Root Structure and Transport

Root Structure and Transport

• Diffusion is the net movement of molecules from high to low concentration areas
• Osmosis involves water movement across a semipermeable barrier, like a cell's plasma membrane
• Bulk flow is another mechanism for water movement in plants
• Water moves from the soil into the root
• It then enters and ascends through the xylem
• Water travels through the xylem via bulk flow
• Xylem anatomy includes tracheids and vessels
• Cohesion and adhesion of water also aids in xylem transport
• Water moves from the leaf to the air through transpiration
• Transpiration creates tension on the water film around mesophyll cells

Water Movement Pathways

• Apoplastic pathway is when water moves through cell walls without crossing any plasma membranes
• Symplastic pathway is water moving through plasmodesmata and crosses a plasma membrane once
• Transcellular path is where water moves across the plasma membrane multiple times
• The Casparian Strip blocks apoplastic flow, preventing passage between the endodermis into the xylem
• Phloem transports from source to sink tissues, delivering organic molecules
• Phloem translocation uses a Pressure Flow mechanism
• Phloem loading can occur through symplastic and apoplastic pathways
• Maple syrup is xylem sap, showing the flow of sap in maple trees

Plant Reproduction: Floral Organs

• Sepals, petals, stamens (anther with pollens, filament), and carpels (stigma, style, and ovary with ovules) constitute floral organs
• The gynoecium is all the carpels of a flower
• The androecium is all the stamens of a flower
• Perianth is the collective term for all petals of a flower (corolla)
• Calyx is all the sepals of a flower
• Each pistil contains one or more carpels

Organization and Traits of Flowers

• Floral organs originate from the Shoot Apical Meristem (SAM) as modified leaves
• They are arranged concentrically or in spirals
• Organization goes from out to in: sepals, petals, stamens, then carpel(s)
• Symmetry can be radial or bilateral
• Flowers can be unisexual (staminate or pistillate) or bisexual; plants can be dioecious or monoecious
• Fusion between floral organs involves connation and adnation
• Ovary position can be superior (attached above), inferior (attached below)

Flower and Fruit Development

• Hypogynous flowers have other floral parts BELOW the ovary,
• Perigynous flowers have structures that SURROUND the ovary,
• Epigynous flowers have other parts ABOVE the ovary.
• Every flower originates from the Shoot Apical Meristem (SAM)
• Flowering switches the SAM from vegetative to reproductive, forming a floral meristem
• The SAM broadens and flattens during this transition
• Floral organ primordia replace leaf primordia
• Organs develop from lower/outer to upper/inner positions
• Inflorescences are different from solitary flowers

Inflorescences and Fruits

• Inflorescences have a rachis which is the central axis above the peduncle
• A pedicel is a short stalk that connects the flower to the rachis
• The Inflorescence meristem (IM) results from the shoot apical meristem changing
• The inflorescence meristem (IM) is surrounded by flower meristems (FM)
• Simple inflorescences exist
• Compound inflorescences are made
• Indeterminate (Racemose; raceme) inflorescences keep growing and making new flowers and lack a terminal flower
• Determinate (Cymose; cyme) inflorescences have the first flower as the terminal flower
• Capitulum (head) and Spikelet floral arrangements
• Fruits facilitate dispersal via wind, water, animals (attachment or consumption), or explosive dehiscence

Fruit Structure and Classification

• The ovary develops into the fruit
• The ovary wall becomes the pericarp, which consists of exo-, meso-, and endocarp layers
• Each ovule becomes a seed
• The outer ovule layer forms the seed coat (testa)
• Ovules attach to the placenta via the funiculus
• Accessory fruits have a hypanthium that becomes fleshy
• The pericarp is internal to the fleshy part, defining "false fruits"
• Fruit types are classified practically based on if they are fleshy or dry
• Developmental classification depends on floral structure
• Simple fruits come from flowers with a single pistil
• Aggregate from pistils, multiple from many flowers

Simple Fruit Types

• Berry: Flesh = inner pericarp layers
• Drupe: Flesh = mesocarp; Endocarp sclerification
• Pome: Flesh = hypanthium
• Indehiscent fruits do not break open
• Achene and Caryopsis fruits
• Nut has a hard shell (pericarp)
• Dehiscent fruits break open at maturity to release seeds
• Legume: Fruits develop from one carpel and split maturity
• Capsule: Fruits developed form multiple carpels
• Follicle developed from a single carpel
• Silique fruits are developed from two carpels

Plant Reproduction and Life Cycles

• Plants display alternation of generations between sporophytic and gametophytic stages
• Sporophytic generation is diploid and makes spores by meiosis
• Gametophytic generation is haploid and produces gametes by mitosis
• Sporophyte gradually dominates the life cycle and reduces reliance on the gametophyte
• Gametophytes and gametes are not directly exposed to the environment
• Homospory: Appeared first in evolution, present in bryophytes and produces one type of spore
• Heterospory: Present in all seed plants and includes male and female spores

Plant Reproduction

• Pollen sac is the microsporangium and is filled with microsporocytes
• Microsporocytes undergo meiosis to produce four microspores
• Each microspore forms a microgametophyte (pollen grain)
• Nucellus is the megasporangium in angiosperms
• Megaspore mother cell is the megasporocyte
• Embryo sac is the megagametophyte
• Double fertilization results in endosperm cell (3n)
• Angiosperm embryogenesis is the fertilized egg (2n zygote) goes on to become the sporophyte
• Gymnosperm reproduction has female cones (megastrobili) and male cones (microstrobili)

Reproduction Strategies

• Microsporangia differ in appearance and organization between gymnosperms and angiosperms, developmental steps are similar
• Gymnosperm ovules are exposed on the cone and the pollen doesn't require to go on the stigma
• Gymnosperms are naked seeds while angiosperm ovules are protected by the ovary
• Asexual reproduction is fast and cheap but has less diversity
• Sexual reproduction has more diversity but is slower and more expensive

Asexual Reproduction Methods

• Natural asexual reproduction methods include use of solons, tubers, bulbs, buds, adventitious shoots, and apomixis
• Artificial methods involve cuttings, grafting, layering, and tissue cultures
• Asexual reproduction include somatic embryogenesis, embryogenesis from endosperm, and haploid embryogenesis

Plant Nutrition Essentials

• Law of the Minimum is that yield is proportional to amount of limiting nutrient
• Essential elements like C, H, O, N, P, K, Ca, S, Mg are needed in large quantities
• Trace elements like Na, Cl, Fe, B, Mn, Zn, Cu, Mo, Ni are also essential
• Plants absorb mineral nutrients as ions
• Soil health is important for nutrient availability
• Soil particles have negative charges, adsorbing mineral cations and allowing mineral anions
• Nutrients are transferred across plant membranes

Symbiotic Relationships

• Mycorrhizae are symbiotic fungi including Endomycorrhizas which are ~90% of mycorrhizae and Ectomycorrhizas which are ~10% of mycorrhizae
• Arbuscular mycorrhizas involves membrane invaginations
• Membrane invaginations make “huge” surface area in plant cells
• Nitrogen is the most abundant mineral element in plants
• N₂ is converted to NH4+ in Nitrogen fixation
• NH4+ is converted to NO2- and NO3- in Nitrification
• Biological Nitrogen fixation is when Diazotrophs can fix nitrogen

Nodulation and Nutrient Cycles

• Diazotrophs (nitrogen-fixers) = bacteria and archaea that fix N2
• A single bacterium gets endocytosed into the plant cell
• Nodulation is when Rhizobium bacteria infect the root and create an infection thread
• Bacteroids and vesicles develop to assist root cells
• The Phosphorus cycle is where phosphorus does not enter the atmosphere
• Phosphorus is mainly cycling through water, soil and sediments
• Phosphorus compounds reside primarily in rocks

Plant Hormones and Their Functions

• Plants sense, signal, and respond at different levels and time scales
• The plant hormones include Auxin, Giberellin (GA), Cytokinin, Ethylene, Abscisic acid (ABA), Brassinosteroids
• Jasmonic acid (JA), Salicylic acid, Systemin are used for plant defense
• Strigolactones, Secreted peptides are hormone signals

Auxin

• Indole-acetic acid (IAA) most commonly occurring form of auxin in plants that stimulates cell growth
• Auxin concentration promotes growth, expansion, and division
• Movement of auxin away from production sites generates auxin concentration gradients
• Auxin moves long distances through the phloem by way of transport proteins
• Auxin normally moves from the tip of the shoot towards the tip of the root

Auxin and Cytokinin

• Auxin functions include apical dominance, budding, and vascular differentiation
• Auxin results in phototropism, gravitropism and root formation
• Cytokinin is most abundant natural cytokinin and acts in concert with auxin
• Cytokinin are typically synthesized mostly in root tips in tissue
• Cytokinins function in delaying leaf senescence
• Cytokinins and auxins act antagonistically for many plant developmental processes

Gibberellin and Abscisic Acid (ABA)

• Gibberellin functions promote stem elongation and break dormancy
• Gibberellic Acid (GA) is most common and is synthesized in growing tissues
• Gibberellin is detected in seeds
• GA initiates signal transduction of hydrolytic enzymes to break down protein
• ABA triggers seed and bud dormancy but but does act on abscission
• ABA is more commonly synthesized in mature leaves, roots and seeds