Expression profile analysis of early development and gravity response of germinating Ceratopteris richardii spores

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Salmi, Mari L.

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Single-celled germinating spores of the fern Ceratopteris richardii possess a mechanism to detect the directional force of gravity, and they respond to this force by establishing the axis of their development in the direction of the gravity vector prior to their first cell division. In this dissertation, I describe the construction of cDNA microarrays containing almost 3000 different genes expressed in germinating Ceratopteris spores 20 h after they are induced to germinate by light, just after gravity has established the polarity of their growth. Using these microarrays, I have analyzed gene expression changes that occur over time during early development and while the spores were being subjected to the microgravity environment of spaceflight on Shuttle mission STS93. Selected changes detected by microarray analysis were verified by quantitative real-time RT-PCR. In the early development study I used microarrays to evaluate the expression profile of spores during the first 48 h of development as they break dormancy, respond to gravity, and prepare for the first cell division. Spore-expressed genes have been compared to the genes uniquely expressed in two other germinating systems, Arabidopsis seeds and Arabidopsis pollen. My results provide novel data on genes expressed during spore germination, and thus give new insights into the molecular mechanism by which plant cells accomplish the important process of emerging from their dormant phase. This dissertation also presents the first microarray analysis of the changes in gene expression induced by spaceflight in a plant system. Spores flown on NASA flight STS-93 were compared to ground-control samples. Results are discussed in relation to both gene expression changes induced by gravity on earth and to physiological changes observed in other plant systems as a response to microgravity. They are consistent with a model for early cellular responses to gravity perception that include a cytoplasmic pH change and/or calcium mediated polar vesicle transport.



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