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Substantial reductions in yield caused by drought stress can occur when parts of the root system experience water deficit even though other parts have sufficient access to soil water. To identify proteins associated to drought signaling, rice (Oryza sativa L. cv. IR64.) plants were transplanted into plastic pots with an internal wall dividing each pot into two equal compartments, allowing for equal distribution of soil and the root system between these compartments. The following treatments were applied: either both compartments were watered daily ("wet" roots), or water was withheld from both compartments ("dry" roots), or water was withheld from only one of the two compartments in each pot ("wet" and "dry" roots). The substantial differences in physiological parameters of different growth conditions were accompanied by differential changes in protein abundances. Label-free quantitative shotgun proteomics have resulted in identification of 1383 reproducible proteins across all three conditions. Differentially expressed proteins were categorized within 17 functional groups. The patterns observed were interesting in that in some categories such as protein metabolism and oxidation-reduction, substantial numbers of proteins were most abundant when leaves were receiving signals from "wet" and "dry" roots. In yet other categories such as transport, several key transporters were surprisingly abundant in leaves supported by partially or completely droughted root systems, especially plasma membrane and vacuolar transporters. Stress-related proteins behaved very consistently by increasing in droughted plants but notably some proteins were most abundant when roots of the same plant were growing in both wet and dry soils. Changes in carbohydrate-processing proteins were consistent with the passive accumulation of soluble sugars in shoots under drought, with hydrolysis of sucrose and starch synthesis both enhanced. These results suggest that drought signals are complex interactions and not simply the additive effect of water supply to the roots.

Rice (Oryza sativa L. cv. IR64) was grown in split-root systems to analyze long-distance drought signaling within root systems. This in turn underpins how root systems in heterogeneous soils adapt to drought. The approach was to compare four root tissues: (1) fully watered; (2) fully droughted and split-root systems where (3) one-half was watered and (4) the other half was droughted. This was specifically aimed at identifying how droughted root tissues altered the proteome of adjacent wet roots by hormone signals and how wet roots reciprocally affected dry roots hydraulically. Quantitative label-free shotgun proteomic analysis of four different root tissues resulted in identification of 1487 nonredundant proteins, with nearly 900 proteins present in triplicate in each treatment. Drought caused surprising changes in expression, most notably in partially droughted roots where 38% of proteins were altered in level compared to adjacent watered roots. Specific functional groups changed consistently in drought. Pathogenesis-related proteins were generally up-regulated in response to drought and heat-shock proteins were totally absent in roots of fully watered plants. Proteins involved in transport and oxidation-reduction reactions were also highly dependent upon drought signals, with the former largely absent in roots receiving a drought signal while oxidation-reduction proteins were strongly present during drought. Finally, two functionally contrasting protein families were compared to validate our approach, showing that nine tubulins were strongly reduced in droughted roots while six chitinases were up-regulated, even when the signal arrived remotely from adjacent droughted roots.