HOW PLANTS GROW AND RESPOND

Greetings to the entire STEMsocial community. Thanks for the constant support and motivations. I will start today’s post by explaining how plants grow and respond. But before then, I will quickly highlight how some plant hormones were used in the production of biological weapon.

^{Composite image to illustrate the diversity of plants. See below for image and species list. Rkitko, CC BY-SA 4.0}

Biological warfare using plant hormones

One of the most controversial weapons used by the US army in the Vietnam war (1959-1975) wasn’t a bomb or an explosive, but a mixture of two synthetic plant hormones. Named 2,4-D and 2,4,5-T (trust me – you don’t want the full names), these two artificial auxins were given the name Agent Orange after the orange band on the drums they were stored in.

In high concentration, these hormones act as a defoliant – within a few hours leaves start to fall off the broadleafed (dicot) plants. Agent Orange causes plants to outgrow themselves – the rates of respiration and growth of the leaves are so rapid that the stalks can’t bear the weight. This was an attempt to remove the jungle cover from Vietcong guerrillas.

The biggest problem with Agent Orange was that it contained a contaminant – dioxin. This is a particularly nasty chemical that causes birth defects, severe skin problems, leukaemias and other types of cancer. Bot American and Vietnamese people were exposed to dioxin, to the extent that the medical, legal and environmental issues that it caused are still being dealt with till today.

^{U.S. Army Huey helicopter spraying Agent Orange over agricultural land during the Vietnam War. Originally from U.S. Army Operations in Vietnam R.W. Trewyn, Ph.D, Public Domain.}

INTRODUCTION – AN OVERVIEW OF PLANT GROWTH

In this post, I will consider the life story of a flowering plant. First I will look at the different types of plant growth substances, then I will look at their role in the life of a plant.Starting with the seed, I will cover germination, growth, the control of growth and flowering. Then in another post, I will cover plant reproduction until we come full circle to seeds again. Later in this chapter, I will consider the following:

How do seeds germinate? Why do some stay dormant for years?
How do seeds know which way is up, so that roots grow down and stems grow up?
What controls the development of a plant – roots, stem, leaf, flowers and fruits?
How do plants respond to light?
How do plants know when to flower?
What makes the leaves fall in the autumn?

Though plants have no obvious sense organs, muscles or a nervous system, it would be a mistake to think that they are insensitive. Plants respond to their environment in many different ways. They can detect light, gravity, water and other chemicals.

Plants are constantly changing their shape or direction of growth. This movement can result from programmed changes within the plants, e.g. to form flowers, or as a response to an environmental factor such as light or touch.

Growth in plants is different from growth in animals. As you have grown from a small child, all the tissues in your body have grown to some extent, and this has been brought about by cell division (mitosis). In contrast, cell division in plants is restricted to specific areas called meristems, and cells then expand and develop into the different tissues of the plant, such as xylem, phloem and mesophyll. Generally, plants grow and develop by a mixture of cell division, expansion and differentiation. Three factors interact to control the development of a plant: the genes it has inherited; the environment; and plant growth substances.

In the next section I will look at each of the main types of plant growth substances.

PLANT GROWTH SUBSTANCES

The growth and development of plants are controlled by a range of compounds that fall into five main groups: auxins, gibberellins, cytokinins, ethylene and abscisic acid. The first three are families of compounds, while the last two are single, specific compounds. Whether they should be called hormones or plant growth substances is a matter of opinion. The definition of a hormone is ‘a substance that is made by one part of an organism and has an effect on another’. In many cases, the above compounds act as hormones, but in others, they work directly at the site of production. In many cases, we aren’t clear how they work at all, so don’t lose sleep over a name.

^{Lack of the plant hormone auxin can cause abnormal growth (right). William M. Gray, CC BY 2.5}

Generally, the plant growth substances work in combination. Sometimes they work antagonistically, i.e. having opposing effects, and sometimes they work synergistically, i.e. their combined effects are greater than the sum of their individual effects. ‘Two plus two equals fifteen’ is one way to describe synergy.

AUXINS

The first plant growth substances to be investigated were the auxins, starting with work done by Charles Darwin and his son Francis. In 1934, the first auxin to be chemically identified was indoleacetic acid. Since then a variety of related substances have been discovered, including indoles, naphthyls, phenoxyacetic acids and benzoic acids. The defoliants mentioned in the first paragraph at the beginning of this post are phenoxyacetic acids.

Generally, auxins are substances that promote cell elongation and differentiation. They are made in shoot tips, embryos, young leaves, flowers, fruits and pollen. The actual pathway of auxin synthesis is still unknown. Movement of auxin down the plant is polar, i.e. in one particular direction, and results in a concentration gradient from tip to root.

The effects of auxins

Elongation of cells. Auxins stimulate the secretion of hydrogen ions, lowering the pH in and around cell walls. This causes a loosening of the bonds between cellulose fibrils, thus weakening cell walls and allowing the cell to expand in a particular direction.
Gene activation.
Apical dominance. The apical bud produces auxin, which inhibits the growth of lateral buds, so that plants tend to grow tall with minimal branching. If the apical bud is removed, the apical dominance is lifted and more lateral buds become active.
Differentiation of xylem and phloem.
Inhibition of abscission (leaf and fruit
Promotion of fruit development (auxin is released by seeds).
Formation of roots.

From this list of functions it can be seen that artificial auxins are useful in a variety of situations. Examples include: rooting solutions; to encourage fruit development; to inhibit the sprouting of potatoes; and as weedkillers.

GIBBERELLINS

Gibberellins were discovered in 1920 by a group of Japanese scientists who were investigating a disease called ‘foolish seedling’ in which rice plants became tall and pale, and hardly produced any rice. It was found that the seedlings were infected by the fungus Gibberella (now called Fusarium), which was producing a growth-promoting chemical. The active ingredient turned our to be gibberellic acid, and since then about 50 different gibberellins have been isolated.

The principal mode of action of gibberellins is to promote cell elongation. This is seen dramatically in rosette plants such as cabbages and lettuce. These plants have virtually no stem between the leaf nodes, so they appear to be a mass of leaves. If, however, these plants are treated with just a tiny amount of gibberellin, the cells of the stem elongate greatly so that the internodal length (the gap between leaf buds) increases. The overall effect is spectacular.

Gibberellins also have a variety of other functions. They are involved in seed germination and in the production of α-amylase – the enzyme that turns starch into maltose in cell germination. This process is the basis of malting in the brewing industry, so gibberellins are used to speed up the process. They are also used to promote the growth of large seedless grapes.

Interestingly, when gibberellin action is deliberately blocked by synthetic inhibitors, the result is often a sturdy dwarf plant with deep green leaves, greater disease resistance and other desirable characteristics. Many dwarf plants are mutants that are unable to make gibberellin. The exact mode of action of gibberellins is still unclear, but they are produced in tiny amounts, suggesting that they work at a fundamental cellular level, switching genes on and off.

CYTOKININS

Cytokinins were first discovered by two US researchers, Skoog and Miller,when they found that an old sample of DNA from herring sperm would promote cell division in cultures of carrot root tissue. The same effect was seen when the carrot cells were grown in coconut milk, which is actually liquid endosperm. The active ingredient in both cases turned out to be similar to the base adenine, and was given the name kinetin. This turned out to be an artificial cytokinin, because it has never been found in plant tissues. Natural cytokinins identified so far include zeatin (from maize) and isopentenyl adenine.

^{The autumn senescence of Oregon grape leaves is an example of programmed plant senescence. Bureau of Land Management, public domain}

The major effect of cytokinin is to stimulate cell division. When used with auxin, cytokinins can help culture plants. A high cytokinin-to-auxin ratio will stimulate the formation of shoots, buds and leaves, while a low cytokinin-to-auxin ratio will lead to root formation. If you use the two together, you can stimulate the development of whole plantlets from samples of tissue.

ETHENE (OR ETHYLENE)

Ethene is a very simple gas – formula C₂H₄ – whose most widely known role in plants is the ripening of fruit. One of the first clues that ethene could be a plant growth substance came when it was noticed that plants growing near natural gas street lamps abscised (dropped their leaves) earlier than those further away. We now know that ethene, a product of natural gas combustion, was responsible. Overall, ethene promotes ripening and senescence (ageing) of plant tissues.

Ethene is used to control the ripening of fruit – this is the single most important use of a plant hormone in the food business today. The action of ethene is antagonistic to carbon dioxide, so the control of fruit ripening can be reliably inhibited until precisely the right time by using first carbon dioxide and then ethene.

ABSCISIC ACID

The falling of leaves and fruit is called abscission. In the 1960s, two US scientists found a substance that appeared to make the leaves fall off cotton plants. They called this substance abscisin, and when it was isolated and analysed it was given the name abscisic acid. This compound is largely responsible for the fall of leaves in the autumn, and of fruit when ripe.

^{During winter dormancy, plant metabolism comes to a virtual standstill, due in part to low temperatures that slow chemical activity. Bobjgalindo, CC BY-SA 4.0}

Abscisic acid is one compound rather than a class of closely related ones. It is more of a growth inhibitor than a growth promoter, and as such it is antagonistic to the first three classes of growth regulators described above. Abscisic acid has a variety of roles in plant development. In addition to leaf fall, it plays an important part in maintaining seed dormancy and the inhibition of growth during times of physiological stress, e.g. drought. Abscisic acid has also been found to play a role in stomatal closure, where it regulates the opening of anion (negative ion) channels in the guard cell membranes.

In my next post, I will dicuss more on the life of plants; starting with what a seed is, its dormancy, how they germinate, and finally on the movement and growth of plants etc.