Contrasting root traits and native regulation of aquaporin differentially determine the outcome of overexpressing a single aquaporin (OsPIP2;4) in two rice cultivars.

Overexpressing OsPIP2;4 in the two rice cultivars Giza178 and IR64 resulted in contrasting cultivar-dependent physiological attributes under control and drought conditions in the field. In the T3 plants, PIP2;4 expression was significantly higher in the leaves and roots of Giza178 under control but only in the roots under drought condition and higher in the leaves and roots of IR64 under control but not under drought condition compared with that in the corresponding wild types. The transgene improved the plant growth in Giza178 under both growth conditions but had no significant effect in IR64 under either condition. The transgenic lines of Giza178 recovered their leaf relative water content faster than those of the wild type in the afternoon and showed improved gas exchange parameters, water use efficiency, and grain yield, as a result of improved root hydraulic conductivity (Lp _r ) and xylem sap flow. No comparable responses were found in IR64 although Lp _r and xylem sap flow were enhanced in the transgenic lines under the control condition only, suggesting that the positive effect of PIP2;4 on the well-watered leaves of IR64 was offset by the low root/shoot ratio and the inherent expression of other aquaporins. In the transgenic plants of IR64 under drought, PIP2;4 expression was not induced in the roots presumably due to an overriding post-transcription regulatory mechanisms, leading to the lack of changes in the Lp _r and xylem sap flow and consequently, the plant growth, water relations, gas exchange, and grain yield were similar to the wild type. The data suggest that the outcome of overexpressing a single aquaporin gene depends on the plant architecture, internal responses to drought, and native expression of other aquaporins.