Plant Growth and Reproduction

Chapter 9.3 and 9.4

Growth in Plants

• Undifferentiated cells in the meristems of plants allow indeterminate growth– Determinate: growth stops when a certain size is

reached– Indeterminate: when cells continue to divide

indefinitely• Most differentiated cells are totipotent as

well; they have the capacity to generate whole plants

Meristems

• The tissue in most plants containing undifferentiated cells (meristematic cells), found in zones of the plant where growth can take place.

• Primary meristems: found at the tip of stems and roots (apical meristems)- elongation– Root apical meristem- control root growth– Shoot apical meristems- control stem growth

• Secondary meristems: growth that increases diameter (lateral meristems)

Apical Meristem

Shoot Meristem Root Meristem

Role of Mitosis

• Cells in meristems undergo division constantly via mitosis and cytokinesis

• Shoot meristem– Throws off cells needed for growth of the stem– Produces groups of cells that grow into leaves and flowers– With each division a cell remains in the meristem while

others increase in size, differentiates and are pushed from the meristem region

• Each apical meristem can give rise to different tissues– Protoderm= epidermis– Procambium= vascular tissue

Plant hormones

• Plant hormones control growth in the shoot apex• A hormone is a chemical message send from one

region that can alter activity in another region• Auxins – a group of hormones that control growth

in roots, fruits and leaves– The most abundant auxin is indole-3-acetic acid (IAA)– IAA promotes elongation of cells in stems– At high concentrations it inhibits growth

Axillary Buds

• Axillary buds are shoots that form at the junction, or node, of the stem and the base of a leaf

• As the shoot apical meristem grows and forms leaves, regions of the meristem are left behind at the node

• Growth at nodes is inhibited by auxin produced by shoot apical meristem (apical dominance)– Further from a node lower auxin levels– Cytokinins produced in the root promote bud growth– The relative ration of cytokinins and auxins determine if

buds will develop

Experiment

Plant Movement

• Plants can adapt to their environment by changing orientation

• Tropism– Movement is in a direction either toward or away

from a stimulus (e.g. phototropism, gravitropism)• Nastic Movements

– Movements that occur in response to environmental stimuli

– the direction of the response is not dependent on the direction of the stimulus.

Gene Expression

• Auxin influences gene expression, which can influence growth rates

• Phototropism starts by the protein phototropin absorbing light, causing a conformational change

• They then bind to receptors within the cell, which control transcription of genes for glycoproteins (PIN3) that transport auxin cell to cell

Intracellular pumps

• Auxin efflux pumps can set up concentration gradients of auxin

• If phototropins in the tip detect a greater intensity of light on one side of the stem compared to the other, auxin is transported laterally to the shaded side

• Higher concentrations on shadier side increase growth there, so the stem curves toward the light

Phototropism

Gravitropism

• Also is auxin dependent• The upward growth of shoots and downward growth of

roots is due to gravity• If roots are placed on their side, gravity causes

organelles called statholiths to accumulate on the lower side of cells

• This leads PIN3 transporter protein to move Auxin downward

• High auxin levels in this case reduce root cell growth, so the tops elongate and the root bends downward

• NOTE: AUXIN ACTIONS ARE OPPOSITE IN ROOTS AND SHOOTS

Gravitropism

9.4 REPRODUCTION

Angiosperms

• Angiosperms are flowering plants, They are divided into;

• Dicotyledons (dicots):– A flowering plant (angiosperms) that has a seed

with two embryonic leaves or cotyledons.• Monocotyledons (monocots)

– A flowering plant (angiosperms) that has a seed with one embryonic leaf or cotyledons.

Monocots vs. Dicots

Monocots Dicots (Eudicots)

Embryo with single cotyledon Embryo with two cotyledons

Pollen with single furrow or pore Pollen with three furrows or pores

Flower parts in multiples of three Flower parts in multiples of four or five

Major leaf veins parallel Major leaf veins branched

Stem vascular bundles scattered Stem vascular bundles in a ring

New roots are adventitious (stem) New roots develop from radicle ( pre-existing roots)

Terms

• Adventitious root- a root that develops from somewhere other than the root apical meristem

• Cotyledon- A cotyledon is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling.

• Radicle – part of root meristem

Root Structure

Stem Structure

Monocot or dicot?

Reproductive structures

Male parts:StamenAnther

Filament

Female Parts:Pistil

StigmaStyleOvaryOvule

Other:PetalSepal

Dicotyledonous Flower Parts

FLOWER PART FUNCTION

Sepals Protect the developing flower while in the bud

Petals Modified leaves; often colourful to attract pollinators

Stamen The “male” reproductive structure; made of anther and filament

Anther Produces and releases pollen

Filament Stalk of stamen that holds up anther

Carpel The “female” reproductive structure; made of ovary, style, and stigma

Pistil Can refer to a single carpel or a group of fused carpels

Stigma Sticky top of carpel which pollen lands on

Style Supports and holds up the style; gives the stigma exposure to pollen

ovary Base of carpel in which the female sex cells develop; if fertilization occurs, it will turn into a protective fruit

Ovules Found in the ovary; contain female sex cells, eggs

Pollen Contain male sex cells (sperm). Pollen grain is one of the granular microspores that occur in pollen and give rise to the male gametophyte of a seed plant

Flowering

• Flowers are reproductive structures and are produced by the shoot apical meristem

• Flowering involves a change in gene expression in the shoot apex

• Vegetative Phase: When a seed germinates a young plant is formed that grows roots, stems and leaves

• Reproductive Phase: when meristems start to produce flowers instead of leaves

Factors affecting Flowering

• Temperature- limited effect and varies based on plant• Day length (length of darkness)- greatest effect

– Short day plants: flower when darkness lengths increase– Long day plants: flower when there is decreased length of

darkness• Light- can inhibit or activate genes controlling flowering

– Long day plants; the active form of the pigment phytochrome leads to the transcription of the flowering time gene (FT)

• Length of darkness is the trigger NOT length of daylight

PLANT TYPE FLOWERING AND LIGHT EXAMPLES

LONG-DAY PLANTS Bloom when days are longest and nights the shortest (midsummer)

- Require Pfr

Radishes, spinach, lettuce

SHORT-DAY PLANTS Bloom in spring, late summer, and autumn when days are shorter

- Inhibited by Pfr

Poinsettias, chrysanthemums, asters

DAY-NEUTRAL PLANTS

Flower without regard to day length

Roses, dandelions, tomatoes

Phytochrome

• Phytochrome is a photoreceptor and a pigment

• It absorbs light• There are 2 forms of phytochrome

• Pr – absorbs red light • Pfr – absorbs far-red light/darkness

Far-Red Light

• Wavelengths between 700-800nm• At the far end of the visible light spectrum• (Between red light and infrared light)

• During the day, when there is light, red light (wavelength of 660nm) is present

• Pr absorbs red light and is rapidly converted into Pfr

• At the end of the day, after many hours of light, plants will have most of their phytochrome in the form of Pfr

• During the night, when there isn’t light (therefore no red light), Pfr is slowly converted back into Pr

• By morning, most of the phytochrome will be Pr

again

• If there is even a flash of light interrupting the darkness during the night, it will disrupt the process of Pfr turning into Pr

Pr Pfr

_____________________________________________________________

Pfr Pr

• Long-day plants, require Pfr to flower

• Long day = short night!

• At the end of a short night, there will still be lots of Pfr remaining.

• The remaining Pfr at the end of a short night stimulates the plant to flower.

• In short-day plants, the Pfr acts as an inhibitor for flowering.

• So after a short night, the remaining Pfr will prevent the plant from flowering.

• If it was a long night, all the phytochrome will

be in the form of Pr (there will be no Pfr) so flowering CAN occur.

Pollination

• The process in which pollen (which contains the male sex cells –sperm) is placed on the female stigma.

• Can occur via a variety of vectors– Wind– water– Insects– Birds– Bats

• Angiosperms and their pollinators have coevolved (supported by fossil evidence)

• The flowers colours, patterns, odours, shapes and even the time of day it blooms are designed to attract a specific pollinator

• Often, the flower provides the gift of food to the pollinator in exchange for the pollinator unintentionally transporting pollen to the stigma

Mutualism

• Mutualism- a close relationship between two species where both species benefit

• Most flowering plants use a mutualistic relationship with pollinators in sexual reproduction

• Pollinators gain food (nectar), while the plants pollen is distributed to another plant.

Types of pollination

• Self Pollination – when pollen from the anther of a plant falls on its own stigma– A form of inbreeding – thus less genetic variation

• Cross Pollination – pollen lands on the stigma of a different plant.– Increases variation and offspring with different

fitness

Fertilization

• When the male and female sex cells unite to form a diploid zygote.

• The female sex cells are in the ovules. The sperm from the pollen that has attached itself to the stigma must make its way to the ovules in the ovary.

• Pollen attaches to stigma and begins to grow a pollen tube through the style

• Within the growing pollen tube is the nucleus that will produce the sperm.

• The pollen tube completes growing by entering an opening at the bottom of the ovary

• The sperm moves from the tube to combine with the egg of ovule to form a zygote.

Pollen tube

The Seed and Seed Dispersal

• Once the zygote is formed, it develops with the surrounding tissue into the seed

• As the seed is developing, the ovary around the ovule matures into a fruit

• Seed dispersal can be aided by water, wind, animals

• Reduces competition between offspring and parent and helps spread the species

SEED

• Is the means by which an embryo can be dispersed into to distant locations.

• It is a protective structure for the embryo

Seed Part Function

testa Tough, protective outer coat

cotyledons Seed leaves that function as nutrient storage structures

microphyle Scar of the opening where the pollen tube entered the ovule

Embryo root and embryo shoot

Become the new plant when germination occurs

Pre-Germination• Once seeds are formed, a maturation process

follows.• The seed dehydrates until the water content of

the seeds is about 10 -15% of its weight.• At this point, the seed goes into a dormant

period where there is low metabolism and no growth or development.

• Duration is variable for different types of seed• It is an adaptation to environmental conditions

Germination Conditions

• If conditions become favourable, the seed will be germinated.

• GERMINATION – is the development of the seed into a functional plant.

• There are several conditions that must be fulfilled for a seed to germinate.

Germination Conditions

• Water- rehydrates dried seeds, swells the seeds/cracks the seed allowing hydrolytic enzymes to be activated

• Oxygen- needed to perform cellular respiration for growth

• Temperature- important for enzyme activity for growth. Ensures plants don’t germinate in winter (seedlings are fragile).

Metabolic Processes during Germination of a Starchy Seed

1. Seed absorbs water (which leads to many metabolic changes)

2. Gibberellin is released after the uptake of water

Gibberellin – plant growth hormone

3. Gibberellin triggers the release of the enzyme amylase

4. Amylase causes the hydrolysis of the starch into maltose

5. Maltose is hydrolyzed into glucose which can be used for cellular respiration or converted into cellulose to build cell walls for new cells

6. Stored proteins and lipids will also be hydrolyzed to make proteins/enzymes and phospholipids and energy metabolism.

• Germination uses the food stored in cotyledons to grown until it reaches light when it starts to photosynthesize