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

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.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of East Anglia

Year dc:date.issued

2012

Author dc:creator

• Miller, Ben

Language dc:language

en