Root Growth Under Water Deficits: Physiological Complexity and Genetic Diversity.

Under water deficits, the growth responses of different plant parts are varied and complex. Some types of roots continue elongation at low water potentials which completely inhibit shoot growth. Also, root systems may exhibit developmental plasticity, including increases in rooting depth and proliferation of lateral roots, which extend their water-absorbing surface area. The regulation of root growth responses to water deficits is poorly understood, however. Our research on mechanisms of root growth maintenance at low water potentials takes advantage of a kinematic approach; in both maize and soybean primary roots under water deficits, cell elongation is maintained in the apical region of the growth zone but inhibited as cells are displaced further from the apex. This characterization and the physiological knowledge gained to date provide a powerful underpinning for functional genomics studies (Sharp et al., 2004, J Exp Bot 55: 2343-51). We have used proteomics approaches to gain a comprehensive understanding of stress-induced changes in protein profiles within the root growth zone, with particular interest in cell wall proteins. The results reveal major and predominantly region-specific changes in protein composition, and provide novel insights into the complexity of mechanisms that regulate root growth under water deficit conditions. In particular, the results indicate that reactive oxygen species metabolism may be important for modifying cell wall yielding properties as well as protection from oxidative damage. We are also studying genetic diversity in root system developmental plasticity under water deficits in soybean, in order to enable physiological and genetic analyses of the regulatory mechanisms involved. The results reveal substantial diversity in the capacity for increased lateral root development (number and total length of lateral roots produced) under water deficit conditions, with consistent genotypic differences between seedling and more mature plant systems. Supported by NSF, Monsanto/University of Missouri, and the Missouri Soybean Merchandising Council.