Characterization of AtCNGC11/12-induced Cell Death and the Role of AtCNGC11 and AtCNGC12 in Ca2+ Dependent Signalling Pathways
Keywords: cyclic nucleotide gated ion channels
calcium
cpr22
Programmed cell death
cngc12
pathogen defence signalling
dark-induced senescence
gravitropism

The Arabidopsis cyclic nucleotide-gated ion channels (AtCNGCs) form a large family consisting of 20 members. It has been suggested that CNGCs contribute to a wide array of biological functions such as pollen tube growth and pathogen defence signalling. However, the precise mechanisms by which AtCNGCs act, and the extent of their biological roles, have yet to be fully elucidated. AtCNGC11/12, the chimeric CNGC that resulted from the fusion of AtCNGC11 and 12, induces a number of pathogen defence related phenotypes in the Arabidopsis mutant cpr22. Spontaneous lesion formation is one such phenotype. Interestingly, when AtCNGC11/12 is transiently expressed in N. benthamiana it causes cell death which was characterized in this study. Also, AtCNGC11/12 was used to investigate the structural features responsible for the proper function and regulation of AtCNGCs. Electron microscopic analysis of the AtCNGC11/12-induced cell death showed similar characteristics to programmed cell death (PCD), such as plasma membrane shrinkage and vesicle formation. Interestingly caspase-1 inhibitors and the silencing of vacuolar processing enzyme, a plant enzyme with caspase-1 activity, suppressed the induction of cell death. Additionally, pharmacological analyses indicated that the AtCNGC11/12-indiced cell death was also dependent on Ca2+. Furthermore, 3 amino acid residues, R190, A225, and G287, were demonstrated to be essential for AtCNGC11/12-induce cell death. Taken together, these results indicate that the cell death that develops in the cpr22 mutant is indeed PCD and that AtCNGC11/12, is at the point of, or up-stream of, the Ca2+ signal necessary for the development of HR. Furthermore, the functionality of AtCNGC11/12 as a model for AtCNGC structure-function analyses was demonstrated by the identification of several amino acids necessary for cell death development. Yoshioka et al. (2006) demonstrated that the loss of AtCNGC11 or 12 results in decreased resistance to avirulent isolates of the oomycete pathogen, H. arabidopsidis. Thus, the present biological role suggested for AtCNGC11 and 12 is in pathogen defence, specifically within effector triggered immunity (ETI). Like AtCNGC11 and 12, AtCNGC2 has been demonstrated to contribute to pathogen defence signalling but has also been implicated in other physiological responses such as ion stress and senescence. To better understand the roles of AtCNGC11 and 12 in both pathogen defence and other Ca2+ dependent signalling processes, I have investigated promoter:GUS reporter lines, as well as, AtCNGC11 and 12 KO and RNAi silenced lines subjected to various treatments. From this work, I have demonstrated that AtCNGC11 and 12 have similar expression patterns during pathogen defence, development, and dark-induced senescence. Additionally, the findings presented here further characterize AtCNGC11 and 12 as contributors to ETI rather than PAMP triggered immunity. Furthermore, I demonstrated that AtCNGC11 and 12 are likely involved in the endogenous movement of Ca2+, contributing to a range of Ca2+ associated signalling pathways including gravitropism and senescence. Taken together, these results have greatly improved the characterization of AtCNGC11 and 12; significantly contributing to the understanding of a large and increasingly important channel family.