Relative Elemental Growth Rate Research Articles

Separate sites of action for cry1 and phot1 blue-light receptors in the Arabidopsis hypocotyl.

Rapid cell expansion pushes the Arabidopsis hypocotyl (juvenile stem) through the soil until blue light, acting first through phototropin 1 (phot1) and then through cryptochrome 1 (cry1), suppresses elongation to produce a length characteristic of established, photosynthetically capable seedlings. To determine where these two different blue-light receptors act to suppress hypocotyl elongation, we measured relative elemental growth rate, specifically along the hypocotyl midline at 5-min intervals before and during blue light, using a machine-learning-based image analysis pipeline designed specifically for this kinematic analysis of growth. In darkness, hypocotyl material expanded most rapidly (approximately 4% h-1) in a broad zone approximately 1mm below the apical terminus of the hypocotyl (cotyledonary node). Blue light, acting through phot1, rapidly inhibited expansion in this zone, while simultaneously stimulating unexpanded cells in a very narrow, more apical region. Nuclear cry1, and not its cytoplasmic pool, counteracted the phot1-initiated expansion of the small cells in this apical region, preventing them from entering the more basal elongation zone. In a cry1 mutant, expansion of these apical cells proceeded unchecked, reaching rates as high as 6% h-1 to produce the iconic cry1 long-hypocotyl phenotype. The new spatial information shows where to focus future cell and molecular studies of cry1 and phot1 signaling mechanisms and, ecologically, indicates that a seedling may use an apical reservoir of elongation potential to reenter a lit environment should a natural darkening event such as soil disturbance deactivate cry1.

QTL for the Kinematic Traits That Define the Arabidopsis Root Elongation Zone and Their Relationship to Gravitropism.

Cell expansion in a discrete region called the elongation zone drives root elongation. Analyzing time lapse images can quantify the expansion in kinematic terms as if it were fluid flow. We used horizontal microscopes to collect images from which custom software extracted the length of the elongation zone, the peak relative elemental growth rate (REGR) within it, the axial position of the REGR peak, and the root elongation rate. Automation enabled these kinematic traits to be measured in 1575 Arabidopsis seedlings representing 162 recombinant inbred lines (RILs) derived from a cross of Cvi and Ler ecotypes. We mapped ten quantitative trait loci (QTL), affecting the four kinematic traits. Three QTL affected two or more traits in these vertically oriented seedlings. We compared this genetic architecture with that previously determined for gravitropism using the same RIL population. The major QTL peaks for the kinematic traits did not overlap with the gravitropism QTL. Furthermore, no single kinematic trait correlated with quantitative descriptors of the gravitropism response curve across this population. In addition to mapping QTL for growth zone traits, this study showed that the size and shape of the elongation zone may vary widely without affecting the differential growth induced by gravity.

Patch Track Software for Measuring Kinematic Phenotypes of Arabidopsis Roots Demonstrated on Auxin Transport Mutants.

Plant roots elongate when cells produced in the apical meristem enter a transient period of rapid expansion. To measure the dynamic process of root cell expansion in the elongation zone, we captured digital images of growing Arabidopsis roots with horizontal microscopes and analyzed them with a custom image analysis program (PatchTrack) designed to track the growth-driven displacement of many closely spaced image patches. Fitting a flexible logistics equation to patch velocities plotted versus position along the root axis produced the length of the elongation zone (mm), peak relative elemental growth rate (% h-1), the axial position of the peak (mm from the tip), and average root elongation rate (mm h-1). For a wild-type root, the average values of these kinematic traits were 0.52 mm, 23.7% h-1, 0.35 mm, and 0.1 mm h-1, respectively. We used the platform to determine the kinematic phenotypes of auxin transport mutants. The results support a model in which the PIN2 auxin transporter creates an area of expansion-suppressing, supraoptimal auxin concentration that ends 0.1 mm from the quiescent center (QC), and that ABCB4 and ABCB19 auxin transporters maintain expansion-limiting suboptimal auxin levels beginning approximately 0.5 mm from the QC. This study shows that PatchTrack can quantify dynamic root phenotypes in kinematic terms.

High-Resolution Kinematic Analysis of Root Gravitropic Bending Using RootPlot.

Root gravitropic bending is a complex growth process resulting from differential expansion of cells on the upper and lower sides of a gravistimulated root. In order to genetically dissect the molecular machinery underlying root bending, a thorough understanding of the kinetics and spatial distribution of the growth process is required. We have developed an experimental workflow that enables us to image growing roots at high spatiotemporal resolution and then convert XY-coordinates of root cellular markers into 3D representations of root growth profiles. Here, we present a detailed description of the setup for monitoring vertically oriented roots before and after gravistimulation. We also introduce our newly developed custom R-based program RootPlot, which calculates root velocity profiles from root XY-coordinate data obtained using a previously published image processing software. The raw velocity and derived relative elemental growth rate (REGR) curves are then fitted via LOWESS regression for assumption-free data analysis. The resulting smoothed growth profiles are plotted as heatmaps to visualize how different regions of the root contribute to the growth response over time. Additionally, RootPlot provides analysis of overall growth and bending rates based on root XY-coordinates.

Drought Effects On Elongation Kinetics And Sugar Deposition In The Elongation Zone Of Durum Wheat (Triticum durum Desf.) Leaves

The aim of this study was to analyze the effect of drought stress on the kinetics of leaf elongation in relationship to the variation of sugar concentrations and their net deposition rates along the elongation zone of leaf 4 of durum wheat plants. Plants were grown in soil in a naturally illuminated greenhouse, and water was withheld from seedlings for a period of 14 days. Leaf 4 of 26 day-old plants was used for growth measurements and tissue sampling. Relative elemental growth rates (REGR), cell displacement velocity (DV), and elongation zone length (EZL) were significantly reduced by drought treatment. Together, this resulted in a decrease in leaf elongation rate (LER) in drought-stressed plants. Epidermal cell length along the elongation zone was not significantly affected by drought stress, indicating that the decrease in elongation zone length was due mainly to a reduction in cell production rate. The concentration of total soluble sugars (TSS) and non-reducing sugars (NRS) was highest at the leaf base and decreased distally from 10 mm from the leaf base in plants grown under non-stressed (control) conditions. Drought stress caused a significant accumulation of TSS at the leaf base, mainly through an increase in non-reducing sugars. The continuity equation was used to calculate sugar net deposition rates. Drought stress increased the net deposition rates of non-reducing sugars in the first 10 mm from the leaf base. This increase was the principal source for the increase in non-reducing sugars concentrations at the leaf base in response to drought.

The Dynamics of Root Growth: A Geometric Model.

A new model for macroscopic root growth based on a dynamical Riemannian geometry is presented. Assuming that the thickness of the root is much less than its length, the model is restricted to growth in one dimension (1D). We treat 1D tissues as continuous, deformable, growing geometries for sizes larger than 1 mm. The dynamics of the growing root are described by a set of coupled tensor equations for the metric of the tissue and velocity field of material transport in non-Euclidean space. These coupled equations represent a novel feedback mechanism between growth and geometry. We compare 1D numerical simulations of these tissue growth equations to two measures of root growth. First, sectional growth along the simulated root shows an elongation zone common to many species of plant roots. Second, the relative elemental growth rate calculated in silico exhibits spatio-temporal dynamics recently characterized in high-resolution root growth studies but which thus far lack a biological hypothesis to explain them. In our model, these dynamics are a direct consequence of considering growth as both a geometric reaction-diffusion process and expansion due to a distributed source of new materials.

KymoRod: a method for automated kinematic analysis of rod-shaped plant organs.

A major challenge in plant systems biology is the development of robust, predictive multiscale models for organ growth. In this context it is important to bridge the gap between the, rather well-documented molecular scale and the organ scale by providing quantitative methods to study within-organ growth patterns. Here, we describe a simple method for the analysis of the evolution of growth patterns within rod-shaped organs that does not require adding markers at the organ surface. The method allows for the simultaneous analysis of root and hypocotyl growth, provides spatio-temporal information on curvature, growth anisotropy and relative elemental growth rate and can cope with complex organ movements. We demonstrate the performance of the method by documenting previously unsuspected complex growth patterns within the growing hypocotyl of the model species Arabidopsis thaliana during normal growth, after treatment with a growth-inhibiting drug or in a mechano-sensing mutant. The method is freely available as an intuitive and user-friendly Matlab application called KymoRod.

Dynamics of Plant Growth; A Theory Based on Riemannian Geometry

In this work, a new model for macroscopic plant tissue growth based on dynamical Riemannian geometry is presented. We treat 1D and 2D tissues as continuous, deformable, growing geometries for sizes larger than 1mm. The dynamics of the growing tissue are described by a set of coupled tensor equations in non-Euclidean (curved) space. These coupled equations represent a novel feedback mechanism between growth and curvature dynamics. For 1D growth, numerical simulations are compared to two measures of root growth. First, modular growth along the simulated root shows an elongation zone common to many species of plant roots. Second, the relative elemental growth rate (REGR) calculated in silico exhibits temporal dynamics recently characterized in high-resolution root growth studies but which thus far lack a biological hypothesis to explain them. Namely, the REGR can evolve from a single peak localized near the root tip to a double-peak structure. In our model, this is a direct consequence of considering growth as both a geometric reaction-diffusion process and expansion due to a distributed source of new materials. In 2D, we study a circularly symmetric growing disk with emergent negative curvatures. These results are compared against thin disk experiments, which are a proxy model for plant leaves. These results also apply to the curvature evolution and the inhomogeneous growth pattern of the Acetabularia cap. Lastly, we extend the model to anisotropic disks and predict the growth dynamics for a 2D curved surface which develops an elongated shape with localized ruffling. Our model also provides several measures of the dynamics of tissue growth. These include the time evolution of the metric and velocity field, which are dynamical variables in the model, as well as expansion, shear and rotation which are deformation tensors characterizing the growth of the tissue.

Limitation of Cell Elongation in Barley (Hordeum vulgare L.) Leaves Through Mechanical and Tissue-Hydraulic Properties.

The aim of the present study was to assess the mechanical and hydraulic limitation of growth in leaf epidermal cells of barley (Hordeum vulgare L.) in response to agents which affect cellular water (mercuric chloride, HgCl(2)) and potassium (cesium chloride, CsCl; tetraethylammonium, TEA) transport, pump activity of plasma membrane H(+)-ATPase and wall acidification (fusicoccin, FC). Cell turgor (P) was measured with the cell pressure probe, and cell osmotic pressure (π) was analyzed through picoliter osmometry of single-cell extracts. A wall extensibility coefficient (M) and tissue hydraulic conductance coefficient (L) were derived using the Lockhart equation. There was a significant positive linear relationship between relative elemental growth rate and P, which fit all treatments, with an overall apparent yield threshold of 0.368 MPa. Differences in growth between treatments could be explained through differences in P. A comparison of L and M showed that growth in all except the FC treatment was co-limited through hydraulic and mechanical properties, though to various extents. This was accompanied by significant (0.17-0.24 MPa) differences in water potential (ΔΨ) between xylem and epidermal cells in the leaf elongation zone. In contrast, FC-treated leaves showed ΔΨ close to zero and a 10-fold increase in L.

Distribution of linear growth rates in different directions in root apical meristems

Growth of apical meristems in plants may be well described by the growth tensor method. Hejnowicz (Envir. Exp. Bot. 1989, 29) determined growth tensors for roots: one with a minimum and the other with a maximum of the relative elemental growth rate in volume and used them for the description of two types of apices: one with an apical cell and merophytes (I), and the other with files of cells converging towards a quiescent centre, CQ (II). In the present paper the same cases are considered from the point of view of a spatial and directional variation of the relative elemental rate of growth in length, RERG1. Maps of the RERG<sub>1</sub> in two planes: axial and tangential, the latter determined by periclinal-longitudinal (PL) and periclinal-tangential (PT) principal growth directions, are shown. In an apical part of apex i where there is maximum volumetric growth, there also occurs a maximum of RERG<sub>1</sub> for all directions. In regions other than this RERG1 decreases although RERG<sub>1</sub> in the PL direction predominates everywhere. In apex II RERG<sub>1</sub> for all directions has a minimum in CQ and becomes increasingly larger with increasing distance from it - the maximum is in the PL direction in the cylindrical part of the apex. In peripheral parts of both apices, in the place of the root/cap junction, RERG, in the anticlinal direction is significantly small.

Developmentally equivalent tissue sampling based on growth kinematic profiling of Arabidopsis inflorescence stems

• Directional growth in Arabidopsis thaliana during bolting of the inflorescence stem makes this an attractive system for study of the underlying processes of tissue elongation and cell wall extension. Analysis of local molecular events accompanying Arabidopsis inflorescence stem elongation is hampered by difficulties in isolating developmentally matched tissue samples from different plants. • Here, we present a novel sampling approach in which specific developmental stages along the developing stem are defined nonintrusively in terms of their relative elemental growth rate by use of time-lapse imagery and subsequent derivation of growth kinematic profiles for individual plants. • Growth kinematic profiling reveals that key developmental transitions such as the point of maximum elongation rate and the point of cessation of elongation occur over broad and overlapping ranges across individuals within a population of the Columbia (Col-0) ecotype. The position of these transitions is only weakly correlated with overall plant height, which undermines the common assumption that physically similar plants have closely matched growth profiles. • This kinematic profiling approach provides high-resolution growth phenotyping of the developing stem and thereby enables the harvest, pooling and analysis of developmentally matched tissue samples from multiple Arabidopsis plants.

Temperature responses of roots: impact on growth, root system architecture and implications for phenotyping.

Root phenotyping is a challenging task, mainly because of the hidden nature of this organ. Only recently, imaging technologies have become available that allow us to elucidate the dynamic establishment of root structure and function in the soil. In root tips, optical analysis of the relative elemental growth rates in root expansion zones of hydroponically-grown plants revealed that it is the maximum intensity of cellular growth processes rather than the length of the root growth zone that control the acclimation to dynamic changes in temperature. Acclimation of entire root systems was studied at high throughput in agar-filled Petri dishes. In the present study, optical analysis of root system architecture showed that low temperature induced smaller branching angles between primary and lateral roots, which caused a reduction in the volume that roots access at lower temperature. Simulation of temperature gradients similar to natural soil conditions led to differential responses in basal and apical parts of the root system, and significantly affected the entire root system. These results were supported by first data on the response of root structure and carbon transport to different root zone temperatures. These data were acquired by combined magnetic resonance imaging (MRI) and positron emission tomography (PET). They indicate acclimation of root structure and geometry to temperature and preferential accumulation of carbon near the root tip at low root zone temperatures. Overall, this study demonstrated the value of combining different phenotyping technologies that analyse processes at different spatial and temporal scales. Only such an integrated approach allows us to connect differences between genotypes obtained in artificial high throughput conditions with specific characteristics relevant for field performance. Thus, novel routes may be opened up for improved plant breeding as well as for mechanistic understanding of root structure and function.

Dynamics of root growth stimulation in Nicotiana tabacum in increasing light intensity

Light intensity is crucial for plant growth. In this study, the hypothesis was tested whether a sudden increase in light intensity leads to an immediate increase of root growth. Seedlings of Nicotiana tabacum grown in agar-filled Petri dishes were subjected to light intensities of 60 and 300 micromol m(-2) s(-1), respectively. Seedling biomass, sucrose, glucose and fructose concentration as well as primary root growth increased significantly with light intensity. The dynamics of the increase in root growth were analysed here in more detail. In transition experiments from low to high light intensities, root growth increased by a factor of four within 4 d, reaching the steady-state level measured in plants that were cultivated in high-light conditions. The distribution of relative elemental growth rates along the root growth zone retained a constant shape throughout this transition. During the first three hours after light increase, strong growth fluctuations were repeatedly observed with the velocity of the root tip cycling in a sinusoidal pattern between 120 and 180 microm h(-1). These dynamic patterns are discussed in the context of hydraulic and photosynthetic acclimation to the altered conditions. Experiments with externally applied sucrose and with transgenic plants having reduced capacities for sucrose synthesis indicated clearly that increasing light intensity rapidly enhanced root growth by elevating sucrose export from shoot to root.

Towards plant systems biology – novel mathematical approaches to enable quantitative analysis of growth processes

Towards plant systems biology – novel mathematical approaches to enable quantitative analysis of growth processes

The effect of gibberellic acid on the response of leaf extension to low temperature

The effect of cooling on leaf extension rate (LER) and on relative elemental growth rate (REGR) was measured in both gibberellic acid (GA)-responsive dwarf barley and in the same barley variety treated with GA. Seedlings were maintained at 20 degrees C while their leaf extension zone (LEZ) temperature was reduced either in steps to -6 degrees C in short-term cooling experiments, or to 10 degrees C for 48 h in long-term cooling experiments. Short-term cooling resulted in a biphasic response in LER, with a clear inflection point identified. Below this point, the activation energy for leaf extension becomes higher. The short-term response of LER to cooling was altered by the application of GA, which resulted in a lower base temperature (Tb), inflection point temperature and activation energy for leaf extension. Both GA-treated and untreated seedlings were less sensitive to cooling maintained for a prolonged period, with LER making a partial recover over the initial 5 h. Although long-term cooling reduced maximum REGR, it resulted in a longer LEZ and an increase in the length of mature interstomatal cells in GA-treated and untreated seedlings. These changes in overall physiology appear to enhance the ability of the leaves to continue expansion at suboptimal temperatures. In both GA-treated and cold-acclimated tissue, the occurrence of a longer LEZ was associated with a lower temperature sensitivity in LER.

Phosphorus nutrition and mycorrhiza effects on grass leaf growth. P status‐ and size‐mediated effects on growth zone kinematics

This study tested whether leaf elongation rate (LER, mm h(-1)) and its components--average relative elemental growth rate (REGRavg, mm mm(-1) h(-1)) and leaf growth zone length (L(LGZ), mm)--are related to phosphorus (P) concentration in the growth zone (P(LGZ) mg P g(-1) tissue water) of Lolium perenne L. cv. Condesa and whether such relationships are modified by the arbuscular mycorrhizal fungus (AMF) Glomus hoi. Mycorrhizal and non-mycorrhizal plants were grown at a range of P supply rates and analysed at either the same plant age or the same tiller size (defined by the length of the sheath of the youngest fully expanded leaf). Both improved P supply (up to 95%) and AMF (up to 21%) strongly increased LER. In tillers of even-aged plants, this was due to increased REGRavg and L(LGZ). In even-sized tillers, it was exclusively due to increased REGRavg. REGRavg was strictly related to P(LGZ) (r2 = 0.95) and independent of tiller size. Conversely, L(LGZ) strictly depended on tiller size (r2 = 0.88) and not on P(LGZ). Hence, P status affected leaf growth directly only through effects on relative tissue expansion rates. Symbiosis with AMF did not modify these relationships. Thus, no evidence for P status-independent effects of AMF on LER was found.

Aphid infestation causes different changes in carbon and nitrogen allocation in alfalfa stems as well as different inhibitions of longitudinal and radial expansion.

Alfalfa (Medicago sativa) stem elongation is strongly reduced by a pea aphid (Acyrthosiphon pisum Harris) infestation. As pea aphid is a phloem feeder that does not transmit virus or toxins, assimilate withdrawal is generally considered as the main mechanism responsible for growth reduction. Using a kinematic analysis, we investigated the spatial distributions of relative elemental growth rates of control and infested alfalfa stems. The water, carbon, and nitrogen contents per unit stem length were measured along the growth zone. Deposition rates and growth-sustaining fluxes were estimated from these patterns. Severe short-term aphid infestation (200 young adults over a 24-h period) induced a strong and synchronized reduction in rates of elongation and of water and carbon deposition. Reduced nitrogen content and associated negative nitrogen deposition rates were observed in some parts of the infested stems, especially in the apex. This suggested a mobilization of nitrogen from the apical part of the growth zone, converted from a sink tissue into a source tissue by aphids. Calculation of radial growth rates suggested that aphid infestation led to a smaller reduction in radial expansion than in elongation. Together with earlier observations of long-lasting effects of a short-term infestation, this supports the hypothesis that in addition to nutrient withdrawal, a thigmomorphogenesis-like mechanism is involved in the effect of aphid infestation on stem growth.

The decrease in growth of phosphorus‐deficient maize leaves is related to a lower cell production

ABSTRACTThe spatial distribution of leaf elongation and adaxial epidermal cell production in leaf 6 of maize (Zea mays L. cv. Cecilia) plants grown in a growth chamber under two contrasting availabilities of P in the soil was investigated. Lower displacement velocities from 32.5 mm from leaf base and a shorter growth zone were found in low P (LP) leaves compared with control leaves. P deficiency significantly diminished maximum relative elemental growth rate and shifted its location closer to the leaf base. Cells were significantly longer in LP than in control leaves for all positions from the leaf base except at the end of the growth zone. For both treatments it took a similar time for a cell situated at the leaf base to reach the limit of the growth zone. The average length of the cell division zone was decreased by 21% in LP leaves. Significant differences were found in cell production and cell division rates from 12.5 mm from the leaf base although maximum values were similar between P treatments. A shorter zone of cell division with lower cell production rates along most of its length was the regulatory event that decreased cell production, and ultimately leaf elongation rates, in P‐deficient maize plants.

Expansion dynamics, metabolite composition and substance transfer of the primary root growth zone ofZea maysL. grown in different external nutrient availabilities

ABSTRACTA combined analysis of growth and metabolite composition was performed in primary roots ofZea maysL. (Var. Alexander). The seedlings were hydroponically cultivated either in pure water or in complete nutrient solution. The overall root growth performance was similar in both treatments. Yet, digital image sequence processing methods resolved, that growth distribution and oscillatory movements within the growth zone depended strongly on external nutrient availability. Metabolite concentration profiles were similar in both treatments for most investigated metabolites, indicating a thorough mobilization of nutrient resources from the seed, but concentrations of glutamine, glutamic acid, NO3–, NH4+, malate and citrate showed pronounced differences between treatments. No diurnal variations in metabolite concentrations were found. Deposition rate profiles were in general more similar to relative elemental growth rate profiles than concentration profiles and were not affected by the treatment. Major ions were deposited maximally in front of the centre of growth activity, while greatest hexose deposition was found behind that. Relative to their abundance in the root growth zone, net rates of transfer from mature tissue were highest for sucrose, glutamic acid and aspartic acid, whereas glucose, fructose and most amino acids inversely showed high net rates of transfer out of the root growth zone, indicating a high catabolic rate for those substances there. NO3–, but not other nutrients, was transferred to a great extent from the root growth zone to the mature tissue in nutrient solution. Overall, the results show, that a careful analysis of growth dynamics allows quantifying and interpreting a number of important flux parameters in the growing organ and that the performance of the primary root does not depend strongly on external nutrient availability.

Components of leaf elongation rate and their relationship to specific leaf area in contrasting grasses

Summary In grasses the leaf growth zone is the main site of shoot growth where anatomical and chemical characteristics of leaves originate. Yet, there is insufficient information to generalize as to whether the leaf growth zone reflects habitat characteristics, whether leaf growth traits are regularly interrelated, and whether they coincide with characteristics of mature leaves. Here the contribution of both length of the leaf growth zone (region where cell division and expansion occurs) and relative elemental growth rate to the variability in leaf elongation rate (LER) were examined in eight grass species from habitats with different soil fertility. Further, we examined the relationship of the above traits with specific leaf area (SLA) and its components. Growth zone length differed significantly among species (P < 0.05) and it was the trait contributing the most to LER. Using LER and derived components it was possible to classify seven out of eight species into two groups related to soil fertility. Leaf elongation rate exhibited a positive correlation to SLA and a negative correlation to leaf dry matter content. A significant relationship existed between size of the growth zone and leaf dry matter content. The results suggest that the leaf growth zone is a critically important leaf trait that explains inherent differences in LER and other plant characteristics of grass species.