Plants, too, have hormones. Ooooooooooooooooooh. They control plant growth such as whether they grow to the left or to the right in response to light. Some key hormones are auxin (more specifically indoleacetic acid/IAA), cytokinins and gibberellins. Gibberellin is a fun word, just say it, geeee-berrrrrrr-elllllllll-eeeeen-brbrlrbrlrbbbrlrlrllllllllllll.
Auxin stimulates cell elongation in the roots and shoots in flowering plants. It accumulates away from light which simulates growth on the dark side, bending the shoot in the opposite direction, where the light is.
This ensures plants respond to their environment in a way that best maximises their survival prospects. More light enables better photosynthesis, while growing upwards exposes their leaves to more sunlight. Auxin also suppresses lateral bud growth, so only the apical bud is growing, as it secretes auxin that travels downwards. This is called apical dominance and it ensures that the shoot grows tall rather than wide. If the apical bud is removed, lateral buds sprout.
Cytokinins are made in the roots and travel upwards to promote lateral plant growth. In this way, they’re working opposite to auxin. Overall they strike a balance to drive good growth both upwards and laterally.
Auxin also controls root growth (vertical growth, either shoots above ground or roots in the soil), so these two opposite-direction-travelling hormones affect different types of plant growth.
Gibberellins regulate seed germination, stem elongation, leaf growth, producing pollen and flowering.
Since many of these growth and flowering processes are light-dependent, their development is photomorphogenesis which means development of structure driven by light.
This is achieved via a plant receptor that is light-sensitive called phytochrome. It’s a protein-based molecule with a chromophore group at the centre called bilin. It absorbs light in the red and far-red end of the light spectrum, and hence acts as a detection mechanism which informs the plant of the duration of day and night to control flowering, as well as for the circadian rhythm.
The red (r) and far-red (fr) forms of phytochrome transition between states according to the presence of sunlight. When the far-red, active form becomes abundant, it initiates chemical reactions that start the processes of growth and flowering. The top half of phytochrome is the receptor part sensitive to light, while the bottom, rounded half is the part that catalyses chemical reactions to enact the changes based on the detection of light.