Plant Defence 3 - Acquired Resistance

In posts one and two of this mini-series I explored how plants can defend against bacteria by releasing dangerous chemicals and by killing off cells. This post looks at how surviving one bacterial attack can make plants more able to survive subsequent ones with both local and systemic acquired resistance.

Locally acquired resistance is the simplest to manage, and provides a clear advantage. If cells have been attacked once it makes sense to defend them in case of a second attack. Plants achieve this by strengthening the cell walls in cells that have survived the bacterial attack. Experiments adding elicitors (bits of bacteria that stimulate the plant pathogen receptors) to plant cells showed that proteins in the cell wall became oxidatively cross-linked as they sense the bacteria. Interestingly the molecule responsible for this is hydrogen peroxide, one of the molecules also involved in the cell-death response discussed in post two. If it doesn't kill the plant, it makes it stronger.

The cell membrane is the blue box at the bottom, whereas the cell wall is the light blue rods in the middle. It is the cell wall which is strengthened. Image from wikimedia commons.

This response is all very well for plant cells which happened to be near the site of infection, but what about the rest of the plant? Is it possible for cells on the other side of the plant to be warned and ready for a pathogen attack? Despite the inability of plant cells to move, the answer surprisingly is yes. Cells at the site of infection can release a chemical called salicylic acid which moves through the plants vascular system (the system which also delivers sugars and other important nutrients to all parts of the plant).

The chemical structure of salicylic acid, which is chemically similar to the active component of aspirin.

Following an infection, the levels of salicylic acid were found to rise dramatically in cells around the zone of infection, before spreading through the rest of the plant. This isn't a species specific response either but one found in many different species; grafting parts of one plant onto another did not stop either plant from acquiring resistance. In response to the salicylic acid signal cells start accumulating small amounts of hydrogen peroxide, which can lead to the same cell wall strengthening seen around the area of infection.

As well as salicylic acid it has also been suggested that infected areas of the plant can release the volatile molecule methyl salicylate, commercially known as oil of wintergreen. Rather than travelling through the plant this signal is airborne, allowing transmission not just to other parts of the plant, but to neighbouring (and therefore likely to be related) plants as well. As the only difference between these two signalling molecules is the addition of a small CH3 group, the methyl salicylate can easily be converted back into salicylic acid once it reaches the cells where it can cause the same downstream response.

If anyone was wondering quite why I've suddenly been into plants part of the reason is that the BBC is showing a program called "Botany - a blooming history" and I've been catching the episodes. Despite the slight naffness of the title, it's actually a really good program showcasing experiments, personalities, and the scientific method as it unfolds the history of plant science. You can catch the episodes here on iPlayer.

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Brisson, L., Tenhaken, R., & Lamb, C. (1994). Function of Oxidative Cross-Linking of Cell Wall Structural Proteins in Plant Disease Resistance The Plant Cell, 6 (12) DOI: 10.2307/3869902

Durrant, W., & Dong, X. (2004). SYSTEMIC ACQUIRED RESISTANCE Annual Review of Phytopathology, 42 (1), 185-209 DOI: 10.1146/annurev.phyto.42.040803.140421

Shulaev, V., Silverman, P., & Raskin, I. (1997). Airborne signalling by methyl salicylate in plant pathogen resistance Nature, 385 (6618), 718-721 DOI: 10.1038/385718a0

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Plant defence 2 - Honourable Suicide

The first post of this mini-series covered how plants can defend themselves against bacterial attack by releasing chemicals, either on a regular basis or as a specific response to the attack. This post will explore the hypersensitive response, which allows plants to rapidly kill of cells around the area of infection, starving the bacteria of nutrients to prevent it spreading. The end result is a small area of dead plant matter, with the rest of the organism unaffected.

One of the main differences between plants and animals that I flagged up in the last post is that plant cells don't move. The use of the hypersensitive response shows another; plants have a very non-determinant structure. Animals will grow towards a clear well defined shape and once they get it, they stick with it. Your body does change as you grow older, but it's not about to grow an extra leg. Plants on the other hand may have determinant structures within them, such as leaves or flowers, but the overall organism can just keep growing for as long as it needs to. If a leaf is lost through disease, the plant can just grow a new one, or several new ones.

These leaves are expendable. Your arm is not. Image from wikimedia commons.

Because of its non-determinant nature it is a lot easier for the plant to kill parts of itself off in order to stop an infection spreading. One way that the hypersensitive response does this is by the production of large numbers of reactive oxygen species in cells surrounding the site of infection. These include hydrogen peroxide, and various hydroxide and oxygen containing free radicals. Free radicals are species with one unpaired electron and therefore are extremely reactive and extremely dangerous. These free radicals lead to chain reactions that can break down lipids in the membrane, inactivate enzymes and generally roll around like a loose canon causing havoc within the cell.

As well as reactive oxygen species, the cell also experiences large ion fluxes, as potassium and hydroxide ions flood into the cell and hydrogen and calcium ions flood out. These result in the cell releasing any stored toxic compounds it might have (which may also help to kill the bacteria) and may serve to integrate the mitochondria into the process of cell death (see reference one). As mitochondria are crucial in coordinating the programmed cell death of animal cells it would be surprising if they did not play some part in the controlled destruction of plant cells. The actual sequence of destruction varies from plant to plant, but the overall result is the same, and area of dead plant tissue within the still healthy surviving plant.

A leaf infected with tobacco mosaic virus, showing lighter areas of dead leaf interspersed with the green areas of normal growth. Image from wikimedia commons.

The plant hypersensitivity response can (if you want it to) be considered analogous to the human innate immune response, in that it occurs directly in response to a bacterial attack, and it occurs only at the site of bacterial infection. Plants, however, also have ways of making more long-term changes to protect against bacterial attacks in the future both at the site of the old reaction and throughout the whole plant. How the plant achieves this, without any cellular movement, will be the topic of the final post in this mini-series.

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Lam, E., Kato, N., & Lawton, M. (2001). Programmed cell death, mitochondria and the plant hypersensitive response Nature, 411 (6839), 848-853 DOI: 10.1038/35081184

Pontier, D., Balagué, C., & Roby, D. (1998). The hypersensitive response. A programmed cell death associated with plant resistance Comptes Rendus de l'Académie des Sciences - Series III - Sciences de la Vie, 321 (9), 721-734 DOI: 10.1016/S0764-4469(98)80013-9

Taiz, Zeiger, Plant Physiology, third edition Sinauer Associates 2002.
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The MolBio Carnival is here!

The fifth issue of the MolBio carnival is here! We've got loads of great entries in this edition, all focusing on the mysterious world inside cells, so take some time out to take a look and comment on them. The inside of cells is such a fascinating place to explore - and many of these posts were written by the people whose job it is to explore them. I've used a fairly broad interpretation of molecular biology, so in this carnival you'll see everything from the atomic details of protein interactions, to the (comparatively) far bigger world of bacterial colonies in the gut.

All good explorations should start with a map - and you don't get much better than the truly gorgeous pictures spotlighted by E. Campbell of the HighMag Blog. This beautiful picture shows a cell with the actin-binding proteins stained purple in order to see how they interact with a mutant actin motor.

Once we head inside the cell, we can start to explore the many complex and fascinating interactions that help to control it. While the DNA might encode all the information needed to create cellular proteins, it isn't just the DNA that is responsible for cellular behaviour, as explained by Christopher Dieni in "How I Learned to Stop Worrying and Love Epigenetics" (which comes second place in the Lab Rat award for best post name). As well as proteins, DNA expression is also controlled by fragments of RNA, explained beautifully clearly by student blogger Khalil A. Cassimally who looks into whether miRNA might be used to control cocaine addiction. And while we're at the level of molecular interactions for cellular control, we can look at control mechanisms for protein folding as well, as the Computational Biology blog takes us through the consequences of entanglement during protein folding.

Picture from Robert Ezra showing protein (green) binding to DNA (gold)

But the cell does not consist solely of DNA and proteins - it also relies on metabolites such as sugars and fats. These metabolites pass through a complex series of reactions in order to convert them to energy, and research on how these reactions occur and are controlled has been going on for many years. Sigmabioblogs has a wonderful interview with the biolegend Dr. Donald Nicholson, who is now over 80 years old and has been working on metabolic pathways for pretty much his entire life! On the subject of nutrients, there is also a great bilingual post on Knedliky about how flavour-enhancers work at a molecular level.

These small and focused intramolecular reactions aren't just used to control the cell, but also to control far bigger systems, or cell-cell interactions and communication. Memoirs of a Defective Brain explains how the bacteria Strep pyogenes uses intramolecular interactions to prevent the immune system recognising an infection. His post "The SpyCEP who cleaved me" not only wins the Lab Rat award for best post name, but also features the BEST diagram I've ever seen for explaining the subtle and complex interactions between cells of the immune system:

While we're on the theme of bacteria (yay!) we'll head over to the stomach. James, of (currently...) Disease of the Week, has written a great two part series on those bacteria in our gut, focusing on the question of how they actually get into our gut, and what they do when they get there. Part 1 deals with babies, and Part 2 with adults. There's also a lovely post from Lucas Brouwers, of Thoughtomics, which looks at the evolution of cyanobacterial toxins - and why a bacteria that lived millions of years before humans were even thought of would need to produce such a powerful neurotoxin.

And lets not forget the plants! They rely on intracellular interactions as much as any other organism. There's an old (but very good) post from Denim and Tweed about how nitrogen fixing bacteria made the leap from being intracellular parasites to mutualistic helpers. We've also got a post from It Takes 30 - about how sex is specified in plants. Unlike humans, who rely on chromosomes, hormones, and a whole host of social norms and pressures to distinguish the sexes, plants might need no more than a single amino acid insertion.

Those are basically just brightly painted sexual organs on display

We'll finish the exploration on a slightly larger, but no less fascinating level, the reproductive systems of marsupials by the amazing piratey Captain Skellet. By labelling gene markers for the development of organs researchers have come to the (not unexpected) conclusion that marsupials are Just Weird, and no one is quite sure why...

The next edition of the MolBio carnival will be hosted at PHASED, so if you've missed out this time, go submit your posts here by the 3rd of January. Blog carnivals are a great way to share information and to get new readers, so it's highly recommended!

Iron and Stress

Iron is a metal that is essential for all living things as it is heavily involved in cell redox reactions and the electron transport chain (a major part of aerobic respiration). However it is strongly reactive with oxygen - outside of living organisms this leads to rust, inside it can lead to the production of dangerous reactive oxygen species - and therefore needs to be controlled and contained within the cell. In order to provide this control, all living organisms (apart from yeast, weirdly enough) use a protein called ferritin. Multiple subunits of ferritin proteins (usually 24, although occasionally only 12) form an outer shell, with a central cavity that can contain around 2000-400 individual ferric ions (iron ions) keeping them safely out of harms way.

Diagram shows the shell created by ferritin (iron, or ferrous, ions help inside). Image from wikimedia commons.

In plants, the ferritin proteins are found in non-chlorophyll containing plastids, and occasionally in mitochondria (although no one is quite sure why). Many plants contain a number of different genes coding for ferritin proteins which, due to a high similarity in sequence identity and functional redundancy (i.e. most of them do similar things) makes individual ferritins difficult to study. The small weed Arabidopsis thaliana is a good model organism in this case as it contains only four ferritins, imaginatively names AtFer1-4.

These four ferritins are expressed differently at different stages of the cell lifecycle, and in responses to different materials. The diagram below shows which genes are upregulated in response to the oxidative compound H2O2, free iron (Fe) and the plant hormone abscisic acid (ABA):

Upstream of the AtFer1 gene is a 15 base pair sequence named IDRS (iron-dependent regulatory sequence) which is used to repress the gene under iron deficient conditions. This is thought to be upregulated by a phosphatase (which would remove phosphate from a DNA binding protein bound to the IDRS) which in turn is upregulated by the plant hormone NO (nitric oxide). The kinetics of AtFer3 are very similar to AtFer1 and it is therefore thought to be regulated by a similar system. AtFer2 may be activated in a different manner, and it shows very different kinetics to the other three genes. It does contain an IDRS sequence upstream of the gene, but it is not certain whether this is functional.

Despite containing a large source of iron, ferritins are likely to function more to prevent the damage caused by reactive oxygen species (caused by reactions of free iron) rather than as an iron store. Mutant plants containing no ferritin do not have an immediately obvious phenotype (outward appearance) although if extra iron is added they have to produce a large number of energetically wasteful detoxifying enzymes, in order to combat the dangers of oxidative stress. Ferritins therefore seem to have evolved not as an iron storage system, but as a buffering mechanism, to allow increases in iron within a plant to have a beneficial, rather than damaging, effect.

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Briat JF, Ravet K, Arnaud N, Duc C, Boucherez J, Touraine B, Cellier F, & Gaymard F (2010). New insights into ferritin synthesis and function highlight a link between iron homeostasis and oxidative stress in plants. Annals of botany, 105 (5), 811-22 PMID: 19482877

Evolving Molecular Machines: The Plant Edition

Over at Thoughtomics, Lucas has a post up about the evolution of mitochondrial import systems. He starts by going back in time two billion years:

"Life was well underway at the time, with proto-bacteria already populating the oceans for over hundreds of millions of years. One of the cells alive at the time, swallowed an alpha-proteobacterium. Something remarkable happened: the alpha-proteobacterium did not die but survived in the host cell. Over time, the host and symbiont became to be dependent on each other." That symbiont became a mitochondria.

He gets massive brownie point for writing 'proto-bacteria' rather than bacteria, and it is a very remarkable event to have happened. However from the point of view of a plant, it's only half the story, because plants carry two endosymbionts within them: the mitochondria and the chloroplast.

Their stories are remarkably similar. After becoming engulfed by the surrounding cell, two major things happened to them: First (and it had to be first otherwise major problems would have arisen!) a protein import mechanism arose, creating more communication between the symbiont and the host and allowing things to pass between them. Second, the symbiont lost bits of its genome, transferring them into the nucleus of the surrounding cell to create the cooperative arrangement seen today:

Picture above from the amazing science illustration gallery by California state University. Nucleus is purple, chloroplasts are green, and the mitochondria are orange.

Lucas's post covered the evolution of the import mechanism for the mitochondria. I'm going to write about the same thing, but for chloroplasts. After all, the plants already have mitochondria so they can't use exactly the same import process, they have to be able to differentiate between the two.

Like mitochondria, the chloroplasts are surrounded by two membranes, and outer membrane and an inner membrane. Two transporters are therefore required to get proteins across. In the mitochondria these are called TOM and TIM (Transport of Outer, and Inner Membrane respectively) and in the chloroplasts they are called TOC and TIC, just to keep things simple (Transport of Outer and Inner Chloroplast membrane). They look fairly similar to TIM and TOM, but recognise different sequences attached to the proteins. While the mitochondrial transport machines recognise sequences that contain a lot of the amino acid arginine and form a specific helical shape, the chloroplast machines (TOC and TIC) recognise sequences rich in serine and proline:

TOC and TIC. The proteins of the TOC machine are coloured green, and the TIC machine proteins are all the rest. Diagram from here.

One of the questions that Lucas asks in his post is: where did all of these proteins come from? After all, before you have an endosymbiont, you don't need any kind of apparatus to transport proteins into them. Once you start looking closer at the transport machinery it starts looking suspiciously like a rather rushed and last minute job. Different proteins with different functions have been cobbled together, and while there's still a bit of a debate as to whether these proteins came from the surrounding cell or the endosymbiont I suspect that it may be a bit of both. The cell needs to communicate with the little alien inside it, and once the endosymbiont started loosing genes, it needed a way to keep resources coming in.

So how do you make a protein importer? What do you assemble it from? Plants had a slightly easier task with the chloroplasts as they already had a perfectly serviceable TIM/TOM transporter present. Looking at the TIC complex, the first two components to come in contact with the imported protein (TIC22 and 20, shown in the diagram above in dark purple and orange) show homology to components of the TIM machinery; TIM23 and 17 for anyone interested in the detail. However TIC22 also has far stronger homologues in cyanobacteria, which means it is likely to be a protein owned by the chloroplast, similar to the proteins owned by the mitochondria that got roped in to help with protein import.

The TOC proteins (all green in the diagram above) all appear to have no other function in modern plants other than protein transport. Toc 34 is the GTPase, and as there are many GTPases in cells (used to provide energy) it could have arisen from any one of them. The other TOC proteins are involved in membrane and may have arisen from ancestral membrane receptor proteins, while some components, (including TOC64, not shown above) appear to be rather redundant, as the machinery works perfectly well without them.

The research on this is a little sketchy, there are no good solid biochemical means as yet to discover what might of happened somewhere around two billion years ago in order to create a transport mechanism between the cell and it's organelles. There are plenty of different views out there as well, about where the different subunits might have come from. The only thing that seems clear is that like the mitochondrial import system, this was clearly pulled together from bits of old machinery lying around. It had two billion years to get better after all, and reach the efficiency of the modern-day protein import machines.

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Gross J, & Bhattacharya D (2009). Revaluating the evolution of the Toc and Tic protein translocons. Trends in plant science, 14 (1), 13-20 PMID: 19042148

Carbon carbon everywhere...

A while ago I wrote about the Great Oxidation Event, the point way back in the history of the earth where a lot of little blobby organisms suddenly discovered the trick of using sunlight as energy, and in the process producing a reaction that liberated oxygen (photosynthesis). This (as I discussed in the previous post) had a major impact on all the rest of the life on earth, but it also rather majorly effected the photosynthesising organisms themselves, not so much because of the increase in oxygen, but because of the semi-simultaneous decrease in carbon dioxide.

When photosynthetic organisms first developed, there was a lot more carbon dioxide in the environment for them to use (with the world so new and all...) and therefore they weren't particularly bothered about getting hold of it. However when the carbon dioxide levels dropped (along with a potential rise in temperature) they were suddenly in very real danger of suffocating. These were marine organisms, and there just isn't that much carbon dioxide in seawater. There's plenty of carbon floating around, certainly, but it's all in bicarbonate form (HCO3- rather than CO2) and Rubisco, the main enzyme involved in photosynthesis, doesn't know how to use bicarbonate, it relies exclusively on carbon dioxide.

Our lecturer called it the 'Ancient Mariner paradox'; the ocean is full of carbon, but the photosynthesis machinery just couldn't use it:

"Water, water, everywhere,

And all the boards did shrink;

Water, water, everywhere,

Nor any drop to drink."

This left the little suffocating blobs with three options. They could stay remain tiny (as the picoplankton did) to minimise diffusion differences and therefore still survive dispite the low carbon dioxide levels. Or they could try to change the way Rubisco worked, but Rubisco has a rather compromised active site as it is, having to both distinguish between carbon dioxide and oxygen and trying to keep carbon dioxide processing levels high. Rubisco is often criticised as being an 'inefficient' enzyme, and compared to other enzymes it is, but with carbon dioxide levels at the level they are in the sea it's only ever working at half of its maximum speed. Carbon dioxide is the clear limiting factor.

So instead, these photosynthetic organisms started to develop ways to get carbon dioxide into the cell and concentrating it around the Rubisco. The main factor in this was the enzyme carbonic anhydrase, which converts bicarbonates back into carbon. However doing that inside the cell just leads to the carbon dioxide diffusing right back out again and therefore today almost all photosynthetic bacteria (and chloroplasts inside plants) contain a special internal compartment, a protein coat surrounding the Rubisco, and all the carefully hoarded carbon dioxide:

Figure above shows TEM of bacteria with carboxysomes pointed out by arrows. The scale bar on the bottom right is 100nm

Photosynthesising bacteria (apart from the picoplankton) contain a compartment called a carboxysome, which consists of a protein coat which contains carbonic anhydrase enzyme and Rubisco, allowing carbon dioxide to be produced right where it's most needed. The addition of a number of bicarbonate transporters on the outside of the cell allows bicarbonate to be brought into the cell, and the whole assembly is known as a Carbon Concentrating Mechanism, or CCM.

When these photosynthesising proto-bacteria were then picked up by free-moving proto-algae to become chloroplasts, they kept their CCMs. The CCM of eukaryotic chloroplasts is called a pyraniod, and can be seen in the picture below (from Dartmouth College) as the dark black blob in the upper left hand corner. The white things that it's surrounded by are starch grains. The big fuzzy blob below it is the cell nucleus, and the little grey membrane-filled circles are the mitochondria. The long black threads are either thylakoid membranes (inside the chloroplast) or endoplasmic reticulum:

The first algae would have been marine as well, and would have needed the CCMs in their chloroplasts in order to produce energy. Sea water tends to be alkaline, which means that the biocarbonate: carbon dioxide ration is insanely large. Gasses don't diffuse very well in water either, carbon dioxide takes about about 10 000 times longer to get anywhere in liquid compared to air.

In fact the best thing to do to get as much free carbon dioxide as possible is to leave the water altogether, and head out onto the land. But that is different story.

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Tanaka S, Kerfeld CA, Sawaya MR, Cai F, Heinhorst S, Cannon GC, & Yeates TO (2008). Atomic-level models of the bacterial carboxysome shell. Science (New York, N.Y.), 319 (5866), 1083-6 PMID: 18292340

Dancing through life

I'm heading off for a weekend away as soon as my other half wakes up, so no time for a proper paper analysis today, just a quick video of some algae dancing:



The organisms shown above are microalgae called Volvox carteri. The large circle is a surrounding membrane which holds within it normal volvox cells (the little white spots) and larger germ line cells (the large white spots) which later hatch to form new Volvox. The reason the three cells above look like they're dancing is because they move by beating little hair like structures called cilia all in time, causing them to move or spin. The three algae shown above have manage to get their cilia moving together at the membrane surface, and are momentarily stuck together because of it.

Here's another video of them dancing on a surface:



Move videos, along with some of a swimming Chlamydomonas, can be found at the Goldstein Labs Youtube channel.

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Drescher K, Leptos KC, Tuval I, Ishikawa T, Pedley TJ, & Goldstein RE (2009). Dancing volvox: hydrodynamic bound states of swimming algae. Physical review letters, 102 (16) PMID: 19518757

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Paleoatmosphere

The first signs of life on earth appeared about 4.5 Ga (1 Ga is an American billion, ie. 10⁹ years) ago. It's not yet completely certain exactly how this life arose; hot volcanic mineral springs have been suggested, as have the more traditional lightning-struck primordial soups and (rather wonderfully) radioactive beaches. At any rate something happened (and there was certainly plenty of time for it to happen in) which lead to a little membrane-bound ball with internal nucleic acids which, crucially, could replicate...

Awesome little gif from here

And then it was all over really, bar the evolution.

What kind of world did these first little blobs of life appear in? The surrounding temperature is pretty much unknown, hypothesises for both warm and cold have been put forward and all that can really be agreed on is somewhere between 4-100 degrees. It was most definitely wet; water was in liquid form when life first started, a fact that was probably vitally important for the formation of life as we know it.

The atmosphere would have been very different, little oxygen and an abundance of carbon dioxide with plenty of methane being released into the atmosphere once the first life forms (appropriately called methanogens) started eking out an entropy-defying existence. In order to get energy to power cellular processes you need to set up redox pathways, which involve cycles of electron donors and acceptors. The main electron donors around at the time were H2, H2S and CH4 and the main acceptor probably nitrogenous. Water, the electron donor used for photosynthesis, was around in abundance, but none of the little proto-life-blobs quite had the energy required to split it (or the physical proteins required back then either) so it mostly stayed unused.

Carbon dioxide levels went down, methane levels went up, the planet warmed up a little due to global warming. Things stayed like that for a billion years or so (1 Ga) and then something quite special happened, something that would have mindblowingly devastating affects on the life surrounding it.

Photosynthesis. The uptake of carbon dioxide into the cell, and the reactions that stick it onto a carbon chain, effectively 'fixing' it as sugar; turning air into food. And as everyone hopefully was taught back in primary school, this process releases oxygen, which is good for us but was almost fatal for the life-forms around 3Ga (probably was fatal for some of them). When oxygen isn't being used for respiration, it can be highly toxic to cells. It screws up the internal redox potential, it creates dangerous free-radicals and it precipitates ions out into soluble forms.

Of course the oxygen produced by the photosynthesising proto-bacteria didn't go straight up into the atmosphere right away. There were too many ions floating around in solution to bind to it, and this caused a huge precipitation event; in common terms, everything rusted. Iron was pulled out to form large rust beds, which set down iron ore deposits to be dug up by humanity 2.5 billion years later and used in the Industrial revolution.

The arrival of this new resource (oxygen) lead to a change in the way organisms respired as well. Up until what is sometimes called the Great Oxidation Event (when oxygen started being released into the atmosphere by all the photosynthesising blobs) most respiration was anoxic, probably similar to anaerobic respiration (or fermentation) in anaerobic bacteria around today. This process, while enough t0 keep life going, is around sixteen times less efficient than aerobic respiration. The proto-life-blobs that managed to use the oxygen would therefore have gained a major energy boost.

Over the next 1.5 billion years the atmosphere changed from a highly reducing state (where the early proto-life-blobs developed) to a more oxidising environment. Endosymbiosis and the formation of mitochondria and chloroplasts allowed the first eukaryotes to specialise their metabolism even more. Rather than have the whole cell as a bundle of metabolic redox reactions, releasing potentially dangerous radicals into the cytoplasm, the energy production could be specialised inside it's own compartment, churning out enough energy for the cell to get bigger. Complex intracellular tubules allowed nutrients to be diffused all over this larger cell which would then commit what was from a bacterial point of view the biggest evolutionary mistake ever, and package the cell nucleus away in an inaccessible membrane. (Eukaryote cells then had to develop squishy things like sex in order to regain enough genetic plasticity to actually evolve.)

The effect of oxygen was not just limited to respiration; nowadays many metabolic pathways involve oxygen at some point, including those necessary for the production of sterols (used in signalling molecules and cholesterol, which is an important membrane component), indoles (found in the amino acid tryptophan) and several antibiotics. Oxygen can be an important resource if used correctly.

It's occasionally speculated just why life took so long to move out of the blob phase and into multicellularity. Spending over three billion years as blobs seems a little odd considering that the last billion years involved the branching out of multicellular organisms in a a whole myriad of forms and features, from velociraptors to cockroaches to annelid worms to highly specialised bacteria capable of forming complex networks of bacterial hunting packs. My personal opinion is that all that time was needed simply to get the metabolic background necessary for more complex cellular arrangements. Without the biochemical pathways necessary to generate reasonable amounts of energy, cells have severe limitations placed on their abilities. And biochemically, most organisms are remarkably similar. Differences between the eukaryotes, bacteria and archaea maybe, and plants and fungi have a few different bits of metabolic pathways, but otherwise the internal cellular reactions are remarkably conserved. Not just metabolic ones either; the finely tuned DNA replication machinary, protein synthesis, and even several signalling pathways remain conserved throughout the Kingdoms.

All those internal pathways had three billion years of self organisation and optimisation before they even had to begin to think about making multicellular creatures. No wonder they all fit together so well!

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Falkowski PG (2006). Evolution. Tracing oxygen's imprint on earth's metabolic evolution. Science (New York, N.Y.), 311 (5768), 1724-5 PMID: 16556831

Half plant, half predator, all weird.

I was planning on brushing up my knowledge of chloroplasts today, as next week I'm starting a plantsci course for my options lectures, but I got sidetracked by Captain Skellet alerting me to Hatena. I've heard of several organisms containing proto-plasmids; symbiotic chloroplasts which haven't completely been endosymbiosed, but Hatena was a new one so I went to look it up. And I'm very glad I did, because it's pretty amazing.

Hatena, taken from the reference. Green blob is the symbiont. Scale bar is 10um

Quick background: chloroplasts are little membrane enclosed vesicles in plants which carry out photosynthesis. Current theory for how they developed is that they were once free-living bacterial type organisms (cyanobacteria) which were engulfed by a larger cell and over time lost their own identity and became little photosynthesising factories inside the larger cell. (I've got another post on it here for anyone particularly interested in the subject.)

Hatena arenicola doesn't have a chloroplast, but it does have a symbiotic relationship with another organism; nephroselmis. The nephroselmis is always found in the same place in the Hatena, and carries out photosynthesis to provide energy for both of them. Unlike regular chloroplasts, nephroselmis has it's own proper nucleus and even it's own mitochondria although most of the internal cellular organisation and any kind of motile apparatus (such as flagella) has been lost.

The weirdest thing about these two organisms though, is their replication cycles. When Hatena replicates, the nephroselmis doesn't, and as a result only one of the offspring gets the photosynthesising symbiont. The other organisms remains colourless and develops a complex feeding apparatus at the apex of the cell, presumably as it can no longer rely on the symbiont for food. This wierd 'half plant, half predator' lifecycle is shown below. (Picture taken from the reference, scale bar 10um):


That's just weird. Seriously odd. The Hatena is able to move seemingly freely between being a predator consuming other cells for food, and being a plant-like organism, once it settles down with it's symbiotic partner. The grey non-symbiont organisms can be induced to take up free-moving nephroselmis and (in the words of the paper) "tentitavely" maintain a symbiotic relationship with them.

The paper suggests that Hatena cycles between these two modes of living, depending on circumstance. Thus the 'predator' grey cell shown above will continue eating fellow cells until it consumes a nephroselmis, at which point it degrades its complex feeding apparatus, accepts energy from the symbiont until it's ready to divide. One of the daughter cells will then go through the whole cycle again while the other remains as a non-predating plant. The authors freely admit that there is little evidence for much of these stages, but it seems a reasonable way to explain what is going on.

As this is clearly a very early stage in symbiotic capture it has important implications for the endosymbiotic theory of chloroplast evolution. Along with various other 'intermediate' symbionts (such as Karenia mikimotoi and Lepidodinium viride) the Hatena helps to show how chloroplasts might have first formed in the cellular ancestor of plants. Hatena and its symbiont have already acquired an intimate structural association, only the coordination of their cell cycles would be required to turn the nephroselmis into an internally replicating plastid.

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OKAMOTO, N., & INOUYE, I. (2006). Hatena arenicola gen. et sp. nov., a Katablepharid Undergoing Probable Plastid Acquisition Protist, 157 (4), 401-419 DOI: 10.1016/j.protis.2006.05.011