Apical dominance – when a growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot. So if you break the shoot tip (the source of auxin) off a plant, the plant starts to grow side branches from lateral buds that were previously dormant.
In AS/A-level Biology, auxin is constantly made by cells at the tip of the shoot. It is then transported downwards, from cell-to-cell. This auxin accumulates in the nodes between the lateral buds. Somehow, its presence here inhibits their activity. Two simple experiments provide evidence for this mechanism:
- If we cut the tip off two shoots and apply IAA (synthetic auxin) to one of them, the one with IAA will continue to show apical dominance and the side shoots will not grow. The one without IAA will branch out sideways.
- If a growing shoot is tipped upside down, apical dominance is prevented and the lateral buds start to grow out sideways. This can be explained by the fact that auxin is not transported upwards against gravity, but only downwards. So in the upside-down shoot, the auxin produced in the apical meristem does not reach the lateral buds and therefore cannot affect them.
Gibberellin and Stem Elongation
Gibberellin – a group of plant hormones that stimulate cell elongation, germination and flowering.
In Japan, a plant disease called Bakanae is caused by a fungus and makes rice grow very tall. Attempts to isolate the fungal compounds involved identified a family of compounds called gibberellins. One of these was gibberellic acid (GA3). Scientists began applying GA3 to dwarf varieties of plants (e.g. maize, peas), which made these plants grow taller. These results seem to suggest that gibberellic acid is responsible for plant stem growth, but such a conclusion is too hasty.
In AS/A-level Biology, scientists compared GA1 concentrations of tall pea plants (homozygous for the dominant Le allele), and dwarf pea plants (homozygous for the recessive le allele), which were otherwise genetically identical. They found that plants with higher GA1 concentrations were taller. However, to show that GA1 directly causes stem growth, the researches needed to know how GA1 is formed. They worked out that the Le allele was responsible for producing the enzyme that converted GA20 to GA1.
They also chose a pea plant with a mutation that blocks gibberellin production between ent-Kaurene and GA12-aldehyde. Those plants produce no gibberellin and only grow to about 1cm in height. However, if you graft a shoot onto a homozygous le plant (which cannot convert GA20 to GA1), it grows tall. The shoot has no GA20 of its own, but it has the enzyme to convert GA20 to GA1 - this confirmed that GA1 caused stem elongation. Dwarf varieties of plants lack the dominant allele for an enzyme needed for synthesis of gibberellins.
In AS/A-level Biology, further studies have shown that gibberellins cause growth in the internodes by stimulating cell elongation (by loosening cell walls) and (by stimulating production of a protein that controls the cell cycle). Internodes of dwarf peas have fewer cells and shorter cells than those of tall plants, and mitosis in the intercalary meristems of deep-water rice plants increases with gibberellin treatment.
This is the end of this topic!