Defining the mechanisms of specificity in the symbiosis signalling pathway of Medicago truncatula

Miller, Ben (2012) Defining the mechanisms of specificity in the symbiosis signalling pathway of Medicago truncatula. Doctoral thesis, University of East Anglia.

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Legume plants are able to form symbiotic interactions with rhizobial bacteria and
arbuscular mycorrhizal fungi. The establishment of these two symbioses depends
upon signalling between the plant host and the microorganism, of which
lipochitooligosaccharide (LCO) signals are essential. Perception of symbiotic LCOs
induces a signalling pathway which is common to both mycorrhization and
nodulation, and oscillations of calcium in the nucleus (so-called calcium spiking) are
central to this common symbiosis signalling pathway. Detailed analysis of Medicago
truncatula gene expression in response to rhizobial and mycorrhizal LCOs reveals
relatively little overlap in gene induction. The nodulation-specific marker NIN was
induced by both rhizobial and sulphated mycorrhizal LCOs. However, the
mycorrhization-specific marker MSBP1 was only induced by non-sulphated
mycorrhizal LCOs. Importantly, this differential induction of NIN and MSBP1 by
LCOs was dependent on components of the common symbiosis signalling pathway.
Immediately downstream of calcium spiking lies a calcium- and
calcium/calmodulin-dependent protein kinase (CCaMK) which is believed to decode
calcium spiking. It has been hypothesised that CCaMK activates differential
signalling outputs in response to specific calcium signatures associated with each
symbiosis. A model for the activation of CCaMK via autophosphorylation has
previously been proposed, but is unable to explain symbiosis signalling specificity.
A mutational approach was therefore undertaken to determine the importance of
calcium- and calmodulin-binding during the activation of CCaMK. Interestingly,
most mutations which blocked nodulation also blocked mycorrhization. However,
the identification of a mutation which impairs mycorrhization but permits
nodulation suggests that CCaMK may mediate specificity via this putative
phosphorylation site.
A multidisciplinary study based on this mutational analysis has proposed a new
model for CCaMK activation which relies upon differentially regulated
autophosphorylation of CCaMK in response to calcium and calmodulin. A full
mechanism to explain how CCaMK determines specificity has yet to be elucidated.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Users 2259 not found.
Date Deposited: 02 May 2013 13:19
Last Modified: 02 May 2013 13:19


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