Root Anatomy and Transport

Root Anatomy (Cross Section)

• The parts of the root include: Epidermis, root hairs, cortex (including endodermis), and stele (vascular cylinder).

Lateral Transport (Short Distance - Radial Axis)

• 3 routes exist
• Transcellular: occurs across plasma membranes and cell walls.
• Symplast: occurs within the cytoplasm and is continuous among cells via plasmodesmata.
• Apoplast: occurs within cell walls and intercellular spaces.

Absorption of H2O and Minerals at Root

• Root hairs, branching patterns, and mycorrhizae all increase surface area.
• Plasma membranes of root hairs are permeable to water and allow for lateral pathways through roots.
• Aquaporins are integral transmembrane pore proteins increasing water permeability.
• Aquaporins are impermeable to protons, maintaining membrane potential.
• Aquaporin numbers vary according to cell type and environment.
• These are also found in animal, bacterial, and fungal cells.
• Minerals undergo selective crossing at the plasma membrane and accumulate.
• The endodermis acts as the final selective passage point.
• The Casparian strip is a belt of suberin (hydrophobic wax).
• Substances traveling through the apoplast meet a dead end at the Casparian strip, and must cross a plasma membrane twice to reach the stele, creating a selective passage.
• Minerals move into the apoplast of the cells in the stele after passing the endodermis.
• Transfer cells help move solutes from symplast to apoplast using large mitochondria to provide ATP for active transport, and finger-like cell wall extensions to increase surface area.
• Water follows via osmosis when solute concentration increases in the apoplast.

Anatomy of Xylem

• The xylem is dead at maturity.
• Secondary walls are secreted with spiral or ring patterns, to allow stretching during growth.
• It carries H2O from the roots to the shoot, and provides structural support.
• There are two types of cells: tracheids and vessel elements.
• Tracheids are spindle-shaped with long, tapered ends.
• H2O moves from cell to cell through pits, areas with only primary walls and plasmodesmata.
• Vessel elements have wider, shorter, and thinner walls and are less tapered than tracheids
• Vessel elements are aligned end to end with perforations for H2O passage.
• Walls are strengthened and waterproofed with lignin.
• Lignin forms crosslinks with polysaccharides in the cell wall, providing structural strength.
• Lignin is more hydrophobic than cellulose and hemicellulose, providing the ability to use xylem as a H2O transport system.

Ascent of Xylem Sap

• Tracheids and vessel elements have a small diameter to help overcome the effect of gravity (capillarity).
• The walls of xylem vessels are pulled together, decreasing diameter.
• Root pressure and transpiration/tension-cohesion/adhesion account for the ascent of xylem sap.

Root Pressure

• Solutes accumulate in the stele, causing osmosis and increased pressure.
• Not the most important mechanism, although water and solutes move up following a pressure gradient (high to low pressure areas).
• Most important during the night when transpiration decreases.
• Guttation is an effect, which releases H2O droplets at leaves through hydathodes.
• Hydathodes evolved from modifications of stomata and can be used as salt glands in some plants
• Guttation is mainly at night, with increased humidity or in waterlogged soil.

Transpiration/Tension-Cohesion/Adhesion

• Transpiration occurs when H2O(g) diffuses out of spongy mesophyll air spaces via stomata.
• This is replaced by evaporation of H2O(l) film covering the cell walls of spongy mesophyll.
• A meniscus then forms on the H2O film.
• Tension increases as the meniscus concavity increases, pulling H2O from the xylem to the mesophyll.
• Adhesion to cell walls and cohesion between water molecules pull the water column up.
• Cavitation is a potential problem of this mechanism, concerning the formation of a H2O(g) pocket in xylem that breaks flow and needs refilling.
• Pits allow for detours, bringing in water to redissolve the gases, new xylem is added yearly, root pressure (only possible in small plants), and solutes moved to xylem draw water by osmosis.

Guard Cells and Stomata

• The waxy cuticle on leaves reduces H2O(g) loss by evaporation, but is also impermeable to CO2
• The solution is stomata
• A secondary problem is that H2O exits through stomata.
• Spongy mesophyll increases the surface area for CO2 absorption.
• The problem with this is increased surface area for evaporation.
• A secondary benefit is evaporative cooling, maintaining a temperature range for photosynthetic enzymes.
• Stomata open through the increase of guard cell turgidity.
• A proton pump moves H+ out of the cell.
• K+-gated channels open, allowing K+ in, and Cl- in, making water potential more negative.
• H2O then flows in.
• Swelling of guard cells occurs due to radial cellulose microfibrils, and cells buckle outwards as they swell.
• Light and photosynthesis trigger stomata openings by stimulating a blue receptor in the plasma membrane.
• The consumption of CO2 (stomata open if placed in container without CO2) and the internal clock of guard cells also affect the opening
• Even when placed in a dark room, the stomata will open.
• Stomata close due to water deficiency causing loss of turgor pressure.
• Abscisic acid is a hormone in spongy mesophyll, alerting the stomata to close since it is produced when there is a water deficiency.
• High temperatures also cause stomata to close, stimulating cell respiration and increasing [CO2].

Anatomy of Phloem

• Phloem cells must be alive for transport to occur, requiring ATP.
• Transport occurs in both directions (up and down the stem and roots)
• It carries photosynthesis products or hydrolysis of storage polymers, as well as other solutes like amino acids and minerals.
• The two types of cells are sieve tube members (elements) and companion cells.
• Sieve-tube members have a mix of central vacuole contents and cytosol, forming phloem sap where the tonoplast, nucleus, and some organelles disappear or partially degrade
• A layer of cytoplasm with the remaining organelles surrounds the centrally located sap.
• End walls are between sieve-tube members with sieve plates perforated for flow, forming a sieve tube.
• Companion cells are next to sieve-tube members and are connected by plasmodesmata.
• Nucleus and ribosomes also serve sieve-tube member, helping to load or unload sugar into or out of the sieve-tube member.

Phloem Transport

• Phloem sap moves by bulk flow because of pressure differences between a source and a sink.
• The source is the location where sugars are produced (photosynthesis or hydrolysis of stored polymers), allowing loading.
• The sink is the location where sugars are consumed, allowing unloading.
• Solutes are actively transported from source cells to sieve tube members.
• This could also happen by secondary transport (coupling of sucrose+protons) and help from companion cells, resulting in increased [solute]
• H2O from xylem then follows by osmosis, building hydrostatic pressure at the source end,
• Solutes are then unloaded at the sink by active transport diminishing hydrostatic P at the sink end by water flow
• This causes H2O to flow back to the xylem and sap flows from the source to sink, based on water potential

Plant Nutrition - Essential Nutrients

• Essential nutrients are directly required for growth and reproduction, and cannot be replaced by another element.
• Macronutrients are needed in large amounts: greater or equal to 1 g/kg dry weight, like N and P.
• Micronutrients are needed in smaller amounts: mainly as enzymatic co-factors: less than 100 mg/kg dry weight, examples are Fe and Cu.

Nitrogen Cycle

• Nitrogen is the most limiting element.
• Required for proteins, nucleic acids, chlorophyll, etc.
• N2 cannot be used by plants and must be converted to NH4+ or NO3.
• It involves several processes, including nitrogen fixation, nitrification, denitrification, ammonification, anaerobic ammonium oxidation (anammox), and nitrogen reduction.
• Nitrogen fixation is exclusive prokaryotic metabolism, involving symbiotic and free-living bacteria and archaea (e.g., Rhizobium and Cyanobacteria).
• N2 is reduced to NH3, which is metabolically expensive, requires nitrogenase, and is anaerobic
• Lightning can also turn N2 to NO.
• Nitrification is oxidation: NH4+ converts to NO2 via bacteria (e.g., Nitrosomonas and Nitrosococcus) and archaea, and NO2 converts to NO3 via bacteria (e.g., Nitrobacter).
• Both NH4+ and NO2 are toxic in high concentrations.
• Denitrification is the reduction of NO3 to N2, which is anaerobic and performed by bacteria (e.g., Pseudomonas and Clostridium).
• Ammonification converts organic N to NH4+ from decay by bacteria and fungi.
• Anaerobic ammonium oxidation (anammox) is a process where NO2 + NH4+ becomes N2 + 2H2O via bacteria.
• It requires a special organelle called an anammoxosome, removes ammonium in wastewater, and acts as a sink for fixed N, responsible for 30-50% of N2 in oceans
• Nitrogen reduction is when NO3 converts to NH4+ and is incorporated into cells via nitrate reductase in plants.

Plant Hormones - Cytokinins

• Cytokinins stimulate cytokinesis when auxin is present.
• The ratio of auxin/cytokinin controls cell differentiation.
• The effect of [cytokinin] and [auxin] are as follows: When their concentrations are equivalent, cells grow but do not differentiate, forming a callus. If [cytokinin] > [auxin] shoot buds develop and when [cytokinin] < [auxin] roots form.

Plant Hormones - Gibberellins

• Gibberellins are a family of over 100 hormones
• Discovered in "foolish seedling" disease in rice, causing seedlings to grow too tall and topple over was found to be a fungus Gibberella fujikuroi (Ascomycota).
• Mendel's tall v. short can relate to gibberllins
• the le (length) gene codes for enzyme 3b-hydroxylase, this enzyme converts GA20 to GA1 (GA = Gibberellic Acid).
• Mutants do not make the enzyme and are short, but when mutant plants treated with GA1à tall
• It is produced mainly in roots and young leaves, and is responsible for stem elongation, specifically in reproductive growth where there will be flowers at terminal position on stems
• Responsible for fruiting and is involved in germination, beginning with intake of H2O by seed (imbibition), the releasing gibberellins by embryo, production of enzymes (a-amylase) by aleurone layer, and lastly, digestion of starch and proteins from storage (cotyledons or endosperm).

Plant Hormones - Abscisic Acid (ABA)

• ABA is a "stress" hormone which accumulates when plants are dehydrated, closing stomata.
• It prepares plants for winter, slowing growth, induces leaf primordia to develop into scales and inhibits cell division in vascular cambium
• It controls seed dormancy (balance is on ratio of gibberellins/ABA)
• Controls storage of proteins during seed formation, and first rain washes ABA in desert plant seeds, leading to flowering.
• It has an antagonistic effect on aleurone layer, relative to gibberellins.
• vp mutants are deficient in ABA, leading to seeds germinating while on the parent.

Plant Hormones - Ethylene

• People in ancient China discovered that incense in rooms sped up the ripening of fruits
• 1800s - noticed trees near street gas lamps drop leaves prematurely because it diffuses as a gas through intercellular air spaces and can also move in cytosol
• Promotes fruit ripening, leaf abscission, and flower fading
• It inhibits cell elongation, and aging cells will produce more in a chain reaction.
• Fruits ripen as ethylene gas signal spreads from fruit to fruit
• Leaf abscission is controlled by ethylene and auxin and is a wintering adaptation that prevents desiccation when roots cannot absorb H2O from frozen ground.
• Layers of abscission are as follows: Nutrient shunt to storage places Autumn leaf stops producing new chlorophyll Formation of abscission layer Addition of enzymes. The leaf falls of and a cork layer forms a protective scar.

Plant Hormones - Photoreceptors - Phytochromes

• It's a pigment protein containing a chromophore, which causes conformational changes of the chromophore (cis to trans) when exposed to light.
• Many are encoded by several genes, existing in two alternating forms: Pr (absorbs red; 660 nm) converts to Pfr in presence of red light and Pfr (absorbs far-red; 730 nm) which converts to Pr.
• Based on the ratio of light: greater ratio of red/far-red causes increase in [Pfr] and shade where lower ratio of red/far-red causes decrease in [Pfr]
• It synchronizes biological clock with sunset and sunrise causing [Pr] > [Pfr] in the darkness, since Pfr slowly converts to Pr at sudown,.
• Seed germination can also be affected by red and far-red light.

Plant Hormones - Flowering/Photoperiodism

• Determined in part by flowering night length, so the plants measure the length of a continuous dark period.
• Phytochromes are involved in flowering since LDP flower when the last flash of series is red, and the SDP flower when the last flash of series is fr
• Florigen is a small protein associated with flowering that occurs in 3 processes: An SDP is kept in light, but a leaf is covered where the plant flowers, where each leaf is capable of detecting dark period.
• Since signal is synthesized in the leaf and moves out to other parts of the plant, it can be stated that photoperiodic stimulus causes permanent changes in the leaf