When there is little rainfall and upper soil layers are depleted of moisture, plants rely on the ability of root systems to proliferate throughout the soil prole to extract water. e patterns of root growth and soil drying are spatially heterogeneous, such that regions of the soil are dried unevenly, leaving patches or layers of moisture not yet explored by roots (Hodge et al. 2009; White and Kirkegaard 2010; Kano et al. 2011; Miyazawa et al. 2011). In some circumstances, to reach moisture, roots must pass through soil that is already dry because other roots-perhaps of neighboring plants-have previously extracted the soil water. In other circumstances, seed sown in dry soil germinates when surface layers are wetted, but seedling roots must penetrate through dry soil to nd moisture in deeper layers. In such situations, roots must be able to continue growing through a soil matrix that is oen at water potentials (ψw) that may be inhibitory to growth. For example, under drought conditions, the nodal root axes of maize (which are produced from the stem nodes) are challenged to grow into and through surface soil that may have become very dry. In these circumstances, the water for continued root growth can be supplied to the root tip via the phloem (Boyer et al. 2010). In addition, it was demonstrated that maize nodal roots are able to continue growing at tissue ψw lower than those that are inhibitory for the growth of leaves, stem, and

reproductive structures (Figure 35.1; Sharp and Davies 1979; Westgate and Boyer 1985). Similarly, the primary root of several crop species is able to continue growing at low soil ψw that completely inhibits shoot growth (Sharp et al. 1988; Spollen et  al. 1993; Yamaguchi et al. 2010). ese ndings indicate some form of internal regulation within the root growth zone that allows the maintenance of cell elongation under these conditions. e mechanisms underlying primary root growth maintenance at low ψw have been studied extensively, as reviewed previously (Sharp et al. 2004; Ober and Sharp 2007; Yamaguchi and Sharp 2010). However, large gaps remain in our understanding of how the growth of roots in response to low ψw is controlled and how the molecular physiology that regulates root growth and development at high ψw is modulated to enable continuation of growth when water is limiting.