On the Inside: Figure 1.

Germinating spores of the fern Ceratopteris richardii respond to gravity. The earliest detected response in this system is a gravity-directed Ca2+ current that rapidly reverses direction when the cells are turned upside-down. The orientation of this current predicts the direction of cell polarization, including the direction of nuclear migration, and, 48 h later, the direction of primary rhizoid growth, which is indicated by the site where Ca2+ entered the cell. Salmi et al. (pp. 94–104) present pharmacological results that support a role for nitric oxide (NO) and cGMP in mediating the responses of germinating Ceratopteris spores to gravity. The authors report that Viagra, a cGMP phosphodiesterase inhibitor that increases NO-dependent cGMP levels, disrupted the gravity-directed changes in cell polarity in a dose-dependent manner. The effects of Viagra effects were antagonized by application of NO scavengers. To identify other components of the signaling system, the authors analyzed gene expression changes induced by Viagra treatment using microarrays and quantitative real-time reverse transcription-PCR. Preliminary microarray analyses revealed that the expression of several genes, possibly involved in actin dynamics and chaperone function, was significantly altered by Viagra treatment. Thus, NO and cGMP appear to act as downstream effectors linking the gravity stimulus to polarized growth in C. richardii spores.