nematodes and parasitic bacteria Flashcards
What is a nematode?
small animals, no larger than 100 cells
they have a flexible cuticle and soft body and are extremely abundant
what percent of nematodes are plant parasites?
15% are plant parasites
Tell me about the biology of nematodes
they moult their cuticle, going through 4 moults as juveniles. they can also enter a state of cryptobiosis (hidden life) where metabolic activity is suspended. this makes them hard to kill.
how do nematodes get about?
They can move a little in the soil, however rely on passive dispersion
how do nematodes feed on plants?
they feed on all parts of the plant using a stylet (a hollow retractable spear-like mouth) to withdraw content of cells
Feeling causes lesions in the plant tissue and enlargement of cells, tricking the plant into continued growth in the area, feeding the nematode
Tell me about root feeding nematodes
when nematodes feed on the root, they damage the root system and reduce the plants ability to absorb water, causing reduction in root mass/structure.
Root damage provides opportunity for nematode to further invade roots
What do nematodes do when they have degraded the plant too much?
drop back into soil and undergo cryptobiosis until the next generation of plants
Tell me about the different feeding styles of nematodes
Ectoparasites: remain outside of plant and feed on root cells using stylet - they have a wide range of hosts
Semi-endoparasites: burrow head in roots to form a permanent feeding cell. they swell and remain
Endoparasite: completely enter root and feed on internal tissue
What are the two types of endoparasitic nematode?
Migratory endoparasite: penetrate root and move within root, causing vast necrotic regions
Sedentary endoparasite: J2 invades the root tip and migrates to the vascular tissue, where it penetrates head and remains. secretions induce formation of large feeder cells
It is not easy for bacteria to survive on the surface of a leaf, what passive feature do they have to help them
Gram positive bacteria cell membrane is enveloped in a thick murein wall
Gram negative bacteria cell wall enveloped in thin wall and outer membrane
Bacteria also surround themselves with a coat of EPS (exopollysacchiride) slime called a capsule. Helping them to retain water, nutrients and adhere to surfaces
How do bacteria decide whether to express traits for survival on leaf surface or to bypass leaf epidermis?
Quorum sensing (QS)
involves the exchange of signal molecules (low molecular weight metabolites) between the population.
IMPORTANT: this requires the population to be dense enough for the message to accumulate and meet threshold
when threshold is met, the signal binds to receptor, triggering gene expression in all members of population
give me some examples of quorum signalling
production of toxins - to overcome plant immunity
bacteria mobility
siderophores - absorption of iron from host
how do bacteria migrate?
via activation of flagella and production of pili (extending via polymerisation and retracts, dragging bacteria)
how does bacteria bypass epidermis
penetrate the stomata
wounds in the plant (degraded cell wall due to secreted enzymes)
