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

Abstract

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.

Figure 1.

Maintenance of root growth under water deficit conditions. A) Comparative responses of elongation rate in different organs of maize to the development of water stress during soil drying. Nodal root elongation continued at growth zone water potentials that caused complete inhibition of elongation in vegetative and reproductive shoot tissues. Because growth responses were determined as a function of the water potentials of the growing tissues, the differential sensitivities reflect inherent differences in how cellular physiology responds to water stress in the different organs. B) More extensive root system development in maize plants when grown under soil drying (dryland) compared with irrigated conditions. A modified from Westgate and Boyer (1985), Figure 1, by permission of Springer Nature. B reproduced from Weaver (1926), Figure 87, p 189, by permission of John Wiley and Sons.

Figure 2.

Cell wall–yielding properties are enhanced in maize primary roots after water stress imposition. Maize seedlings were grown in solution at a water potential of approximately 0 MPa (0.1 mm CaCl₂). Root elongation rate was monitored with a position transducer, and turgor of surface cells in the central region of the growth zone was measured every few minutes using a pressure microprobe. A) A stepwise decrease in media water potential was imposed by addition of −0.42 MPa sorbitol, and after 2 hours the sorbitol was removed. B) The abrupt decrease in water potential caused essentially immediate cessation of root elongation, as well as decrease in root turgor (C). Elongation recovered to the well-watered rate within an hour after the onset of stress, whereas turgor recovery as a result of osmotic adjustment was more gradual and turgor did not reach the well-watered level for the duration of the stress treatment. Full recovery of elongation with only partial turgor recovery indicates that cell wall–yielding properties were rapidly enhanced in response to water stress; in terms of the Lockhart model (see text), either the yield threshold decreased or the extensibility increased, or both. Conversely, removal of water stress caused a short-lived spike in root elongation followed by deceleration to the original rate, again pointing to compensatory adjustments of cell wall–yielding properties. Modified from Hsiao and Jing (1987), Figure 7, by permission of ASPB.

Figure 3.

Kinematic analysis reveals spatially differential responses of tissue expansion to water stress within the growth zone of the maize primary root. Maize seedlings were grown under well-watered conditions (water potential of −0.03 MPa) or at mild (−0.20 MPa), moderate (−0.81 MPa), or severe (−1.60 MPa) water stress (obtained by adjusting the vermiculite media water content). When the primary roots were approximately 5 cm long, the apical 10-mm region was marked at approximately 0.6-mm intervals for temporal analysis of mark displacement away from the apex. A) Displacement of marks during 3.5 h after marking for representative roots growing under well-watered or severe water stress conditions. White lines indicate vertical displacement of the root apices and of marks originally located at 5 and 10 mm from the apex. In well-watered roots, mark separation, and hence tissue expansion, occurred throughout the apical 10 mm, whereas in severely water-stressed roots, mark separation was confined to the apical 5 mm. Water-stressed roots were also substantially thinner than well-watered controls, indicating inhibition of radial expansion. B) Time-lapse analysis of mark displacement during 1 h after marking was used to calculate the distribution of relative elongation rate as a function of distance from the root apex. In all water stress treatments, local elongation rates in the apical 3 mm were maintained at the well-watered rate, whereas elongation was increasingly inhibited with increasing water stress as cells were displaced further from the apex, resulting in progressive shortening of the growth zone. Modified from Sharp et al. (1988), Figures 3 and 5, by permission of ASPB.

Figure 4.

Changes in cell wall–yielding properties and composition in the growth zone of water-stressed maize primary roots. A) Acid-induced extension was enhanced in the apical region (0 to 5 mm from the apex) and almost completely inhibited in the basal region (5–10 mm) of the growth zone in water-stressed roots (WS; vermiculite water potential of −1.6 MPa) compared with well-watered (WW) developmental (roots of the same length) and temporal (roots of the same age) controls. The increased acid-induced extension in the apical region is thought to play an important role in maintaining elongation in this region (Fig. 3) despite substantially decreased turgor (inset) due to incomplete osmotic adjustment (see Fig. 6). Conversely, inhibition of acid-induced extension in the basal region, together with decreased turgor, likely contributes to premature slowing and cessation of elongation as cells are displaced through this region (Fig. 3). The apical region of water-stressed compared with well-watered roots also showed large increases in (B) expansin activity (change in slope of acid-induced extension of heat-killed cucumber hypocotyl wall preparations following addition of maize root tip expansin extract), (C) expansin susceptibility (change in slope of acid-induced extension of heat-killed maize root tip wall preparations following addition of cucumber expansin extract), and (D) XTH activity per unit of cell wall dry weight (CWDW), as well as (E) decreased abundance of galactoside 2-α-1-fucosyltransferase-like protein. F) Principal component analysis (PCA) of cell wall Fourier transform infrared (FTIR) spectra showed that different regions of the growth zone (delineated as mm from the apex) of water-stressed roots (black symbols, water potential of −0.5 MPa imposed with PEG 6000 in solution culture) are compositionally different compared with respective regions of well-watered controls (open symbols). Left and right panels show different wavenumber ranges. A, B, and C modified from Wu et al. (1996), Figures 1, 3B, 6A; A inset modified from Spollen and Sharp (1991), Figure 2B; D modified from Wu et al. (1994), Figure 3C; E modified from Voothuluru et al. (2016), Figure 6; F reproduced from Fan et al. (2006), Figure 3; A–D and F by permission of ASPB, E by permission of John Wiley and Sons.

Figure 5.

Osmotic adjustment in maize nodal roots under soil-drying conditions. Water potential, osmotic potential, and turgor in the growth zone and mature regions of nodal roots and in mature leaf blade tissues (green shading) of maize plants growing under well-watered or soil-drying conditions. In roots growing through dry soil, a steep “growth-induced” water potential gradient developed between the growth zone and adjacent mature region due to axial delivery of water and the hydraulic resistance of nonvascularized root tip tissues. This necessitates a high capacity for osmotic adjustment to maintain turgor in the root growth zone. The leaf blade was completely wilted in the same plants. Modified from Sharp and Davies (1979), Figure 6, by permission of Springer Nature.

Figure 6.

Different solutes contribute to osmotic adjustment in the apical and basal regions of the maize primary root growth zone. A) Spatial distribution of osmotic potential in the apical 10 mm of roots growing under well-watered conditions (water potential of −0.03 MPa) or at mild (−0.20 MPa), moderate (−0.81 MPa), or severe (−1.60 MPa) water stress. In the apical region where elongation is maintained in water-stressed roots (Fig. 3), increased proline concentrations (B) resulted primarily from increased net rates of proline deposition (D). In the basal region of growth inhibition (Fig. 3), conversely, increased hexose concentrations in water-stressed roots (C) occurred despite decreased net rates of hexose deposition (E) because tissue expansion, and hence water deposition, decreased to a greater extent. A, C, and E modified from Sharp et al. (1990), Figures 2, 4A, 6C; B and D modified from Voetberg and Sharp (1991), Figures 1A, 2A; by permission of ASPB.

Figure 7.

Increased endogenous ABA levels are necessary for maintaining root growth under water deficit conditions. A) Maize primary root elongation rate as a function of ABA content in the growth zone (apical 10 mm) for various genotypes growing in vermiculite under well-watered (water potential of −0.03 MPa, open symbols) or water-stressed (water potential of −1.60 MPa, closed symbols) conditions. In well-watered roots, the growth zone ABA content of hybrid (cv. FR27 × FRMo17) seedlings was raised above the normal level by adding various concentrations of ABA (A) to the vermiculite, which caused progressive inhibition of root elongation. Conversely, in water-stressed roots, the growth zone ABA content was decreased below the normal level by treatment with fluridone (F) or by using the vp5 or vp14 mutants (ABA deficient), which resulted in inhibition of root elongation with a common relationship of growth inhibition to ABA deficiency. Data are plotted as a percentage of the rate for the same genotype at high water potential. B) Recovery of elongation in water-stressed roots of the vp5 maize mutant when growth zone ABA content was restored by applying exogenous ABA. C)  Arabidopsis primary root growth was greatly reduced in the aba2 mutant (ABA deficient) but not in the Col-0 wild-type when grown under conditions of low aerial relative humidity (40% RH) compared with a high humidity control (Mock) treatment. The roots were growing in well-watered soil. D) ABA accumulation in the growth zone of wild-type roots in the low humidity treatment was visualized using the ABACUS2s ABA biosensor. E) Relative quantification of the emission ratio signal in (D) in various regions of the root showed that the elongation zone (EZ) accumulates more ABA when grown in the low humidity treatment. DZ, differentiation/maturation zone; RHair, root hair zone. A reproduced from Sharp (2002), Figure 2, by permission of John Wiley and Sons; B modified from Sharp et al. (1994), Plate 2 and Table 1, by permission of Oxford University Press; C to E, modified from Rowe et al. (2023), Figure 4, CC BY 4.0.

Figure 8.

Schematic summarizing the effects of ABA accumulation on diverse cellular processes in different regions of roots growing under water stress conditions.

Figure 9.

Impact of water availability on lateral root development in the maize primary root system. A) Average length, (B) cell flux, and (C) cortical cell length profiles along the growth zone of the 10 longest first-order lateral roots from the upper 15 cm of the primary root system of maize (cv. FR697) seedlings during 11 days after transplanting (DAT) to well-watered (WW) or mild water deficit (MWD, water potential of −0.28 MPa) conditions. Promotion of lateral root length in the MWD treatment was associated with delayed determinacy compared with WW roots, as evident from sustained rates of cell flux (the rate within a file that cells leave the growth zone, which under steady growth conditions equals the rate of cell production from the meristem) and repression of changes in cortical cell length profile, final cell length, and length of the growth zone that occurred in the WW roots over the course of the experiment. D) A xerobranching response is triggered in wild-type (WT) maize when growing root tips lose contact with water, for example, when growing across an air gap, causing repression of lateral root formation until the roots reenter moist conditions. E) The ABA-deficient mutant vp14 produced a significantly higher number of lateral roots in the air gap compared with the wild-type. A to C modified from Dowd et al. (2020), Figures 3A, 5, 4, by permission of John Wiley and Sons. D and E reproduced from Mehra et al. (2022), Figure S3, by permission of AAAS.

Figure 10.

Schematic summarizing the effects of water stress on diverse cellular processes in different regions of roots growing under water stress conditions that eventually determine root system architecture.