpSRP) pathway ( Schünemann, 2004). In the case of LHCPs, this works in close cooperation with FtsY and Albino3 (ALB3) ( Tu et al., 1999 Moore et al., 2000 Woolhead et al., 2001), with ALB3 being responsible for cpSRP-dependent LHCP integration into the thylakoid membrane ( Bals et al., 2010 Falk et al., 2010). The importance of ALB3, and hence of the LHCPs, for the biogenesis of thylakoid membranes is striking. In the alb3 mutant, a significant loss in thylakoid membrane and grana stacking is observed ( Sundberg et al., 1997). As mentioned above, the impact on grana formation of a specific lack of LHCII trimers or alterations in their subunit composition is less severe.

The assembly of the LHCIIs also relies on the incorporation of chlorophyll b ( Horn et al., 2007). In a mutant devoid of chlorophyll b (ch1-3), the concentration of LHCII was decreased, with no LHCII trimers detectable ( Kim et al., 2009). This led to smaller chloroplasts and a 30% decrease in numbers of grana per chloroplast area. A cross of ch1-3 and lhcb5 showed a further decrease in LHCII monomers and resulted in a loss of >60% grana area ( Kim et al., 2009). This decrease in grana stacking due to reduced LHCII levels was attributed to a decline in van der Waals attraction, lower electrostatic interaction between opposite charges across the partitioning gap, and impaired formation of PSII–LHCII aggregates, which together appear to exert stronger negative effects on grana formation than the positive effects caused by the weaker electrostatic repulsion due to the lack of LHCII ( Chow et al., 1991, 2005 Kim et al., 2009).

Others

The level and stability of the CF1CFo ATP synthase is strongly reduced in the alb4 mutant ( Benz et al., 2009), decreasing the degree of appression in grana stacks ( Gerdes et al., 2006). Mutants affected in the assembly of the Cyt b6f complex ( Lennartz et al., 2001 Dreyfuss et al., 2003 Maiwald et al., 2003 Xiao et al., 2012) have not yet been characterized with respect to thylakoid ultrastructure. Nevertheless, ultrastructural data are available for a mutant with markedly reduced levels of Cyt b6f ( Manara et al., 2014). However, these lines show normal thylakoid formation, indicating that the Cyt b6f complex does not play a significant role in the establishment of the thylakoid ultrastructure ( Manara et al., 2014).

Interdependency of protein and lipid supply

The composition of thylakoid membranes varies little between photosynthetically active organisms ( Siegenthaler, 1998 Vigh et al., 2005). Lipid membranes serve as habitats for the proteins involved, playing important roles in their stability and functionality ( Mizusawa and Wada, 2012 Boudière et al., 2014). Liposomes consisting of DGDG and MGDG are able to stabilize LHCII trimers, while the absence of DGDG slightly destabilizes the complex ( Yang et al., 2006). Interestingly, increasing MGDG content in LHCII–PSII liposomes increases the antenna cross-section and boosts photosynthetic activity ( Zhou et al., 2009). SQDG may be similarly involved in stabilizing PSI ( Sugimoto et al., 2010).

Conversely, proteins can modulate the phase behaviour of MGDG. Thus, by increasing the amounts of LHCII, the inverted hexagonal phase can be progressively transformed into ordered lamellar structures ( Simidjiev et al., 2000). It has been hypothesized that the amount of thylakoid-incorporated non-bilayer-forming lipid is controlled by the current state of the membrane ( Garab et al., 2000), such that changes in protein content and distribution cause excess MGDG to be forced into a non-bilayer phase or be recruited from there ( Garab et al., 2000). The existence of such a non-bilayer phase and its exchange with the membrane was shown by Krumova et al. (2008) (see above). Only the tight packing of proteins into the membrane is compatible with the high concentration of MGDG, and vice versa. Consequently, the protein-rich appressed grana stacks were found to have a higher MGDG:DGDG ratio than stroma lamellae ( Gounaris et al., 1983, 1986), in agreement with the idea that non-bilayer-forming lipids mediate stacking ( Lee, 2000).


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