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

Abstract

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.

A Hydraulic Model Is Compatible with Rapid Changes in Leaf Elongation under Fluctuating Evaporative Demand and Soil Water Status

“…The plant hydraulic conductance increased during the morning, consistent with earlier reports (Lopez et al, 2003;Sakurai-Ishikawa et al, 2011;Hachez et al, 2012). This limited the decline in LER due to stomatal opening, consistent with the model that could not predict experimental data of LER without an appreciable increase in plant conductance.…”

“…Diurnal change of gene and/or protein expression of most PIPs was previously reported in maize roots and mature parts of the leaf, but not in the leaf elongation zone (Lopez et al, 2003;Hachez et al, 2008Hachez et al, , 2012. However, PIP phosphorylation, resulting in increased water channel activity, is also likely to occur in the early morning, as it is ubiquitous in the control of PIP activity (Maurel et al, 2008;Van Wilder et al, 2008).…”

Aquaporin structure–function relationships: Water flow through plant living cells

“…Two aquaporin-encoding transcripts were found to be down-regulated during the first 15 and 60 min of a salt treatment, respectively. The expression level of the two transcripts then recovered and, after 7 d, expression had become higher than in plants grown in standard conditions [118].…”

“…These changes include a coordinated transcriptional down-regulation and subcellular relocalization of both PIPs and TIPs [117]. On the other hand, salt and water stresses induced the accumulation of ZmTIP2-3 transcripts [118]. Similar findings on the dominance of symplastic water transport were obtained using transgenic tobacco plants expressing an antisense construct of the tobacco NtAQP1 gene that encodes another PIP1b isoform [25], while Barley HvPIP2;1 is a plasma membrane aquaporin and its expression was downregulated after salt stress in barley [38].…”

Rapid changes in root hydraulic conductivity and aquaporin expression in rice (Oryza sativaL.) in response to shoot removal – xylem tension as a possible signal

“…Water flow rate through AQPs, as an alternative signal regulating root AQP expression, could also explain diurnal changes in root AQP expression (e.g. Henzler et al, 1999;Lopez et al, 2003;Hachez et al, 2008;Vandeleur et al, 2009;Sakurai-Ishikawa et al, 2011). The question needs to be addressed of at which time scale protein levels of rice root AQPs change in response to shoot removal.…”