Why does a plant grow?

FTDM growing plants

Why does a plant grow a new leaf? Or start to flower? Or send out a branch of new roots? What tells a plant to - go?

For centuries these underlying questions have employed plant physiologists; Prior to plant physiology even becoming a field of study. In the latter part of the 19th century Julius von Sachs presented evidence that plants produce ‘organ forming substances.’ That is, there is something inside all plants that moves around, controlling growth and development. 

In the early 1900’s Frits Warmolt Went, a Dutch biologist, demonstrated the existence of one of these substances as part of his dissertation. At the same time Nikolai Cholodny, a Russian microbiologist, came up with the same model. These two scientists are known for finding the plant hormone auxin. 

Auxin is a plant hormone

Hormones are chemical messengers that send instructions from one place within an organism to another. Once at its target, the hormone modulates cellular processes. Plant and animal hormones are not exactly the same. In animals, glands secrete hormones. They are transported through the circulatory system and regulate the animal’s physiology and behavior. In plants, all cells are capable of producing hormones. Transportation happens through the cytosol if the hormone is traveling a short distance, or via the vascular tissue for long distance travel. 

Auxin is one such group of plants hormones. Called the growth hormone, auxin has many roles within a plant. Influencing growth, promoting apical dominance, vascular development, phototropism, gravitropism, prevention of leaf abscission and fruit drop are all a result of auxin. It’s a lot for one group of molecules to do. 

I’m interested in how auxin promotes lateral and adventitious root growth. Lateral roots are those that stick out from the primary root. Advantageous roots are those that develop from non-root tissue, like stems, or leaves.  

At home, I promote adventitious root growth. If a houseplant is getting too big I chop off a stem, stick it in water and wait. The technical term for this is taking a cutting. After a few weeks the submerged stem begins sending out roots. Once they start swirling around the bottom of the jar I pull out the new plant and repot it in soil. This has worked admirably for geraniums, wandering jew, purple heart and basil.

Above - basil cuttings rooted in water at 10 and 20 days after cutting. Since these are stem cuttings, the roots are advantageous. Eventually, they were planted in my garden and rewarded me with lots of pesto!

All these new plants are the result of auxin

(as well as water, light, nutrients, and other hormones of course).

Once in the root of a plant, above the area where root cells are already dividing, elongating and maturing, auxin signals that it is time to send a root out to the side! Stimulated cells start dividing, eventually bursting through the epidermis as the new root grows.

The most prevalent form of auxin is indole-3 acetic acid (IAA). It’s a relatively simple molecule with a negatively charged carboxyl group on one end and a positively charged aromatic ring on the other. Auxin molecules are produced primarily in shoot apical meristems and young leaves. This makes sense since much of the work they do is focused around areas where plants are growing.

Once the auxin molecule was isolated, it wasn’t long before synthetic versions were being produced. Horticultural applications were the primary function, but perhaps not in the way you might think from ‘the growth hormone.’ One major use of auxin is as an herbicide. In high doses auxin stimulates ethylene production, another plant hormone that inhibits plant growth. Ultimately, ethylene causes leaves to fall off and the plant dies. Two synthetic auxin herbicides mixed together, 2,4-D and 2,4,5-T, are more commonly known as Agent Orange. 

Not all synthetic auxins are quite as controversial or dangerous. Indole-3 butaic acid (IBA) is one such auxin. Structurally, IBA is very similar to IAA. Looking at the molecular diagrams of the two you can see IBA has an extra zig and zag. Although IBA originated as solely a synthetic auxin, it has since been isolated in maize and other plant tissues. 

IBA is more effective than IAA at initiating root formation. For the home houseplant enthusiast, rooting powder is available to help root your plant cuttings. The one I am familiar with is a grey powder. You dip the plant cutting into the powder prior to planting. Elevated amounts of auxin around the cutting stimulate advantageous root growth. Faster root formation leads to nutrient uptake, plant growth and new leaves.

Begonias are houseplants that are propagated by cutting on a commercial scale. See the chopped off leaf in the center of the picture? That is the original cutting. Once rooted new leaves grew from it. Photo taken by Claire Collie

Begonias are houseplants that are propagated by cutting on a commercial scale. See the chopped off leaf in the center of the picture? That is the original cutting. Once rooted new leaves grew from it. Photo taken by Claire Collie

Auxin is just one of five classes of plant hormones. Other classes regulate plant growth along with auxin, as well as play roles in bud formation, opening and closing stomata, cell division, seed germination, and a whole host of other plant 'decisions.' 

It sounds easy to fill the world with plants. 

In Kenny’s post many months ago he warned against putting all one’s eggs in the same basket. It doesn’t allow you to plan for problems that might arise in the future. Although he was writing about microbes, the same adage is true of all species. Taking a cutting of a plant is an easy way to propagate new plants, but it is making an exact copy of said plant. If there were future environmental changes or disease pressures all the cloned plants would have the same reaction. On my windowsill this isn’t a big deal, but on a large agricultural scale no genetic variation could be devastating.