The physiology of maize primary root growth under water deficits has been studied extensively (reviewed in Sharp et al., 2004, J Exp Bot 55: 2343-51). The research has taken advantage of a kinematic approach, which revealed different responses of cell elongation to water deficit in distinct regions of the growth zone, with elongation being maintained preferentially toward the apex. Previous work showed that longitudinal cell wall extensibility is enhanced in the apical region of the growth zone in water-stressed compared to well-watered roots, thereby facilitating the maintenance of cell elongation despite reduced turgor pressure. The biochemical basis of this response is poorly understood. Cell wall proteins (CWP) are believed to play important roles in controlling cell wall extension, and in this study, we extracted water soluble and loosely ionically-bound CWP using a vacuum infiltration-centrifugation technique, and examined protein profiles using 2-D gel electrophoresis and mass spectrometry. The results reveal major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 121 water-deficit responsive proteins were identified and categorized into six groups related to cell wall function: ROS metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, protein turnover, and other/unknown. In particular, the protein identifications, combined with in-situ assessment of apoplastic ROS levels, suggest that generation of apoplastic ROS is increased in the apical region of water-stressed roots, which could directly cause wall loosening. Integration of the results with microarray analysis of CWP gene expression will also be discussed. (Supported by NSF Plant Genome Program grant no. DBI-0211842.)