Kinetics of Adaptation to Osmotic Stress in Lentil (Lens culinaris Med.) Roots

Not so hidden anymore: Advances and challenges in understanding root growth under water deficits

Limited water availability is a major environmental factor constraining plant development and crop yields. One of the prominent adaptations of plants to water deficits is the maintenance of root growth that enables sustained access to soil water. Despite early recognition of the adaptive significance of root growth maintenance under water deficits, progress in understanding has been hampered by the inherent complexity of root systems and their interactions with the soil environment. We highlight selected milestones in the understanding of root growth responses to water deficits, with emphasis on founding studies that have shaped current knowledge and set the stage for further investigation. We revisit the concept of integrated biophysical and metabolic regulation of plant growth and use this framework to review central growth-regulatory processes occurring within root growth zones under water stress at subcellular to organ scales. Key topics include the primary processes of modifications of cell wall-yielding properties and osmotic adjustment, as well as regulatory roles of abscisic acid and its interactions with other hormones. We include consideration of long-recognized responses for which detailed mechanistic understanding has been elusive until recently, for example hydrotropism, and identify gaps in knowledge, ongoing challenges, and opportunities for future research.

Water stress and cell wall polysaccharides in the apical root zone of wheat cultivars varying in drought tolerance

Glycosyl composition and linkage analysis of cell wall polysaccharides were examined in apical root zones excised from water-stressed and unstressed wheat seedlings (Triticum durum Desf.) cv. Capeiti ("drought-tolerant") and cv. Creso ("drought sensitive"). Wall polysaccharides were sequentially solubilized to obtain three fractions: CDTA+Na(2)CO(3) extract, KOH extract and the insoluble residue (alpha-cellulose). A comparison between the two genotypes showed only small variations in the percentages of matrix polysaccharides (CDTA+Na(2)CO(3) plus KOH extract) and of the insoluble residues (alpha-cellulose) in water-stressed and unstressed conditions. Xylosyl, glucosyl and arabinosyl residues represented more than 90 mol% of the matrix polysaccharides. The linkage analysis of matrix polysaccharides showed high levels of xyloglucans (23-39 mol%), and arabinoxylans (38-48 mol%) and a low amount of pectins and (1-->3), (1-->4)-beta-D-glucans. The high level of xyloglucans was supported by the release of the diagnostic disaccharide isoprimeverose after Driselase digestion of KOH-extracted polysaccharides. In the "drought-tolerant" cv. Capeiti the mol% of side chains of rhamnogalacturonan I and II significantly increased in response to water stress, whereas in cv. Creso, this increase did not occur. The results support a role of the pectic side chains during water stress response in a drought-tolerant wheat cultivar.

Change in XET activities, cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (psi(w)=-0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low psi(w). The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-psi(w) vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.

Turgor regulation in osmotically stressed Arabidopsis epidermal root cells. Direct support for the role of inorganic ion uptake as revealed by concurrent flux and cell turgor measurements

Hyperosmotic stress is known to significantly enhance net uptake of inorganic ions into plant cells. Direct evidence for cell turgor recovery via such a mechanism, however, is still lacking. In the present study, we performed concurrent measurements of net ion fluxes (with the noninvasive microelectrode ion flux estimation technique) and cell turgor changes (with the pressure-probe technique) to provide direct evidence that inorganic ion uptake regulates turgor in osmotically stressed Arabidopsis epidermal root cells. Immediately after onset of hyperosmotic stress (100/100 mM mannitol/sorbitol treatment), the cell turgor dropped from 0.65 to about 0.25 MPa. Turgor recovery started within 2 to 10 min after the treatment and was accompanied by a significant (30-80 nmol m-2 s-1) increase in uptake of K+, Cl-, and Na+ by root cells. In most cells, almost complete (>90% of initial values) recovery of the cell turgor was observed within 40 to 50 min after stress onset. In another set of experiments, we combined the voltage-clamp and the microelectrode ion flux estimation techniques to show that this process is, in part, mediated by voltage-gated K+ transporters at the cell plasma membrane. The possible physiological significance of these findings is discussed.

Modification of expansin transcript levels in the maize primary root at low water potentials

We previously demonstrated that maintenance of cell elongation in the apical region of maize primary roots at low water potentials (psi(w)) was associated with an increase in expansin activity and extractable expansin protein. Here, we characterized the spatial pattern of expansin gene expression along the growing maize root and studied the effect of low psi(w) on expansin gene expression. Roots were divided into three segments: apical 0 to 5 mm, subapical 5 to 10 mm, and non-growing 10 to 20 mm. Of the five expansin genes expressed in control roots, two alpha-expansins (Exp1 and Exp5) and two beta-expansins (ExpB2 and ExpB8) are expressed specifically in the growing region, whereas expression of beta-expansin ExpB6 is shifted basipetally. After seedlings were transplanted to vermiculite with a psi(w) of -1.6 MPa, transcripts for Exp1, Exp5, and ExpB8 rapidly accumulated in the apical region of the root. These mRNA changes correlated with the maintenance of root elongation and increases in wall extensibility found previously. The beta-expansins ExpB2 and ExpB6 showed distinctive patterns of expression and responses to low psi(w,) indicative of distinctive functions. Inhibition of abscisic acid (ABA) accumulation at low psi(w) (by fluridone treatment) had no effect on expansin expression, except that ExpB2 transcript level showed a minor dependence on ABA. Gene-specific regulation of alpha- and beta-expansin mRNA pools likely contributes to growth alterations of the maize (Zea mays) root as it adapts to a low psi(w), but these changes do not appear to be mediated by changes in ABA content.

Turgor, temperature and the growth of plant cells: using Chara corallina as a model system

Rapid changes in turgor pressure (P:) and temperature (T:) are giving new information about the mechanisms of plant growth. In the present work, single internode cells of the large-celled alga Chara corallina were used as a model for plant growth. P was changed without altering the chemical environment of the wall while observing growth without elastic changes. When P: was measured before any changes, the original growth rate bore no relationship to the original P. However, if P of growing cells was decreased, growth responded immediately without evidence for rapid changes in wall physical properties. Growth occurred only above a 0.3 MPa threshold, and increasing P caused small increases in growth that became progressively larger as P rose, resulting in a curvilinear response overall. The small changes in growth close to the threshold may explain early failures to detect these responses. When T was lowered, the elastic properties of the cell were unaffected, but growth was immediately inhibited. The lower T caused P to decrease, but returning P to its original value did not return growth to its original rate. The decreased P at low T occurred because of T effects on the osmotic potential of the cell. At above-normal P, growth partially resumed at low T Therefore, growth required a P-sensitive process that was also T-sensitive. Because elastic properties were little affected by T, but growth was markedly affected, the process is likely to involve metabolism. The rapidity of its response to P and T probably excludes the participation of changes in gene expression.

Adaptation of roots to low water potentials by changes in cell wall extensibility and cell wall proteins

It is common for the root/shoot ratio of plants to increase when water availability is limiting. This ratio increases because roots are less sensitive than shoots to growth inhibition by low water potentials. The physiological and molecular mechanisms that assist root growth under drought conditions are reviewed, with a focus on changes in cell walls. Maize seedlings adapt to low water potential by making the walls in the apical part of the root more extensible. In part, this is accomplished by increases in expansin activity and in part by other, more complex changes in the wall. The role of xyloglucan endotransglycosylase, peroxidase and other wall enzymes in root adaptation to low water potential is evaluated and some of the complications in the field of study are listed.

Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport

Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (psi) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of psi gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low psi. In contrast, in leaves under reductions in psi of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low psi in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone psi is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that psi of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of psi in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment.

Separating growth from elastic deformation during cell enlargement

Plants change size by deforming reversibly (elastically) whenever turgor pressure changes, and by growing. The elastic deformation is independent of growth because it occurs in nongrowing cells. Its occurrence with growth has prevented growth from being observed alone. We investigated whether the two processes could be separated in internode cells of Chara corallina Klien ex Willd., em R.D.W. by injecting or removing cell solution with a pressure probe to change turgor while the cell length was continuously measured. Cell size changed immediately when turgor changed, and growth rates appeared to be altered. Low temperature eliminated growth but did not alter the elastic effects. This allowed elastic deformation measured at low temperature to be subtracted from elongation at warm temperature in the same cell. After the subtraction, growth alone could be observed for the first time. Alterations in turgor caused growth to change rapidly to a new, steady rate with no evidence of rapid adjustments in wall properties. This turgor response, together with the marked sensitivity of growth to temperature, suggested that the growth rate was not controlled by inert polymer extension but rather by biochemical reactions that include a turgor-sensitive step.

The control of cell expansion in roots

The expansion of roots is considered at the level of the single cell. The water relations of cell expansion are discussed. Water entry, solute import and cell wall properties are considered as possible regulatory points. It is argued that root cell expansion can be understood in terms of cell turgor pressure and the physical properties of the cell wall, provided solute supply is not limiting. Various measurements of cell wall properties in roots are presented and the assumptions underlying their measurements are presented. It is concluded that cell wall properties must be measured over short time periods to prevent alterations in wall properties during the experiment. The radial location of the load-bearing layers is discussed and it is concluded that, unlike aerial tissue, growth is limited by the properties of the inner layer of the root cortex. Evidence is presented to show that cell wall properties can change both during development and following turgor perturbation. In general, however, turgor itself is tightly regulated, particularly towards the root tip. A number of environmental situations are presented in which root growth is altered. The mechanism of the alteration is discussed at the single cell level. These 'stresses'include osmotic stress, low temperature and soil compaction. In many cases the alteration of root growth is consistent with changes in the ceil wall properties of the growing ceils. Severe stress, resulting in near cessation of root cell extension, can result in a change (usually an increase) in turgor pressure. The change in turgor pressure of the cells in the growing zone is smaller than that which would be expected from a continuation of an unstressed solute import rate. This exemplifies both the change in cell wall properties and the tight turgor homeostasis of root tips. The biochemical processes which underlie the modulation of cell wall properties are presented as they are currently understood in roots. Measurements of the chemical composition of the wall have not revealed any useful differences which can explain the developmental or stress-induced changes in cell wall properties. Recent work on cell wall enzymes and proteins may provide information about control of cross-linkages within the wall. In the last section the relative importance of apoplastic and symplastic solute transport to the expanding cells is considered. At present the consensus appears to favour the symplastic route, but the apoplastic pathway may also operate, possibly as a scavenging mechanism for leaked ions. The regulation of turgor pressure by linking solute import with wall loosening is discussed. Contents Summary 3 I. Introduction 4 II. Factors controlling cell expansion 4 III. Wall extensibility and yield threshold in roots 6 IV. Environmental effects on root cell expansion 10 V. Modification of cell wall biochemistry 15 VI. Linkage of growth with solute import 18 VII. Future prospects 21 VIII. Acknowledgements 22 IX. References 22.

Transient changes in length and growth of wheat coleoptile segments following treatments with osmotica and auxin

The dependence of elongation on the osmotic potential of the medium was investigated, using coleoptile segments (CS) of Triticim aestivum L. (cv. Hartri) and an optoelectronic device. The study aimed at separating the osmoelastic response from the irreversible growth response when an osmoticum (mannitol) was added, and to compare both processes in order to consider the possibility of growth-induced reduction in turgor pressure. The prompt inhibition of elongation registered just after addition of 50 mM mannitol as well as the subsequent resumption of the original elongation rate could be quantitatively explained by the extent and the kinetics of the osmoelastic relaxation. An initial reduction in the irreversible elongation component by mild osmotic stress could not be demonstrated. Above a critical value, the irrevesible growth was insensitive to a further increase in water potential. The minimum turgor pressure required to drive steady growth was not far from zero in both the presence and absence of auxin. The rate (r) of osmotically caused shortening per unit change of water potential [Formula: see text] was determined from the kinetics of CS shortening induced by addition of mannitol at nearly isotonic concentration (300 mM). This parameter relates a fractional change in length to the difference in water potential between inside and outside, and was assumed to depend largely on the hydraulic resistance of the tissue and cuticle. It was found to be independent of IAA. The relatively low value of Γ suggests significant reduction of turgor at high growth rates. In accordance with this conclusion, the extent of osmoelastic shortening after a transfer to 300 mM mannitol (dependent on wall strain) was significantly decreased in the presence of IAA. Addition of 100 μM IAA to CS growing at a constant rate induced pronounced oscillations in the rate of elongation, which may be connected with the change in elastic cell wall strain. Whereas the steady state growth rate before the addition of IAA was the same in the presence and in the absence of 50 mM mannitol, the maximum growth rate found after addition of IAA was substantially reduced in the mannitol variant.

Wall extensibility: its nature, measurement and relationship to plant cell growth

Expansive growth of plant cells is controlled principally by processes that loosen the wall and enable it to expand irreversibly. The central role of wall relaxation for cell expansion is reviewed. The most common methods for assessing the extension properties of plant cell walls ( wall extensibility') are described, categorized and assessed critically. What emerges are three fundamentally different approaches which test growing cells for their ability (a) to enlarge at different values of turgor, (b) to induce wall relaxation, and (c) to deform elastically or plastically in response to an applied tensile force. Analogous methods with isolated walls are similarly reviewed. The results of these different assays are related to the nature of plant cell growth and pertinent biophysical theory. I argue that the extensibilities' measured by these assays are fundamentally different from one another and that some are more pertinent to growth than others.

Spatial distribution of turgor and root growth at low water potentials

Spatial distributions of turgor and longitudinal growth were compared in primary roots of maize (Zea mays L. cv FR27 x FRMo 17) growing in vermiculite at high (-0.02 megapascals) or low (-1.6 megapascals) water potential. Turgor was measured directly using a pressure probe in cells of the cortex and stele. At low water potential, turgor was greatly decreased in both tissues throughout the elongation zone. Despite this, longitudinal growth in the apical 2 millimeters was the same in the two treatments, as reported previously. These results indicate that the low water potential treatment caused large changes in cell wall yielding properties that contributed to the maintenance of root elongation. Further from the apex, longitudinal growth was inhibited at low water potential despite only slightly lower turgor than in the apical region. Therefore, the ability to adjust cell wall properties in response to low water potential may decrease with cell development.

Water Relations of Growing Maize Coleoptiles : Comparison between Mannitol and Polyethylene Glycol 6000 as External Osmotica for Adjusting Turgor Pressure

Water relations of growing segments of maize (Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Psi(o)) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (pi(i)). The nonpenetrating osmoticum PEG affects pi(i) in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Psi(o) range permitting growth (0 to -5 bar) nor in the Psi(o) range inducing osmotic shrinkage (-5 to -10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in pi(i) accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Psi(o) which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles.

Dynamic Relation between Expansion and Cellular Turgor in Growing Grape (Vitis vinifera L.) Leaves

Measurements of the growth and water relations of expanding grape (Vitis vinifera L.) leaves have been used to determine the relationship between leaf expansion rate and leaf cell turgor. Direct measurement of turgor on the small (approximately 15 micrometer diameter) epidermal cells over the midvein of expanding grape leaves was made possible by improvements in the pressure probe technique. Leaf expansion rate and leaf water status were perturbed by environmentally induced changes in plant transpiration. After establishing a steady state growth rate, a step decrease in plant transpiration resulted in a rapid and large increase in leaf cell turgor (0.25 megapascal in 5 minutes), and leaf expansion rate. Subsequently, leaf expansion rate returned to the original steady state rate with no change in cell turgor. These results indicate that the expansion rate of leaves may not be strongly related to the turgor of the leaf cells, and that substantial control of leaf expansion rate, despite changes in turgor, may be part of normal plant function. It is suggested that a strictly physical interpretation of the parameters most commonly used to describe the relationship between turgor and growth in plant cells (cell wall extensibility and yield threshold) may be inappropriate when considering the process of plant cell expansion.

Effects of NaCl and CaCl(2) on Cell Enlargement and Cell Production in Cotton Roots

In many crop species, supplemental Ca(2+) alleviates the inhibition of growth typical of exposure to salt stress. In hydroponically grown cotton seedlings (Gossypium hirsutum L. cv Acala SJ-2), both length and weight of the primary root were enhanced by moderate salinities (25 to 100 millimolar NaCl) in the presence of 10 millimolar Ca(2+), but the roots became thinner. Anatomical analysis showed that the cortical cells of these roots were longer and narrower than those of the control plants, while cortical cells of roots grown at the same salinities but in the presence of only 0.4 millimolar Ca(2+) became shorter and more nearly isodiametrical. Cell volume, however, was not affected by salinities up to 200 millimolar NaCl at either 0.4 or 10 millimolar Ca(2+). Our observations suggest Ca(2+)-dependent effects of salinity on the cytoskeleton. The rate of cell production declined with increasing salinity at 0.4 millimolar Ca(2+) but at 10 millimolar Ca(2+) was not affected by salinities up to 150 millimolar NaCl.

Estimation of osmotic parameters accompanying zeatin-induced growth of detached cucumber cotyledons

Water potential (psi), the osmotic potential (psi(pi)), and the pressure potential (psi(p)) of detached cotyledons isolated from Cucumis sativus L. cv Marketer seedlings after 0, 1.5, and 3 days growth with and without zeatin were determined. From zero time to 3 days, cotyledons incubated without exogenous zeatin exhibited a slight decrease in psi (from -0.4 to -1.0 bars), while those grown with zeatin developed even more negative values (about -4 bars). Both groups showed rising psi(pi) values (decreases in solutes per unit volume), but this rise was more dramatic in those treated with zeatin. These data indicate that the capacity of zeatin-treated cotyledons to take up water more rapidly than controls and thus expand faster must be due to wall loosening, as reflected in psi(p) values which declined during 3 days from about +11 bars to about +1.4 bars.It was also found that freshly detached cotyledons or those grown without exogenous zeatin exhibited osmoregulation in polyethylene glycol (PEG) solutions. That is, while cotyledons initially lost H(2)O into certain PEG solutions, their psi values decreased over time and they began absorbing water after 1 to 4 hours. After 3 days growth, zeatin-treated cotyledons had lost most of this capacity of osmoregulate. It seems likely that osmoregulation in cotyledons not treated with zeatin is due to wall loosening rather than changes in psi(pi). Zeatin-treated cotyledons with already loosened walls may not have this option to deal with water stress and thus simply come to equilibrium with external PEG solutions.