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Penetrometer resistance (MPa)

mechanical impedance. Root tip traits beneficial to root penetration include traits that decrease cavity expansion pressure, frictional resistance, or axial cell wall tension. In soil containing macropores and channels, the ability of roots to exploit such channels may also be of significant importance, and root hairs are probably sufficiently strong to aid root tip anchorage significantly. Thus, it is essential to consider root responses to soil strength when developing strategies to breed drought-resistant crops and, in order to address this adequately, it may require to develop some novel screening approaches.

3.2 Root Conductivity

Water enters into roots through the epidermis, exodermis, cortex, endodermis, the pericycle, stele parenchyma, and finally into the xylem vessels (Fig. 2.4). The radial conductance of roots is about two orders of magnitude lower than the axial conductance which is largely determined by the dimensions, and the number of xylem vessels (Bramley 2006; Bramley et al. 2007; Tyerman et al. 2009). The composite transport model which comprises apoplastic, symplastic, and transcellular flow-paths operating in parallel has

Fig. 2.4 Transverse root sections indicating the root and passage cells and (b) stele and endodermis consisting apoplastic and cell-to-cell pathways of radial water move- with cells showing suberin (s) lamellae are shown ment to the xylem. (a) The outer cortex with exodermis

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Fig. 2.4 Transverse root sections indicating the root and passage cells and (b) stele and endodermis consisting apoplastic and cell-to-cell pathways of radial water move- with cells showing suberin (s) lamellae are shown ment to the xylem. (a) The outer cortex with exodermis been widely used to describe the flow of water through roots (Steudle 2000). According to this model the apoplastic flow-path consist of water movement outside of the cells' plasma membrane, the symplastic flow-path is through the cytoplasm of cells connected by plasmodesmata, and the transcellular flow-path is across cell membranes (Tyerman et al. 2009). The combination of symplastic and transcellular flow-paths is known as the cell-to-cell pathway. The movement of water through the apoplast is driven only by hydrostatic gradients, while across a membrane-delimited (transcellular) pathway both hydrostatic and osmotic gradients are involved. When plants transpire the hydrostatic gradient dominates due to the tensions developed in the xylem. Water moves via both the apoplastic and the cell-to-cell pathway driven by hydrostatic gradients, the proportion depending on the relative hydraulic conductances of the two pathways (Tyerman et al. 2009). When transpiration rate is slow, as normally occurred during the night or under drought conditions, osmotic flow may dominate, because without large hydrostatic-driven water flows ions in the stele are not diluted, creating an osmotic gradient. However, under normal transpiration conditions the water flow-path taken is mainly influenced by root anatomy. In particular, the apoplastic pathway can be inhibited by the presence of Casparian bands, which are deposits of suberin or lignin in the cell wall. Casparian bands occur in radial and transverse walls of the endodermis and exodermis (Steudle and Peterson 1998; Tyerman et al. 2009). Suberin lamellae may also occur on the tangential walls to further inhibit apoplastic flow (Fig. 2.4). Suberin lamellae can also restrict movement of water along the transcellular pathway. The formation of these barriers to water movement is often associated with the imposition of stress such as water deficits and aging of the plant (Vandeleur et al. 2008). A possible role of this enhanced formation of suberized layers might be correlated to reduction of excess water losses to soil which might occur under drought conditions. On the other hand, in certain species, the transcellular path seems to play a major role as it is efficiently facilitated by water channel proteins named aquaporins. These proteins belong to the ubiquitous super family of Major Intrinsic Proteins (Maurel et al. 2008). The structure of several aquaporins (Tornroth-Horsefield et al. 2006) enables them to insert as tetramers in the membrane forming four individual pores which allow the passage of water or of small neutral molecules (Maurel et al. 2008, 2010). In plants, aquaporins fall into four or five homology subfamilies, among which the Plasma membrane Intrinsic Proteins (PIPs) represent the most abundant aquaporins at the plasma membrane. Because this membrane is a potential obstacle to transcellular water flow, PIPs can control a large part of the root water permeability or hydraulic conductivity (Lpr; Tournaire-Roux et al. 2003) . A large array of environmental and hormonal stimuli are known to trigger short-term (minutes to hours) adjustments of Lpr. Among them drought and salinity stress, usually induces a significant decrease in hydraulic conductivity, whereas Absciscic acid (ABA) can exert either an up- or a downregulating effect, depending on time, dose or species (Parent et al. 2009). Soil compaction or flooding which restrict oxygen diffusion in the soil, result in root anoxia which, in turn, downregulates hydraulic conductivity in certain plant species (Tournaire-Roux et al. 2003; Bramley et al. 2010). There is now substantial pharmacological and genetic evidence that most of the short-term changes in root hydraulics caused by abiotic environmental stress are mediated through the regulation of root aquaporin expression and activity. A variety of mechanisms involving transcriptional control (Alexandersson et al. 2005), stimulus-induced internalization of PIPs (Boursiac et al. 2008) , or regulated channel opening and closing (gating) by cytosolic calcium, cytosolic protons, or aquaporin phosphory-lation has been revealed (Boursiac et al. 2008; Verdoucq et al. 2008). It is important to note that most of the soil stress conditions, including water, nutrient, or oxygen deprivation, all influence hydraulic conductivity and induce an accumulation of ROS in root tissues. A conserved signaling chain involving ROS and acting downstream on aquaporin phosphorylation and subcellular relo-calization mediates, in part, the downregulating effects of these stresses on hydraulic conductivity (Boursiac et al. 2008) . A ROS-induced stimulation of hydraulic conductivity has also been reported in certain plant species (Benabdellah et al. 2009; Maurel et al. 2010).

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