Diurnal Regulation of Water Transport and Aquaporin Gene Expression in Maize Roots: Contribution of PIP2 Proteins

In maize (Zea mays) roots, xylem water transfer supported by root pressure occurs during the day and is less important at night. Diurnal modifications of osmotic pressure gradient between medium and xylem could not explain the oscillation of water flux in young maize roots during the day-night cycle. We observed a high turgor pressure of root cortical cells associated with a high flux. In maize roots, ZmPIP transcripts oscillate during the day-night cycle exhibiting some characteristics of genes regulated by a circadian mechanism. The PIP protein level profile is different from the mRNA pattern. Moreover, ZmPIP1 and ZmPIP2 protein levels are differentially regulated during the light and dark period and in response to continuous darkness suggesting different roles for both classes of PIP. Finally, our results suggest that aquaporins from ZmPIP2 subgroup may contribute to root water transfer by cellular pathway that occurs during the light and the dark period of the day-night cycle.