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
