Let it Go: Abscission

Frozen

Up next in the Frozen series: Plants let it go! Here’s the Disney version in case you’ve been living under a rock for the past several months.

 

The cell walls grow thin in the zone tonight

No chlorophyll to be seen

The leaf is about to fall

Thanks to all the ethylene

In the winter weather those cells would have died

Wouldn’t have mattered how many genes I transcribed!

Turn them red, set them free,

I can’t move I’m just a tree

Breakdown, recycle, not just a show

But, here’s your show!

Let it go, let it go

Not holding on any more

Let it go, let it go

Away they fall, away they soar!

I don’t care

What they’re going to say

Let my limbs lay bare,

The cold never bothered me anyway!

It’s funny how abscission

Makes mincemeat of cell walls

And the genes that once controlled me

Can’t get to my leaves at all!

It’s time to see what I can do

Other organs can shed too

No petals, no flowers for me, I set them free!

Let them go, let them go

Already pollinated, that’s why!

Let them go, let them go

Useless now to me, so let them fly!

Here I stand

And here I’ll stay

I can’t move on!

My fruit flutters through the air onto the ground

My seeds spiraling on the wind all around

The next generation moving forward at last

But I’m never leaving

My roots hold me fast.

Let it go, let it go

It’s really better for me and my spawn

Let it go, let it go

That springtime form is gone!

Here I stand

In the light of day

I can’t move on,

I’m better at biochemistry anyway!

 

 

It’s hard to tell if plants have any inner emotional turmoil like the one Elsa had to fuel this song. However, it is safe to say that no matter how stoic they appear, there’s a lot going on inside them. Since they can’t move, plants have to roll with environmental changes using biochemistry and a flexible developmental program. Plants may not run away to the North Mountain, but they know how to ‘let it go’ when necessary. They do this in a process called abscission, a regulated way of shedding parts of themselves for the good of the plant or the next generation.

One of the most spectacular shows of abscission is the autumnal color change and shedding of leaves in deciduous trees. We call it ‘Fall’ because of the dramatic leaf drop before winter. Even though plants can take some measures to protect themselves from the cold, keeping some tissues over the winter requires too much energy of the plant. The trees do what they can to recycle the valuable contents of the leaf cells, which results in their colorful display. Then they coordinate a way to drop their leaves that doesn’t leave open wounds on the plant.

It’s not just letting loose of leaves before winter, abscission is an important plant process for other organs as well. Plants don’t need to waste their energy on maintaining flower petal tissue after they’ve already been pollinated. If seeds are on their way to forming, no need to keep the lush colorful tissues that can’t photosynthesize enough to support themselves. Further down the line, once fruit has formed to encapsulate the seeds, it must also be let go from the mother plant in order for the next generation to find a hospitable place to germinate. These examples represent normal developmental progessions for plant tissues, but sometimes plants have to improvise. When plant tissues become infected with bacteria, viruses, or fungi, they can kick the abscission process into high gear in the hopes that shedding the infected or damaged parts will prevent the death of the whole plant.

The abscission process creates an area of tissue designated the abscission zone, in which the cells take on distinctive characteristics. These cells are smaller and have an extensive network of endoplasmic reticulum and Golgi membranes with connections to the plasma membrane. The specialized cells of the abscission zone acquire the ability to respond to certain triggers to induce cell separation. These triggers can be a combination of environmental (ex. defense proteins induced upon infection) and internal (ex. the plant hormone ethylene) stimuli. When these triggers are perceived, the cells of the abscission zone up-regulate enzymes that breakdown the cell walls making cell separation easier. Ultimately, the abscission zone cells that remain on the main body of the plant differentiate into a protective layer so there is no open wound on the plant. The abscission process requires the coordinated activity of a large number of genes that must straddle the intersections of developmental pathways and environmental sensory integration.

Plant scientists are still working out the details, but the confluence of so many processes during abscission makes it a difficult problem to attack. However, understanding the abscission process remains a high-priority pursuit in the world of plant science. Agriculturally and horticulturally important plants have been heavily selected with respect to their abscission properties over many generations. In some cases, preventing early abscission may increase yields of certain crops (think- use those leaves longer, don’t drop them). Slowing down or halting the abscission process is also important for aesthetic reasons (keep those flowers and leaves around longer). Facilitating the abscission process may be equally useful and aesthetically pleasing. If plants could be engineered to abscise sooner or more completely, harvests would be easier because fruit would require less force to remove from the plant.

Consider the case of the gardenia- a striking example of a plant that needs to learn to let it go. It is perhaps the most perfect garden shrub- beautiful white blooms that coordinate with any background palette and a lovely scent. However, the blooms past their prime are truly one of the most pitiful sights in the botanical world- shriveled and brown and hanging on for dear life many days longer than anyone would care to look upon them. I’m not sure what inner turmoil is raging within this species, but biochemically, the processes of programmed cell death within the petals and their abscission from the plant are not coordinated in a way that is pleasing to the eye. If there were a gardenia variety that timed these processes more closely with one another, then gardeners would not be forced to look at the crumpled brown blooms.

 

 

Johnna

References and Links:

http://en.wikipedia.org/wiki/Abscission

http://lifeofplant.blogspot.com/2011/03/leaf-abscission.html

http://www.loyno.edu/lucec/natural-history-writings/abscission-reason-why-leaves-fall

http://www.ncbi.nlm.nih.gov/pubmed/24151306

http://www.ncbi.nlm.nih.gov/pubmed/24138310

http://www.ncbi.nlm.nih.gov/pubmed/23265318

http://www.ncbi.nlm.nih.gov/pubmed/12221970

http://www.ncbi.nlm.nih.gov/pubmed/11402180

http://jxb.oxfordjournals.org/content/48/4/821.full.pdf

http://www.plantcell.org/content/15/10/2296.abstract

Advertisements

One thought on “Let it Go: Abscission

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

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s