Ethylene–auxin interactions regulate lateral root initiation and emergence in Arabidopsis thaliana

Plant root systems display considerable plasticity in response to endogenous and environmental signals. Auxin stimulates pericycle cells within elongating primary roots to enter de novo organogenesis, leading to the establishment of new lateral root meristems. Crosstalk between auxin and ethylene in root elongation has been demonstrated, but interactions between these hormones in root branching are not well characterized. We find that enhanced ethylene synthesis, resulting from the application of low concentrations of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), promotes the initiation of lateral root primordia. Treatment with higher doses of ACC strongly inhibits the ability of pericycle cells to initiate new lateral root primordia, but promotes the emergence of existing lateral root primordia: behaviour that is also seen in the eto1 mutation. These effects are correlated with decreased pericycle cell length and increased lateral root primordia cell width. When auxin is applied simultaneously with ACC, ACC is unable to prevent the auxin stimulation of lateral root formation in the root tissues formed prior to ACC exposure. However, in root tissues formed after transfer to ACC, in which elongation is reduced, auxin does not rescue the ethylene inhibition of primordia initiation, but instead increases it by several fold. Mutations that block auxin responses, slr1 and arf7 arf19, render initiation of lateral root primordia insensitive to the promoting effect of low ethylene levels, and mutations that inhibit ethylene-stimulated auxin biosynthesis, wei2 and wei7 , reduce the inhibitory effect of higher ethylene levels, consistent with ethylene regulating root branching through interactions with auxin.     

Auxin reflux between the endodermis and pericycle promotes lateral root initiation

Lateral root (LR) formation is initiated when pericycle cells accumulate auxin, thereby acquiring founder cell (FC) status and triggering asymmetric cell divisions, giving rise to a new primordium. How this auxin maximum in pericycle cells builds up and remains focused is not understood. We report that the endodermis plays an active role in the regulation of auxin accumulation and is instructive for FCs to progress during the LR initiation (LRI) phase. We describe the functional importance of a PIN3 (PIN‐formed) auxin efflux carrier‐dependent hormone reflux pathway between overlaying endodermal and pericycle FCs. Disrupting this reflux pathway causes dramatic defects in the progress of FCs towards the next initiation phase. Our data identify an unexpected regulatory function for the endodermis in LRI as part of the fine‐tuning mechanism that appears to act as a check point in LR organogenesis after FCs are specified.     

Auxin fluxes in the root apex co-regulate gravitropism and lateral root initiation

Root architecture plays an important role in water and nutrient acquisition and in the ability of the plant to adapt to the soil. Lateral root development is the main determinant of the shape of the root system and is controlled by external factors such as nutrient concentration. Here it is shown that lateral root initiation and root gravitropism, two processes that are regulated by auxin, are co-regulated in Arabidopsis. A mathematical model was generated that can predict the effects of gravistimulations on lateral root initiation density and suggests that lateral root initiation is controlled by an inhibitory fields mechanism. Moreover, gene transactivation experiments suggest a mechanism involving a single auxin transport route for both responses. Finally, co-regulation may offer a selective advantage by optimizing soil exploration as supported by a simple quantitative analysis.     

Phenotypic plasticity of the maize root system in response to heterogeneous nitrogen availability

Mineral nutrients are distributed in a non-uniform manner in the soil. Plasticity in root responses to the availability of mineral nutrients is believed to be important for optimizing nutrient acquisition. The response of root architecture to heterogeneous nutrient availability has been documented in various plant species, and the molecular mechanisms coordinating these responses have been investigated particularly in Arabidopsis, a model dicotyledonous plant. Recently, progress has been made in describing the phenotypic plasticity of root architecture in maize, a monocotyledonous crop. This article reviews aspects of phenotypic plasticity of maize root system architecture, with special emphasis on describing (1) the development of its complex root system; (2) phenotypic responses in root system architecture to heterogeneous N availability; (3) the importance of phenotypic plasticity for N acquisition; (4) different regulation of root growth and nutrients uptake by shoot; and (5) root traits in maize breeding. This knowledge will inform breeding strategies for root traits enabling more efficient acquisition of soil resources and synchronizing crop growth demand, root resource acquisition and fertilizer application during crop growing season, thereby maximizing crop yields and nutrient-use efficiency and minimizing environmental pollution.     

Short-term physiological and developmental responses to nitrogen availability in hybrid poplar

Nitrogen fertilization induces dramatic changes in the growth and development of plants, including forest trees. In this study we examined short-term responses of hybrid poplar, Populus balsamifera ssp. trichocarpa x deltoides, to N fertilization. Glasshouse-grown saplings subjected to limiting, intermediate, and luxuriant levels of ammonium nitrate over a 28 d time course demonstrated rapid changes to whole-plant architecture and biomass accumulation. Nitrogen-associated shifts in allocation occurred in temporally distinct stages. Nitrogen availability modulated parameters that affect carbon gain, including light-saturated net photosynthesis and leaf area. These parameters were affected by N-induced changes to leaf maturation and senescence. Leaf area was also affected by N-induced sylleptic branch development. Genes encoding vegetative storage proteins and starch biosynthetic enzymes exhibited contrasting patterns of expression under differential N availability. A gene encoding a previously uncharacterized putative pectin methylesterase inhibitor displayed expression patterns comparable to the starch biosynthetic genes. The results of this study illustrate the phenotypic plasticity that P. balsamifera ssp. trichocarpa x deltoides exhibits in response to differential N availability.     

The ecological significance of plasticity in root weight ratio in response to nitrogen: Opinion

We analyzed data on root weight ratio from a range of experimental studies documenting plant allocation changes in response to altered nitrogen availability. Our goal was to determine the degree to which plasticity in allocation between roots and shoots exists and to search for patterns in such plasticity among species. Our survey included 77 studies representing 206 cases and 129 species. As expected, we found that root weight ratio decreased with increased nitrogen availability in the majority of cases examined, and this response was most consistent when plants were grown individually or in intraspecific competition (versus interspecific competition). Surprisingly, however, we found no evidence to support existing hypotheses that fast-growing species adapted to high soil fertilities exhibit the highest levels of morphological plasticity, or that plasticity is positively associated with competitive ability. Rather, we found that average amounts of plasticity in root weight ratio in response to nitrogen availability were similar among species grouped by maximum relative growth rate and habitat fertility. Similar results were obtained for species categorized by life form, life history or root weight ratio itself, and plasticity in root weight ratio also had no consistent relationship with competitive ability. Numerous difficulties are associated with the attempt to search for pattern using independent studies, however our results lead to the conclusion that strong patterns in plasticity of root weight ratio in response to nitrogen availability among species do not exist. We discuss two reasons for this: (1) the costs of plasticity relative to its benefits are lower than previously predicted and (2) plasticity in traits other than root weight ratio is more important to plant foraging ability.     

Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the formation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to examine the role of auxin, cytokinin, ethylene and nutrients in the regulation of root architecture. Surprisingly, extensive dichotomous and coralloid branching of lateral roots occurred spontaneously in Pinus taeda , P. halepensis and P. muricata . In P. sylvestris , P. ponderosa and P. nigra , treatment with auxin transport inhibitors (ATIs), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the ethylene-releasing compound 2-chloroethylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branching and coralloid organ formation. Formation of both spontaneous and ATI-induced coralloid structures was blocked by treatment with an ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)glycine; this inhibition was reversed by either ACC or CEPA. In addition, the induction of this unique morphogenetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomous and coralloid structures, as well as the regulation of their formation by nutrient levels, show a striking similarity to those of ectomycorrhizas.     

Kernel set in maize genotypes differing in nitrogen use efficiency in response to resource availability around flowering

Environmental conditions affect grain yield in maize (Zea mays L.) mainly by altering the kernel number per plant (KNP). This number is determined during a critical period of about 2 weeks around silking. The objectives of this study were to assess how the rate and timing of nitrogen (N) fertilizer applications affect biomass partitioning and KNP in two genotypes with different N use efficiency, and to compare kernel set of these genotypes under varying regimes of carbohydrate and N availability during the critical period for kernel set. In the first field experiment, plant density and the rate of N supply per plant were varied independently. In the second field experiment, N availability was controlled via the application of N fertilizer, and carbohydrate availability was controlled by shading or thinning at silking. In both experiments, low rates of N supply reduced KNP more strongly in the non-efficient genotype when compared to the efficient genotype. The genotypic differences in kernel set were neither associated with N uptake into the above-ground biomass at maturity, nor above-ground biomass at silking. In the non-efficient genotype, application of N fertilizer at silking increased KNP. This increase was not associated with an increase in plant growth but with increased partitioning of biomass towards the reproductive organs during the critical period for kernel set. The genotype which had been selected for its high N use efficiency also showed higher kernel set at high plant density and shading during flowering when compared to the non-efficient genotype. Under conditions of restricted resource availability per plant, plant and ear growth rates during the critical period of about 14 days after onset of flowering declined compared with non-limiting conditions. However, these growth rates were less reduced in the efficient genotype. Pooling treatments of different plant density and different available N, each hybrid showed linear responses of KNP to plant growth rate and to ear growth rate. Furthermore, in the efficient genotype KNP was reduced to a lesser extent in response to decreasing growth rates. We conclude that higher kernel set of the efficient genotype compared to the non-efficient genotype under stressful conditions was associated with low sensitivity of plant growth and dry matter distribution towards reproductive organs to low assimilate availability during the critical period of kernel set, and particularly with low sensitivity of kernel set to decreasing plant and ear growth rates.     

Relationships between root morphology and nitrogen availability in a recent theoretical model describing nitrogen uptake from soil

Abstract. The effect upon potential maximum nitrogen uptake rate of root morphology and nitrogen availability in soil was investigated using a simple nutrient transport model. Parameter values appropriate to an ecological or an agricultural context were introduced from the literature. The model predicted that the maximum uptake rate of nitrate was morphology-dependent only at extremely low concentrations. For ammonium, this was so for all realistic concentrations, assuming a high potential maximum uptake rate. The important concentration range for ammonium was two orders of magnitude greater than that for nitrate. With a lower potential maximum uptake rate of ammonium, root morphology was important below 15/igNg' soil, the concentration range in this case being a single order of magnitude greater than that for nitrate. The effects of root hairs were to decrease the threshold concentration for morphology-dependence, and to minimize root dry weight per unit volume of soil needed to maintain maximum nitrogen uptake rate. The effects of simultaneous mass flow of solution were negligible. The possible significance of these effects upon plant growth are discussed in relation to nitrogen availability.     

Relationship between root elongation rate and diameter and duration of growth of lateral roots of maize

The objective of this work was to describe the relationship between elongation rate and diameter of maize roots and to estimate the length and growth duration of lateral roots of maize. Diameters and elongation rates of roots were measuredin situ on plants grown 5 weeks in small rhizotrons under greenhouse conditions. At the end of the experimental period the roots were harvested and diameters of axile and lateral roots were measured. The frequency distribution of diameters of harvested roots was bimodal with a minimum at 0.6 mm; 97% of axile roots were larger than this value and 98% of the lateral roots were smaller. Root elongation per day increased as diameter increased but the slope of the relationship with lateral roots was about 2.5 times that with axile roots when separate linear regressions were fitted to the two populations. The length of lateral roots found on axillary roots between the base and about 30 cm from the apex was approximately 2.2 cm. All of the data was consistent with the hypothesis that the lateral roots grew for about 2.5 days and then ceased growing. The axillary roots continued to grow throughout the experimental period at a rate of about 3 cm day⁻¹. Contribution from the Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Agronomy paper No. 1661. This research is part of the program of the Center for Root-Soil Research.     

Plant response to variations in nitrogen availability in a desert shrubland community

Spatial variations in nitrogen availability were studied in a desert community codominated byLarrea tridentata (DC.) Cov. andProsopis glandulosa Torr. Measurements of natural ¹⁵N values in tissues suggested thatProsopis obtains approximately half of its nitrogen through direct symbiotic fixation. Soils were collected under 1)Prosopis shrubs, 2)Larrea shrubs 2 m fromProsopis (LP), and 3)Larrea 2 m from otherLarrea but> 5 m from the nearestProsopis (LL).Prosopis soils showed significantly higher rates of nitrogen mineralization than LL soils in both A and B horizons. Rates of mineralization in LP soils were significantly higher than rates in LL soils only in the B horizon and were not significantly different from rates inProsopis soils. Leaf nitrogen concentrations were significantly higher in LP shrubs (2.06%) than in LL shrubs (1.78%), although ¹⁵N values did not differ between the two shrub types. Nitrogen concentrations inPerezia nana Gray, a perennial herb, were greater in plants underProsopis shrubs (2.09%) than under LP shrubs (1.93%) or LL shrubs (1.67%). Despite apparent differences in nitrogen availability, biomass ofLarrea and density ofPerezia did not differ significantly among these sites.     

Signalling mechanisms underlying the morphological responses of the root system to nitrogen in Arabidopsis thaliana

Plants display considerable developmental plasticity in response to changing environmental conditions. The adaptations of the root system to variations in N supply are an excellent example of such developmental plasticity. In Arabidopsis, four morphological adaptations to the N supply have been characterized: (i) a localized stimulatory effect of external nitrate on lateral root elongation; (ii) a systemic inhibitory effect of high tissue nitrate concentrations on the activation of lateral root meristems; (iii) a suppression of lateral root initiation by high C:N ratios, and (iv) an inhibition of primary root growth and stimulation of root branching by external L-glutamate. These responses have provided valuable experimental systems for the study of N signalling in plants. This article will highlight some recent progress made in this direction from studies using the Arabidopsis root system. One recent development of note has been the emerging evidence of a regulatory role of nitrate transporters in some of the responses. It has been reported that the AtNRT1.1 (CHL1) dual-affinity nitrate transporter acts upstream of the ANR1 MADS box gene in mediating the stimulatory effect of a localized nitrate supply on lateral root proliferation. The AtNRT2.1 high-affinity nitrate transporter seems to be involved in the repression of lateral root initiation by high C:N ratios. The systemic inhibitory effect of high nitrate supply on lateral root development, which is mediated by abscisic acid (ABA), may be linked to the recently identified ABA receptor, FCA. The newly discovered root architectural response to external L-glutamate potentially offers a valuable experimental tool for studying the biological function of plant glutamate receptors and amino acid signalling.     

Branching patterns of root systems: quantitative analysis of the diversity among dicotyledonous species

Background and Aims

Root branching, and in particular acropetal branching, is a common and important developmental process for increasing the number of growing tips and defining the distribution of their meristem size. This study presents a new method for characterizing the results of this process in natura from scanned images of young, branched parts of excavated roots. The method involves the direct measurement or calculation of seven different traits.

Methods

Young plants of 45 species of dicots were sampled from fields and gardens with uniform soils. Roots were separated, scanned and then measured using ImageJ software to determine seven traits related to root diameter and interbranch distance.

Results

The traits exhibited large interspecific variations, and covariations reflecting trade-offs. For example, at the interspecies level, the spacing of lateral roots (interbranch distance along the parent root) was strongly correlated to the diameter of the finest roots found in the species, and showed a continuum between two opposite strategies: making dense and fine lateral roots, or thick and well-spaced laterals.

Conclusions

A simple method is presented for classification of branching patterns in roots that allows relatively quick sampling and measurements to be undertaken. The feasibilty of the method is demonstrated for dicotyledonous species and it has the potential to be developed more broadly for other species and a wider range of enivironmental conditions.     

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

Global nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.     

Regulatory components involved in altering lateral root development in response to localized iron: Evidence for natural genetic variation

On the search for sparingly available nutrients, plants may alter their root architecture to improve soil exploration. So far, the examples for root system modifications induced by a heterogeneous availability of nutrients have been reported for the macronutrients nitrogen (N) and phosphorous (P). In an attempt to extend this type of knowledge to other nutrients, we recently provided evidence that Arabidopsis roots are able to sense a local availability of the micronutrient iron (Fe) and to respond with lateral root elongation into the Fe-containing patch. This specific root response was caused by enhanced elongation of cells leaving the root meristem and was dependent on an AUX1-mediated auxin accumulation in the lateral root apices. In this report, we compare mechanisms underlying this response with those known for other nutrients and show that a substantial genotypic variation exists among accessions of A. thaliana in the responsiveness of lateral roots toward localized Fe supplies.     

Grain yield and kernel weight of two maize genotypes differing in nitrogen use efficiency at various levels of nitrogen and carbohydrate availability during flowering and grain filling

Grain yield per plant (GYP) and mean kernel weight (KW) of maize (Zea mays L.) are sensitive to changes in the environment during the lag phase of kernel growth (the time after pollination in which the potential kernel size is determined), and during the phase of linear kernel growth. The aim of this study was to assess genotypic differences in the response to environmental stresses associated with N and/or carbohydrate shortage at different phases during plant development. The rate and timing of N and carbohydrate supply were modified by application of fertilizer, shading, and varying the plant density at sowing, at silking or at 14 d after silking. The effects of these treatments on the photosynthetic capacity, grain yield and mean kernel weight were investigated in two hybrids differing in N use efficiency. The total above-ground biomass and grain yield per plant of the efficient hybrid responded little to altered environmental conditions such as suboptimal N supply, enhanced inter-plant competition, and shading for 14 d during flowering, when compared to the less efficient genotype. We conclude that grain yields in the efficient genotype are less sensitive not only to N stress, but also to carbohydrate shortage before grain filling. Shading of N deficient plants from 14 d after silking to maturity did not significantly reduce grain yield in the non-efficient genotype, indicating complete sink limitation of grain yield during grain filling. In the efficient genotype, in contrast, grain yield of N-deficient plants was significantly reduced by shading during grain filling. The rate of photosynthesis declined with decreasing foliar N content. No genotypic differences in photosynthesis were observed at high or low foliar N contents. However, at high plant density and low N supply, the leaf chlorophyll content after flowering in the efficient genotype was higher than that in the non-efficient genotype. Obviously, the higher source capacity of the efficient genotype was not due to higher photosynthetic N use efficiency but due to maintenance of high chlorophyll contents under stressful conditions. In the efficient genotype, the harvest index was not significantly affected by N fertilization, plant density, or shading before the grain filling period. In contrast, in the non-efficient genotype the harvest index was diminished by N deficiency and shading during flowering. We conclude that the high yielding ability of the efficient genotype under stressful conditions was associated with formation of a high sink capacity of the grains under conditions of low carbohydrate and N availability during flowering and with maintenance of high source strength during grain filling under conditions of high plant density and low N availability.     

Role of cytokinin and auxin in shaping root architecture: regulating vascular differentiation, lateral root initiation, root apical dominance and root gravitropism

BACKGROUND AND AIMS: Development and architecture of plant roots are regulated by phytohormones. Cytokinin (CK), synthesized in the root cap, promotes cytokinesis, vascular cambium sensitivity, vascular differentiation and root apical dominance. Auxin (indole-3-acetic acid, IAA), produced in young shoot organs, promotes root development and induces vascular differentiation. Both IAA and CK regulate root gravitropism. The aims of this study were to analyse the hormonal mechanisms that induce the root's primary vascular system, explain how differentiating-protoxylem vessels promote lateral root initiation, propose the concept of CK-dependent root apical dominance, and visualize the CK and IAA regulation of root gravitropiosm. KEY ISSUES: The hormonal analysis and proposed mechanisms yield new insights and extend previous concepts: how the radial pattern of the root protoxylem vs. protophloem strands is induced by alternating polar streams of high IAA vs. low IAA concentrations, respectively; how differentiating-protoxylem vessel elements stimulate lateral root initiation by auxin-ethylene-auxin signalling; and how root apical dominance is regulated by the root-cap-synthesized CK, which gives priority to the primary root in competition with its own lateral roots. CONCLUSIONS: CK and IAA are key hormones that regulate root development, its vascular differentiation and root gravitropism; these two hormones, together with ethylene, regulate lateral root initiation.     

Geometrical properties of simulated maize root systems: consequences for length density and intersection density

The spatial distribution of root length density (RLD) is important because it affects water and nutrient uptake. It is difficult to obtain reliable estimates of RLD because root systems are very variable and heterogeneous. We identified systematic trends, clustering, and anisotropy as geometrical properties of root systems, and studied their consequences for the sampling and observation of roots. We determined the degree of clustering by comparing the coefficient of variation of a simulated root system with that of a Boolean model. We also present an alternative theoretical derivation of the relation between RLD and root intersection density (RID) based on the theory of random processes of fibres. We show how systematic trends, clustering and anisotropy affect the theoretical relation between RLD and RID, and the consequences this has for measurement of RID in the field. We simulated the root systems of one hundred maize crops grown for a thermal time of 600 K d, and analysed the distribution of RLD and root intersection density RID on regular grids of locations throughout the simulated root systems. Systematic trends were most important in the surface layers, decreasing with depth. Clustering and anisotropy both increased with depth. Roots at depth had a bimodal distribution of root orientation, causing changes in the ratio of RLD/RID. The close proximity of the emerging lateral roots and the parent axis caused clustering which increased the coefficient of variation.