Spatial separation of light perception and growth response in maize root phototropism

Although the effects of gravity on root growth are well known and interactions between light and gravity have been reported, details of root phototropic responses are less documented. We used high-resolution image analysis to study phototropism in primary roots of Zea mays L. Similar to the location of perception in gravitropism, the perception of light was localized in the root cap. Phototropic curvature away from the light, on the other hand, developed in the central elongation zone, more basal than the site of initiation of gravitropic curvature. The phototropic curvature saturated at approximately 10 micromoles m-2 s-1 blue light with a peak curvature of 29 +/- 4 degrees, in part due to induction of positive gravitropism following displacement of the root tip from vertical during negative phototropism. However, at higher fluence rates, development of phototropic curvature is arrested even if gravitropism is avoided by maintaining the root cap vertically using a rotating feedback system. Thus continuous illumination can cause adaptation in the signalling pathway of the phototropic response in roots.     

Changes in response to drought stress of triticale and maize genotypes differing in drought tolerance

Direct effects and after-effects of soil drought for 7 and 14 d were examined on seedling dry matter, leaf water potential (ψ), leaf injury index (LI), and chlorophyll (Chl) content of drought (D) resistant and sensitive triticale and maize genotypes. D caused higher decrease in number of developed leaves and dry matter of shoots and roots in the sensitive genotypes than in the resistant ones. Soil D caused lower decrease of ψ in the triticale than maize leaves. Influence of D on the Chl b content was considerably lower than on the Chl a content. In triticale the most harmful D impact was observed for physiologically younger leaves, in maize for the older ones. A period of 7-d-long recovery was too short for a complete removal of an adverse influence of D.     

Phytochelatin synthesis in maize seedlings in response to excess zinc

Buthionine sulfoximine (BSO) specifically inhibits γ-glutamylcysteine synthetase and decreases a cellular level of glutathione (GSH) in maize seedling roots. Exogenous GSH restores Zn-phytochelatins synthesis in BSO-treated maize plants.     

Effects of water stress on growth, osmotic potential and abscisic acid content of maize roots

Under water stress conditions, induced by mannitol solutions (0 to 0.66 M ) applied to the apical 12 mm of intact roots of Zea mays L. (cv. LG 11), a growth inhibition, a decrease in the osmotic potential of the cell sap and a significant accumulation of abscisic acid (ABA) were observed. When the roots were placed in a humid atmosphere after the stress, the growth rate increased again, even if elongation had been totally inhibited. Under a stress corresponding to an osmotic potential of -1.09 MPa in the solution, growth was totally inhibited, which means that the root cell turgor pressure was reduced to the yield threshold. These conditions led to the largest accumulation of ABA. The effect of water stress on the level of ABA was studied for three parts of the root. The greatest increase in ABA (about 10 fold) was obtained in the growth zone and this increase was apparently independent of the hydrolysis of the conjugated form. With a mannitol treatment of 1 h equivalent to a stress level of -1.39 MPa, a 4-fold increase in ABA efflux into the medium was obtained. These results suggest that there are interactions between water stress, root growth, osmotic potential and the ABA level. The growth under conditions of stress and the role of endogenous ABA in the control of plant metabolism, specially in the growth zone, are discussed.     

Xylem pressure response in maize roots subjected to osmotic stress: determination of radial reflection coefficients by use of the xylem pressure probe

The absolute pressure in conducting xylem vessels of roots of 2-week-old, slowly transpiring intact maize plants (bathed in nutrition medium) was determined to be +0·024 ± 0·044 MPa using the xylem pressure probe. When the roots were subjected to osmotic stress (NaCI, KCI or sucrose), the xylem pressure decreased immediately and became more negative. However, the response of xylem pressure to osmotic stress was considerably attenuated, indicating that the radial reflection coefficients, σ₁₃ of the maize root for these solutes were rather low (between 0·2 and 0·4 depending on the concentration of the osmoticum). The low values of a, may be caused (partly) by unstirred layer effects. In repeated osmoticum/nutrition regimes a complex pattern of changes in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loading of the xylem. These processes were not observed when the roots were subjected to osmotic stress after excision. In this case, a biphasic response was observed comparable to that found for excised roots using the root pressure probe.     

Genetic variation in ZmTIP1 contributes to root hair elongation and drought tolerance in maize

Drought is a major abiotic stress that threatens maize production globally. A previous genome‐wide association study identified a significant association between the natural variation of ZmTIP1 and the drought tolerance of maize seedlings. Here, we report on comprehensive genetic and functional analysis, indicating that ZmTIP1, which encodes a functional S‐acyltransferase, plays a positive role in regulating the length of root hairs and the level of drought tolerance in maize. We show that enhancing ZmTIP1 expression in transgenic Arabidopsis and maize increased root hair length, as well as plant tolerance to water deficit. In contrast, ZmTIP1 transposon‐insertional mutants displayed the opposite phenotype. A calcium‐dependent protein kinase, ZmCPK9, was identified as a substrate protein of ZmTIP1, and ZmTIP1‐mediated palmitoylation of two cysteine residues facilitated the ZmCPK9 PM association. The results of this research enrich our knowledge about ZmTIP1‐mediated protein S‐acylation modifications in relation to the regulation of root hair elongation and drought tolerance. Additionally, the identification of a favourable allele of ZmTIP1 also provides a valuable genetic resource or selection target for the genetic improvement of maize.     

Pattern of sulfate uptake during root elongation in maize: its correlation with productivity

The efficiency of sulfate uptake was evaluated in excised roots of 22 maize genotypes, 12 inbreds and 10 hybrids, in order to study the relationship between the kinetic characteristics of the uptake and the grain productivity. During root elongation, the uptake capacity showed a pulse which appeared when the root reached 1/3 to 1/2 of its final length. The size of the accumulated pool of sulfate was significantly correlated with the productivity. The kinetic parameters of the uptake, Vmax and Km, followed the same trend, showing pulses, whoxe maximum had the same position for Vmax and Km in each genotype. The variability with the genotype of the size and duration of the Vmax pulse was not strictly connected with that of Km. The main correlation between Vmax and Km patterns was the following; inbreds were generally characterized by low Vmax and low Km; hybrids by high Vmax and high Km. As a consequence, in most cases, the benefit of the heterotic stimulation of Vmax was contrasted by the loss of affinity of the transport system or the nutrients.     

Response of maize seedling roots to changing ethylene concentrations

Maize roots (Zea mays, cv. DK 626) growing in aerated solutions showed striking variations in the amount of ethylene produced during different stages of development. As endogenous ethylene increases, root elongation decreases. Exogenous 1-aminocyclopropane-1-carboxylic acid (ACC) supplied to these roots also inhibited their elongation and increased both the fresh weight of the apex and the ethylene produced. The inhibitor of ethylene biosynthesis, 2-aminoethoxyvinyl glycine (AVG), and the inhibitor of ethylene action, silver thiosulfate (STS), also reduced growth and increased swelling. As growth diminishes at reduced ethylene concentrations or with impeded ethylene action, these results support the view that ethylene is necessary for root growth. As ACC treatment also inhibited root elongation, it appears that ethylene was inhibitory at both low and high concentrations. Whereas ACC stimulated ethylene production 4 h after the beginning of treatment, inhibition of root elongation and promotion of fresh weight advanced slowly and needed 24 h to be established. At that time, root elongation reached a maximum response of 60% inhibition and 50% increase in weight. At 48 h, higher doses of ACC were required to provoke the same response as at 24 h. This suggests that the root growth progressively accomodates to higher ethylene concentrations. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 539–545. This text was submitted by the authors in English.     

Root cortical aerenchyma improves the drought tolerance of maize (Zea mays L.)

Root cortical aerenchyma (RCA) reduces root respiration in maize by converting living cortical tissue to air volume. We hypothesized that RCA increases drought tolerance by reducing root metabolic costs, permitting greater root growth and water acquisition from drying soil. To test this hypothesis, recombinant inbred lines with high and low RCA were observed under water stress in the field and in soil mesocosms in a greenhouse. In the field, lines with high RCA had 30% more shoot biomass at flowering compared with lines with low RCA under water stress. Root length density in deep soil was significantly greater in the high RCA lines compared with the low RCA lines. Mid‐day leaf relative water content in the high RCA lines was 10% greater than in the low RCA lines under water stress. The high RCA lines averaged eight times the yield of the low RCA lines under water stress. In mesocosms, high RCA lines had less seminal root respiration, deeper rooting, and greater shoot biomass compared with low RCA lines under water stress. These results support the hypothesis that RCA is beneficial for drought tolerance in maize by reducing the metabolic cost of soil exploration.     

Appearance of bulges on maize roots as affected by 6-benzylaminopurine and α-naphthylacetic acid

The thickening that appeared on maize roots under the influence of 6-benzylaminopurine and α-naphthylacetic acid (concentration 10⁻⁵, 10⁻⁶, 10⁻⁷ and 10⁻⁸ M) were analysed. The changes in length and width of maize roots at the edge of elongation zone after 24,48 and 72 h of treatment were studied. The growth in length of cells at the edge of elongation zone stopped abruptly but the growth in width slowly continued. So, the growth of cells in length and width under the influence of growth regulators was not simultaneous. They had distinct time limits.     

Root hair formation and elongation of primary maize roots

In maize ( Zea mays L. cv. LG 11) roots cultured in humid air, the presence of hairs was not related to root growth. However, maximum hair length and length of the hair zone could be correlated to the elongation rate of the primary root. Under the growth conditions used, the emergence of root hairs always took place in the extending zone. In more basal regions, rhizodermal cells could not give rise to root hairs. Results were similar for roots preincubated in a buffer solution.     

Influence of silicon pretreatment on aluminium toxicity in maize roots

The influence of Si pretreatment on Al toxicity in an Al sensitive maize variety (Zea mays L. var. BR 201 F) was investigated using root elongation rates (RER) and hematoxylin staining as stress indicators. Plants pretreated with 1 mt M Si (+ Si) and then exposed for 24 h to Al in nutrient solution without concurrent Si supply in the rooting medium exhibited higher RER than plants that were not pretreated with Si (-Si). The ameliorative effect of Si was due to lower Al uptake and to the exclusion of Al from the root tips in + Si plants. Lower Al uptake in + Si plants was not a consequence of decreased Al availability in the bulk solution. The possible mechanisms of Si-induced increase of Al resistance are discussed     

Water uptake profile response of corn to soil moisture depletion

The effects of soil moisture distribution on water uptake of drip‐irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady‐state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.     

Water relations and root activities of Buchloe dactyloides and Zoysia japonica in response to localized soil drying

Effects of localized soil drought stress on water relations, root growth, and nutrient uptake were examined in drought tolerant ‘Prairie’ buffalograss [Buchloe dactyloides (Nutt.) Engelm.] and sensitive ‘Meyer’ zoysiagrass (Zoysia japonica Steud.). Grasses were grown in small rhizotrons in a greenhouse and subjected to three soil moisture regimes: (1) watering the entire 80-cm soil profile (well-watered control); (2) drying 0–40 cm soil and watering the lower 40 cm (partially dried); (3) and drying the entire soil profile (fully dried). Drying the 0–40 cm soil for 28 days had no effect on leaf water potential (Ψ _leaf ) in Prairie buffalograss compared to the well-watered control but reduced that in Meyer zoysiagrass. Root elongation rate was greater for Prairie buffalograss than Meyer zoysiagrass under well-watered or fully dried conditions. Rooting depth increased with surface soil drying; with Prairie buffalograss having a larger proportion of roots in the lower 40 cm than Meyer zoysiagrass. The higher rates of water uptake in the deeper soil profile in the partially dried compared to the well-watered treatment and by Prairie buffalograss compared to Meyer zoysiagrass could be due to differences in root distribution. Root ¹⁵N uptake for Prairie buffalograss was higher in 0–20 cm drying soil in the partially dried treatment than in the fully dried treatment. Diurnal fluctuations in soil water content in the upper 20 cm of soil when the lower 40 cm were well-watered indicated water efflux from the deeper roots to the drying surface soil. This could help sustain root growth, maintain nutrient uptake in the upper drying soil layer, and prolong turfgrass growth under localized drying conditions, especially for the deep-rooted Prairie buffalograss. This revised version was published online in June 2006 with corrections to the Cover Date.     

Responses of maize roots to drying – limits of viability

Turgid pieces of mature maize roots were dried in air and progressive changes in their relative water content (RWC) determined. Viability was tested by reproducibility of the drying curves after dehydration to successively lower RWCs. After reaching a chosen RWC, the pieces were rehydrated (approximately 2 h), and a 2nd and 3rd dehydration curve measured. Each drying curve was characterized by two parameters (a scale parameter λ, and a shape parameter β) of a survivorship function, which is a linear function of time. The parameter λ is more informative, and does not change in successive dehydrations for RWC > 0·4, suggesting no irreversible damage to the roots. Damage and death were indicated by divergences of λ in successive dehydrations to RWC = 0·35–0·15. Cryo-analytical microscopy confirmed these data while indicating specifically death of 50 and 100% of cortical cells at RWC 0·30 and 0·15, respectively, and survival of 50% or more of sieve tubes, pericycle and vascular parenchyma cells at root RWC as low as 0·15. This pattern of stelar cell survival may allow roots to preserve their capacity for renewal of axial conductivity and branch root development following periods of severe water stress.     

Changes in cellular osmotic potential and mechanical properties of cell walls during light-induced inhibition of cell elongation in maize coleoptiles

The growth rate of maize ( Zea mays L. cv. Cross Bantam T51) coleoptiles in the dark was highest at the basal zone and decreased towards the tip. Growth was strongly inhibited by white fluorescent light (5 W m⁻²), especially in the basal zone of coleoptiles. Light irradiation caused an increase in the values of stress-relaxation parameters, the minimum stress-relaxation time and the relaxation rate and a decrease in the extensibility (strain/stress) of the cell walls at all zones. In addition, during growth, the accumulation of osmotic solutes was strongly inhibited by white light irradiation, resulting in an increased osmotic potential. The influences of white light on the mechanical properties of the cell wall and the osmotic potential of the tissue sap were most prominent in the basal zone. Significant correlations were observed between the increment of coleoptile length and the mechanical properties of the cell walls or the osmotic potential of the tissue sap and osmotic solutes content. Furthermore, light inhibited the outward bending of split coleoptile segments. These facts suggest that white light inhibits elongation of maize coleoptiles by modifying both the mechanical properties of the cell walls and cellular osmotic potential, which control the rate of water uptake.     

Xylem tension affects growth-induced water potential and daily elongation of maize leaves

Diurnal rates of leaf elongation vary in maize (Zea mays L.) and are characterized by a decline each afternoon. The cause of the afternoon decline was investigated. When the atmospheric environment was held constant in a controlled environment, and water and nutrients were adequately supplied to the soil or the roots in solution, the decline persisted and indicated that the cause was internal. Inside the plants, xylem fluxes of water and solutes were essentially constant during the day. However, the forces moving these components changed. Tensions rose in the xylem, and gradients of growth-induced water potentials decreased in the surrounding growing tissues of the leaf. These potentials, measured with isopiestic thermocouple psychrometry, changed because the roots became less conductive to water as the day progressed. The increased tensions were reversed by applying pressure to the soil/root system, which rehydrated the leaf. Afternoon elongation immediately recovered to rapid morning rates. The rapid morning rates did not respond to soil/root pressurization. It was concluded that increased xylem tension in the afternoon diminished the gradients in growth-induced water potential and thus inhibited elongation. Because increased tensions cause a similar but larger inhibition of elongation if maize dehydrates, these hydraulics are crucial for shaping the growth-induced water potential and thus the rates of leaf elongation in maize over the entire spectrum of water availability.     

Control of soil physical properties and response ofBrassica rapa L. seedling roots

A system was designed, constructed, tested, and used to growBrassica rapa L. seedling roots which were exposed to O₂ concentrations from 0 to 0.21 mol mol⁻¹, water potentials from 0 to −80 kPa, temperatures from 10 to 34°C, and mechanical impedance from 0 to 20.8 kPa. The experimental design was a central composite rotatable design with seven replications of the center point. Measurements were taken of taproot length, taproot diameter at the point of initiation of root hairs (diameter 1), and one cm above the first measurement (diameter 2), and total length and number of first-order laterals. Temperature had the greatest effect on seedling root growth, with linear and quadratic temperature effects significant for all root measurements except taproot diameter 2 which just had a significant linear effect. Water potential had a significant linear effect on lateral length and number of laterals and a significant quadratic effect on taproot diameter 1. Mechanical impedance had a significant effect only on taproot diameter 2. Oxygen was not significant for any root measurement. The mechanical impedance by water potential interaction was significant for taproot length and taproot diameter 1. A temperature optimum was found for taproot length, taproot diameter 1, lateral length, and lateral number, at 26.0, 42.5, 26.5, and 26.4°C, respectively. Taproot diameter 1 had a water potential optimum at −36.5 kPa, whereas taproot diameter 2 had a mechanical impedance optimum at 12.5 kPa. A growth cell designed for this study allows independent control of soil strength, water potential, oxygen concentration, and temperature. Thus, the cell provides the capability which was demonstrated forBrassica rapa L. to grow seedling roots under complete control of the soil physical properties.