Do you want to make a plastid?

FrozenNext up in the Frozen series… Do you want to make a plastid? Here’s the snowman version from Frozen for your reference.

Plant cells…

Do you want to make a plastid?

Come on, differentiate

Time to photosynthesize

Special things to metabolize

C’mon they need a fate

Those used to be proplastids

And now they’re not

We hope genetics tells us why!

Do you want to make a plastid?

It doesn’t have to be a chloroplast

Do you want to make a plastid?

Some grana stacks green and tall?

I think some development is overdue…

Environment gives you cues

From the nucleus transcription calls!

(Induce there GLKs)

It gets a bit confusing

All these proteins whizzing by



Chromoplasts get lots of color

With all these carotenoids

Amyloplasts store starches

For all the tissues underground!

Do you want to make a plastid?

It could even be a tannosome

Yes, etioplasts are in there

In the dark since who knows when

Shine the light to start,

The nucleus is there for them,

No longer inside meristems,

Time for some changes to begin!

They do so many functions,


What can’t plastids do?

Do you want to make a plastid?

Well, if you are a plant cell, the answer to that refrain is always, “Yes!” Plastids are specialized organelles in plants that perform a wide array of specialized biochemistry. The most well-known and recognized of these are chloroplasts. These subcellular compartments are packed full of the thylakoid membranes and enzymes necessary to convert light into sugars. Chloroplasts perform other important biochemical reactions including the synthesis of starches and fatty acids as well as the assimilation of nitrogen into amino acids.

The chloroplast by User:Miguelsierra, adapted by User:Vossman via Wikipedia

There are other types of plastids made in plant cells for other specialized purposes. There are chromoplasts, which are full of carotenoid and other pigments giving them distinctive colors. The cells of tomato flesh, carrots and rose petals contain this type of plastid. Of course, pigments don’t do much good in root tissues. You may not find chloroplasts or chromoplasts there, but they do contain leucoplasts (loosely defined as non-green plastids used for storage of starches and oils). Storage-type plastids aren’t relegated to the roots. Recently other types of plastids have been identified, which specialize in the synthesis and storage of distinctive metabolites. Tannosomes have recently been characterized as specialists in tannin storage. A phenyloplast was also recently described in vanilla fruit as the storage location for vanillin precursor metabolites. In fact, the range of biochemical specialization that plastids perform strains their traditional nomenclature and definitions. As scientists learn more about them, the divisions between traditional types seems to blur. It seems that plants can make them as specialized as they need to be along a gradient of biochemical functions.

Types of plastids by Mariana Ruiz Villarreal LadyofHats via Wikipedia

Plant cells have definitely capitalized on the versatility of the plastid organelle structure to ensure dedicated locations for all manner of biochemical reactions and/or storage of their products. This is no small task. It requires the coordination of genes in two different genomes (that of the plant cell nucleus as well as a small genome in the plastid), trafficking of proteins throughout the cell, sensing of environmental cues, and synchronization with the overall plant developmental program. It’s enough to make any project manager run out of color-coded post-its and lose track of scheduled task times. Here are the highlights:

First of all, plastids do not exist in their mature forms in plant seeds and developing meristem tissues, but as immature and developmentally-flexible proplastids. These proplastids may not be special biochemists in their own right, but they have the potential to become any kind of plastid the plant cell needs. Consequently, proplastids have to know whether they are inside a root cell, petal cell or leaf cell. This means that plastid differentiation (aka development to maturity) must be coordinated with the development of the rest of the plant. Scientists are beginning to learn more about how this works, but this research is difficult since we’re talking about tiny, unremarkable pro-organelles within small portions of tissue. It’s a universal rule of cell biology that studying on small things is much more difficult than studying large things. Genetics has provided some new insights as to which genes have control over this developmental process. Transcription factors like the Golden-like (GLK) family of proteins have been identified as major players in this process, turning on suites of other genes to trigger proplastid development into mature plastids.

Of course, a lot of plant cell and plastid development is tied to environmental cues like light intensity. For example, plants can develop up to a point in darkness, but that doesn’t mean those cells have the same mature chloroplasts present in light-grown plants. Dark-grown plant cells contain plastids termed etioplasts, which are somewhat more differentiated than proplastids but not quite chloroplasts. Upon illumination, etioplasts quickly whip up thylakoid membranes and the photosynthetic machinery and get to the work of photosynthesis.

Plastid development and maintenance requires a large protein migration within plant cells. While plastid genomes make a number of their proteins, they do not make all of their resident proteins. Many essential proteins must come from genes transcribed in the plant cell nucleus and translated in the cytoplasm or endoplasmic reticulum. Thus, plant cells have an elaborate system of trafficking proteins to the proper cellular destination. This is true for all kinds of eukaryotic cells, but plant cells have extra organelles to distinguish among relative to animal cells. For those proteins with final destinations in the chloroplast, they have signals within their sequences that target them there. When they arrive at the chloroplast, they must pass through the surrounding double membranes using the Translocon of Outer Chloroplast Membrane (TOC) and the Translocon of Inner Chloroplast Membrane (TIC). In addition to the large influx of proteins that must be deposited upon initial development, the nucleus continues to provide essential proteins to the chloroplasts throughout their lifetime. It’s a constant flurry of activity maintained by two-way communication between the nucleus and chloroplast such that each organelle knows the biochemical status of the other.

Development into a mature plastid type isn’t the end of the line. There is still some developmental plasticity allowing certain plastids to convert into other plastid types. I’ve mentioned before that chloroplasts convert to chromoplasts in ripening fruit, as can be seen in the characteristic color change of green, unripe fruits into bright yellows, oranges and reds. Chloroplasts also provide the starting point for the development of other plastids specializing in storage of certain biomolecules. Plant cells also have ways of recycling the valuable contents of their plastids when cells and tissues reach the end of their lives (think autumn leaf color-change). As you might expect, this process requires more changes in gene transcription, protein synthesis and membrane structure orchestrated by the cell nucleus in response to more environmental and developmental cues.

Finally, proplastids and plastids of all types have ways of replicating themselves within plant cells. In this way, as plant cells divide each new daughter cell will contain roughly equal numbers of plastids. It wouldn’t be very good if a certain cell line within a tissue ran out of plastids. As much as plastids rely on the nucleus to full development and function, the nucleus cannot spontaneously create plastids from nothing. This wrinkle adds another layer of complexity to plastid biology. It means plastids must have some way of dividing themselves within the cells, and when plant cells divide, they must have some way of equally segregating the multiple plastids between the two new cells.

Basically, plastids are a lot of work for plant cells to make and maintain. However, these high-maintenance organelles are worth it given the biochemical rewards they offer. Scientists are only beginning to understand how plants make, maintain and recycle their plastids. Pieces of the complex system are studied by plant biologists all over the world and will likely keep them busy for many years to come.




References and Links:


One thought on “Do you want to make a plastid?

  1. Pingback: Frozen: A Plant Science Parody | New Under The Sun Blog

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