Video Transcript
Which of the following tables correctly summarizes the major plant hormones and some
of their roles in a plant?
The question asks us about plant hormones, which are chemical messengers that travel
throughout the plant in order to help it respond to stimuli. Although there are many different hormones in plants, the tables we are given include
just three of the major types. So, let’s have a quick discussion of these.
Auxins are plant hormones that are produced in the tips of roots and shoots. They have many roles, but their main function is to promote tropisms through cell
elongation. Tropisms are directional growth responses toward or away from a stimulus. The most dominant tropism is phototropism, the plant’s movement in response to
light.
When exposed to light, most plant species will accumulate auxin on the shaded side of
a plant shoot. The high concentration of auxin promotes cell elongation in that region of the shoot,
causing the plant to bend towards the light. This is termed positive phototropism, and it allows more light to enter the shoots,
which in turn can increase the plant’s rate of photosynthesis.
Next, let’s review gibberellins. These hormones are first produced by seeds when they are under ideal growth
conditions. For most plant species, this means warm, wet, well-oxygenated soil.
Gibberellins break seed dormancy and stimulate germination by helping the starchy
food stores in the seed to break down into simple sugars and amino acids. The sugars allow the cells in the plant embryo to initiate cellular respiration, and
the amino acids allow them to build proteins, leading to germination. The energy from respiration and the proteins from amino acids allow roots to begin
growing and the stem to reach above the soil so photosynthesis can occur.
Gibberellins continue to play a role throughout the plant’s life by encouraging stem
elongation and cell division so that the plant grows taller.
The fruit of a plant, such as the unripe strawberry shown here, contains its
seeds. In order for its genes to be passed down to a new generation, a plant must ensure
that its seeds are dispersed. Ethylene is a gaseous hormone that promotes the ripening of fruit. Ripe fruit is more likely to be eaten by animals, which increases the chances of seed
dispersal.
Now that we have discussed the three plant hormones in the tables, let’s check which
one correctly summarizes their roles.
Auxins are not involved in seed germination and do not directly promote fruit
ripening, so we can rule out tables (A) and (C). We can see that tables (B) and (D) both give the correct description of auxins as
promoters of cell elongation and tropisms. However, table (B) gives incorrect descriptions for the role of both gibberellins and
ethylene.
The correct summary of the roles played by these plant hormones is found in table
(D). Auxins promote cell elongation and mediate positive phototropism in shoots,
gibberellins promote stem elongation and initiate seed germination, and ethylene
promotes fruit ripening.
