Plant Development

March 28

Developmental Biology, Spring 2016

Migration from water to land

• Life evolved in aquatic environments for 3 billion years

before migrating to land

• Evolutionary pilgrimage of complex organisms onto land

began only 500 million years ago

• Terrestrial colonies founded by plants completely

transformed the biosphere

Adaptation to land

• Advantages

– Access to sunlight

– CO2 as gas

– (initially) few predators/herbivores

• Challenges

– Dessiccate environment

– Mobility

– Transport nutrients

– Supportive structures

– Reproduction

Adaptation to land

• Advantages

– Access to sunlight

– CO2 as gas

– (initially) few predators/herbivores

• Challenges

– Dessiccate environment (waxy leaves, seed coats)

– Mobility (leaf structure, cell shape)

– Transport nutrients (vasculature)

– Supportive structures (roots, shoots)

– Reproduction (embryos, seeds, flowers, dispersal)

Ancestral green algae

(450 million years ago)

Land plants

Vascular plants

Seed plants

Last common ancestor of

animals and plants was a

single-celled organism

The plant body

• Plants have three basic

organs

– Roots

– Stems

– Leaves

• Plant organs are

composed of three tissue

systems

– Dermal

– Vascular

– Ground

Angiosperms

• Derived from Greek: ‘enclosed seed’

• Largest phylum of land plants; >300,000 species

• Vascular seed plants

• Ovule is fertilized

• Ovule develops into a seed inside an enclosed ovary

• Ovary is enclosed inside a flower

• Flower contains the male or female organs, or both

• Flower develops into a fruit

• Gymnosperms (the other phylum of seed-bearing plants)

do not develop their seeds inside an ovary (seeds are on

the surface, e.g. cones)

Life cycle

Pollen

• Male gamete

• Mature pollen contains two cells

• Tube cell: guides pollen

germination and growth of the

pollen tube

• Generative cell: divides to

produce 2 sperm

• One sperm will fertilize the egg

• One sperm will participate in the

formation of the endosperm (a

structure that provides

nourishment for the embryo)

Carpel

• Contains the stigma, style, and

an ovary containing one or more

ovules

• Ovules are attached by a

placenta to the ovary wall

• Fully developed ovules are called

seeds

• Integuments are layers of cells

that enclose the spores

• Pollen tube will grow through the

micropyle

• There are four female

gametophytes inside the

megaspore – one of these is the

egg

Self incompatibility

• Pollination does not

guarantee fertilization

• Interspecific incompatibility

(between species)

• Intraspecific incompatibility

(within species)

• Recognition of self depends

on the self-incompatibility

locus (S locus)

Self incompatibility

Multiple alleles of the S locus dictate incompatibility

Double Fertilization

• The pollen tube enters the embryo sac and two sperm cells are

released

• The ovule has to undergo meiosis and produces 4 haploid

megaspores

• 3 megaspores degenerate, and one haploid megaspore remains

• This megaspore undergoes mitosis to form 8 nuclei (embryo sac)

• Cell walls start to form

• Egg cell and polar nuclei are fertilized

Embryogenesis

• Radial patterning: produces three tissue systems – Ground

– Dermal

– Vascular

• Axial patterning: produces apical-basal axis (root-shoot) – Auxin hormone

– Activates transcription by degrading a repressor of transcription

• Assymetric cell division

• Set aside meristems for post-embryonic development

• Establish accessible food reserve for germinating embryo – Development of cotyledons from the endosperm

Embryogenesis

Dormancy

• At the end of embryogenesis there is a shift from constructing

the body plan to creating a food reserve

• Genes coding for seed storage proteins start to get activated

• Metabolism slows, and the seed desiccates

• A seed coat forms from the integuments

• Gene called viviparous triggers dormancy

• Viviparous mutants act like ferns – goes directly to

postembryonic development

• Abscisic acid (hormone) maintains dormancy

• Gibberelins (hormone) block dormancy

• Seeds can stay dormant for years

Plant development

Embryonic vs. Post-embryonic development

Most animal development is embryonic

Plant development

Embryonic vs. Post-embryonic development

In plants, the mature embryo and young plant have only two leaves

Plant development

Embryonic vs. Post-embryonic development

New organs are made in response to environmental cues

Regeneration and extreme flexibility

The plant body

Where does growth occur?

Meristems: Stem cell population and the source of new plant tissues

Pluripotent

Shoot apical

meristem (SAM)

Leaves,

branches,

flowers

Root apical

meristem (RAM)

Root organs

Shoot apical meristem

Root apical meristem

• Meristem

grows in two

directions

• The meristem

itself doesn’t

give rise to

appendages

(roots)

• Lateral roots

grow out of

mature tissue

Stem cells

Stem cell niche

Specifying fate

Flowering or Inflorescence

• Inflorescence signals a reproductive transition in plants

• Signals from leaves trigger flowering

• In some species this is dependent on photoperiodicity (day length)

• Phytochrome pigments transduce signals from the environment

• Vernalization (period of cold) enhances the flowering signal

• Vegetative to reproductive state

A derivative of a stem cell (yellow) is

displaced to the periphery of the shoot

meristem, becomes part of a floral

meristem and is incorporated into a flower

Transition of Shoot Apical Meristem

from vegetative to floral meristem

Flower

• Specialized shoot with four circles of modified leaves

• Sepals

• Petals

• Stamens (male)

• Carpels (female)

Structure of a flower

Floral identity

• Specification of floral meristems

• Genes like LEAFY, APETALA, CAULIFLOWER are floral

meristem identity genes

Wild type Mutant

Floral identity

Wild-rose

Simple flower with

one row of 5 petals

Hybrid tea rose

A genetic mutant with

(35 or 40 petals)

selected by breeders

• What kind of mutation is it?

• Homeotic mutations

• In a developing flower, the order of each primordium’s emergence determines its fate: sepal, petal, stamen, or carpel

• Plant biologists have identified several organ identity genes (plant homeotic genes) that regulate the development of floral pattern

• These are MADS-box transcription factor genes

• Not HOX genes

• A mutation in a plant organ identity gene can cause abnormal floral development

Floral identity

In a developing flower, the order of each

primordium’s emergence (outer to inner) determines

its fate: sepal, petal, stamen, or carpel

© 2011 Pearson Education, Inc.

Developing

leaves

Shoot Apical Meristem (SAM) Inflorescence

Transition from shoot apical meristem to infloresence

Rule 1. Each gene acts in two whorls

(e.g. A is expressed in whorls 1 & 2)

A A

B B

C C

Three rules of the ABC model

Rule 1. Each gene acts in two whorls

(e.g. A is expressed in whorls 1 & 2)

Rule 2. Combinations of gene products determine organ identity

(e.g. activity A + B specify a petal)

A A

B B

C C

Three rules of the ABC model

Rule 1. Each gene acts in two whorls

(e.g. A is expressed in whorls 1 & 2)

Rule 2. Combinations of gene products determine organ identity

(e.g. activity A + B specify a petal)

Rule 3. A and C activities are mutually exclusive

(e.g. when A is mutated, C is expressed in that whorl)

A A

B B

C C

Three rules of the ABC model

For Rule 3, if the products of gene A and gene C gene have

antagonistic action, will A and C ever be expressed in the same

whorl?

Whorls

Organs

For Rule 3, if the products of gene A and gene C gene have

antagonistic action, will A and C ever be expressed in the same

whorl?

Answer: No

Whorls

Organs

For Rule 3, if the products of gene A and gene C gene have

antagonistic action, will A and C ever be expressed in the same

whorl?

Answer: No

Follow-up question:

If gene A is normally expressed in whorls 1 and 2, which gene

will be expressed in those whorls 1 and 2 in a mutant of gene A?

Whorls

Organs

For Rule 3, if the products of gene A and gene C gene have

antagonistic action, will A and C ever be expressed in the same

whorl?

Answer: No

Follow-up question:

If gene A is normally expressed in whorls 1 and 2, which gene

will be expressed in those whorls 1 and 2 in a mutant of gene A?

Answer: Gene C

Whorls

Organs

What are the organs in a gene A mutant?

Why is it called Apetela?

Whorls

Organs

Gene A is missing

When A is missing, then C is expressed in the whorl where A used to be…

Remember Rule 3 of ABC model

A and C activities are mutually exclusive

(e.g. when A is mutated, C is expressed in that whorl)

C C C C B B

Apetala 2 (no petals) = mutation in Gene A

Wild-type flower

A A B B

C C

What types of floral organs are made in a mutant in gene B?

Whorls

Organs

Apetala 3 and Pistillata (carpel is also called pistil, thus Pistillata)

= Gene B mutant

Wild-type flower

A A B B

C C

A A

C C

What types of floral organs are made in a mutant in gene C?

Why is it called Agamous?

Whorls

Organs

Agamous (no gametes) = Gene C mutant

Wild-type flower

A A B B

C C

A A A A B B

Normal flower

Gene A Apetala 2 mutant (no petals) Only carpels and stamen)

Gene B Apetala 3 and Pistalata mutant (no petals or stamen) Only sepals and carpels

Gene C Agamous mutant (no gametes) Only sepals and petals

What types of floral organs are made in a double mutant of gene B

and gene C?

Whorls

Organs

What types of organs are made in a triple mutant A, B and C?

Whorls

Organs

The triple ABC mutant makes leaves in the 4 whorls

What does this tell you about the relationship between

leaves and flowers?

This result supports the theory that flowers evolved from leaves,

This theory dates to 1790, and was proposed by Goethe the existentialist

Goethe proposed that the different parts of a plant result from the

“metamorphosis (meaning transformation) of a basic organ, the “ideal leaf”

into other more specialized organs.

The forth whorl does not contain carpels

(female organ), but instead contains extra

stamen (male organs) hence the name

superman.

The superman mutant

Follow up question:

What is the normal function of

wild-type Superman gene?

Whorls

Organs

In the superman

mutant

the B gene is

expressed

in 4th whorl, so whorl 4

becomes a stamen

instead of a carpel

Organs

B

stamen

Wild-Type superman

Suppresses B gene in whorl 4

Answer: Wild-type superman gene normally suppresses

gene B activity in 4th whorl

Superman Gene

Fruiting

• Mature ovary

• Helps seed dispersal for reproduction

• Wall of the ovary becomes the pericarp which is the thick

wall of the fruit

Type of fruit Floral origin Example

Simple Single ovary of one

flower

Aggregate Many ovaries of one

flower

Multiple Many ovaries of many

clustered flowers

Midterm exam

Class average: 89

Class average without extra credit: 83

For next class

In-class quiz (10 questions, 15 minutes)

• Plant Development

Paper discussion

Kareem et al. Plethora genes control regeneration

by a two-step mechanism.

There are two supplemental readings to help with

the material

Email two questions about the paper