Recently we have had many posts with gorgeous vandaeceous blooms. Everyone noted and mentioned a particular characteristic about these flowers. The flowers keep growing in size even after buds open.
How does this happen? Why does it happen? And does it happen in other orchids or even other plants?? These are all the questions I will try to answer in this article.
All the floral organs are ultimately made up of neatly arranged and organized cells.
The microscopic images are from a Phalaenopsis orchid and by Brian Johnston from Canada
The ‘growth’ we observe is very different from say the normal growth of leaves, roots or developing buds. Growth can be defined as the increase in mass (weight) and volume (size) which also corresponds to the increase in the number of cells. We have all studied mitosis in various details at some or the other point in our academic lives. Basically growth occurs by the multiplication of the cells termed as mitosis. Here is a diagrammatic representation of all the stages in Mitosis to brush up.
In essence a cell enlarges in size, duplicates its DNA, and then divides into two sister cells each with an identical copy of the original DNA. This goes on until the desired growth is achieved. What is important to note is that cell division or mitosis cannot control the shape and size, these have to be regulated my plant hormones and organ identity genes that organize these cells into proper structures. Mitosis can only increase mass and volume, it cannot give the necessary shape without the help from hormones and other factors.
From Both the above images you can see that on duplication it is impossible for the cells to organise into the same square shape as that of the original group of cells, so along with growth in size, there is change of organisation and shape.
This is essentially what happens during tissue culture when cells are allowed to multiply to form callus. Callus is a mass of multiplying cells which is unorganized, i.e. lacking arrangement in the form of organs like roots, leaves, etc. Shown here is an image of tobacco callus from Wikipedia.
Another important aspect is that this organization and division can only occur in meristematic cells. Meristimatic cells are the precursor for all cells. After division and organization by the hormones and regulatory genes the cells differentiate into cells that are meant and programmed for a particular function, in doing so they lose their meristimatic identity. It is just like young students out of school, they can decide pretty much whatever they want to do and study in their lives, but once they choose a path of say medical studies, they can only become a doctor, not an engineer.
When an orchid bud opens, it essentially is completely developed, all the structures are organized, there are petals, column etc and there is no unorganized meristimatic tissue that can multiply and yet orchid flowers grow in terms of mass and volume without losing the integrity of their organization and shape, how is that even possible?
The answer to this question is an event called Endoreduplication. It is a very fancy term for a simple phenomenon. The cells instead of multiplying and increasing in number, they simply increase in size. This provides two benefits the organ (in this case flower) can still grow inspite of not having meristimatic tissue, and still retain the shape and organization. And how do the cells undergo endoreduplication? They simply duplicate their genome or the DNA but do not divide into sister cells. Because they have double the DNA they need to expand and increase in mass and volume. The following picture illustrates the process of Endoreduplication.
As can be seen from the following image Endoreduplication yields increase in size without changing the shape as against growth by mitosis.
This brings in a new variable into play. Polyploidy. Usually cells have two identical copies of their DNA. It is to ensure that the cell has one extra copy in case one of the copy goes bad and cannot function due to a mutation. It is like keeping two sets of pens, an extra one in case the other does not work . So usually cells are diploid (2n). But now after endoreduplication the cells are tetraploid (4n) i.e. the cells have 4 copies of the same DNA. It has its own benefits, I will be writing about polyploidy later.
So does this happen in all orchids or even in other plants. And the answer is yes. It does occur in all orchids studied so far. It might not be as dramatic as observed in Vandaeceous flowers though. It also happens in other plants, an exciting example is that from an apple variety reported here by Peter Hirst from the Purdue University.
Cells in the fruits of Grand Gala apples on the right, undergo endoreduplication, which causes them to grow larger than the regular Gala apples, on the left.
There are many benefits of endoreduplication. Firstly as I mentioned previously it allows an organ to enlarge in size even when it has lost the capacity for multiplication. It also does not need reorganization and patterning of the tissue. Another important thing is that endoreduplication is much more energy efficient. It involves much less protein synthesis and energy expenditure on the part of the cells. And lastly the resulting cells are polyploidy which has many benefits, about which I will talk in another article. I hope you like this article and understand and are in awe of the multitude ways the orchids delight us.