Ch. 32 Plant Defense and Behavior

Question 1

what is a plant’s first line of defense against pathogens?

Answer 1

epidermis (thick walls and waxy cuticle)

Question 2

how do pathogens most commonly enter the plant?

Answer 2

they cannot penetrate the cuticle, so they enter through:

1. wounds
2. piercing mouthparts of insects/nematodes
3. stomata

Question 3

how do pathogens spread throughout the plant once inside?

Answer 3

they use existing transport proteins

Question 4

define biotrophic pathogens

Answer 4

pathogens that obtain resources form living cells

Question 5

define necrotrophic pathogens

Answer 5

pathogens that kill cells before colonizing them

Question 6

what type of pathogen is a virus?

Answer 6

biotrophic

Question 7

what are parasitic plants?

Answer 7

plants that obtain resources by infecting other plants and tapping into their host plant’s vascular system

Question 8

what are heteroparasites?

Answer 8

Parasitic plants, tap into the host plant’s tissue to retrieve water and minerals (mistletoe)

Question 9

what are holoparasites?

Answer 9

Parasitic plants, tap into the host plant’s tissue to retrieve water and minerals AND also sugars (cuscuta and rafflesia)

Question 10

define virulent pathogen

Answer 10

pathogens that are able to overcome the host plant’s defenses and lead to disease

Question 11

define avirulent pathogen

Answer 11

pathogens that damage only a small part of the host plant because the host is able to contain the infection

Question 12

what are the two parts of a plant immune system?

Answer 12

1. basal (general)

2. specific

Question 13

what are characteristics of the basal immune system?

Answer 13

• plasma membrane receptors recognize molecules produced by broad classes of pathogens
• “first line of defense”

Question 14

what are characteristics of the specific immune system?

Answer 14

• depends on R genes
• allow plant cells to identify and deactivate AVR proteins produced by specific pathogens
• directly activates defensive genes
• described as the “arms race” because AVR proteins get smarter and R proteins must also get smarter to be able to keep defending the plant

Question 15

define “R” proteins

Answer 15

• receptors on cell membranes that detect pathogens

- each is coded by an R gene (resistance)

Question 16

define AVR proteins

Answer 16

• avirulence proteins
• produced by pathogens
• enter into plant cells and facilitate infection

Question 17

what happens when an specific R protein binds with a specific AVR protein?

Answer 17

1. prevents AVR protein from blocking the plant’s defenses

2. activates additional defenses

Question 18

define hypersensitive response

Answer 18

• uninfected cells surrounding the infection rapidly produce large numbers of oxygen reactive species
• triggers wall reinforcements and causes cells to die
• death of surrounding dead cells = barrier of dead tissue
• blocks biotrophic pathogens and slows the growth of necrotrophic pathogens

Question 19

define a chemical response

Answer 19

plant produces antimicrobial compounds that attack bacterial/fungal cell walls

Question 20

define a physical response

Answer 20

strengthening cell walls, plugging xylem, closing stomata

Question 21

define vascular wilt disease

Answer 21

• sometimes leads to plant suicide

- can occur when plant defenses end up blocking water sources to leaves (blocking xylem)

Question 22

define systematic acquired resistance (SAR)

Answer 22

• acquired immunity
• ability to resist future infections
  (ex. healthy tobacco leaves can acquire resistance to a pathogen if other leaves on the same plant have already been exposed)
• occurs in response to a wide range of pathogens ONLY when an infection results in necrosis (vascular wilt disease or hypersensitive response)

Question 23

can pathogens alter genomes?

Answer 23

YES! some bacteria have found a way to insert some of their genome into the host cell and trigger a certain behavior in the host cell
(ex. Rhizobium radiobacter)

Question 24

what happens usually when bacteria insert a Ti plasma of their own DNA into the host cells genome?

Answer 24

• Ti genes cause host cells to divide
• a tumor forms
• cells produce compounds that the bacteria can metabolize

Question 25

how do caterpillars overcome the chemical defense of milkweed leaves?

Answer 25

• they “dig trenches”
• larger caterpillars sever major veins and block toxic “latex” flow
• milkweed leaves also have a mechanical defense (many hairs)

Question 26

what are some examples of mechanical defenses?

Answer 26

• spines (on stems and branches)

- hairs (some hairs secrete chemical defenses, too)

Question 27

characteristics of chemical defenses (alkaloids)

Answer 27

• nitrogen bearing compounds (very costly)
• damage nervous system of animals
• bitter tasting
• (ex. caffeine, morphine, nicotine)
• very effective in small concentrations

Question 28

characteristics of chemical defenses (terpenes)

Answer 28

• do not contain nitrogen
• volatile/evaporate easily
• make up many of the essential oils of plants
• spread easily
• may serve as warning signals/strong smell/imitate insect hormones

Question 29

characteristics of chemical defenses (phenols)

Answer 29

• (tannins)

- bind to proteins and reduce digestibility of the plants (make them less easy to eat)

Question 30

define protease inhibitors

Answer 30

defensive proteins that bind to the active site of enzymes in the herbivore’s digestive system

• reduces the nutritional value of plant tissue
• keeps proteins from being broken down

Question 31

what is a very common example of a biotic defender? what is the cost of employing this defender?

Answer 31

• ants
• cost = nectar
• removing ants leads to higher herbivory

Question 32

what are myrmecophiles?

Answer 32

• ant plants
• provide food and shelter to plants
• plants emit chemical that repels ants away from flowers so that other pollinators can still be involved

Question 33

how are grasses adapted to protect themselves from predation?

Answer 33

• apical meristems are close to the ground (they wont get bitten off)
• leaves tightly imbricated
• leaves grow from the base

Question 34

define constitutive defense

Answer 34

defenses that are always active

-occurs when a threat is constant or generalized

Question 35

define inducible defense

Answer 35

defenses that are only active when the plant senses a threat
-occurs when threat is uncommon

Question 36

what happens when a plant is attacked by herbivores?

Answer 36

• it expresses a new set of genes

- genes increase production of chemical defenses, strengthen cell walls, and trigger production of jasmonic acid

Question 37

define jasmonic acid. what does it ultimately do?

Answer 37

• production is triggered when a plant is attacked by herbivores
• signal that is transmitted through the phloem
• induces transcription of defensive genes
• volatile chemicals released from damaged plants trigger defensive response in undamaged plants*

Question 38

what is the janzen-connell hypothesis?

Answer 38

Vulnerability of a population to
disease is dependent on the
density of its individuals.

Many, low-density species are
more likely to survive than a
few, high-density species.

Dispersal of seeds is critical not
simply to reduce competition
between generations but also to
reduce exposure of the offspring
to pathogens.

Question 39

how do we protect crops from herbivores and pathogens?

Answer 39

• Applying pesticides/herbicides
• Understanding the ecology of the farm
• Crop breeding