Re: How are plants able to respond to external stimuli without a nervous system?

Hi Adam,

Plants have multiple ways of responding to the environment, and even in animals, not all 
responses require that the organism is consciously aware of what is happening.  For example, 
human skin produces more melanin in response to sun exposure, but this is independent of the 
nervous system or being aware of the sunlight…surely blind people tan just as well as the rest of 
us!  As another example, our bodies can respond to warm or cold conditions even when we are 
asleep or otherwise unconscious.

While animals rely on their nervous system to relay information from one part of their body to 
another, plants rely largely on hormones.  These molecules are synthesized at the part of the 
plant that is exposed to an external stimulus, such as light, injury or lack of water, and can travel 
to another part of the plant to relay information.

Let’s consider a few stimuli that plants respond to.

Light:
We’ve all seen plant on a windowsill bend toward the light.  This is an example of phototropism.  
It has long been known that the plant hormone auxin plays an important role in this 
phenomenon.  But how does the plant “see” the light and what controls auxin production?  Plants 
contain pigments known as phytochromes that play an important role in light sensing and 
response.  When they absorb light of a particular wavelength, they change conformation, which 
changes how they interact with the rest of the cell.   For example, in one conformation a 
phytochrome may be located in the cytoplasm, but when in a different conformation it will be 
transferred to the nucleus, where it can affect gene expression and auxin biosynthesis. 

Drought
For the purpose of gas exchange, the leaves of plants contain many pores known as stomata.  
The pores are formed by two crescent shaped guard cells, kind of like this: ( ) 

However, the crescent shape depends on a high turgor pressure within the cells.  When the plant 
has plenty of water, the water content of the guard cells is high, resulting in high turgor 
pressure, maintaining their crescent shape, keeping the pore open.  If the plant is subjected to 
drought, the guard cells lose water and therefore turgor pressure, and lose the crescent shape.  
As a result, the pore closes, kind of like this: ||

This is advantageous to the plant, since closed pores lose less water to the atmosphere by 
evaporation.

In addition, the roots of a plant produce a hormone, abscissic acid (ABA) in response to drought.  
ABA is then transported to the rest of the plant, signaling the other tissues that the roots do not 
have enough water.  Guard cells are very sensitive to ABA, and respond by closing the stomata.

Injury:
When plant tissue is wounded, by chewing insects for example, molecules released from the 
damaged cell walls signal the production of a small polypeptide called systemin.  This molecule 
acts as a hormone, traveling to other parts of the plant and inducing the expression of wound 
response genes.  In this way, the whole plant “knows” when one part of the plant has been 
injured.

Dr. Alex Brands