Growth in plants Topic 9.3

U: Undifferentiated cells in the meristems of plants allow indeterminate growth Most plants have indeterminate growth = cells divide indefinitely (most animals have determinate growth) Most plant cells are totipotent = ability of a single cell to divide and produce all of the differentiated cells in an organism (only an animal zygote/early embryo is totipotent) Meristem = undifferentiated cells that are dividing (where plant growth takes place) Primary meristems = Root apical meristem = tip of root where growth occurs Shoot apical meristem = tip of stem where growth occurs Secondary meristems = Lateral meristem = cause stem to grow laterally (in diameter); in dicots

U: Mitosis & cell division in the shoot apex provide cells needed for extension of stem & development of leaves

U: Plants hormones control growth in shoot apex Hormone = chemical message produced & released in one part of organism to have an effect in another part Auxins = plant growth hormones IAA = indole-3-acetic acid (type of auxin) Promotes elongation of cells in stems Made in shoot apical meristem & transported down stem to stimulate growth High concentrations can inhibit growth

Axillary buds = side stem Apical dominance = axillary buds closer to shoot tip inhibited by auxin Can be disrupted by pruning shoot tip Cytokinins = hormones produced in root; promote axillary bud growth Gibberellins = hormones that contribute to stem growth

If you want bushier, thicker plants, cut them back!

auxins vs cytokinins

U: Plants respond to environment by tropisms Plant hormones control growth rate and direction of growth Direction influenced by light & gravity Stems grow towards brightest light In absence of light, plants grow up Tropism = directional growth in response to external stimuli (light & gravity) Phototropism = growth towards light Gravitropism = growth in response to gravity

U: Auxin influences cell growth rates by changing the pattern of gene expression 1st stage in phototropism = light absorption by photoreceptors called phototropin (blue-light receptors) Phototropins bind to receptors, which control transcription of genes that code for PIN3 proteins that transport auxin from cell to cell

U: Auxin efflux pumps can set up concentration gradients of auxin in plant tissue Auxin is transported to shady side of stem Cells on shady side elongate, curving stem towards light Leaves attached to stem will receive more light and photosynthesize faster Auxin transported to bottom of cells Auxin inhibits root cell elongation Top cells elongate faster than bottom cells, causing root to bend downward