Gravity and light: integrating transcriptional regulation in roots.

Light and gravity give clues about time and space which plants use to direct their growth. Roots grow towards the vector of gravity (positive gravitropism) to access water and nutrients in the soil and to provide stability. Their response to light depends on its wavelength and intensity. Roots grow towards red light (positive phototropism) and away from blue and white light (negative phototropism) (Kiss et al. 2003). Plants respond to these signals via differential cell elongation which results in directional growth. Light and gravity are simultaneous stimuli so plants must respond to both at the same time and integrate them into a single response. Our hypothesis is that integration of light and gravity responses in roots can be observed at the level of gene expression. One of the fastest known signal transduction elements described for gravity and light responses is inositol- 1,4,5 triphosphate (InsP3). Cellular levels of InsP3 increase within 15s to 30s in response to stimulation by gravity or light, respectively (Morse et al. 1987; Perera et al. 1999). We therefore analyzed light induced changes in transcript abundances of genes known to respond specifically to gravity in roots of wild type and transgenic plants dampened in their InsP3-signaling (Kimbrough et al. 2004; Perera et al., in preparation.).