Plant Morphology and Physiology Midterm 1

Lectures 1-4

Basic Plant Organs

stems, roots, leaves

Simple Primary Plant Tissues

parenchyma, collenchyma, sclerenchyma, root endodermis, pericycle

Specialized Plant Cells

parenchyma cells, collenchyma cells, fibres, sclereids, endodermal cells, pericycle cells

parenchyma 

• thin primary cell wall (usually undignified), nucleolus, cytoplasm, and one (or more) vacuoles 

• often intracellular spaces, shape varies, aerenchyma

• primary wall usually stains pink with toluene blue O

• photosynthesis storage, pith, local transport

• can undergo certain cell deaths

collenchyma 

• differentiate from parenchyma (alive at maturity)

• thickened primary cell wall (usually not lignified)

• primary wall stain darker pinky-purple with toluene blue O

• support, protection

sclerenchyma

• fibres and sclerosis, normally dead at maturity

• thickened primary and secondary cell walls (usually lignified)

• cell wall usually stains blue/blue green with Toludine blue O (lignin)

• support, protection from microorganisms

complex primary plant tissues

leaf mesophyll, xylem, phloem, epidermis, secretory tissues 

leaf mesophyll

spongy mesophyll cells, palisade mesophyll cells

xylem - specialized plant cells

fibres, parenchyma cells, trachieds, vessel elements (primarily flowering plants), transfer cells

phloem - specialized plant cells

fibres, parenchyma cells, sclereids, sieve tube members and companion cells (flowering plants) or sieve cells and albuminous cells (germs, gymnosperms) transfer cells

epidermis - specialized plant cells

shoot and root epidermal cells (including root-hair cells and root cap cells), trichomes, cells associated with pores (stomata): guard cells and subsidiary cells 

secretory tissues - specialized plant cells

transfer cells, secretory cells 

complex secondary plant tissues

secondary xylem, secondary phloem, periderm

secondary xylem 

fibres, parenchyma cells, trachieds, vessel members (primarily flowering plants) 

secondary phloem

fibres, parenchyma cells, sclereids, sieve-tube members and companion cells (flowering plants) or sieve and albuminous cells (ferns, gymnosperms) 

periderm

cork cells, cork cambium, parenchyma cells 

xylem 

• transport, transpiration stream, water, and minerals (from soil), long distance signaling molecules 

• xylem parenchyma, protoxylem, metaxylem 

• vessels. vs trachieds 

• contains many cell types and composition often changes with maturity 

• hollow surrounded by a secondary cell wall

types of cells in xylem

• xylem parenchyma, protoxylem, metaxylem, tracheite and vessels 

• undignified and alive initially, high lignin field and often dead at maturity 

• different secondary cell wall thickening’s reflect maturity and function 

• tylose (parenchyma) and fibres 

Phloem

• vascular tissues develop from procambium

• transport, fixed carbon (predominantly sugars), long distance signaling molecules

• contains several cell types

  • angiosperms: sieve elements (or sieve tube members), parenchyma (incl. companion cells) and fibre cells

• pressure systems

• not necessarily dead but not expressing genes

epidermis

• protective (biotic and abiotic factors), gas exchange and transportation (shoot), secretory, structural

• many cell systems, differences between different organs

  • leaves: trichomes, socket cells, meristemoids, guard cells, pavement cells, glands, etc 

  • roots: root hairs/trchiomes formed by tricholbacsts, atrichoblasts, root cap, etc roots

meristems: cell division and fate acquisition 

• plant growth and development product of cell division and cell expansion 

  • increase in size often more dependent on elongation/expansion

• cells largely immobile

  • positional signals are import at in organizing and determining fate

• constant break down and remaking of cellulose

types of meristems

apical, lateral, intercalare

apical meristems

• at or near the tips of roots and roots

• elaborate adult plant

• specified in embryo

  • primary SAM and RAM

  • elaborate adult plant

• reproductive transition

• lateral root and shoot meristems

  • auxiliary meristems

  • lateral root meristems

• adventitious meristems (green)

  • arise ectopically

  • ‘differentiated’ tissues

  • always shoot meri

lateral meristems

• responsible for secondary growth (increases girth)

• vascular cambium and cork cambium

intercalare meristems 

• have continuously growing leaves (indeterminate) 

• plants without secondary meristems, monocots 

• growth of cut grass

growth in plant cells vs animal cells

vacuoles and cell walls allows plants to grow faster then animals

Vacuole(s)

• often 1 large central vacuole, but there may be many e.g. during and after cell division

• regulating turgor

  • expansion/elongation

  • growth with fewer resources

  • ‘hydroskeleton’

• storage/isolation waste or harmful materials (heavy metals, defence compounds, pigments)

• regulating pH

  • can cause different colours, moving protons in and out 

• resource storage 

  • e.g. ‘protein bodies’ are modified vacuoles 

transvacuolar  cytoplasmic strands 

• internal actin filaments support them 

• important role in cell division (with microtubules) 

• distribution routes for organelles and metabolites 

• golgi and ER move inside the secretion system 

• dynamic, appear and disappear

plastids

proplasitds, chloroplasts, chloroplast, chromoplasts, leucoplasts, amyloplasts 

proplastids

progenitors of other plastids

• colourless, found in meristematic cells of shoots, roots, embryos, and endosperm, no distinctive morphology, vary in shape

chloroplasts

site of photochemical apparatus, distinctive internal membrane organization (thylakoid discs) chlorophyll pigments and light reactions of photosynthesis associated with thylakoid membranes. Green, lens-shaped. Present all photosynthetic tissue and organs

e.g. leaves, storage cotyledons, seed coats, embryos, outer layer of unripe fruits

chromoplasts

red- orange- and yellow-coloured

high levels of carotenoid pigments, often found in flowers, fruits, senescing leaves and certain roots. Chromoplasts often develop from chloroplasts but may also form from proplatids and amyloplasts 

leucoplasts

colourless plastids, distinct from proplastids having lost their progenitor function e.g. amyloplasts, elaioplsts/oleoplasts and proteinoplsts, the sites of synthesis and storage of starch, lipids and proteins respectively

amyloplasts

stored fixed carbon reserves as starch. starch granules, found in roots (detection of gravity) and storage tissues

e.g. cotyledons, endosperm, and tubers

amyloplasts can re differentiate into other plastid types

cell wall

middle lamella, primary cell wall, secondary cell wall, cellulose, hemicellulose, pectin, logins

middle lamella

between adjacent cells, forms first

primary cell wall

often relatively thin and flexible, allows for expansion

secondary cell wall

forms inside primary wall after full expansion, forms insider primary wall after full expansion, may be multiple secondary cell wall layers in some cells 

cellulose 

5,000 - 18, 000 glucose monomers polymerized β(1 → 4) linked glucose is not readily degraded/hydrolyzed, contrast with starch 

hemicelluloses

e.g. xylan, heterogenous sugar polymer (500 - 3, 000 monomers, less resistant to hydrolysis) cross-links cellulose micro fibrils

pectin

cross linking, particularly in middle lamella, complex mix of sugar monomers often rich in galacturonic acid

logins

strengthen and water proof secondary cell wall

plasmodesmata essentials 

plants are a mosaic of simplistically connected cells 

plasmodesmata - symplastic connection, regulated movement of larger molecules through cytoplasmic sleeve

• some movement through desmotubule (ER)

• smaller molecules e.g. ions, sugars, amino acids pass freely

  • some small proteins e.g. GFP, also pass through

• larger protein (>10kDa) movement prevented or regulated

• primary = formed during cell division (endoplasmic reticulum trapped across the middle lamella as new cell wall made)

• secondary = formed during between mature cells

• limited functional similarity to Gap junctions between animal cells, for transport of small molecules

  • development

  • cardiac conductance and morphogenesis

The Shoot Apical Meristems (SAM)

• tunica-corpus: one way of mapping the SAM

• tunica: one or more peripheral layers of cells which divide in planes perpendicular to the surface of the meristem (anticlinal divisions)

• corpus: body of cells several layers deep in which cells divide in two planes (anticlinal and periclinal)

• CZ slowly dividing stem cells

• PZ rapid cell division. cells incorporate into lead and flower primordial

• RZ davidson and subsequent elongation of these cells generates stem cells

• the WUSCHEL (WUS) gene is expressed in the organizing center and is needed to maintain stem cell fate in cells of the CZ

  • the protein moves 

• the CLAVATA 3 gene expressed in stem cells of CZ, where it specifies stem cell fate 

  • produces a small peptide 

  • binds CLV1 receptor 

  • down regulates WUS

similarities between SAM vs RAM 

SAM: stem cells above OC generating, L1, L2, L3 layers 

RAM: stem cells around QC generating central root cap, cortex/endodermis, stele (vascular cylinder) 

SAM: WUSCHEL (WUS)  expressed in the OC

RAM: WUSCHEL-RELATED HOMEOBOX GENES (WOX5) expressed in the QC

arabidopsis (eudicot) vs mailer (monocot) 

• genetically: the story cannot be moved wholesale into other plant taxonomic groups 

• very similar organization 

• similarities in gene expression 

• but: knockout WUS homologous in MAIZE. no phenotype type 

arabidopsis vs ferns

• in bryophytes and ferns the entire shoot system is elaborated from a single apical cell

• at what point in the angiosperm development does a single cell generate all the tissues of the shoot?

• apical cell resulting from 1st division of zygote

importance of photosynthesis 

• biological conversion from an inorganic state to an organic state (plants, phytoplankton, cyanobacteria)

• 100-200 billion meteoric tonnes of C are converted per year 

• by product of C fixation is the product of oxygen 

• every C molecule in our bodies passed through the outer membrane of a chloroplast, the site of fixation of C in plants 

photosynthesis 

conversion of C from CO₂ to organic C (CH₂O) using light as the source of energy 

light must be captured by a pigment

• the pigment which absorbs light for photosynthesis is chlorophyll, contained within chloroplasts

• chlorophyll a and b exist

• otero pigments in the chloroplasts also absorb light (carotenoids)

chloroplasts in photosynthesis

• chlorophyll is associated with the inner membrane region of the chloroplast, the thylakoids (grana), part of the membrane system, the liquid matrix between the grana is the stroma

thylakoids 

• hollow ‘balloons’ 

• grana = thylakoids stacked on each other 

• membrane structures that absorb light and use energy for light to do the work 

light

• released from the sun as electromagnetic radiation

• short wavelengths (X- UV-rays waves are about 1 nm in length)

• long wavelengths (radar, micro-, radio-waves can be km in length)

• light has wavelike properties and the distance between wave peaks differ

light: spectrum

• visible wavelengths occur between about 400 and 700 nm

• small part of the spectrum of light coming from the sun

light: photons and energy 

• a convenient way to think of light as packets of light energy -photons 

• each photon has a specific amount of energy (quantum) that is related to the wavelength of light (shorter wavelengths have more energy) 

• sunlight is a rain of photons of different energy s

• the quantum energy contained within a photon depends upon its frequency 

modified planck law

energy of a photon = speed of light / wavelength

• the speed of light is constant at ~30 000 km/sec, so the longer the wavelength, the smaller the value of E

• y- UV- and X-rays have very high energy and collide with other molecules to ionize and damage/destroy them

• infrared and microwaves have low energy but can vibrate molecules and cause them to warm up

light: reflected vs. absorbed 

• much of light that reaches the earth is in the visible (lambda) 

• light that is visible to us is not absorbed by the pigment, but reflected 

• if all the wavelengths are absorbed, then the object is black 

• if all are reflected the object is white 

blue light

higher energy photons of blue light can be absorbed by G, Y, and R but not by B

red light

lower energy photons of red light can be absorbed by G, Y, and B but not by R

yellow light 

mid-energy photons of yellow light can be absorbed by G, R, and B but not by Y 

green light

green light is absorbed by all but G pigments

absorption spectrum of a green leaf

least absorption in the green wavelengths of the visible spectrum

• chlorophyll a absorbs in the blue and red wavelengths

• chlorophyll b absorbs in the blue

how do we know chlorophyll is the most importantly pigments for photosynthesis 

action spectrum shows the only R and B light are efficient at driving photosynthesis: absorption spectrum of chlorophyll’s a and b (& carotenoids) 

what happens when a pigment absorbs light?

• light is a package of energy (quantum)

• causes a reaction when it is absorbed by a pigment

• electron moved to a higher energy orbit (excited) by light and usually fall back again, releasing heat

the fate of excited electrons 

moleucle fluoresces as photons of a lower energy state are released, utilized as useful energy or released as heat 

• leaves a positive and negative charge

excitation of chlorophyll by light 

high energy blue light drives electrons into an outer orbit: excited state 2 

lower energy red light drives electrons into an inner orbit: excited state 1 

photo systems

chlorophyll is concentrated int eh thylakoids of chloroplasts in photo systems

about 250-400 pigment molecules (mostly chlorophyll a and b) are arranged in an antenna (pigment) complex

these gather the light and funnel energy towards a reaction centre

there are two photo systems

the two photosystems 

• PSI is richer in chlorophyll a and absorbs most at ~700 nm (P700) 

• PSII is richer in chlorophyll b and absorbs most at 680 nm (P680) 

what happens when light is absorbed by the photo systems

the electrons pass through an electron transport chain in the thylakoids membrane and are passed to PSI (which is a slightly higher energy state)

this is known as the Z-scheme, or non-cyclic photophosphorylation c

charge problem

the two electrons which leave the PS II must be replaced in the chlorophyll otherwise it would assume a positive charge and be unstable

these electrons come from water

in the PSII complex water is ‘split’ to give H+ and ½ O in the light reaction photolysis of water

photolysis of water 

2e- released are used to replace those lost from the chlorophyll and oxygen is released (H+ function later)

now we can account for two parts of the photosynthesis equation 

electrons excited from PSI are captured by NADP to make NADPH (chemical reducing power) via an electron transport chain 

hydrogen ions (protons) from photolysis

• line up the schemes to show how they occur in the thylakoids membrane of the chloroplast

• H+ produced by PSII and during the movement of the e- from PSII to PSI are released into the lumen of the thylakoids

• H+ causes an ionic imbalance between the inside and the outside of the thylakoids

• the ions are released to the outside through special channels (down a concentration gradient)

• as this occurs an enzyme associated with them utilizes the energy towards produce ATP from ADP + Pi

products of the light reactions of photosynthesis 

• oxygen form the photolysis of water, released by the plant 

• NADPH: powerful reducing agent that takes part in biochemical reactions 

• ATP: a source of energy for biochemical reactions 

reactions of photosynthesis that do not require light: the dark reactions

• the utilization of the chemical products of the light reaction, NADPH and ATP occur in the stroma of the chloroplast

• the dark reactions are those in which the CO₂ is converted to sugars

the stroma (dark) reactions

• it has been known for a long time that the first organic production of photosynthesis is a 3C compound, 3-phosphoglycerate (3 PGA)

• for many years, researches looked for 2C compound to which CO₂ was though to be added 2 C + CO₂ → 3C but none was ever found 

• in the 1940s and 50s Melvin Calvin and Andrew Benson discovered that the CO₂ was added to a 5C compound, in what is now known as the calvin benson cycle 5 C → 2 CO₂ + 2×3C

calvin cycle

in the 1950s Melvin Calvin and Andrew Benson discovered that the CO2 was added to a 5 C compound in what is now known as the Calvin-Benson cycle or the C3 mode of photosynthesis - noble prize 1961 

the enzyme that produces 3 PGA from RuBP is ribulose bisphosphate carboxylase (RuBP, carboxylase, Rubisco) 

• over abundant enzyme that makes up >30% of all protein on the planet 

• the most prevalent final product of photosynthesis is sucrose (glucose-fructose), the transport sugar within plants

the calvin cycle has to turn 3 times 

1. the fixation 

2. the reduction → NAPH and ATP make G3P 

3. resetting 

RubisCo is not efficient 

• rubisCo forms 3-PGA most efficiently when there is low O₂ and high CO₂ 

  • carboxylase reaction

• but our atmosphere has high 02 (~20%) and low CO2 (~400 ppm)

• in normal atmosphere the enzyme can also undergo a 2nd reaction

  • an oxidative one to cleave

• 5C RuBP → (2C) glycolate + (3C) 3PGA

• the glycolate is then decarboxylated to release CO2, making photosynthesis less efficient

• about 25% of fixed CO2 is lost at normal temps and up to 50% at higher temps

are there plants that have evolved a more efficient means of fixing CO₂? 

to take in CO₂ for photosynthesis, plants must open their stomata which leads to loss of water transpiration 

this affects water use efficiency 

why does earth reflect green light

green is a less abundant source, no one else was utilizing green

too much light

how can too much light damage plants and how can plants respond to protect themselves from too much light

• plants need methods to reduce the amount of light they get

• light energy is captured by chlorophyll pigments in the Thylakoids membranes within the chloroplasts

the latter of too much light includes

• physiological and biochemical responses 

• organ movement 

• organelle movement 

  • chloroplast reposition

• grana rearrangements

evolution and impact of plants: importance 

• accumulated adaptations to life on land across ‘deep time’ 

  • spread and diversified

  • terrestrial ecosystems

• primary production O2, human civilization

hadean 

• no oxygen 

• molten 

• 0 plants (nothing living) 

archean 

• 1st life - apex chert fossils of western australia 

  • oldest fossils

proterozoic

• the great oxygenation event (cyanobacteria)

• eukaryotes are common fossils in this era

• first multi cellular eukaryotes

• free living in diverse (incl, extreme) environments

• glaciation event caused by plants

archean and proterozoic

1st life, cyanobacteria

• ‘ancestors’ of chloroplasts

• generate l2 and fix nitrogen

• stomalite communites

cryogenian “snowball earth” 

photosynthetic eukaryotes 

• acritachs dip-photosynthetic protists microfossils 

• small communities in fresh water 

  • green algae present, algal mats (pond scum)

ediacaran (end of the pre-cambrian) 

• red and green algae

• chytrid fungi

cambrian 

• plants only made the transition to land once 

• red and green algae begin to come forward as primary producers 

  • red algae (rhodophyta) important builders of limestone reefs (solenopores)

  • green algae ancestors of embryophytes

• small plants break down rocks and make soil very slowly

ordovician

algae dominant

• 1st embryophytes

  • evolved from order: chorales ex. chara (a pond weed)

  • fossil spores (liverwort origins)

• fungi spores told us about the first plants, plants are dependent on waater

liverworts (marchantiophyta) 

• thallus or leafy 

• single celled rhizoids

• oil bodies (including secondary metabolites) 

  • responsible for smell/taste of a lot of plants

• gemtophyte dominate life cycle - short lived sprophyte

ordovician - silurian mass extinction 

• ice age 

• we can’t know for sure but we think plants are to blame 

silurian

• sea levels rise as climate warms

• plants establish communities on edges of water

• bryophytes diversity

  • liverworts (nepatophyta), homoworts (anthoceratophyta), mosses (dryophta)

• land plants represent growing resource

• important adaptations by the end of silurian

  • polysporangiaphytes (branching)

  • tracheophyta (vasculature), earliest lycophyte

• plant approaches colonizing land

  • stay in consistently damp places (bryophytes)

  • tolerate desiccation and resurrect

  • reduce water-loos with cuticle (vascular plants and a few bryophytes)

  • improve efficiency of water gathering and distribution (vascular plants)

• life cycles

  • gametophyte (n) dominate (brytophytes)

  • meeting of gametes dependent on water

• sporophyte (2n) becomes less dependent on gametophyte and eventually dominant in vascular plants lineage

bryophytes

• gametophyte (n) dominant

• sporophyte (2n) dependent

  • sporophyte 1st cuticle 

  • sporophyte 1st stomata 

• balancing CO2 uptake with water loss 

  • waxy cuticle 

  • stomata 

horneophytopsida 

• no leaves, roots (they have rhizoids) or true vascular (water-conducting cells) 

• later silurian - gone by mind devonian

rhyniophyta (div) or rhyniopsida (class) 

• no leaves or roots 

• indeterminate rhizomes, determinate aerial stems 

• true vasculature (s-type tracheas) 

devonian 

• diversication of lycophyte 

• diversification of other vascular plant groups 

  • origins of seed plants and trees

• lycopodiophyta

• euphyllophytes

• spermatophytes

lycopodiophyta 

• oldest vascular plant division 

• advanced forms had ‘roots’ 

• evolved tree-size species prevalent in carboniferous 

• included club mosses 

  • small-scale like leaves (scale trees)

• spores

• earliest tree

  • wattieza, cladoxylopsida ancestors of ferns