Root Growth Under Water Deficits: Physiological Complexity and Coordination.

The root system is critical to plant adaptation to drought conditions. Some roots can maintain elongation under water stress levels that completely inhibit shoot growth, and the physiological mechanisms underlying this ability are important to understand. In the maize primary root under low water potential conditions, cell elongation is maintained in the apical region of the growth zone but progressively inhibited further from the apex. These responses involve spatially differential and coordinated regulation of cellular growth processes, including modifications of cell wall extensibility, and provide a powerful foundation for functional genomics studies. This presentation will focus on two areas of current investigation. 1) Proteomic analysis revealed region-specific changes in cell wall protein composition. In particular, the abundance of proteins related to reactive oxygen species (ROS) generation increased in the apical region, including oxalate oxidases (OxO), which produce hydrogen peroxide. To investigate the role of OxO/apoplastic ROS in root elongation, transgenic maize lines expressing a wheat OxO gene are being characterized. Interestingly, results indicate that cell production rather than expansion processes are regulated. 2) Ferulates are important in cross-linking cell wall components, and microarray analysis suggested that regulation of ferulate metabolism may contribute to the differential changes in wall extensibility between the apical and basal regions of the growth zone under water stress. To evaluate whether ferulate accumulation in the basal region prohibits wall extension, root segments were treated with feruloyl esterase (FE) to release wall-bound ferulates. An extensometer system showed that FE treatment restores expansin-induced wall extension in the basal region of water-stressed roots, while having a minimal effect in the same region of well-watered roots. These studies provide novel insights into the complexity and coordination of root growth regulation and adaptation under water deficit conditions. This research was supported in part by a grant from Monsanto.