Plants Playing Hard to Get

By: Jason Johns

We all know the negative effects of inbreeding, so there's no need for you to read about the dull details here. Bottom line...it causes major birth defects and reduces genetic variability. Both of these are generally considered to be BAD things. If you don't belong to this school of thought, I don't want to meet you.

As organisms with the ability to move around, most of us humans have been given the gift of choice with our mates. Stop for 10 seconds and think about it really quick...if you were stuck in the same spot for your entire life, without the ability to move any part of yourself:

How would you eat? 

How would you drink? 

More importantly, how would you...you know...DO IT?

Conveniently as humans, we do have the ability to move, which contributes to our ability to choose a mate. Our conscious allows us to make decisions such as, "Hey...I'm related to that person. I'm not going to have sex with them." It may seem obvious, but think about plants. They can't move at all. So how do they choose who they will share genetic material with?


Quick plant reproduction rundown:
- pollen = plant semen
- stigma = female reproductive organ

Pollen lands on the stigma and makes itself a tube down the style to transport its genetic material down to the plant's ovary (yep, plants call them ovaries too) where it can fertilize an egg and make a new plant baby (a seed)
* see diagram on the right

Here are some ways that plants could get their own pollen on their own stigma (A.K.A. have sex with themselves):

1. Their flowers could have both male and female productive parts. An insect could be crawling around inside a flower and transport some pollen to the stigma. Guess what...that plant just had sex with itself.

2. The same insect that still has pollen on it could crawl to a whole other flower on the same plant and deposit some of this pollen onto the stigma of the new flower. Whoops...still inbreeding.

3. They could have male parts and female parts on different flowers, but on the same plant. Sophisticated term-loving botanists would call this plant monoecious. An insect crawling around on one of these plants could first visit a male flower, get pollen all over itself, then crawl over to a female flower and accidentally deposit some of the pollen from the same plant onto the stigma. Ewww...it's having sex with itself again.

You get the point. If not, read it again.

Assuming plants can't somehow stop the bees from facilitating inbreeding, we would assume they have no choice in the matter. If we did that, however, we would be making an a** out of you and me...twice.

Remember, plants have been surviving on this planet longer than any other organism without the ability to move...they're probably some pretty creative sons of guns. OK, if you're still with me, take another sip of coffee and buckle up for a little science. We're about to learn how plants recognize their own pollen and basically zap the thing so that it doesn't make an inbred mutant.

Wait...seriously? Plants can do that? They don't have brains...or 9mm's.

Let's get a quick science lesson first. I promise it will be fun.

When genetic material (DNA) from the male meets genetic material from the female, a cascade of chemical reactions occur that lead to the pollen tube and the eventual fertilization of an egg, which leads to a new plant baby (illustrated above). This is the way it works in many plants, regardless of where the pollen comes from.

However, plants that have a self incompatibility mechanism built-in are able to reject their own pollen. SERIOUSLY. When the male DNA meets the female DNA (from the same species), they shake hands and match up their genes. Ok here comes another science word...allele. An allele is just a variation of a gene. For example, as humans we have genes on our DNA that give us eyes, but the specific makeup of our alleles determine whether these eyes will be blue, brown, green, etc.

The alleles in the diagram below illustrate an occurrence know as heterozygosity. If you've never heard that word, don't be scared of it, learn it. It just means that the two alleles it has for a certain gene (one from mom, one from dad) are different. This leads to genetic variation, which is a good thing. The alleles on this diagram code for flower color. This is a nice and simple example, but what we are interested in are the alleles that code for self incompatibility. 

Geneticists have found the alleles that code for these self incompatibility genes and have named them the S-loci. When the two alleles that match up at this locus are different, it's all gravy. The plant will start making a pollen tube so that it can reach those eggs for a little fertilización. However, if these alleles happen to match up, genes that code for the self incompatibility mechanism are turned on. Hold that thought...
                                         
Cells use DNA to make RNA to make proteins that make everything. We can think about it that simply for now. So, when pollen DNA matches up with stigma DNA, the pollen cells make RNA that will eventually make the proteins necessary to make a pollen tube that will allow the pollen to fertilize the egg and make the baby (a seed).


















If you haven't taken any science (other than from Morgan Freeman) in the last 5 years read those last few paragraphs again.


Back to self incompatibility. As I was saying, when the male DNA meets the female DNA they match up their genes. These genes are expected to match up because they're the same species. However, in some extra creative plants, if the male/female alleles match up, they initiate the SI defense mechanism.

This diagram shows the inability of 'self' pollen to grow a pollen tube,
while 'cross' pollen is able to do so successfully

In plants with an SI mechanism, when certain alleles match up, some of the cells in the female reproductive part of the flower make proteins (called S-RNases) that essentially target the "self" pollen and blast the RNAs that the pollen is making to try and make itself a tube to go fertilize a seed. When these RNAs get degraded, the proteins necessary to make a pollen tube can no longer be synthesized, and the plant has successfully prevented inbreeding. (If you want a much more scientific and thorough explanation of this process, see the papers referenced at the end of the post.)


See, plants are more than just pretty petals and healthy vitamins. They are complex, creative, sultry machines programmed to survive and thrive in one spot for their entire lives. And that's just the way they like it.

     

References
Silva, N. D. Goring. 2001. Mechanisms of self-incompatibility in flowering plants. Cellular and Molecular Life Sciences 58:53-67.
Takayama, S., A. Isogai. 2005. Self-Incompatibility in Plants. Annual Review Plant Biology 56:467-87.
"Self-Incompatibility: Avoiding Inbreeding." Self-Incompatibility. N.p., n.d. Web. 19 Jan. 2015.

Photo Credits
http://www.demotivationalposterz.com/2010/11/inbreeding.html
http://guildwars.incgamers.com/forums/showthread.php?489598-Does-Animal-Inbreeding-constitute-Animal-Cruelty
http://vicvapor.com/desert-flower-new-hd-wallpapers-2014

http://leelasbiopage.com/Xnotes.html

http://www.cell.com/trends/genetics/fulltext/S0168-9525(01)02613-0

http://followpics.me/hipster-scientists-take-on-the-central-dogma-of-molecular-biology/

http://www.lhsc.on.ca/Patients_Families_Visitors/Genetics/Inherited_Metabolic/Mitochondria/DiseasesattheMolecularLevel.htm