RNA-seq analysis identifies potential modulators of gravity response in spores of Ceratopteris (Parkeriaceae): evidence for modulation by calcium pumps and apyrase activity.

Gravity regulates the magnitude and direction of a trans-cell calcium current in germinating spores of Ceratopteris richardii. Blocking this current with nifedipine blocks the spore's downward polarity alignment, a polarization that is fixed by gravity ∼10 h after light induces the spores to germinate. RNA-seq analysis at 10 h was used to identify genes potentially important for the gravity response. The data set will be valuable for other developmental and phylogenetic studies. De novo Newbler assembly of 958 527 reads from Roche 454 sequencing was executed. The sequences were identified and analyzed using in silico methods. The roles of endomembrane Ca(2+)-ATPase pumps and apyrases in the gravity response were further tested using pharmacological agents. Transcripts related to calcium signaling and ethylene biosynthesis were identified as notable constituents of the transcriptome. Inhibiting the activity of endomembrane Ca(2+)-ATPase pumps with 2,5-di-(t-butyl)-1,4-hydroquinone diminished the trans-cell current, but increased the orientation of the polar axis to gravity. The effects of applied nucleotides and purinoceptor antagonists gave novel evidence implicating extracellular nucleotides as regulators of the gravity response in these fern spores. In addition to revealing general features of the transcriptome of germinating spores, the results highlight a number of calcium-responsive and light-receptive transcripts. Pharmacologic assays indicate endomembrane Ca(2+)-ATPases and extracellular nucleotides may play regulatory roles in the gravity response of Ceratopteris spores.