Morphological responses of plant roots to mechanical stress

Abstract

Background

Roots are continuously exposed to mechanical pressure and this often results in their morphological modification. Most obvious are changes in the overall form of the root system as well as in the shapes of particular roots. These changes are often accompanied by modifications of the cell pattern and cell morphology.

Scope

This review focuses on the morphological responses of roots to mechanical stress. Results of early and recent experiments in which roots have been exposed to mechanical pressure are assembled, analysed and discussed. Research applying different experimental sets, obstacles, media of various compactness and structure are reviewed. An effect of the combination of mechanical stresses with other abiotic stresses on roots, and results of estimating the force exerted by the roots are briefly discussed. Possible consequences of the cell pattern rearrangements are considered.

Conclusions

Several modifications in root morphology are commonly reported: (1) decreased root size, (2) radial swelling accompanied by increased radial dimension of the cortex cell layers and (3) enhanced cap cell sloughing. Nevertheless, because of differences between species and individual plants, a universal scenario for root morphological changes resulting from externally applied pressures is not possible. Thus, knowledge of the root response to mechanical impedance remains incomplete. Studies on the mechanical properties of the root as well as on possible modifications in cell wall structure and composition as the elements responsible for the mechanical properties of the plant tissue are required to understand the response of root tissue as a biomaterial.

INTRODUCTION

In its natural environment – soil – a growing root tip encounters obstacles and soil particles that in some cases might be displaced while in other cases might inhibit further growth. In either case, however, the tip experiences mechanical pressure that to some degree affects all of root development and morphology. Root development and the general condition of the root system clearly have a crucial influence on whole plant growth and productivity (Lynch, 1995). For maximum use of plants we still need to learn more about the roots and about the ways to utilize this knowledge in agriculture and other areas of human activity. We therefore need to understand the different processes leading to proper development of the root, including its response to mechanical stresses.

As both a living object and a physical body the root may respond to mechanical pressure in a complex way. In mechanics a physical body deforms under mechanical stress. For example, an elastic rod undergoes buckling and/or becomes thicker in response to compression while it becomes thinner in response to stretching during application of an axially orientated force. The body is termed elastic if the change of the form is reversible – as long as the change is instant. A viscous body also deforms under mechanical stress, but in this case deformation is not immediate. If the change in form is not reversible after removing the source of the mechanical stress the physical body is termed plastic (Meyers and Chawla, 2009). Most materials, plant tissues among them, are neither completely plastic nor completely elastic or viscous (Niklas, 1992) but have features of all three kinds of materials. However, we should see the difference between deformation of an inanimate object as a passive reaction to mechanical stress and the morphological response of a living plant organ as an active process that involves adaptive and/or defensive mechanisms. Usually, it is difficult to determine if the reaction is passive or active, and in most cases it may be a combination of the two. Here we attempt to consider the character of the root response.

There are no literature reviews focusing on the morphological aspects of roots as either biological or physical objects, especially those reported in recent years. In their comprehensive review, Barley and Greacen (1967) considered soil and its properties as a source of mechanical impedance and discussed soil’s influence on the growth of roots. The excellent review by Atwell (1993) covers morphological and physiological responses of roots. However, it refers to studies from more than 20 years ago. The work by Clark et al. (2003) only briefly reviews morphological responses. Contemporary surveys mostly consider other aspects, such as different techniques for studying the influence of soil compaction on root growth (Tracy et al., 2011), the effect of various types of stress on root elongation (Bengough et al., 2011) or correlation between soil conditions and cereal root system architecture (Rich and Watt, 2013).

Roots are physical bodies; however, as living plant organs they sense any change in the environment and are able to activate various processes of adaptation. Our goal here is to assemble data on morphological modifications of roots in response to mechanical stress with respect to the dual character of roots. We discuss the results concerning changes in the morphology of roots and the root system, modifications to the root internal structure, the root cap response to mechanical stress, the role of the growth regulatory factor ethylene, interactions between mechanical stress and other abiotic stress factors and estimation of root growth pressure. Finally, we draw conclusions on the role and possible advantages of the change in root form and anatomy.

MECHANICAL FORCE CHANGES THE PHYSICAL BODY FORM: EFFECT ON MORPHOLOGY OF ROOT AND ROOT SYSTEM

Morphological traits of the root system, such as its size, the number of branches and their spatial distribution depend on the plant species and environmental conditions. But these features also influence the acquisition of soil resources. Moreover, a developed and properly branched root system provides the whole plant with stability.

Medium density and structure

Experiments in which plants had been grown in compact medium showed that changes usually involved a reduction in the speed of axial growth (Abdalla et al., 1969; Azam et al., 2014; Colombi et al., 2017), in the length of individual roots (Cook et al., 1996; Konôpka et al., 2009; Bécel et al., 2012; Lipiec et al., 2012; Loades et al., 2013; Chen et al., 2014; Colombi et al., 2017) and consequently in root system size (Mulholland et al., 1996a; Grzesiak, 2009). Commonly observed was radial swelling of the roots (Atwell, 1988; Bengough and Mullins, 1991; Sarquis et al., 1991; Kirby and Bengough, 2002; Alameda et al., 2012; Azam et al., 2014; Chen et al., 2014; Loades et al., 2015; Colombi et al., 2017) and occasionally a decrease in the number of roots occurred (Iijima and Kono, 1991; Grzesiak, 2009) (for details see Table 1). These morphological responses of the root to dense medium play an important role in facilitating growth under such unfavourable conditions. For example, swelling of the root leads to reducing stress in the front of the root apex and suppresses root buckling (Abdalla et al., 1969; Bengough et al., 2006). Some specific effects of compacted soil on root system architecture and individual root morphology depend on the plant species and the variant of the experiment applied. In various crop plants (Tsegaye and Mullins, 1994; Grzesiak, 2009; Konôpka et al., 2009; Bécel et al., 2012) and in lupin (Atwell, 1988; Chen et al., 2014) an altered pattern of root branching was observed. In maize individual roots formed local bends (Konôpka et al., 2009) and in wheat various genotypes showed significant diversity in root tip geometry under mechanical stress (Colombi et al., 2017). As was shown by Materechera et al. (1991), the roots of dicotyledons elongate faster and penetrate a dense soil more easily than those of monocotyledons, possibly because roots of the latter are characterized by smaller diameters. In general, the elongation rate is usually negatively correlated with the density of the medium (Atwell, 1988; Croser et al., 1999, 2000; Clark et al., 2001; Benigno et al., 2012) and with root penetration resistance (Bengough and Mullins, 1991; Tsegaye and Mullins, 1994).

Table 1.

Detailed specification of the effects of mechanical stress on root and root system morphology

Effect	Stress conditions	Species	References
Reduced root system size	compacted soil	Avena sativa	Schuurman (1965)
	compacted soil	Oryza sativa, Sorghum bicolor, Zea mays, Coix lacryma-jobi	Iijima and Kono (1991)
	compacted sand	Lolium perenne, Trifolium repens, Agrostis capillaris	Cook et al. (1996)
	compacted soil	Hordeum vulgare	Mulholland et al. (1996a)
	compacted soil	Z. mays, ×Triticosecale	Grzesiak (2009)
	compacted soil	Triticum aestivum, Secale cereale, Triticosecale Wittmack, Z. mays	Lipiec et al. (2012)
	compacted soil	Pisum sativum, Acacia salicina, Eucalyptus camaldulensis, E. leucoxylon, E. kochii	Azam et al. (2014)
	compacted soil	T. aestivum	Chen et al. (2014)
	glass beads	Z. mays	Groleau-Renaud et al. (1998)
	large soil aggregates	Z. mays	Donald et al. (1987); Alexander and Miller (1991)
	pressurized glass beads	H. vulgare	Goss and Drew (1972)
Reduced individual root length	compacted soil compacted sand compacted soil compacted soil compacted soil compacted sand compacted soil large soil aggregates glass beads pressurized soil/glass beads pressurized glass beads pressurized soil pressurized glass beads compression (apparatus) changed orientation	O. sativa, S. bicolor, Z. mays, C. lacryma-jobi L. perenne, T. repens, A. capillaris Z. mays Prunus persica H. vulgare Lupinus angustifolius T. aestivum Z. mays Z. mays H. vulgare, O. sativa H. vulgare, T. aestivum, Z. mays, Beta vulgaris P. sativum Z. mays Z. mays Arabidopsis thaliana	Iijima and Kono (1991) Cook et al. (1996) Konôpka et al. (2009) Bécel et al. (2012) Loades et al. (2013) Chen et al. (2014) Colombi et al. (2017) Donald et al. (1987) Moss et al. (1988), Groleau-Renaud et al. (1998) Abdalla et al. (1969) Goss (1977) Castillo et al. (1982) Veen (1982) Barley (1962) Okamoto et al. (2008)
Increased root diameter (root swelling)	compacted soil/sand compacted soil compacted soil compacted soil/sand compacted soil compacted soil compacted soil compacted soil compacted soil large soil aggregates glass beads pressurized soil/glass beads pressurized glass beads pressurized soil/glass beads pressurized sand axial loading (apparatus) rigid tubes changed orientation	L. angustifolius Z. mays 22 monocotyledonous and dicotyledonous species P. sativum Glycine max Nicotiana tabacum H. vulgare P. sativum, A. salicina, E. camaldulensis, E. leucoxylon, E. kochii T. aestivum Z. mays Z. mays H. vulgare, O. sativa H. vulgare Z. mays L. angustifolius Z. mays Z. mays A. thaliana	Atwell (1988); Chen et al. (2014) Bengough and Mullins (1991); Konôpka et al. (2009) Materechera et al. (1991) Croser et al. (1999); Kirby and Bengough (2002) Ramos et al. (2010) Alameda et al. (2012) Loades et al. (2013, 2015) Azam et al. (2014) Colombi et al. (2017) Donald et al. (1987); Logsdon et al. (1987) Moss et al. (1988) Abdalla et al. (1969) Goss and Russell (1980) Veen (1982); Sarquis et al. (1991) Hanbury and Atwell (2005) Kuzeja et al. (2001) Potocka et al. (2011) Okamoto et al. (2008)
Flattened roots	compacted soil compacted soil rigid tubes	G. max H. vulgare, Triticosecale Wittmack Z. mays	Ramos et al. (2010) Lipiec et al. (2012) Potocka et al. (2011)
Root bending/buckling	compacted soil two-layer phytagel medium two-layer gel medium artificial obstacle (horizontal barrier) rigid tubes artificial obstacle (vertical barrier)	Z. mays (bending) A. thaliana (bending) Medicago truncatula (helical buckling) A. thaliana (bending) Z. mays (buckling) Populus deltoides × P. nigra (bending)	Konôpka et al. (2009) Yamamoto et al. (2008); Yan et al. (2017) Silverberg et al. (2012) Massa and Gilroy (2003) Potocka et al. (2011) Bizet et al. (2016)
Wavy root phenotype	changed orientation	A. thaliana	Okada and Shimura (1990); Rutherford and Masson (1996); Buer et al. (2000); Thompson and Holbrook (2004)
Altered branching Increased number of lateral roots Increased branching density Reduced number of lateral roots Decreased branching density Altered distribution of lateral roots Laterals formed close to root tip	pressurized glass beads bending bending bending compacted soil compacted soil pressurized glass beads compacted sand compacted soil pressurized glass beads compacted sand compacted soil slope soil conditions compacted soil compacted soil pressurized glass beads	Z. mays Fraxinus ornus P. nigra A. thaliana P. sativum Z. mays H. vulgare L. perenne, T. repens, A. capillaris Z. mays, ×Triticosecale H. vulgare T. repens L. angustifolius Spartium junceum L. angustifolius G. max H. vulgare	Veen (1982) Chiatante et al. (2007) Scippa et al. (2008) Ditengou et al. (2008); Richter et al. (2009) Tsegaye and Mullins (1994) Konôpka et al. (2009) Goss and Drew (1972) Cook et al. (1996) Grzesiak (2009) Goss (1977) Cook et al. (1996) Chen et al. (2014) Chiatante et al. (2003); Lombardi et al. (2017) Atwell (1988) Ramos et al. (2010) Goss and Drew (1972); Goss (1977)
Reduced cap size	compacted sand pressurized glass beads artificial obstacle (horizontal barrier)	Z. mays H. vulgare Z. mays	Iijima et al. (2003a) Wilson and Robards (1979) Souty and Rode (1987)

Application of different density of topsoil and subsoil showed that roots could grow even in a very dense topsoil, although they did not penetrate into a very dense subsoil (Schuurman, 1965). Similar results were obtained by Yamamoto et al. (2008), who used two-layer phytagel medium of different concentration. In their experiment the roots did not penetrate into the high-concentration lower layer, but bent along the boundary between the layers (as is typical of an inanimate physical body at a solid barrier), while the roots penetrated the lower layer when its concentration was low (Yamamoto et al., 2008; Yan et al., 2017). Introducing another layer of phytagel of moderate concentration between the upper soft and the lower hard layers increased the degree of root penetration (Yan et al., 2017). This suggests that the moderate-concentration layer may function as an acclimation zone which allows the root tip to grow into harder layers (Yan et al., 2017). Roots of Medicago truncatula grown in two-layer hydrogel (softer upper layer and stiff lower layer) formed helices above the boundary between the layers (Silverberg et al., 2012). Such a helical shape results from a combination of mechanical buckling of the root whose growth has been halted by the hard surface and twisting of the root tip trying to penetrate the stiff medium. This specific morphological response allows axial loads to be converted to transverse loads (Silverberg et al., 2012).

Another important factor affecting root morphology is structure of the medium and its aggregate size. In a loose and porous medium roots grow rapidly and maintain their cylindrical shape, although such a medium may limit contact of the root with nutrients. In a hard medium, root growth is slower and the root itself becomes deformed, although access to potential sources of water and nutrients is easier (Passioura, 1991). Moreover, in the natural environment the structure of the medium may undergo dynamic changes due to weather, and animal and human activity. Deep ripping leading to decompaction of soil and to the change of its structure will clearly influence root architecture (Chen et al., 2014). In general, roots growing in coarse soil aggregates usually have greater diameters and are shorter than those growing in media consisting of finer aggregates (Donald et al., 1987; Logsdon et al., 1987; Alexander and Miller, 1991).

In some experiments soil has been replaced by glass beads, application of which not only simulates mechanical impedance (Groleau-Renaud et al., 1998), but also allows researchers to maintain unchanged aggregate and pore sizes as well as to quantify both impedance and growth rate (Goss and Drew, 1972). Root system architecture depends strongly on the beads’ diameter (and consequently on pore size): roots growing in larger beads are longer and form shorter laterals while those growing in smaller beads are shorter and develop a greater number of laterals (Goss and Drew, 1972). In some cases, the roots curve around the beads and lateral root primordia then form on the convex side and root hairs on the concave side (Goss and Russell, 1980). Pressure from individual beads on the root may result in local distortion on the root’s surface (Wilson and Robards, 1978). Unfortunately, the results do not allow us to infer if this local change in root shape is reversible (elastic) or permanent (plastic). Application of a controlled external mechanical pressure through the media or directly to the roots results in similar effects as for compacted soil, i.e. slower axial growth (Hanbury and Atwell, 2005) and consequently reduced length of roots (Barley, 1962; Goss, 1977; Castillo et al., 1982; Veen, 1982; Lindberg and Pettersson, 1985) as well as in root swelling (Abdalla et al., 1969; Veen, 1982; Kuzeja et al., 2001; Hanbury and Atwell, 2005).

The morphological changes, especially those concerning root system architecture (reduced root system size, modified branching pattern), but also modifications of the form of individual roots (increased diameter) probably lead to better adaptation of the living organ to unfavourable environmental conditions. On the other hand, deformations such as swelling or bending to some degree resemble deformation of a rod under compression. The root thus responds to mechanical pressure in a way similar to an inanimate physical body.

Artificial obstacles

Over 60 years ago, Wiersum (1957) stated that roots are not able to grow into pores that are smaller than the root tip diameter. However, later experiments showed a high morphological adaptability of roots to extremely unfavourable growth conditions, namely pipes thinner than the roots (Scholefield and Hall, 1985; Potocka et al., 2011). In the Scholefield and Hall (1985) experiment roots of grasses were grown through sheets of steel mesh and through glass capillaries that formed a system of rigid pores. The transverse dimension of the roots was about twice the pore size, yet the roots were able to penetrate the pores, although local constriction was observed in the region where it passed through the pore. Potocka et al. (2011) forced maize root tips to grow either into tight plastic tubes of conical endings or into plastic tubes whose cross section was locally changed to an oval by use of a clip. In both variants the root tips managed to push through the narrowest part, although the morphology of the root apex was strongly modified (Fig. 1A, B). Arabidopsis roots whose vertical growth was impeded by a glass barrier underwent bending (Massa and Gilroy, 2003) as in the above-described application of a two-layered medium (Yamamoto et al., 2008). According to Bizet et al. (2016) a zone of mechanical weakness critical to the bending process is localized in the region between the growing and the mature zones of the root.

Fig. 1.

Morphology of maize root growing through tight tubes with circular (A) or oval (B) cross sections. Arrows indicate the narrowest region of the tubes. Root tip in A is covered with mucilage (asterisk); schematic drawing in the left lower corner shows the real shape of the apex (root proper + root cap). In B the front (left) and the side (right) view of the same root are shown, and root buckling is indicated by asterisks. (C) Axial section of the maize root. The root-cap boundary is broken by the RAM cells growing onto the cap side (arrow). Scale bars: 0.5 mm (A), 1 mm (B), 50 μm (C).

Changed orientation in the gravity field

Significant alterations in root system morphology occur when a growing plant experiences a changed orientation in relation to the gravity vector. Seedlings of Spartium junceum grown in containers tilted at 45° formed asymmetrical root systems whose branches failed to develop horizontally, growing up-slope and down-slope instead. Total root length increased in comparison with control plants (Chiatante et al., 2003). Arabidopsis roots respond to a changed orientation of 45° by forming a wavy growth pattern (Okada and Shimura, 1990), whose character is modulated by the growth conditions (Rutherford and Masson, 1996; Buer et al., 2000, 2003; Thompson and Holbrook, 2004). Lateral root primordia are always formed on the convex side (Fortin et al., 1989; Lucas et al., 2008), as found in mechanically bent roots (Goss and Russell, 1980; Ditengou et al., 2008; Richter et al., 2009). When grown horizontally Arabidopsis roots exhibit traits typical of organs exposed to mechanical force: they become thicker and shorter (Okamoto et al., 2008). As in Arabidopsis (Ditengou et al., 2008; Richter et al., 2009), bending tap roots of seedlings of woody plants (Chiatante et al., 2007; Scippa et al., 2008) results in a greater number of laterals. From a mechanical point of view, the convex side of the waving or bent root undergoes tension while the concave side undergoes compression. This means that when the curvature is formed the distribution of mechanical stresses within the tissue changes locally (Szymanowska-Pułka, 2013), which is followed by a chain of reactions on the cellular and molecular levels (see Richter et al., 2009 and Laskowski et al., 2008, respectively).

Effect on plant morphology

The non-impeded organs as well as whole plants whose roots are mechanically impeded do not remain unaffected (Tardieu, 1994). The plant becomes smaller (Iijima and Kono, 1991) and shoot growth slows (Cook et al., 1996; Mulholland et al., 1996a; Roberts et al., 2002), explaining the reduced shoot length (Kobaissi et al., 2013) and shoot dry weight (Donald et al., 1987; Alexander and Miller, 1991; Iijima and Kono, 1991; Grzesiak, 2009). The most frequently reported changes concern leaves. Numerous characteristics such as leaf number (Iijima and Kono, 1991; Grzesiak, 2009), leaf area (Alexander and Miller, 1991; Cook et al., 1996; Mulholland et al., 1996b; Grzesiak, 2009; Bingham et al., 2010; Kobaissi et al., 2013) and leaf elongation rates (Young et al., 1997) are decreased and stomata closure is observed (Roberts et al., 2002). The last was shown to be regulated by a root-sourced abscisic acid signal (Mulholland et al., 1996a, b; Roberts et al., 2002). Only the most important responses have been discussed in this section; the effect of mechanical stress on the morphology of above-ground plant parts is another wide topic that needs a separate review paper.

INTERNAL STRUCTURE OF THE PHYSICAL BODY CHANGES UNDER MECHANICAL STRESS: EFFECT ON CELL MORPHOLOGY AND CELL PATTERN

An internal structure of a deformed physical object undergoes modification through displacement of atoms and changes in their mutual distances. The specific character of the modification depends strictly on the material type. Similarly, alterations in root morphology are accompanied by internal structural changes (for details see Table 2). Swollen roots either have an increased number of cell layers (Wilson et al., 1977) or their cells show modified sizes (Atwell, 1988; Materechera et al., 1991; Hanbury and Atwell, 2005). Reduced root elongation usually accompanies decreasing length of cells (Bengough et al., 2006; Okamoto et al., 2008) and slower cell production (Croser et al., 1999). In some cases changes in the size or cellular organization of the root apical meristem (RAM) are observed (Wilson and Robards, 1979; Potocka et al., 2011).

Table 2.

Detailed specification of the effects of mechanical stress on cell morphology and cell pattern

Effect	Stress conditions	Species	References
Increased radial size of cortical cells	compacted soil compacted soil compacted soil pressurized glass beads pressurized glass beads pressurized sand	22 monocotyledonous and dicotyledonous species Lupinus angustifolius Triticum aestivum Hordeum vulgare Zea mays L. angustifolius	Materechera et al. (1991) Atwell (1988) Colombi et al. (2017) Wilson et al. (1977) Veen (1982) Hanbury and Atwell (2005)
Increased number of cortical layers	compacted sand compacted soil compacted soil pressurized glass beads	P. sativum Z. mays, P. sativum, Gossypium hirsutum T. aestivum H. vulgare	Croser et al. (1999) Iijima and Kato (2007) Colombi et al. (2017) Wilson et al. (1977)
Decreased radial size of cortical cells	rigid pores	Lolium perenne Z. mays (plus epidermis) Cicer arietinum	Scholefield and Hall (1985) Bengough et al. (1997) Kolb et al. (2012)
Increased cell length	rigid pores	L. perenne (cortex)	Scholefield and Hall (1985)
Decreased cell length	compacted soil compacted sand pressurized glass beads pressurized glass beads pressurized sand changed orientation	L. angustifolius (epidermis, cortex) P. sativum (cortex) H. vulgare (epidermis, cortex, endodermis) Z. mays (cortex) L. angustifolius (epidermis) Arabidopsis thaliana (epidermis)	Atwell (1988) Croser et al. (1999) Wilson et al. (1977) Veen (1982) Hanbury and Atwell (2005) Okamoto et al. (2008)
Deformed cells	compacted soil compacted soil pressurized glass beads rigid pores	Glycine max (cortex, root hairs) T. aestivum (cortex), H. vulgare (cortex, vascular cylinder), Secale cereale (cortex), Triticosecale Wittmack (cortex, vascular cylinder), Z. mays (cortex) H. vulgare (epidermis, cortex, endodermis) C. arietinum (cortex)	Ramos et al. (2010) Lipiec et al. (2012) Goss and Drew (1972) Kolb et al. (2012)
Increased diameter/area of the stele	compacted soil compacted soil compacted soil pressurized glass beads axial loading (apparatus)	Z. mays, P. sativum, G. hirsutum H. vulgare, S. cereale, Triticosecale Wittmack, Z. mays T. aestivum H. vulgare Z. mays	Iijima and Kato (2007) Lipiec et al. (2012) Colombi et al. (2017) Wilson et al. (1977) Kuzeja et al. (2001)
Decreased diameter/area of the stele	compacted soil rigid pores	T. aestivum Z. mays	Lipiec et al. (2012) Bengough et al. (1997)
Altered organization of the vascular tissue Increased xylem thickness Increased xylem fibre area Increased diameter of xylem vessels Decreased diameter of xylem vessels	bending slope soil conditions compacted soil pressurized glass beads rigid pores	Populus nigra Spartium junceum Z. mays, P. sativum H. vulgare Z. mays	De Zio et al. (2016) Lombardi et al. (2017) Iijima and Kato (2007) Wilson et al. (1977) Bengough et al. (1997)
Enhanced cell proliferation	compacted sand pressurized glass beads bending	Z. mays (lateral cap) H. vulgare (stele, pericycle) P. nigra (cambium)	Iijima et al. (2003a) Wilson et al. (1977); Wilson and Robards (1978) De Zio et al. (2016)
Aberrantly orientated divisions	pressurized glass beads rigid tubes	H. vulgare (endodermis, root cap) Z. mays (root cap)	Wilson and Robards (1978, 1979) Potocka et al. (2011)
Reduced size of the meristem	compression (apparatus) artificial obstacle (horizontal barrier)	Z. mays Z. mays	Barley (1962) Souty and Rode (1987)
Meristem opening	pressurized glass beads rigid tubes	H. vulgare Z. mays	Wilson and Robards (1979) Potocka et al. (2011)
Ectopic root hair formation	pressurized glass beads pressurized sand rigid tubes changed orientation	H. vulgare L. angustifolius Z. mays A. thaliana	Goss and Drew (1972); Goss and Russell (1980) Hanbury and Atwell (2005) Potocka et al. (2011) Okamoto et al. (2008)
Enhanced sloughing of root cap cells and mucilage secretion	compacted sand/soil rigid tubes	Z. mays Z. mays	Iijima and Kono (1992); Iijima et al. (2000, 2003a); Somasundaram et al. (2008) Potocka et al. (2011)
Ultrastructural changes in root cells	compacted soil pressurized glass beads	Z. mays (root cap) H. vulgare (endodermis, pericycle, root meristem, root cap)	Iijima and Kono (1992) Wilson and Robards (1978, 1979)
Cell wall modification	compacted soil compacted soil pressurized glass beads pressurized glass beads	Z. mays H. vulgare H. vulgare Z. mays	Degenhardt and Gimmler (2000) Bingham et al. (2010) Wilson and Robards (1978) Veen (1982)

The most significant morphological changes are found in the cortex cells, of which some biometric traits are thought to correspond to the mechanical properties of the root tissues (Chimungu et al., 2015) as well as in the stele cells (Wilson et al., 1977). In response to high soil compaction and densely packed glass beads, the cortex becomes thicker due to an increased number of cells (Colombi et al., 2017), an increased radial dimension of cells (Atwell, 1988) or both (Wilson et al., 1977). In the stele the number of cells increases related to its larger diameter (Wilson et al., 1977). In some cases, the stele diameter remains unchanged independently of the level of compaction (Atwell, 1988). A similar effect occurs in roots that have been constricted radially (Scholefield and Hall, 1985) or from the sides (Kolb et al., 2012) whose cortex cells become significantly compressed while the dimension of the stele does not change. In axially loaded roots the vasculature increases (Kuzeja et al., 2001) as in roots grown in a dense medium (Wilson et al., 1977). The above results suggest that the direction of the mechanical force exerted on the root influences the character of the change in cell dimension and consequently in the cell pattern.

Other anatomical changes may be seen in endodermis and pericycle cells. In barley both tissues demonstrated secondary features earlier than in unimpeded roots (Wilson and Robards, 1978). The pericycle formed two layers which resulted from tangential divisions in the protoxylem pole mother cells. In the endodermis atypical oblique divisions occurred in cells that had entered the later stages of development (state III cells; Wilson and Robards, 1978). Aberrantly orientated division walls were also reported in the root cap columella of barley (Wilson and Robards, 1979) and maize roots (Potocka et al., 2011). Bent roots of woody plants showed significant differences in cambial cell number, xylem and phloem thickness, and vessel area on both the convex and the concave side of the bend (De Zio et al., 2016).

There have been few attempts to examine the potential effect of mechanical stress on the organization of the RAM. Barley (1962) and Souty and Rode (1987) mentioned that the RAM of mechanically impeded maize root apices was reduced in size. There are also reports concerning the lack of a distinct boundary between the root meristem and cap (Wilson and Robards, 1979). In the above-mentioned experiment by Potocka et al. (2011) the boundary was broken and the cells localized in the root proper pole grew onto the cap side, so that the typical closed organization of the RAM in this species changed to open (Fig. 1C) which led to strong modification to the cell arrangement. When the mechanical force had been removed the arrangement of the cells became more regular and returned to the closed type (Potocka et al., 2011). Nevertheless, the question arises, is the change from closed to open organization a simple consequence of the applied force or is it an element of the root adaptation? It appears that the latter is the case as the cell pattern eventually recovers.

A change in cell pattern meaning de facto a rearrangement of the cell wall network may indicate a tendency to adjust to the new distribution of stress within the root. Being a highly organized structure stiffening the whole plant body and responsible for protection of the interior of cells, it is the cell wall network that mostly responds to and transduces mechanical stimuli affecting the living plant organ. Unfortunately, there are few reports on the effect of mechanical stress on root cell wall structure and composition. One of the few observations concerns barley roots whose endodermal cells were characterized by suberin lamellae and thick a tertiary cellulosic wall due to an atypically early transition to later stages of development (Wilson and Robards, 1978). Another report comes from Veen’s (1982) study on crown roots of maize exposed to mechanical stress, in which there was reorientation of some cellulose microfibrils of cortex cells from transverse to longitudinal. Such a change may be correlated with radial thickening of the root because of a smaller number of transverse microfibrils that typically inhibit growth in the radial direction. Relatively recent research on barley roots revealed significant decreases of cellulose and hemicellulose content and increased lignin concentration in root tissue (Bingham et al., 2010). The last was also observed in endodermal cell walls of impeded maize roots (Degenhardt and Gimmler, 2000). To understand the mutual relationship between the cell pattern and root growth more research on both the reorganization of the cell wall network and possible changes in the structure and chemical composition of the cell walls in roots is needed.

Some authors report the increased exudation of deformed roots (Groleau-Renaud et al., 1998; Oliveros-Bastidas et al., 2012), suggesting that there might be a correlation between deformation and exudation as the effects of the same factor, namely mechanical impedance (Oliveros-Bastidas et al., 2012). Root exudation leads on to consideration of root cap behaviour when affected by mechanical stress.

ROOT CAP FACING MECHANICAL IMPEDANCE

The root cap covers the root tip and thus it is the region of the first contact of the root with soil. It plays multiple roles in root biology and in root interactions with the environment (Barlow, 2003; Iijima et al., 2008) and its functioning depends on the type of RAM (Hamamoto et al., 2006). Among the most evident functions of the root cap are protecting the root tip from environmental impulses, determining the direction of root growth and reducing the friction between soil and the growing root by sloughing of peripheral root cap cells and secretion of mucilage (Bengough and McKenzie, 1997). The lubricating effect of the root cap was shown by Iijima et al. (2003b, 2004) who estimated the contribution of mucilage to the decrease in root penetration resistance of maize roots grown in compact soil (Iijima et al., 2004). Decapped roots showed significantly reduced elongation either with or without mucilage treatment (Iijima et al., 2004). Interestingly, in experiments with decapped Zea roots grown in ballotini no effect of the decreased elongation rate was observed (Goss and Russell, 1980). Decapping not only changes the shape of the apical part of the root, which clearly results in an altered distribution of mechanical stress in the region, but it also may change the strategy of adaptation under conditions of a temporal lack of root tip protection.

In mechanically impeded roots mucilage production is enhanced (Potocka et al., 2011) and the pattern of its accumulation is modified (Wilson and Robards, 1979), possibly correlated with some ultrastructural effects, such as the increased number of dictyosomes and secretory vesicles in cap cells (Iijima and Kono, 1992). Enhanced cap exudation is also accompanied by a significant increase in the number of cap cells sloughed into the medium (Iijima et al., 2000, 2003a; Somasundaram et al., 2008). The size of the root cap and the number of cells of which it is composed change in response to mechanical impedance. The caps of stressed roots are usually shorter and narrower at the base (Wilson and Robards, 1979; Souty and Rode, 1987; Iijima et al., 2003a) and their total cell number is reduced despite the fact that the rate of cell production in flank areas is slightly enhanced (Iijima et al., 2003a). As was shown by Bengough and McKenzie (1997), the enhanced release of the outermost cells lessens mechanical impedance to root penetration. We thus infer that this kind of response (not observed in inanimate physical objects) may be adaptive. Decrease in the transverse dimension of the cap may also play the adaptive role as it makes the cap thinner and thus better adjusted to penetration through a dense medium. On the other hand, the decreased length of the root cap resembles shortening of an inanimate elastic rod under axial stress.

There is little information about the anatomical changes that occur in the cap under mechanical stress. In caps of impeded roots of barley, Wilson and Robards (1979) observed vacuolation of meristematic cells, a decrease in the number and atypical location of amyloplasts in the columella cells, and an atypical orientation of the cellular divisions. The last observation was also reported by Potocka et al. (2011) in the maize root cap.

ETHYLENE IN RELATION TO THE MORPHOLOGICAL RESPONSE OF ROOTS

Morphological alteration in mechanically impeded roots is preceded by enhanced ethylene production 1 h after stress application (Kays et al., 1974; Sarquis et al., 1991, 1992; He et al., 1996). In maize roots subjected to a cycle of alternating mechanical impulses, ethylene production increased and decreased rapidly during the on and off phase of the impulse, respectively (Sarquis et al., 1991). Inhibition of the action of ethylene partially restores the length and diameter of the impeded roots (Sarquis et al., 1991;Zacarias and Reid, 1992) which indicates a mediating role of this hormone in the morphological response of the root. Yamamoto et al. (2008) suggested that ethylene may regulate root responses to stress through softening or hardening the root tip (thus changing its mechanical properties) and through altering the growth rate of the root. In support of this are the recent results by Santisree et al. (2011), who showed that roots are unable to penetrate growing medium under conditions of lower ethylene production.

Application of ethylene results in root deformation similar to that induced by mechanical stress. In response to exogenous ethylene, unimpeded maize roots show increased diameter and decreased length (Moss et al., 1988) while Arabidopsis roots change their wavy growth pattern (Buer et al., 2000, 2003). Interestingly, in ethylene-treated decapped roots, elongation growth is not inhibited indicating that the cap may be a site of perception and response to this hormone (Hahn et al., 2008).

The response of treated roots to ethylene is accompanied by changes in the auxin response. In impeded Arabidopsis roots enhanced ethylene synthesis affects the pattern of auxin distribution (Okamoto et al., 2008) while in tomato roots in the presence of the ethylene inhibitor 1-methylcyclopropene a reduced ethylene synthesis along with a decrease of the DR5::GUS reporter activity were observed (Santisree et al., 2011, 2012). It has been also shown that ethylene regulates auxin distribution in the root cap and the interaction of the two hormones regulates the size of the cap (Ponce et al., 2005).

EFFECT OF A COMBINATION OF MECHANICAL STRESS AND OTHER ABIOTIC STRESSES ON ROOT MORPHOLOGY

In its natural environment a root is exposed to long-lasting or permanent stresses of various types to which it needs to adapt. Thus, the total of the environmental inputs to which a root adapts are shaped by both the stress experienced at that point and the impulses that have been previously experienced (Bengough et al., 1997). In lab experiments roots are generally subjected to one or more types of stress applied in a controlled way, for a set and usually short time in relation to the life of the plant. The roots adapt to new conditions ranging from morphological changes to modification of the composition of the cell wall (Degenhardt and Gimmler, 2000).

Various abiotic stresses such as low/high water availability (Cairns et al., 2004; Iijima and Kato, 2007; Konôpka et al., 2008; Kobaissi et al., 2013) or hypoxia (Barley, 1962; Iijima and Kato, 2007), applied together with mechanical stress, may induce various morphological root responses. The character of the responses usually resembles a typical reaction of roots to mechanical stress, namely decreased length and increased diameter. The interaction among the stresses alters root morphology depending on the character and the levels of particular stresses (Alameda et al., 2012), but it is the mechanical stress that seems to have the strongest influence (Cairns et al., 2004; Konôpka et al., 2008; Benigno et al., 2012; Kobaissi et al., 2013). There are few reports on alleviating the effects of mechanical stress on root deformation in combination with other abiotic stresses. A high aluminium (Al) concentration decreases root morphological parameters such as length and number, while application of mechanical stress along with Al partially overcomes the effect and induces enhanced mucilage production, which eventually decreases Al accumulation in the root tip (Horst et al., 1990; Okamoto and Yano, 2017).

The anatomical effects of combined stresses are mainly seen in the root cortex. Mechanical stress applied with drought and low oxygen supply result in a changed thickness of the tissue associated with an altered number and size of cortical cells (Iijima and Kato, 2007) and in aerenchyma formation (He et al., 1996), although the specific response depends on the plant species (Iijima and Kato, 2007). Effects at the cellular level are rarely reported and relate to altered tissue composition observed in roots grown in highly compacted soil with different types of stress such as low nitrogen (Bingham et al., 2010), high salinity, high pH and high concentrations of heavy metals (Degenhardt and Gimmler, 2000). The inputs of different abiotic characters may then interact with mechanical stress although even when applied in a controlled way precise interpretation of their effects is difficult (Alameda et al., 2012; Suzuki et al., 2014).

Mechanical stress may affect growth conditions by inducing stresses of different character, as with soil compaction that may reduce access to water and/or air (Drew et al., 2000; Tracy et al., 2011; Chimungu et al., 2015). An opposite action may also take place: drought stress may increase mechanical stress to the root (Iijima and Kato, 2007). As consequence of wetting and drying weather cycles, cyclical swelling and shrinkage of the soil occur (Striker et al., 2007) which eventually results in an alternating distribution of mechanical stress sensed by the roots. For more information on the impact of other abiotic stresses on root development, the reader is reffered to the recent reviews by Franco et al. (2011) and Sánchez-Calderón et al. (2013) and to the latest works in the field (Kadam et al., 2015; Líška et al., 2016).

ESTIMATION OF ROOT GROWTH PRESSURE

The level of root growth pressure in soil clearly affects the degree of root tip deformation. That is why in some cases study of the morphological effects of mechanical stress on roots involves estimation of the root force. For technical reasons it is very difficult to measure this force and some researchers use penetrometer probes whose readings, however, appear significantly overestimated (Bengough and Mullins, 1990). Direct measurements show that root pressure for various plant species ranges from 0.10 to 0.65 MPa (Table 3) and depends strongly on growth conditions (Whiteley et al., 1981; Misra et al., 1986; Bengough and Mullins, 1991; Bengough et al., 1994; Bengough and McKenzie, 1997; Clark and Barraclough, 1999) as well as on the cultivar and the measurement technique (reviewed by Clark et al., 1999). Potocka et al. (2011) showed that the force exerted by the root growing through a plastic pipe depended on the space available above the tightest region of the pipe, which may be related to variability in the structure of the soil and its porosity in the field conditions. Another method was proposed by Kolb et al. (2012) who made a growing root push between photoelastic discs. Application of this material enabled visualization and registration of the optical fringes on the basis of which the value and direction of radial mechanical force exerted by the root might be estimated (Table 3). In studies determining root growth pressure a mathematical approach has also been applied. Kirby and Bengough (2002) presented a predicted distribution of the stresses around the tip of a root penetrating soil, based on the critical-state finite-element model. The model showed that the stress depends on the distance from the root tip, on soil conditions and on root diameter.

Table 3.

Measured root pressure

*Depending on soil conditions.

†Data from the plot in Figure 4 of the reference.

CONCLUDING REMARKS AND PERSPECTIVES

Based on the many descriptions of the root responses to mechanical stimuli of various types we may draw a portrait of the root as an easily adapting plant organ, quickly responding to changes in the environment. Nevertheless, a universal scenario of root responses is not possible as roots of various species at various stages of development may react in various ways to the same stimulus. Typically, the most often observed features of the impeded roots are (1) reduced axial growth and consequently decreased root length, (2) radial swelling in many cases associated with increased radial dimension of the cortex cell layers and (3) enhanced root cap cell sloughing (Table 1, Table 2). A reduction in root elongation seems to be a direct effect of the axial components of the applied force (Kolb et al., 2012; for detailed analysis see Bengough, 2012). This is explainable from the point of view of kinematics where a growing root may be considered a moving physical body whose movement is reduced or even stopped by a potential obstacle. Root thickening may be an element of both the physical reaction and the adaptation process while enhanced cap cell sloughing seems to be (a part of) an adaptation to stress conditions. The morphological response of the root to mechanical stress is more than a reaction of the physical object because being a living organ the root is able to sense and adapt to altering conditions of the environment.

The changed form is often accompanied by a rearrangement of the cell pattern in which either time (Goss, 1977) or the level of the applied mechanical impulse (Cook et al., 1996) might have a significant effect. The most spectacular examples concern modifications in RAM organization (Potocka et al., 2011) that may dramatically change cell fate within the root apex. Eventually, reorganization of the RAM is reversible, but the further fates of the cells have not been studied. To explore the issue, additional studies concerning the possible relationship between cell fates and experimental conditions are needed.

To give a sense of the significance of modification to the cell pattern in the meristematic area we need to consider the character of growth of the root apex. As with other plant organs, root apices grow in a highly coordinated way, designated ‘symplastic’ (Erickson, 1986). This coordination is recognizable through a stable and regular cellular pattern that is preserved during growth. Hejnowicz (1984) assumed that within the growing plant organ the so-called principal directions of growth (directions of either minimal or maximal growth rates) are present. These directions are tangential to periclines and anticlines (von Sachs, 1887) that are visible in the cell wall pattern. In plants growing in soil it is the root apex that experiences the strongest mechanical impulse from the environment. This usually alters its morphology and cell arrangement, changing the pattern of periclines and anticlines and consequently the principal directions of growth. As shown by Lynch and Lintilhac (1997) there is a direct relationship between the principal directions of growth and the principal directions of stress in the plant organ. This explains why in the morphologically changed root there is a rearranged distribution of mechanical stress.

The pronounced ability of roots to adapt to different stimuli and to moderate their effects is often described as root plasticity (Suralta et al., 2018; Riedelsberger and Blatt, 2017). This colloquial term is not precise from the point of view of mechanics because it does not fully mirror the mechanical properties of this plant organ. The mechanical properties of a material, such as Young’s modulus or viscosity coefficient, are estimated through rheological and other mechanical tests. Rheological tests on pea lateral roots (Tanimoto et al., 2000) show a viscoelastic and plastic characteristics of the root tissue material. Some recent work (Loades et al., 2013, 2015) has shown that environmental conditions, such as mechanical impedance coming from compacted soil, alter mechanical properties and their distribution along the root axis. More experiments on the mechanical properties of roots are needed to advance our understanding of their reaction as either physical objects or living organs.