Plant root growth is affected by both gravity and mechanical stimulation . A coordinated response to both stimuli requires specific and common elements. To delineate the transcriptional response mechanisms, we carried out whole-genome microarray analysis of Arabidopsis root apices after gravity stimulation (reorientation) and mechanical stimulation and monitored transcript levels of 22,744 genes in a time course during the first hour after either stimulus. Rapid, transient changes in the relative abundance of specific transcripts occurred in response to gravity or mechanical stimulation, and these transcript level changes reveal clusters of coordinated events. Transcriptional regulation occurs in the root apices within less than 2 min after either stimulus. We identified genes responding specifically to each stimulus as well as transcripts regulated in both signal transduction pathways. Several unknown genes were specifically induced only during gravitropic stimulation (gravity induced genes). We also analyzed the network of transcriptional regulation during the early stages of gravitropism and mechanical stimulation.Plants adapt their growth in response to environmental cues. Gravity is a constant force that guides the direction of plant growth. Mechanical stimuli such as wind, rain, and obstacles in the soil trigger changes in growth patterns (Braam and Davis, 1990;. Gravitropic and mechanical stimulation in the root are interactive processes, mutually influencing differential growth (Mullen et al., 2000;Fasano et al., 2002;. The mechanisms of sensing and signal transduction for either stimulus are not well understood, but it has been shown that the transcript levels of specific genes are regulated early during signal transduction after either stimulus Braam and Davis, 1990).It is widely accepted that gravitropic stimulation by reorientation is perceived by sedimentation of starchcontaining plastids (statoliths) in the columella cells of the root tip (Kiss et al., 1989(Kiss et al., , 1996Juniper et al., 1966;Blancaflor et al., 1998). Columella cells show differences in their contribution toward gravity perception and in the velocity of sedimentation of their statoliths (Blancaflor et al., 1998). While complete sedimentation of the statoliths requires at least 5 min (Blancaflor et al., 1998;MacCleery and Kiss, 1999), the presentation time (duration of the stimulus that is required to establish a response) was estimated at only approximately 1 min for wild-type Arabidopsis root tips (Blancaflor et al., 1998). Statolith sedimentation is postulated to disrupt the actin-based cytoskeletal network and its links to plasma membrane receptors in some regions of the cell cortex (Yoder et al., 2001). Other groups hypothesized that statoliths are directly connected to the cytoskeleton, activating membrane proteins anchored to the filaments upon dislocation or activation of mechanosensitive ion channels (Evans et al., 1986;Pickard and Ding, 1993;Collings et al., 2001;Blancaflor, 2002).The transduction of the physical alt...