life11e_ch36

key concept 36.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action

The discovery of two key plant hormones exemplifies the experimental approaches that plant biologists have used to investigate the mechanisms of plant development.

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Gibberellins have multiple roles in plant growth and development.
The first discovered function of auxin related to its role in phototropism.
Polar transport of auxin distributes the hormone down the shoot; lateral transport distributes it from side to side.

Early in the twentieth century, two plant hormones were discovered: gibberellins (there are several active forms) and auxin (there are several forms, but one predominates). Initially, the discoveries came from observations of natural phenomena:

Gibberellins: In rice plants, a disease caused by the fungus Gibberella fujikori resulted in plants that grew overly tall and spindly.
Auxin: Biologists and indoor gardeners noted that seedlings would bend toward the light when placed near a light source.

A chemical substance was then isolated that could cause each phenomenon:

Gibberellic acid (see Table 36.2) made by the G. fujikori fungus caused rice plants to overgrow. Later it was found that plants make gibberellic acid as well, and that applying it to plants caused growth.
Indole-3-acetic acid (see Table 36.2) applied asymmetrically to the growing tips of seedlings caused cell elongation on the side away from the light, which resulted in the shoot bending toward the light.

Finally, mutant plants that do not make each hormone exhibit a phenotype expected in the absence of the hormone, and adding the hormone reverses that phenotype (Figure 36.3):

Tomato plants that do not make gibberellic acid are very short; supplying them with the hormone results in normal growth.
Arabidopsis thaliana individuals that do not make auxin are also short; supplying them with that hormone reverses that phenotype.

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Figure 36.3 Hormones Reverse a Mutant Phenotype (A) The two mutant dwarf tomato plants in this photograph were the same size when the one on the right was treated with a gibberellin solution. (B) The short phenotype of this Arabidopsis mutant was reversed in the plant on the right by supplying auxin.

Note that the phenotype involved—short stature, or dwarfism—is similar in both mutant plants even though different hormones are involved. This observation exemplifies a concept that is important to keep in mind when studying plant hormones: their actions are not unique and specific, as is the case with animal hormones (see Table 36.1).

The approaches outlined above—observation, hormone isolation, hormone treatment, and analysis of mutant plants—are just some of the methods used to identify plant hormones and understand their roles in plant development. Plant biologists have also studied hormones using chemical inhibitors and using plant transformation experiments that alter hormone levels or the plants’ responses to hormones.