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Plant hormones

Auxins and cytokinins

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 and phytochromes

The micropyle was initially the opening through which the pollen entered the ovule for pollination, and now can serve to direct the emerging seedling out into the ground during germination. This can happen after a period of dormancy during which the seedling does not develop. This can help the plant save energy, and only develop in good conditions. Hydration of the seed can kickstart germination.

The plant hormone gibberellin has a role in ending dormancy, hence regulating germination. 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 (P660 and P730) 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.

The periodic removal or non-removal of P730 determines the flowering of short-day plants and long-day plants respectively. Short-day plants flower when the day length is less than 12 hours, as opposed to long-day plants that flower when it is longer i.e. summertime in the northern hemisphere.

This phenomenon can be exploited by growers to artificially control whether a plant flowers. This enables out-of-season plant growth in farming.

Light stimulates the action of gibberellin, which stimulates the breakdown of stored starch into active sugars which enable seedling growth and together with water encourage the continued expression of the relevant protein products in the seedling such as amylase which breaks down starch.

The radicle section of the embryo is first to emerge and start downwards growth, while the plumule continues its upward growth above soil.

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