L2- rhizobia Flashcards
what symbiotic relationship do plants have with rhizobia
rhizobia enter a symbiotic association with legumes, nodules develop (usually on the root) in which atmospheric N2 fixation to NH3 occurs
economically very important
is development host specific
it is but perhaps not as much as was once thought
what is the implication of rhizobia symbiosis
only prokaryotes contain nitrogenase, eukaryotic N fixation is symbiotic
the nitrogenase consists of two proteins, both are extremely sensitive to O2, local low 02- oxygen poisons nitrogenous enzyme
the reaction is energetically expensive - approx 48 ATP molecules are used for each N2 fixed, upto 25% of fixed carbon from photosynthesis may be used to fix N2
- efficient dicarboxylate transport, high local respiration rate (high o2 flux), much less energetically expensive to use soil N, control over nodulation rates required
as it is so energetically costly,
so all bacteria cant cheat this relationship, but some will
it is much less energetically expensive to use nitrate or ammonia in soil, symbiosis need high oxygen fluxes- but low oxygen tensions
what are different types of nodules
tropical - determinate nodules
all cells at same developmental stage eg soybean, french bean
temperate- indeterminate nodules
all developmental stages in one nodule
alfa a, pea, vetch, clover
what are the stages of symbiotic nitrogen fixation
symbiosis need to be somewhat specific as rhizobia end up inside plant cells, this is rare in nature
what are some signals sent by roots
alfalfa produces luteolin which attracts rhizobium mellitoti
peta and vetch produce naringenin which attracts rhizobium leguminosarum by viciae
soybean produces daidzein which attracts bradyhizobium japonicum
what shows host specificity
bacterial protein nodD binds flavonoids and activates bacterial nodulation gene expression
the responses are host specific
nodD acts as a detector of the host plant
the other nod genes have a variety of roles including the synthesis of nod factors
mutations in nod genes stop infection
the attachement is HIGHLY SPECIFIC and acts to limit plant defence responses, localised hydrolysis of the cell wall occurs in the protected environment formed by root hair curling
the rhizobia (and plant) form an infection thread that grows down the root hair and towards the primordium.
what microbial signals restrict initial plant defence
rhizobia have lipopolysaccharides (LPS) and extra cellular saccharides (EPS) on their surface that act as specific signals
- microbial associated molecular patterns (MAMPS) could trigger basal defence responses
- reactive oxygen species produced by host in limited amounts - ROS scavengers in Rhizobia detoxify
-T3/T4 effectors limit basal defence responses but trigger R gene mediated responses
the host responds to rhizobial signals and dampens it defence systems, plants with mutations in these signalling pathway elicit strong defence repsonces
what nod genes do bacterial genes involve in nodulation process
common nod genes (Nod A,B,C) genes can be swapped between strains without affecting host specificity
host-specific nod genes, ( exo (exopolysaccharide), lps (lipopolysaccharide), ndv ( cylic glucans) ) - swapping genes affects host specificity
the sequences of these genes is closely related between strains but they cannot be interchanged without stopping nodulation on their normal host
nodH- other host-specific nod genes may be present or absent
are nod genes required for synthesis of Nod factors
the N-actyl glucosamine backbone with a lipophilic tail
is the basis, a common feature of bacteria. this mean when it goes into membranes itll be in membrane and tail will anchor it it in membrane and the head will be above
different rhizobia produce subtly different Nod factors, will have slight differences in structur, these compounds induce root hair curling at very low concentrations, the common nod gene (nodABC) direct synthesis of the nod factor backbone, host specific nod genes (H) make subtle alterations
what is effect of nod factors on CA2+
nod facors induce Ca2+ spiking
spiking is only seen in plants which nodulate, some non-nodulating mutants dont exhibit spiking, species which never nodulate (eg tomato) dont show spiking. spiking will take place over hours, not short term
what can be used to quantity spike frequency and correlate this with nodulation gene expression
chameleon sensors are used to quantify spike frequencies and correlate this with nodulation gene expression
can see root hair cells and cortical cells respond differently to queues
spiking frequency correlates with host ENOD11 (early nodulation gene 11) gene expression (fused to beta-glucuronidase)
author of study proposed that 36 consecutive spikes are sufficient to trigger ENDO11 expression
what is unique about medicago truncatula
medicago truncatula is a useful system for studying plant-microbial symbiosis
it has many of the advantages of arabidopsis but can form associations with mycorrhizal fungi and rhizobium
in both symbiosis, invasion of plant tissues by symbiont must involve a suppression of host defence responses
mutagenesis has allowed the unique and shared elements of the signal transduction pathway to be defined
what do arbuscular mycorrhiza secrete
a lipochitooligosaccharide
a mycorrhiza factor
it has a actyl glucosamine backbone and a lipophilic tail
what is the interaction between nod and receptors
bacteria produce nod factors, plant has receptors for these factors, these proteins are involved in all sorts of external sensing, once the nod factors bind the receptors they interact with another receptor sitting on the surface, this generares an internal signal pathway that transmits the signal into nucleus - causing calcium spiking this triggers infection
if there is calcium spiking and no infection= mutation downstream
if there is no calcium spiking the mutation is upstream of this
for mycorrhizal, the factors interact with a different receptor but the downstream routes/receptors become common
when do host cell proliferate
rhizobia induce root hair curling and infection thread formation in epidermal cells
inner cell layers are activated, pericycle cells opposite a proto-xylem pole undergo a limited number of cell divisions
inner cortical cells lose identity and enter the cell cycle a nodule primordium in the inner cortex forms
outer cortical cells enter the cell cycle but are arrested and will form pre infection threads
what is root nodule differentiation
cells of the nodule primordium obtain their final identity after rhizobia infection
cells at the apex of the primordium form a meristem
peripheral tissues limit gas exchnge
central tissue contains cells that host the rhizobia
what do bacteria differentiate into
bacteriods
differentiation of bacteria involves peptide signals from the host plant
a large number of bacterial genes are required for successful nodulation
bacterial genes for N2 fixation
nif- nitrogen fixation (similar to gene in free living N2 fixers)
fix- these genes are only found in nodulating strains
these include the structural genes for the nitrogenase, expression of these genes is regulated by the nif A gene product which is O2 sensitive. the genes for N2 fixation are only switched on when the O2 level drops late in nodule development
if the bacteria lack either the nif or fix genes, they will still form nodules but will not fix N2
what about plate genes in nodulation
plant genes can be split into EARLY and LATE nodulation genes
EARLY nodulation genes are involved in the development of the symbiosis - PsENDO12- pisum sativum early nodulation gene 12
LATE nodulation genes are involved in meeting the requirements of nitrogen fixation, they contain transport / metabolism genes such as glutamine synthase, uricase, sucrose synthase
leghaemoglobin, 25% of the nodule protein, binds o2 very tightly and releases it only at low o2 levels
What does oxygen act as
oxygen acts as a signal controlling bacterial development
once the plant late nodulation genes have been switches on, bacterial nif genes are switched one, bacterial nif A genes acts as o2 sensor, when o2 levels falls the nif A gene product switches on the expression of the genes encoding nitrogenase
what controls the number of nodules
N2 fixation is energetically expensive, over nodulation would limit plant growth, plant limits nodulation
mutations in HAR1 gene lead to over nodulation
plant gene encodes a serine threonine receptor kinase closely related to CLAVATA1
CLV1 regulated meristem development- HAR1 is proposed to fulful the same role in the symbiosis
also not all nodulation is caused by nod factors, such as in bradyhizobial species
what can mutation in cytokinin signalling lead to
mutations in cytokinin signalling can lead to nodule formation, mutations lead to spontaneaous nodule formation in the absence of rhizobia, the mutation leads to activation of the cytokinin singalling
crossing the snf2 mutation with a har mutant increases nodule formation further, control is thought to occur via cytokinin’s produced in the shoots
what happens between legumes and rhizobia
legumes can conditionally sanction rhizobia
legumes produce bigger nodules with more effective strains, if strains are mixed then the host plant provides more nutrients to the nodules containing the better strain
