Lecture 10: Plants III (angiosperm reproduction and defense)

flower

reproductive organ in angiosperms and contains both the male and female gametophytes; can contain two types of sex organs: carpels and stamens

carpel

the female sex organs which contains the female gametophytes (Within ovules), contains the stigma, style, ovary, and ovule

stamens

the male sex organs containing the male gametophyte, made up of an anther and a filament

monoecious

plants that have both male and female reproductive structures

dioecious

plants that are either male or female, with male and female reproductive organs on separate plants

megagametophyte

female gametophyte, made up of seven cells with 8 nuclei

female gametophyte development

the ovule contains a cell called the megasporocyte that undergoes meiosis → one of the resulting haploid cells survives and is called the megaspore → the megaspore mitotically divides three times and gives rise to 8 nuclei (an egg + 2 synergids + 3 antipodal cells + a large central cell with two nuclei)

megasporocyte

the cell in the ovule that undergoes meiosis

megaspore

one of the resulting haploid cells that survives

synergids

attract the pollen tube

antipodal cells

provide nutrients, but degenerate after fertilization

male gametophyte development

microsporocytes in the anther undergo meiosis, yielding microspores → these microspores undergo one mitotic division and yield pollen grains containing two cells (have two different functions: generative cell and tube cell)

generative cell

divides by mitosis yielding two sperm cells that will participate in double fertilization

tube cell

synthesizes the pollen tube that delivers the sperm to the female gametophyte after a pollen grain arrives at the stigma

self-fertilization

produce egg and sperm, act as mother and father of next generation

interbreeding depression

the loss in fitness a population experiences due to inbreeding exposing deleterious recessive alleles, high rate when individuals who are heterozygous for deleterious recessive mutations self

Strategies to avoid self-fertilization

1. dioecious plants cannot self-fertilize/separation of male and female flowers in monoecious plants 2. genetic self-incompatibility

genetic self-incompatibility

plants make use of certain genes encoded in a region of the genome called the S-locus, if the pollen and style alleles are the same, pollen is rejected and either fails to germinate or the pollen tube is prevented from growing through the style

double fertilization

when the pollen tube reaches the female gametophyte, one synergid breaks down and two sperm cells are released into its remains → one sperm cell fertilized the egg, forming a diploid zygote that will develop into a sporophyte and one sperm cell fertilizes the central cell in the gametophyte, the nuclei fuse, forming a triploid cell. The cell then rapidly divides and gives rise to the endosperm

Post fertilization events

integuments develop into the seed coat and the ovary develops into a fruit

Fruits function

protect the seeds from animals and plant pathogens, aid in dispersal

tannins

antinutrients that plants produce to discourage herbivory

Seed dispersal

prevents parent-offspring competition for resources, mitigates risk if local conditions worsen

Seed dispersal mechanisms

ingestion by animals, adhesion to animals, wind, water, shattering, ballistic dispersal

Reasons for asexual reproduction in plants

vegetative reproduction and apomixis

Vegetative reproduction

totipotent cells of a plant give rise to new plants, typically occurs in stable environments where conditions are not suitable for seed germination, leaves population vulnerable to pathogens, ex. strawberries

apomixis

seed/embryo production from diploid cells rather than fertilization, gives rise to clones, has potential for yielding hybrid crops as clones (often display hybrid vigor)

hybrid vigor

improved fitness of individuals derived from a cross of genetically distinct varieties within the same species

reproductive growth

flowering → drive by changes in gene expression

vegetative growth

growth of roots, stems, and leaves

annuals

live one year and die after flowering

biannual

live two years and die after flowering

perennials

live three or more years, flower each year but continue to grow

photoperiod

can drive a plant to begin flowering

phytochrome

light-sensitive photoreceptor protein in plants that regulates growth and development by detecting red and far-red light

constitutive defenses

defenses that are always present and nonspecific (cuticle, cell wall)

induced defenses

defenses that are activated in the presence of a pathogen and can be general or specific → activated by elicitors

elicitors

molecules derived from pathogens for which plants have receptors to detect (two types)

PAMPs

pathogen associated molecular patterns → typically bind cell surface receptors (chitin, flagellin)

Avr proteins

pathogen produced molecules that typically act as virulence factors, facilitating infection. bind specific cytoplasmic receptors encoded by plants called R proteins

Plant immune response

formation of nitric oxide (NO) and reactive oxygen species (ROS) → locally toxic to pathogens, and act to signal an immune response in the rest of the plant; strengthening of the cell wall by depositing various polymers and blocking plasmodesmata to prevent pathogen movement through these channels; changes in gene expression leading to the production of phytoalexins

plasmodesmata

channels that connect the cytoplasm of plant cells

phytoalexins

antimicrobial molecules produced by a plant in response to a pathogen

Avr genes

binds an R protein → triggers plant immune response and the plant resists the pathogen

R proteins

cytoplasmic receptors of Avr proteins

gene-for-gene resistance

Avr protein binds an R protein → the plant immune response is triggered, and the plant resists the pathogen → if no corresponding R protein, then plant is susceptible to the pathogen → selection pressure on the pathogen to evade the R gene repertoire of the plant it infects and pressure on the plant for R genes to match Avr genes

hypersensitive response

prevents the spread of infections: infected cells undergo apoptosis, this deprives the pathogen of nutrients → surrounding cells close their plasmodesmata via lignin synthesis → can sequester an infection to a localized area and leave only a necrotic lesion

secondary metabolites

molecules that are not used for basic cellular processes → how plants protect themselves chemically (serve a function to deter or harm herbivores)

non-protein amino acids

common in the seeds of plants, near universally toxic (can be mis-incorporated into polypeptides at the ribosome), canavanine

alkaloids

can be neurotoxic, can also deter herbivory from insects

phenolics

can inhibit digestive enzymes and cause stomach distress (tannins)

calcium oxalate

presumed to play a role as a deterrent to herbivory based on the “needle” like crystal structures found in many plants, can cause inflammation of the mouth and throat but are generally insoluble, dietary intake of soluble oxalates binds calcium in the kidneys and can cause acute kidney failure

Plants call for help

when plants detect elicitors, the release volatiles that attract insect predators, ex. volicitin is secreted by herbivorous beet armyworm caterpillars

Self toxicity prevention

separate storage of precursors of cyanogenic glycosides → compartmentalization of toxins in the vacuole sequesters them from the main metabolic activities of the cell, production of cyanogenic glycosides (form cyanide when acted upon by enzymes), precursors are separated from the enzymes that lead to the production of cyanide

hyperaccumulator

plants that specialize in absorbing and storing exceptionally high concentrations of specific metals or trace elements → able to live in soils with high concentrations for heavy metals → offer opportunity for phytoremediation of soils

phytoremediation

grow tolerant hyperaccumulator plants in contaminated soils