Roles of Morphology, Anatomy, and Aquaporins in Determining Contrasting Hydraulic Behavior of Roots  

Abstract

The contrasting hydraulic properties of wheat (Triticum aestivum), narrow-leafed lupin (Lupinus angustifolius), and yellow lupin (Lupinus luteus) roots were identified by integrating measurements of water flow across different structural levels of organization with anatomy and modeling. Anatomy played a major role in root hydraulics, influencing axial conductance (L ax ) and the distribution of water uptake along the root, with a more localized role for aquaporins (AQPs). Lupin roots had greater L ax than wheat roots, due to greater xylem development. L ax and root hydraulic conductance (L r ) were related to each other, such that both variables increased with distance from the root tip in lupin roots. L ax and L r were constant with distance from the tip in wheat roots. Despite these contrasting behaviors, the hydraulic conductivity of root cells (Lp c ) was similar for all species and increased from the root surface toward the endodermis. Lp c was largely controlled by AQPs, as demonstrated by dramatic reductions in Lp c by the AQP blocker mercury. Modeling the root as a series of concentric, cylindrical membranes, and the inhibition of AQP activity at the root level, indicated that water flow in lupin roots occurred primarily through the apoplast, without crossing membranes and without the involvement of AQPs. In contrast, water flow across wheat roots crossed mercury-sensitive AQPs in the endodermis, which significantly influenced L r . This study demonstrates the importance of examining root morphology and anatomy in assessing the role of AQPs in root hydraulics.

The effects of the combination of salinity and excess boron on the water relations of tolerant tomato (Solanum lycopersicum L.) cv. Poncho Negro, in relation to aquaporin functionality

“…This has been demonstrated by the fact that the tomato Poncho Negro, moderately tolerant of high salinity and excess B, grows in the Lluta valley in northern Chile under these conditions. Therefore, our results suggest that the influx of water in tomato Poncho Negro may be determined by the extent to which water flow predominates in the apoplastic and symplastic pathways according to the composite model (Steudle and Heydt, 1997), where the relative hydraulic conductivity of the pathways and the relative magnitudes of the hydrostatic and osmotic gradients drive the water transport and vary under the stress conditions (Steudle and Heydt, 1997;Bramley et al, 2009). Thus, this effect was higher in NaCltreated plants, where the regulation by the stomatal conductance was reduced.…”

“…Aquaporins, or major intrinsic proteins (MIPs), are channel-forming membrane proteins with the extraordinary ability to combine a high flux across biological membranes with a high specificity for water (Zhao et al, 2008). These membrane integral proteins form water-conducting channels, which is considered responsible for the variable conductivity of root systems (Javot and Maurel, 2002;Bramley et al, 2009).There are two highly-important aspects of plant aquaporins (Maurel and Chrispeels, 2001). One is their tremendous diversity in plants that can be explained in part by their presence in multiple sub-cellular compartments (Chrispeels and Agre, 1994;Takata et al, 2004) and the other is the fact that some aquaporins are multi-functional channel proteins, allowing some small, neutral solutes, such as CO 2 , urea, B and hydrogen peroxide, to move across cellular membranes (Dordas and Brown, 2001;Hachez et al, 2006).…”

Does Morphological and Anatomical Plasticity during the Vegetative Stage Make Wheat More Tolerant of Water Deficit Stress Than Rice?

“…This is a pilot report showing such a developmental gradient in xylem diameter along the root length in both wheat cultivars, and it was confirmed with calculated axial hydraulic conductance. A large proportion of the lateral roots generally are developed toward the RSJ part compared with the root tip (Bramley et al, 2009). Therefore, under water deficit increase in LMXN near the RSJ can help increase the uptake of water by lateral roots from the top soil layers but strongly decrease it at the root tip (6 cm from the RA) to conserve moisture in lower soil profiles.…”

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

“…This contribution varies with species, time of day and environmental conditions (e.g. Frensch and Steudle, 1989;Zhu and Steudle, 1991;Steudle and Peterson, 1998;Steudle, 2000;Bramley et al, 2009;Fricke, 2010, 2011;Aroca et al, 2012). There is increasing evidence which points to a role for AQPs in also regulating the root hydraulic response to rather sudden changes in shoot transpirational water loss or to 'catastrophic' events such as injury and removal of the shoot (Levin et al, 2009;Almeida-Rodriguez et al, 2011;Sakurai-Ishikawa et al, 2011;Vandeleur et al, 2014).…”