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Pollen tube growth is a dynamic system expressing a number of oscillating circuits. Our recent work identified a new circuit, oscillatory efflux of Cl(-) anion from the pollen tube apex. Cl(-) efflux is the first ion signal found to be coupled in phase with growth oscillations. Functional analyses indicate an active role for Cl(-) flux in pollen tube growth. In this report the dynamical properties of Cl(-) efflux are examined. Phase space analysis demonstrates that the system trajectory converges on a limit cycle. Fourier analysis reveals that two harmonic frequencies characterize normal growth. Cl(-) efflux is inhibited by the channel blocker DIDS, is stimulated by hypoosmotic treatment, and is antagonized by the signal encoded in inositol 3,4,5,6-tetrakisphosphate. These perturbations induce transitions of the limit cycle to new metastable states or cause system collapse to a static attractor centered near the origin. These perturbations also transform the spectral profile, inducing subharmonic frequencies, transitions to period doubling and tripling, superharmonic resonance, and chaos. These results indicate that Cl(-) signals in pollen tubes display features that are characteristic of active oscillators that carry frequency-encoded information. A reaction network of the Cl(-) oscillator coupled to two nonlinear feedback circuits that may drive pollen tube growth oscillations is considered.

Ionotropic glutamate receptors are ligand-gated, multimeric cation channels involved in neuronal transmission in vertebrates. In animals, ligand selectivity is highly specific, and depending on the subunit, channels open in response to glycine/D-serine or glutamate. The molecular evolution of the animal glutamate receptor family is an important element in the evolution of the central nervous system. Interestingly, glutamate receptor homologous channels (GLRs) can also be found in plants where they are, for instance, involved in calcium signaling during pollen tube growth or in long distance signaling. While the genome of the moss Physcomitrella patens codes for two GLRs, the GLR family largely diversified in flowering plants. The genome of Arabidopsis thaliana encodes 20 GLRs that subgroup in three clades, one of which is homologous to the moss GLRs, and two clades that are specific for higher plants.Our work consists of studying the molecular specificities of plant GLRs by addressing two main questions: (1) What is the evolutionary meaning of plant GLR diversification? (2) In which signaling pathways are GLRs from the different clades involved in? To address these questions we performed an electrophysiological characterization of GLRs by studying their activity in protoplasts from moss and Arabidopsis GLR knock-plants. To further characterize the function of GLRs, we also expressed and studied them in heterologous systems.Our work reveals unique gating properties of plant GLRs and gives novel insights into the specificity of plant GLRs compared to the animal ones.