Something Old, Something New: Combining Growth Kinematics with Plasma Membrane Proteomics to Understand Maize Primary Root Growth Regulation Under Water Stress.

Maintenance of root growth is critical to plant adaptation to drought conditions. Previous work on the maize primary root revealed distinct responses to water stress in different regions of the growth zone; local elongation rates are preferentially maintained in the apical few mm, whereas elongation is prematurely inhibited as cells are displaced further from the apex, resulting in a shortened growth zone. These responses involve spatially differential and coordinated regulation of cellular growth processes, including modifications of both cell production and cell elongation. As the interface between the cytoplasm and the apoplast (including the cell wall), the plasma membrane (PM) is likely to play major functions in these processes. In addition, PM proteins may be involved in solute uptake for osmotic adjustment, pH regulation, ion homeostasis and other critical processes in roots growing under water-stressed conditions. Proteomic analyses provide a powerful approach to investigate the physiological basis of stress responses. However, to our knowledge, no proteomics studies have focused on the involvement of PM proteins in the response of root growth to water stress. Using a simplified method for enrichment of PM proteins, we compared the developmental distribution of PM proteins in the growth zone of well-watered and water-stressed maize primary roots. The results identified 432 proteins with differential abundances, and the majority of these changes involved distinct, region-specific patterns of response. The identities of the stress-responsive proteins suggest involvement in diverse biological processes including modification of ion transport and homeostasis, accumulation of sugars and other osmolytes, and changes in membrane lipid and cell wall composition. Integration of these findings with results from physiological, transcriptomic, cell wall proteomic and metabolomic studies reveals novel insights into root growth adaptation to water stress.