Biology Exam #2 (Plants)

Why should we care about plants?

• Plants are important for humans

• Plants have proven to be a premier model for the scientific study of diverse aspects of biology

  • Genes (Mendel); Transposons (McClintock)

• Plants solve ecological challenges in really cool ways

• Plants are beautiful

Importance of Plants

• Food (for us and the animals we eat)

• Natural products

  • Building materials, fibers, paper, dyes, flavoring compounds, drugs, preservatives, perfumes, etc…

• Ecosystem services (with other photoautotrophs)

  • Oxygen, ozone, coal (fossilized plants), oil (algal), gas, limestone

• Culture

  • Ornamentals, folklore, natural spaces (can help you de-stress)

What is the story of angiosperms?

Invasion of land

• Began 475 million years ago

• Required specific traits to move onto land

• Other traits evolved very quickly once plants were on land

What made it difficult to invade land?

• Cells may dry out

• Less physical support

• Sperm will have a hard time fertilizing eggs

Conquest of land

• Significant challenges

  • Maintain water in cells

  • Support

  • Sexual reproduction

• Huge advantage

  • Avoids competition for light (no one else was living there)

  • Deeper water = less light = no photosynthesis

• To understand which group invaded land, we can look at groups closely related to land plants (the embryophytes) using a phylogenetic tree

• Traits shared by land plants and their relatives five use clues about what traits were important for moving onto land

Green algae share these homologous traits with land plants

• Cellulose cell wall

• Chlorophyll a & b (same pigments for photosynthesis)

• Sperm with two anterior flagella

Exaptations of Charophyte traits

• Exaptation = process of a trait that an organism already has being used in a different way

  • Ex: dinosaur feather originally for something else, then birds later co-opted them for flight

What features allowed for the common ancestor to transition to living on land?

• Grew in fresh water

• Life cycle that allows dispersal by wind, specifically using spores

  • Adult algae is haploid, some make eggs, some make sperm via mitosis

  • Sperm and egg unite in fertilization and form diploid zygote

  • Zygote undergoes meiosis and creates haploid spores

  • Spores can grow up and become mature organism

• Spores covered in sporopollenin

  • Most stable biopolymer known

  • Extremely decay resistant

  • Dehydration resistant

  • UV protectant

  • Sporopollenin is so decay resistant that scientists couldn’t figure out what it is made of

• Can associate with mycorrhizal fungi – helps get nutrients (extract nutrients from soil and give them to photosynthetic organism, plants give sugars in response = MUTUALISM)

  • Before land plants the earth had no soil

  • Mycorrhizal partners would have been essential for acquiring nutrients

Freshwater charophytes

• Often found growing at edges of small ponds

• Shoreline of these ponds changes with rainfall

• Strong selective pressure to tolerate dry periods

Invasion of land – what did the earliest land plants look like and what were the consequences of their invasion?

• First multicellular plants fossils (~475 Ma)

  • Resemble liverwort thalli

• Radically transformed the land surface

• Many more fossils of early vascular plants ~425 (tetrapods and insects invaded land around this time)

• Adaptation for life on land arose rapidly

How did land plants change the planet?

• Made soil by breaking up rocks and dying (ex: organic material)

  • Compost created soil and breaking of bedrock

    • Plain rock → soil we see today

• Soil created rivers

  • Soil allowed soil to erode more easily, formed riverbed, allowed rivers to form (instead of just sheeting off shoreline)

• Rivers created high nutrient areas in the oceans

Land plant adaptations

arose ~during the invasion of land)

• Cuticle

  • waxy layer on outside of tissue of plant on almost all plants

  • Also inhibits gas exchange

  • stomata/ pores needed

• Pores or stomata

  • Only liverworts have pores (cannot control if open or closed)

  • Stomata = pore that can be adjusted (can open/ close it, ex: during day opens to do photosynthesis, but at night closes so doesn’t lose a lot of water)

• Embryo (land plants = embryophytes)

  • Where does it fit in their lifestyle?

  • Can make 4 spores during zygotic meiosis for each fertilization event (diploid cell makes 4 spores during meiosis)

Alternation of generations – land plants

• Same as before except for next steps

• Zygote undergoes mitosis to produce many sporophytes

• Sporophyte → meiosis → many spores per fertilization

  • Then can produce as many spores as it can supply

The two generations

• Gametophyte (haploid) = makes gametes by mitosis

  • Cannot undergo meiosis

• Sporophyte (diploid) = makes spores by meiosis

• Embryo = a young sporophyte that is nourished by maternal tissue

  • Occurs after zygote undergoes mitosis, produces an embryo, that embryo undergoes mitosis to produce a mature sporophyte

Liverworts, mosses and hornworts (Bryophytes)

• All are very short ~3cm

• Tend to live in moist understories of forests

• Gametophyte is dominant life stage – sporophyte lives on gametophyte and is dependent on it for nourishment

  • Gametophyte dominance

  • Does all the photosynthesis, acquires the nutrients

Liverworts

• ~9000 species

• Do not have stomata – only spores

• What the oldest land plants looked like

Mosses

• ~15000 species

• Have stomata (as do all lineages hereafter)

• Economically important

  • Important in florist trade

    • Holds water well

  • Peat (largely sphagnum moss) forms bogs and is harvested for fuel and whiskey

Hornworts

• ~100 species

• Sporophyte long-lived (longer than in mosses and liverworts)

In all bryophytes…

gametophyte is dominant and the sporophyte grows in and depends on gametophyte for nourishment.

• Ex: If you see a moss, the green moss you see is the gametophyte (haploid) and there sporophytes (diploid) sticking out of it

• Gametophyte provides all nutrients

• Called gametophyte dominance

• Bryophytes do not have a vascular system

Early and basal lineage of land plants were below to the ground

• Over time, sporophytes became larger, more complex, and acquired a vascular system

• Ex: redwood trees are super tall

• Why did plants get taller? Being taller than competitors is important

In order to achieve great heights, plants need…

to transport things within their bodies

• Water needs to be transported from roots to their leaves

• Sugars made in the leaves need to be transported to all other cells – roots and stems

• In the land plants with a vascular system, only the sporophyte generation has one

• No vascular tissue in gametophytes

Why sporophyte dominance?

• Spores, which are dispersed by wind, are dispersed further from greater heights

• Larger, branched sporophytes allow the plant to make more spores per fertilization event

• Gametophytes are constrained by the need for water (and lack a vascular system)

Ferns

• Seedless vascular plants (no seeds)

• 12,000 species

• Gametophyte phase and sporophyte phase grow independently (also true for lycophyte)

  • Sporophyte does its own photosynthesis, acquires own nutrients, disperses spores

  • Gametophyte does its own photosynthesis and acquires own nutrients

• Neither gametophyte dominant not sporophyte dominance

• Gametophyte very small, sporophyte larger

• Gametophyte and sporophyte generation live independently

  • Gametophyte will die after fertilizing for zygote

Gymnosperms

• Have seeds

• About 1000 species, but extremely diverse

• 4 divisions

  • Ginkgoidae (only 1 extant species)

  • Cycadidiae (cycads)

    • Very ancient group

    • One of the most endangered plant groups

    • Biggest threats: climate change and poaching (people want them in their gardens)

  • Gnetidae

    • Mostly tropical group

    • Welwitschia (member of Gnetidae)

  • Pinidae (conifers)

    • Pine trees

    • Ex: Sequoia, Douglas Fir, etc…

Angiosperms

• The most successful land plant group

• ~300,000 species

• Ecologically dominant

• Have flowers

Angiosperms Traits

• Flowers

• ovules/ seeds enclosed in a sporophyll (carpel)

  • Sporophyll = leaf that’s been highly modified for reproduction

  • Spores are no longer dispersed, instead the seed is

• Fruit

• Pollen tubes and double fertilization

Major trends in land plant evolution

• Mosses and other nonvascular plants = gametophyte dominant

• Ferns and other seedless vascular plants = sporophyte independent

• Seed plants (gymnosperms and angiosperms) = sporophyte dominant

Seeds

• Seed is an embryo and its food supply surrounded by a protective coat

• Gymnosperm = naked seed

  • Pollen delivered directly to the ovule

• Ovule = structure of sporophyte in which female gametophyte (including the egg) develops

• Sporophyll = modified leaf specialized for reproduction

• Angiosperm = hidden seed

  • Ovule encased in a sporophyll called a carpel

  • Pollen delivered to carpel tissue

Dispersing with seeds provides some evolutionary advantages over spores

• Seeds remain dormant for days to years, until specific environmental cues trigger conditions are favorable for germination (most spores have shorter lifespan)

• Seeds have a supply of stored food

• They may be transported long distances by animals

• In angiosperms, the maternal plant only invests in fertilized seeds

Review of Evolution of Traits in Plants

1. Alternations of generations (life cycles)

   1. All land plants

2. Stomata

   1. All but liverworts

3. Long-lived sporophyte

   1. hornworts

4. Vascular system

   1. Vascular plants

5. Seeds

   1. Seed plants (gymnosperms and angiosperms)

6. Flowers

   1. Angiosperms

Seeds vs. fruit

• Seeds = fertilized ovules

• Fruit = structures derived from ovary

  • Gymnosperms do not make fruit

Development of female gametophytes in angiosperms

• Female gametophytes = made inside the ovule which is in the ovary

• Carpel = ovary + style + stigma (contains ovules)

• Pistil = 1 or more carpels

• Megaspore mother cell (diploid) undergoes meiosis and produces 4 female spores

  • Only one survives = surviving female spore

• Surviving female spores undergoes mitosis and produces 3 cells, 2 nuclei, 1 egg, and 2 neighboring cells (7 cells with 8 nuclei) → all haploid

• Sporophyte (diploid) undergoes meiosis and produces one haploid daughter spore. THat daughter cell undergoes mitosis to produce haploid gametophytes, which undergoes mitosis to produce game (egg) which is also haploid. Then it will fuse with sperm in fertilization to produce 2n zygote.

Development of male gametophyte in angiosperms

• Male gametophyte = made in anther

• Stamen (anther + filament) = sporophyll with pollen sacs containing pollen. Pollen is the male gametophyte

• The diploid microspore mother cell undergoes meiosis to produce 4 male spores (haploid) and all survive. Each of the spores undergoes mitosis to form tube cell (male gametophyte) which has a generative cell (haploid), which will then undergo mitosis to form 2 sperm

• Sporophyte (diploid) undergoes meiosis and produces four haploid male spore cells. Those spores undergo mitosis to produce haploid gametophytes, which will then undergo mitosis to produce the sperm gamete (haploid), which will fuse with egg in fertilization to form diploid zygote.

Pollination

• A diversity of mechanisms that promote transfer of pollen from a stamen to a stigma

• On an appropriate stigma (species it can fertilize/ have sex with), the pollen will produce a pollen tube

• Pollen tubes grow down the style and find the egg cell

• Ovule produces chemical attractants that attract the pollen tube

Double fertilization

• When pollen grain lands on stigma, it will grow down because of chemical attractant ovule makes. Grows into a hole in the ovule, delivers sperm to egg in ovule, makes zygote.

  • One sperm fuse with egg to produce zygote (2n)

  • Another sperm fuses with polar nuclei to form endosperm nucleus (3n) (2 polar nuclei plus sperm)

• Recap:

  • Egg and sperm = zygote (2n)

  • Polar nuclei and sperm = endosperm (3n)

Endosperm

• Nutritional tissue that feeds growing zygote

• Nutritive tissue that supports the embryo

• Helps it get out of dirt until it can photosynthesize

• In many seeds endosperms disappear

Seedling germination eudicots

Radicle will emerge and begins to grow down, embryonic stem pulls cotyledon up (u shape = eudicots, two cotyledon)

Monocots

Radicle will emerge and so does coleoptile, coleoptile emerges and start photosynthesis (one cotyledon)

Buds - apical and axillary

• Distinction is because of their location on the plant (branch tip or node)

• Both contain meristem tissue (cells are undifferentiated and can form all tissues – can become anything)

• Both produce primary growth

  • Primary growth elongates shoots and roots (in length = makes longer)

  • Primary growth also produces leaves and reproductive structures (ex: flowers, pine cones)

Secondary growth makes things fatter (widens)

Recall Exercise

• Spore becomes gametophyte through mitosis

• Egg becomes zygote through fertilization (fusion with sperm)

• Ovule becomes seed through fertilization

• ovary/ carpel tissue becomes fruit through fertilization

Differences between plant and animal development

1. plant growth is indeterminate, human growth is determinate

   1. Apical and axillary buds have meristem tissues which allow them to keep growing forever until something stops them

2. plant cells’ function is determined by their location

Primary growth

• 3 zones in primary growth start: constant distance behind growing tip

  • All zones stay same distance away from root cap as it moves down (they are grow down)

    • Cells stay where they got made even as zones dragged down

• Zone of cell divisions (including apical meristem): where cells are dividing, some are differentiated, some remain undifferentiated, root tip grows down and zone of cell division moves down with root cap (closest to bottom near root cap)

• Zone of elongation: water moves into cells via osmosis, pushes root cap farther down (middle)

• Zone of differentiation: cells mature, take on functions they will have for the rest of their life (top near surface)

  • Zones do not get bigger (zone of differentiation may be longer)

  • Although we’ve been discussing this is roots, the same processes occur in primary growth of shoots, too

Root elongation zone video

• Zone of cell division: cells same distance apart

• Zone of elongation: cells get farther apart

• Zone of differentiation: cells get farther apart, but not as far as zone of elongation

Primary growth first three tissue types

• Dermal

  • Outside of plant body

• Ground

  • Everything else

  • Any tissue that is not dermal or vascular

• Vascular

  • Xylem = transports water

  • Phyloem = transports sugar

  • Helps transport things throughout plant

Primary root

• Outside = epidermis (cells on outside = dermal tissue)

• Inside epidermis have cortex

• Endodermis = surrounds vascular tissue

• Middle of endodermis = vascular bundle/ tissue

Primary shoots (stems)

• Differences between dicots and monocots

• Dicots:

  • Epidermis

  • Ground tissue connecting pith to cortex

  • Cortex (ground tissue)

  • Pith (ground tissue)

  • Vascular bundle in circle around outside (boundary that separates pith from cortex)

• Monocots

  • Epidermis

  • Vascular bundles are scattered (outside and middle) → no boundary between ground tissue and vascular bundle

  • Just ground tissue (no cortex/ pith)

Leaf

Bundle sheath cell = ground tissue

Secondary growth

• Does not occur in monocots

• Making the plant thicker

• Done by meristems (can differentiate into any plants cells )

  • Takes place in vascular cambium and cork cambium

• Wood is secondary xylem

• Vascular cambium produces wood (largely)

• Cork cambium produces cork that is harvested and put into wine bottles

Vascular cambium

• Cells divide

• If cell differentiates to inside of cambium, it becomes xylem (produced to inside of vascular cambium)

• If cell differentiates to outside of cambium, it becomes phloem (produced to outside of vascular cambium)

• What cell becomes depends on its location

• Produces new tissues in two different locations

  • Oldest cells of secondary xylem on very inside of the cell, oldest cells of secondary phloem on very outside of the plant.

More about primary and secondary growth

• Top of shoot = primary growth only

• Cell divides, one cell remains undifferentiated, one becomes either secondary xylem or phloem

  • What is becomes depends on where it is located

    • To inside = xylem

    • To outside = phloem

• Farther down branch = gets older

  • Primary phloem disappeared = it got crushed because it is soft tissue so got pressed against other cells and crushed

• Increase in girth of woody plants is primarily due to the increase in the production of xylem (wood)

Bark

• Cork, cork cambium, and secondary phloem

  • Inner: pith

  • Then secondary xylem

  • Then vascular cambium

  • Then secondary phloem

  • Then cork cambium

• Ex: you carve your initials 1.5 m above the ground in a 10 meter tall tree. If you return 20 years later and the tree is 25 m tall, your initials will be 1.5 m

• Primary and secondary growth do not happen at the same place at the same time

• Primary and secondary growth can happen at the same time in a plant, but not at the same place in that plant

• Primary growth only happens at tips and roots

Tropisms

• Tropisms are a growth response that results in the curvature of whole plant organs (ex: shoots or roots) toward or away from stimuli due to differential rates of cell elongation

  • Toward = positive tropism

  • Away = negative tropism

• Differences due to elongation on one side vs. other

• Positive phototropism: leaning toward light from window

• Negative phototropism = roots moving away from light in window

Types of tropisms

• Chemotropism = chemical

  • Ex: production of chemical attractants in gametophyte to attract pollen tube (positive)

  • Ex: plant grows away from chemicals in soil (negative)

• Gravitropism = gravity

  • Ex: roots growing down (positive)

  • Ex: cotyledons when they grow up (negative)

• Heliotropism = the sun

  • Ex: leaves of plants growing toward sun (positive)

  • Ex: sunflowers facing same way because before flower bud tracks sun (positive)

• Thigmotropism = touch (something the plant comes into contact with)

  • Ex: stems wrapping around stick in garden (positive)

  • Ex: root hits rock and grows away (negative)

How are stimuli translated into action?

Signal transduction pathway

1. Reception

   1. Hormone or environmental stimulus perceived by receptor

2. Transduction

   1. Receptor changes in some way (ex: structural change) that induces relay proteins and second messengers

3. Response

   1. Activation of cellular response

   2. Plants receive information that leads to pathway that leads to physiological shift

Light

• Growth toward or away from light is known as phototropism

• Growth toward light occurs because cells on dark side elongate more quickly than those on light side

Where is light sensed on phototropism?

• Curvature occurs because cells on dark side elongate more than cells on light side

• Light sensed at tip of plant (not root/ bottom)

What is sensing the light?

• Phototropic = protein that is stimulated by blue light and creates signals for a variety of light-induced processes in plants (including phototropism)

• Cells in the growing tip have lots of phototropin proteins

• In the shade it becomes inactive, and differential activation – some phototropins activated and others not – across the growing tip creates a signal cascade

  • Some phototropins in tip on and others off which leads to signal cascade which leads to differential elongation in zone of elongation

Phototropins are activated by…

blue light

How is that signal transmitted?

• Signal transmitted from one cell to the next

• Cells needed to be able to communicate cell-to-cell

What is the signal transmitted?

• Auxin (indoleacetic acid)

• Auxin is constantly being made while the tip is actively growing and moves down shoot toward roots

• When light is from above, all phototropins are activated and auxin travels down all sides equally

When light is from one side, only lit phototropins are activated and this differential activation alters auxin distribution (only goes down shaded side)

Cell wall

mesh that is made up of many different components, including cellulose microfibrils and cross-linking polysaccharides = help link cellulose microfibrils together, in an aqueous environment

Acid Growth Hypothesis

what auxin is doing to result in elongation in shaded side

1. Auxin travels down stem and binds to a receptor that stimulates a membrane-bound proton pump

   1. Proton pump takes protons from inside cytoplasm and pushes them into the cell wall matrix

2. Cell wall becomes more acidic

3. Low pH activates expansins, which separate polysaccharides from microfibrils (breaks the covalent bond); wall-loosening enzymes cleave polysaccharides, this loosens the cell wall

4. Expansion and enzymes increase cell wall extensibility. Osmosis draws water into cell (because of solute production./ ion uptake) and cell expands because of turgor pressure

   1. Looseness of cell wall allows cell to expand due to water coming into cell

   2. If auxin isn't present, these steps don't happen and cell doesn’t expand (what the lit cell experiences)

Leaf Abscission (Fall Colors)

• Leaves of many temperature woody species change color before falling from the tree or shrub

• The process is instigated by relative changes in hormone concentrations (auxin and ethylene)

• Leaves are responding to changes in light and temperature

• Fall colors are economically important

What happens to get leaves to change colors?

• Three things occur +/- simultaneously to start process

  • Chlorophyll production stops

  • Nutrients and sugars are moved out of leaf to stem/ roots for storage

  • Cells in abscission zone divide, but don’t expand/ elongate

    • Cells get smaller and smaller

• Happen at roughly the same time in response to hormonal changes the leaf is experiencing

• Cell walls are broken down (pieces get degraded and fall apart)

• Cells begin to separate

• Cells below breakpoint suberize their cell walls (reduces water loss) on stem side

  • Wax applied to outside of cells so don’t lose more water

Why do leaves change colors?

• Leaves are green because of chlorophyll

• During growing season, it is continually being produced

• Its production stops in fall

Yellow and orange leaf colors

• Carotenes (orange)

• Xanthaphylls (yellow)

• Carotenoids:

  • aid in light absorption during photosynthesis

  • Are present during the summer, but can’t be seen because of chlorophyll

  • Are always present, so the color of leaves that turn yellow or orange doesn’t vary from year to year

• Lots of species drop yellow and orange leaves

Red leaves

• Some species’ leaves shift from yellow or orange to red

  • Anthocyanins (red) are produced in the leaves after abscission layer forms

  • Not all species make anthocyanins

  • Conditions that favor photosynthesis increase brilliance of reds

    • Ex: fall with lots of moisture in soil, lots of sun, cool days → better reds than bad conditions for plant

  • In species that turn red, leaves will turn yellow or orange and then turn red

  • Reds are not the same from year to year, it depends on the conditions of the fall

Brown leaves

• Brown leaves contain lots of tannins (present all year round)

• Produced to keep insects from eating their leaves

Leaf structure and function

Regardless of shape, primary purpose of all of them is absorption of light and CO2 for photosynthesis

Most leaves are thin

• Diffusion distance within leaf is shorter for CO2

• More light absorbed per investment in leaf tissue

  • Want more surface area per volume (thinner = more light can be absorbed)

Simplified eudicot leaf cross-section

• Top = upper epidermis

• Two layers of mesophyll (palisade first, then spongy)

• Then bottom = lower epidermis (opening is lower epidermis called stoma)

• Mesophyll = parentchyma

Characteristics of mesophyll cells in typical eudicot leaf

Palisade mesophyll

• ~60% of total mesophyll

• 10-40% air volume in layer

• Lots of chloroplasts per cell

• Has more contribution to total photosynthesis (way more than half)

Spongy mesophyll

• ~40% of total mesophyll

• 50-80% air volume in layer

• Fewer chloroplasts

Why do plants have spongy mesophyll?

• All of air spaces in spongy provide pathways for CO2 to diffuse into the leaf

  • Ways for CO2 to get into leaf

  • Air pathways because CO2 diffuses 10,000x more slowly in liquid than in air

    • Air pathways better for diffusion

• Air pathways so important that leaf arrangement with both palisade and spongy results in 8x higher photosynthesis rate than arrangement with two palisade layers (would have too little airspace and CO2 would have to diffuse through water for a longer period of time)

The structure of leaves has evolved to…

maximize absorption of light in the wavelengths that drive photosynthesis

Absorption and reflectance

1. White light (composed of all wavelengths) hits a red box

2. The red box absorbs all wavelengths of light, except red

3. Red light is reflected and that why the box appears red

Based on the color of leaves, what colors of light will drive photosynthesis by green plants ost efficiently?

Red and blue light

Graphs

Top graph:

• Shows absorption spectrum

• How much are the pigments in leaf in chloroplasts absorbing at each of those wavelengths

Bottom graph:

• Shows action spectrum

• Rate of photosynthesis when leaf is exposed to certain wavelengths of light

How does leaf structure maximize absorption of wavelengths that drive photosynthesis?

• When light traveling through air hits water (and vice versa), a small proportion of that light (all colors) is reflected

• Same is true for plant cells

• Some light will reflect off plant

  • Light reflecting back is called “backscatter”

  • The greater number of air-water interfaces in the spongy mesophyll result in more backscatter towards palisade layer

• Light will be absorbed by chlorophyll as it passes through the leaf from above, but most light not absorbed on the first pass, will be reflected back up into leaf

Why is more light reflected when a leaf is illuminated from the bottom vs. the top of a leaf?

The spongy mesophyll has more air-water interfaces that cause more backscatter away from the palisade mesophyll

- ~2.5 times as much light reflects from bottom

Simplification

light is bouncing all over within the leaf, not just at one specific spot

• Even if structure is different, still lots of air-water interfaces

Alternatives to “normal” photosynthesis

• Light = photosynthesis occurring, results in gain in mass

• Dry = no photosynthesis = does not lose mass

• Dark = loses mass = because it is eating the endosperm because it is starving (as it grows, it consumes endosperm and does not replace it)