Regulation of root growth responses to water deficit

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Abstract

The growth and function of roots are essential for crop productivity under water-limiting conditions, but direct improvement of roots by plant breeding has been slow. One difficulty is the observation and quantitative measurement of root systems under conditions that are relevant to field environments. Another challenge is the identification of and selection for specific loci that could improve the acquisition of water from the soil profile. However, advances are being made in the understanding of root growth regulation and development. We review the evidence for the maintenance of root growth by ABA during water deficit, and the interactions with ethylene and other hormones. A biophysical model of cell expansion serves to focus discussion of topics relating to regulation of growth and development. The power of kinematic growth analysis is demonstrated by highlighting changes in growth regulatory processes and associated patterns of gene expression and protein composition that occur specifically in regions of the root where cell expansion is maintained under water deficit conditions. Growth is a complex process; new information adds further insight and further complexity to our understanding of how roots sense and respond to changes in environmental conditions. It is important to unravel these adaptive mechanisms so that it is clear how the manipulation of one process will affect the function of the whole plant, and so that the effect on final yield and water use can be predicted. This complexity makes simple linear models inadequate as explanatory tools, and a systems approach is needed to incorporate the weave of interacting networks of signaling and response pathways. The real challenge is to discover how root growth can be improved, to supply breeders with the practical tools to identify or introduce superior alleles in crop species, and ultimately to ensure that discoveries lead to improvements in productivity in the field