On the origin of chloroplasts

Guest post by Joram Schimmeyer

Chloroplasts in plant cells are easily identified under a microscope by their green colour. Image: J. Schimmeyer.

Of all the biological processes found on Earth, photosynthesis could be considered one of the most important. During photosynthesis, the energy from sunlight is used to build up sugars in the cells of plants, algae and some bacteria. These sugars can then be metabolised by the cells or other organisms that feed on them. Also, photosynthesis produces oxygen gas as a by-product, which is needed by most forms of life on earth. Without photosynthesis, life as we know it would not be possible.

In plants and algae, photosynthesis is carried out in tiny compartments inside cells called chloroplasts. This compartment contains a green pigment called chlorophyll, which is used to harvest light energy and is responsible for plants appearing green in colour. Chloroplasts vary greatly in shape and size, but they are all enclosed by two membranes and filled with even more membranes known as the thylakoid membrane system. The key players of photosynthesis are located within these thylakoid membranes; large groups of proteins use the light energy from chlorophyll to convert carbon dioxide form the atmosphere into sugars. The sugars can then be broken down to provide energy to drive growth and other cellular processes.

The chloroplasts in moss cells. Image by Kelvinsong (CC BY 3.0 via Wikipedia).

For a long time, the structure of the chloroplast held some mystery. Although they are found in plant and algae cells, they have several features that are much more similar to bacteria. For example, chloroplasts have their own DNA that is separate to the rest of the cell’s DNA and is circular, like the DNA found in bacteria, Also, new chloroplasts are made by existing chloroplasts dividing, in a similar way to how bacteria cells reproduce (1). These similarities suggest that chloroplasts were originally photosynthetic bacteria.

It is thought that photosynthesis first evolved in a group of bacteria known as cyanobacteria about 2.1 – 2.7 billion years ago. Then, roughly one billion years ago, a single-celled ancestor of plants and algae engulfed a cyanobacterium. Usually this would have resulted in the cyanobacteria being destroyed by the larger cell, but somehow the bacterial cell survived. The two organisms began a symbiotic relationship, with the smaller cell providing sugars from photosynthesis, and the larger cell provided other molecules that the cyanobacterium needed. Over time the two grew more and more attached to each other. They exchanged genetic information and became inseparable so that they essentially became a new type of photosynthetic organism (2 and 3). Over the course of the next billion years this organism evolved into the algae and plants we know today.

Today, some biotechnology techniques take advantage of the biology of chloroplasts. For example, altering the DNA within chloroplasts makes it possible to use the chloroplast as a bio-factory that produces proteins in large quantities. One advantage of this technique is that the altered DNA stays in the chloroplast and is therefore only inherited along the female line. So, the DNA is not distributed via the pollen, which makes it a lot easier to contain genetically modified plants and avoid spreading modified genes in the environment. A review of recent advances in that field can be found here.

The chloroplast not only has an interesting past, it might also guide us to the future.

About the author: Joram Schimmeyer is a PhD student at the Max Planck Institute for Molecular Plant Physiology where he works on biogenesis processes of the photosynthetic machinery. He writes about his travels, photography and other things on his personal blog (link: www.ohyouhere.de).

 

References:

1: Dennis Venema (2013). Evolution Basics: Endosymbiosis and the Origins of Mitochondria and Chloroplasts http://biologos.org/blogs/dennis-venema-letters-to-the-duchess/evolution-basics-endosymbiosis-and-the-origins-of-mitochondria-and-chloroplasts

2: The Evolution of Chloroplasts: endosymbiosis and horizontal gene transfer http://www.growingpassion.org/2010/04/evolution-of-chloroplasts-endosymbiosis.html

3: Gould, S. B., Waller, R. F., & McFadden, G. I. (2008). Plastid evolution. Annual Review of Plant Biology, 59, 491–517. http://doi.org/10.1146/annurev.arplant.59.032607.092915 http://www.annualreviews.org/eprint/ZmYBmyb8ix5AeW2CuMCA/full/10.1146/annurev.arplant.59.032607.092915