As Many Exceptions As Rules: When Is A Chloroplast Not A Chloroplast?

Biology concepts – gravitropism, plastid, chloroplast, chromoplast, amyloplast, leucoplast, malaria parasite

Believe it or not, the way plant roots know to grow into the dirt is related to photosynthesis! “How can this be?” you ask. Well, let’s talk about it.

The cells in the tips of the plant rootlet respond positively to gravity, called gravitropism (the older word for it is geotropism). If you lay a growing plant on its side, the roots will respond by growing (turning) toward the gravity within 10 minutes. The mechanism for this stimulation involves tension and a plant hormone called auxin.

Auxin is a growth hormone that gets redirected

in the growing plant root. The statoliths settle

and trigger the hormone to some cells more than

others. Auxin means ”to grow” in Greek, but in

some cases, like in gravitropism of roots, it

actually inhibits growth.

The root cap (the cells at the tip of the root) have some specialized cells called statocyte (stat = position, and cyte = cell). Inside the statocytes are dense granules called statoliths (lith = stone). The statoliths are made of densely packed starch and are a specialized type of organelle called an amyloplast, which is used in many plant cells for storing carbohydrate in the form of starch (amylo = starch). The statoliths are denser than the cytoplasm of the cell; they don’t just float around, they settle out according to gravity.

Since the statoliths are connected to the membrane of the cell by the cytoskeletal actin molecules, so when they settle toward gravity, some cells in the membrane are stretched and some are compressed. This tension signals the cells to change the number of receptors for the growth control hormone auxin. More tension (more stretch) causes the auxin to move away, toward cells that are under less tension. Auxin prevents cell enlargement and cell division, so those root tip cells on the bottom receive more inhibition. Those on top enlarge more and divide more, so the root turns down. If the root is already vertical, the tension is equal in all directions, and the growth is equal in all directions – the root gets thicker and longer.

Gravitropism is related to photosynthesis in that both mechanisms involve chloroplasts, sort of. Root cells don’t perform photosynthesis, they are underground, so they don’t have chloroplasts. But they do have the amyloplastid statoliths, and these are related to chloroplasts.

Both amyloplasts and chloroplasts are specialized versions of the plant organelle called the plastid. We asked last week about what defines a plant cell – maybe the plastid is it. All plant cells have some plastids, but in different plant cells they may take different forms, including chloroplasts, chromoplasts, leucoplasts, amyloplasts, elaioplasts, or proteinoplasts, but they all start out as proplastids (pro = early and plastos = form in Greek).

Proplastids are in every new plant cell. From there

they can differentiate into other forms, including

the chloroplast. Other plastids are used for storage

or biochemical production. We will talk about statoliths

again when we discuss proprioception.

When a cell divides, each daughter gets its share of proplastids, and then depending on the chemical signals that the daughter cell receives, the proplastid will differentiate (from latin, means to make separate) into the types of plastids that the cell needs. A proplastid can become any type of plastid, and from time to time can change between forms as the plant cell requires. Think of it as a sort of stem cell inside a plant cell – if the cell happens to be in the stem of the plant, it could be a stem cell inside a stem cell!

Proplastids become etioplasts, chloroplasts or leucoplasts. The etioplast is a sort of pre-chloroplast; a chloroplast without chlorophyll. It is waiting to be stimulated by light energy before it decides to spend all the energy it requires to make the chlorophyll. The old science fair project about growing bean plants in the dark demonstrates the etioplasts. The plants are white when grown in the dark, but bring them into the light and they soon green up. The sunlight stimulates the etioplasts to make chlorophyll, become full-fledged chloroplasts and start photosynthesizing.

This is a photomicrograph of the plastids of a

red flower petal. The chromoplasts hold the

xanthocyanin pigments, but we see it as a

continuous color because they are so small.

If the proplastid does not differentiate toward a chloroplast pathway (etioplast too) then it will become a leucoplast (leuko = white). The leucoplasts don’t have color; they become specialized for the storage of plant materials. If they store starch, they are called amyloplasts. Lipid storing leucoplasts are called elaioplasts, while protein storing plastids are called proteinoplasts. Each type serves a crucial purpose in the cells they inhabit, and they can all interchange, depending on the conditions the plant cell finds itself in.

Even more important, leucoplasts that are not serving as storage organelles have biosynthetic functions. They work in the production of fatty acids and amino acids. Amino acids link together to from proteins, so their synthesis is very important for plants. Plants must manufacture every amino acid it needs, whereas we get many of ours in our diet. There are even some amino acids that humans can’t make, called the essential amino acids. Of the twenty common amino acids, nine of them must be taken in through our diet, and some people with pathologies can’t make up to seven more. Plants don’t have this luxury; all their amino acids must be made on site. Good thing they have leucoplasts.

There is one other type of plastid that we haven’t talked about, the one that is important for the Autumn tourist trade. Etioplasts and chloroplasts can differentiate into chromoplasts, organelles that store pigments (colored molecules) other than chlorophyll. Chlorophyll provides energy through photosynthesis, but they also have a cost. The old saying, “It takes money to make money” applies to plants as well. It takes energy to make chlorophyll, so it only pays to make chlorophyll when there is ample sunlight to put through photosynthesis. When the days get shorter, the profit margin for producing chlorophyll goes down, so the plant just stops making it.

Twin females were imaged after a lifetime of smoking or non-smoking.

Can you guess who was exposed to the oxygen radicals in cigarette

smoke her whole adult life?

The oxygen produced in plant cells during photosynthesis can damage many molecules; oxygen likes to react with other compounds and steal or donate electrons. This oxidative damage can wreak havoc with the cells, just look at the face of a long time smoker – the damage and aging process from the oxidants in cigarette smoke will be evident. The chromoplast pigments, like carotenoids (oranges and yellows) and xanthocyanins (reds and purples), can serve as antioxidants, and protect the other cell structures from the damaging effects of oxygen.

So the chloroplasts lose their chlorophyll in autumn and could be called leucoplasts, but the chromoplasts still have the pigments that had been masked by the greater number of chlorophyll molecules. The trees turn magnificent colors and bring people from the cities to stay in bed and breakfasts, and to purchase handmade scarves and way too much maple syrup and apple butter. Economy and biology are so often interrelated.

Plastids are the quintescential plant organelles – no plant cell is without them in some form (well O.K., there is one exception, we’ll talk about that next week). But that still doesn’t mean that they define a plant cell. Remember that algae are not plants, but they have chloroplasts, and chloroplasts are one type of plastid. There is even a bigger exception in this area; some of the apicomplexans.

Certain protozoal organisms, including the malaria parasite (Plasmodium falciparum) contain an organelle called an apicoplast. P. facliparum or its ancestor obtained an algae cell by secondary endosymbiosis (the primary endosymbiotic event was the algae taking in a cyanobacterium), so the apicoplast has a four, not two, membrane system.

The apicoplast of the malaria parasite is of plastid

origin, but it undergoes some unplant-like changes

during cell division. Image D with the branched

apicoplast is my favorite. Those in panel F will

grow to look the one in panel A.

The apicoplast does not perform photosynthesis; we aren’t exactly sure what it does – but it is crucial for the survival of the parasite. It is located in the front of the parasite (in the direction it moves and invades cells) and is always close to the nucleus and the mitochondrion. This suggests some role(s) in energy production and molecule synthesis.

There is evidence that the apicoplast works in fatty acid and heme synthesis, like the leucoplast or in the production of ubiquinones that are important for the electron transfer chain in the mitochondria. There is also evidence that it is involved in FeS cluster production, like the hydrogenosome and mitosome. Both of these pieces of evidence show the interelationships of the endosymbiosed organelles and the connection between energy production and energy use. Whatever their functions are, if you destroy or inhibit it the malaria bug dies. As such, it has been a popular target for anti-malarial drugs.

Malaria parasites cured of their apicoplasts (cured means freed of) do not die right away. They just can’t invade any new cells and therefore can’t complete their life cycle. This is why anti-apicoplast drugs may be a boon to malaria treatment. The biosynthetic pathways in the apicoplast are the targets of four recent drugs, but the primary way to stop malaria remains the mosquito net. There is strong hope that a new vaccine, called RTS,S is a light at the end of the tunnel for this killer of millions.

The melanosome and the plastid have more in common.

The very rudimentary eye of some dinoflagellates

(dinos = rotating, and flagellum = whip) has a melanin-like

molecule in the pigment cup and the structure is called a

melanosome. However, it is of plastid orgin. The picture

above is of Polykrikos herdmanae. It has 8 transverse flagella,

as well as the pigmented eyespot to detect light sources.

One final thought on the plastid – an addition to the exception of melanosomes. We discussed a few weeks ago that melanosomes were the only organelles that could move from cell to cell. Well, that isn’t exactly so. I held off on adding the plastid to that list until we had discussed what a plastid was.

A 2012 study at Rutgers University tested whether plastids and mitochondria could move between plant cells. There results showed that entire plastid genomes could be seen in recipient cells, and the fact that the whole chromosome passed indicated that the plastid was probably moving from cell to cell intact. But there was no movement of the mitochondria, so it is a plastid (and melanosome) specific event. The researchers hypothesize that this may be a way for plant cells to repopulate damaged cells with working organelles. As such, it would be similar to how mammalian stem cells can move mitochondria into damaged cells during tissue repair. But that is another story.

We have repeatedly talked about how the mitochondrion and plastid can replicate on their own and then are portioned out to the daughter cells when a parent divides. Can it really be that simple? I’ll bet there is a definite mechanism, and I bet that mechanism has exceptions. Let’s look into this next time.

Gregory Thyssena,Zora Svaba, and Pal Maligaa (2012). Cell-to-cell movement of plastids in plants Proc Natl Acad Sci U S A. , 109 (7) DOI: 10.1073/pnas.1114297109

For more information or classroom activties on plastids, gravitropism, or Plasmodium falciparum see:

Plastids –

http://teachersnetwork.org/ntol/lessons/plantcell/index.htm

http://pubs.acs.org/doi/abs/10.1021/ed074p1176A

http://theclassroomguide.com/category/sixth-grade-science-lessons/

http://sln.fi.edu/qa97/biology/cells/cell4.html

http://www.bcb.uwc.ac.za/Sci_Ed/grade10/cells/plastids.htm

http://www.nature.com/scitable/topicpage/the-origin-of-plastids-14125758

http://www.pnas.org/content/109/7/2439.full

http://library.thinkquest.org/27819/ch3_9.shtml

http://www.sivabio.50webs.com/plastids.htm

http://www2.mcdaniel.edu/Biology/botf99/cellstructure/plastids.html