Spatial distribution of transcript changes in the maize primary root elongation zone at low water potential

Background: Previous work showed that the maize primary root adapts to low Ψw (-1.6 MPa) by maintaining longitudinal expansion in the apical 3 mm (region 1), whereas in the adjacent 4 mm (region 2) longitudinal expansion reaches a maximum in well-watered roots but is progressively inhibited at low Ψw. To identify mechanisms that determine these responses to low Ψw, transcript expression was profiled in these regions of water-stressed and well-watered roots. In addition, comparison between region 2 of water-stressed roots and the zone of growth deceleration in well-watered roots (region 3) distinguished stress-responsive genes in region 2 from those involved in cell maturation. Results: Responses of gene expression to water stress in regions 1 and 2 were largely distinct. The largest functional categories of differentially expressed transcripts were reactive oxygen species and carbon metabolism in region 1, and membrane transport in region 2. Transcripts controlling sucrose hydrolysis distinguished well-watered and water-stressed states (invertase vs. sucrose synthase), and changes in expression of transcripts for starch synthesis indicated further alteration in carbon metabolism under water deficit. A role for inositols in the stress response was suggested, as was control of proline metabolism. Increased expression of transcripts for wall-loosening proteins in region 1, and for elements of ABA and ethylene signaling were also indicated in the response to water deficit. Conclusion: The analysis indicates that fundamentally different signaling and metabolic response mechanisms are involved in the response to water stress in different regions of the maize primary root elongation zone

Root Growth under Drought --- Role of Cell Wall Localized Reactive Oxygen Species [abstract]

Only abstract of poster available.Track V: BiomassDrought is the most important cause of crop failure in Missouri and limits crop production in large parts of the US and the world. The root system is critical to plant adaption and crop productivity in drought-prone environments. Some types of roots have the ability to maintain elongation under severe water deficit levels which completely inhibit shoot growth. Previous work on maize primary root growth under water deficit conditions showed that cell elongation is maintained in the apical region of the growth zone but progressively inhibited further from the apex. In association with these growth responses, cell wall extensibility is enhanced in the apical region but decreased in the basal region of the root growth zone. Cell wall proteomic analyses were conducted to identify proteins important for wall extensibility and elongation (Zhu et al. 2007, Plant Physiol. 145: 1533-48). The results revealed predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. Several cell wall proteins related to reactive oxygen species (ROS) generation showed an increased abundance in the apical region of water-stressed roots, prominent among them being putative oxalate oxidases, which result in hydrogen peroxide generation. An increase in cell wall localized ROS in the apical region of water-stressed roots was confirmed by in-situ imaging. ROS could have cell wall loosening or tightening effects and these effects could be region specific. To understand the role of oxalate oxidase/cell wall localized ROS in root elongation, we are studying a transgenic maize line constitutively expressing a wheat oxalate oxidase gene (Ramputh et al. 2002, Plant Sci. 162: 431-440). The results show differential effects on growth and growth-related processes in well-watered and water-stressed roots. Experiments to determine the mechanisms of root growth regulation by oxalate oxidase/cell wall-localized ROS are in progress. This information could be used to develop plants that produce appropriate amounts of ROS to enhance root growth under water deficit conditions. The results may also be applicable to understanding and manipulating plant growth responses to other environmental challenges

Cell Wall Proteome in the Maize Primary Root Elongation Zone. II. Region-Specific Changes in Water Soluble and Lightly Ionically Bound Proteins under Water Deficit1[W][OA]

Previous work on the adaptation of maize (Zea mays) primary roots to water deficit showed that cell elongation is maintained preferentially toward the apex, and that this response involves modification of cell wall extension properties. To gain a comprehensive understanding of how cell wall protein (CWP) composition changes in association with the differential growth responses to water deficit in different regions of the elongation zone, a proteomics approach was used to examine water soluble and loosely ionically bound CWPs. The results revealed major and predominantly region-specific changes in protein profiles between well-watered and water-stressed roots. In total, 152 water deficit-responsive proteins were identified and categorized into five groups based on their potential function in the cell wall: reactive oxygen species (ROS) metabolism, defense and detoxification, hydrolases, carbohydrate metabolism, and other/unknown. The results indicate that stress-induced changes in CWPs involve multiple processes that are likely to regulate the response of cell elongation. In particular, the changes in protein abundance related to ROS metabolism predicted an increase in apoplastic ROS production in the apical region of the elongation zone of water-stressed roots. This was verified by quantification of hydrogen peroxide content in extracted apoplastic fluid and by in situ imaging of apoplastic ROS levels. This response could contribute directly to the enhancement of wall loosening in this region. This large-scale proteomic analysis provides novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions and highlights the spatial differences in CWP composition in the root elongation zone