Emerging role of roots in plant responses to aboveground insect herbivory

Plants have evolved complex biochemical mechanisms to counter threats from insect herbivory. Recent research has revealed an important role of roots in plant responses to above ground herbivory (AGH). The involvement of roots is integral to plant resistance and tolerance mechanisms. Roots not only play an active role in plant defenses by acting as sites for biosynthesis of various toxins and but also contribute to tolerance by storing photoassimilates to enable future regrowth. The interaction of roots with beneficial soil‐borne microorganisms also influences the outcome of the interaction between plant and insect herbivores. Shoot‐to‐root communication signals are critical for plant response to AGH. A better understanding of the role of roots in plant response to AGH is essential in order to develop a comprehensive picture of plant‐insect interactions. Here, we summarize the current status of research on the role of roots in plant response to AGH and also discuss possible signals involved in shoot‐to‐root communication.     

A clonotype nomenclature for T cell receptors

T cell receptor (TCR) nucleotide sequences are often generated during analyses of T cell responses to pathogens or autoantigens. The most important region of the TCR is the third complementarity-determining region (CDR3) whose nucleotide sequence is unique to each T cell clone. The CDR3 interacts with the peptide and thus is important for recognizing pathogen or autoantigen epitopes. While conventions exist for identifying the various TCR chains, there is a lack of a concise nomenclature that would identify both the amino acid translation and nucleotide sequence of the CDR3. This deficiency makes the comparison of published TCR genetic and proteomic information difficult. To enhance information sharing among different databases and to facilitate computational assessment of clonotypic T cell repertoires, we propose a clonotype nomenclature. The rules for generating a clonotype identifier are simple and easy to follow, and have a built-in error-checking system. The identifier includes the V and J region, the CDR3 length as well as its human or mouse origin. The framework of this naming system could also be expanded to the B cell receptor. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Shoot‐derived abscisic acid promotes root growth

The phytohormone abscisic acid (ABA) plays a major role in regulating root growth. Most work to date has investigated the influence of root‐sourced ABA on root growth during water stress. Here, we tested whether foliage‐derived ABA could be transported to the roots, and whether this foliage‐derived ABA had an influence on root growth under well‐watered conditions. Using both application studies of deuterium‐labelled ABA and reciprocal grafting between wild‐type and ABA‐biosynthetic mutant plants, we show that both ABA levels in the roots and root growth in representative angiosperms are controlled by ABA synthesized in the leaves rather than sourced from the roots. Foliage‐derived ABA was found to promote root growth relative to shoot growth but to inhibit the development of lateral roots. Increased root auxin (IAA) levels in plants with ABA‐deficient scions suggest that foliage‐derived ABA inhibits root growth through the root growth‐inhibitor IAA. These results highlight the physiological and morphological importance, beyond the control of stomata, of foliage‐derived ABA. The use of foliar ABA as a signal for root growth has important implications for regulating root to shoot growth under normal conditions and suggests that leaf rather than root hydration is the main signal for regulating plant responses to moisture.     

Osmotic regulation of root system architecture

Although root system architecture is known to be highly plastic and strongly affected by environmental conditions, we have little understanding of the underlying mechanisms controlling root system development. Here we demonstrate that the formation of a lateral root from a lateral root primordium is repressed as water availability is reduced. This osmotic-responsive regulatory mechanism requires abscisic acid (ABA) and a newly identified gene, LRD2. Mutant analysis also revealed interactions of ABA and LRD2 with auxin signaling. Surprisingly, further examination revealed that both ABA and LRD2 control root system architecture even in the absence of osmotic stress. This suggests that the same molecules that mediate responses to environmental cues can also be regulators of intrinsic developmental programs in the root system.     

Pluripotency of Arabidopsis xylem pericycle underlies shoot regeneration from root and hypocotyl explants grown in vitro

We have established a detailed framework for the process of shoot regeneration from Arabidopsis root and hypocotyl explants grown in vitro . Using transgenic plant lines in which the GUS or GFP genes were fused to promoters of developmental genes ( WUS , CLV1 , CLV3 , STM , CUC1 , PLT1 , RCH1 , QC25 ), or to promoters of genes encoding indicators of the auxin response ( DR5 ) or transport ( PIN1 ), cytokinin (CK) response ( ARR5 ) or synthesis ( IPT5 ), or mitotic activity ( CYCB1 ), we showed that regenerated shoots originated directly or indirectly from the pericycle cells adjacent to xylem poles. In addition, shoot regeneration appeared to be partly similar to the formation of lateral root meristems (LRMs). During pre-culture on a 2, 4-dichlorophenoxyacetic acid (2, 4-D)-rich callus-inducing medium (CIM), xylem pericycle reactivation established outgrowths that were not true calli but had many characteristics of LRMs. Transfer to a CK-rich shoot-inducing medium (SIM) resulted in early LRM-like primordia changing to shoot meristems. Direct origin of shoots from the xylem pericycle occurred upon direct culture on CK-containing media without prior growth on CIM. Thus, it appeared that the xylem pericycle is more pluripotent than previously thought. This pluripotency was accompanied by the ability of pericycle derivatives to retain diploidy, even after several rounds of cell division. In contrast, the phloem pericycle did not display such developmental plasticity, and responded to CKs with only periclinal divisions. Such observations reinforce the view that the pericycle is an 'extended meristem' that comprises two types of cell populations. They also suggest that the founder cells for LRM initiation are not initially fully specified for this developmental pathway.     

Carbon cost of root systems: an architectural approach

Root architecture is an important component of nutrient uptake and may be sensitive to carbon allocational changes brought about by rising CO₂. We describe a deformable geometric model of root growth, SimRoot, for the dynamic morphological and physiological simulation of root architectures. Using SimRoot, and measurements of root biomass deposition, respiration and exudation, carbon/phosphorus budgets were developed for three contrasting root architectures. Carbon allocation patterns and phosphorus acquisition efficiencies were estimated for Phaseolus vulgaris seedlings with either a dichotomous, herringbone, or empirically determined bean root architecture. Carbon allocation to biomass, respiration, and exudation varied significantly among architectures. Root systems also varied in the relationship between C expenditure and P acquisition, providing evidence for the importance of architecture in nutrient acquisition efficiency.     

Auxin-induced inhibition of lateral root initiation contributes to root system shaping in Arabidopsis thaliana

The hormone auxin is known to inhibit root elongation and to promote initiation of lateral roots. Here we report complex effects of auxin on lateral root initiation in roots showing reduced cell elongation after auxin treatment. In Arabidopsis thaliana, the promotion of lateral root initiation by indole-3-acetic acid (IAA) was reduced as the IAA concentration was increased in the nanomolar range, and IAA became inhibitory at 25 nM. Detection of this unexpected inhibitory effect required evaluation of root portions that had newly formed during treatment, separately from root portions that existed prior to treatment. Lateral root initiation was also reduced in the iaaM-OX Arabidopsis line, which has an endogenously increased IAA level. The ethylene signaling mutants ein2-5 and etr1-3, the auxin transport mutants aux1-7 and eir1/pin2, and the auxin perception/response mutant tir1-1 were resistant to the inhibitory effect of IAA on lateral root initiation, consistent with a requirement for intact ethylene signaling, auxin transport and auxin perception/response for this effect. The pericycle cell length was less dramatically reduced than cortical cell length, suggesting that a reduction in the pericycle cell number relative to the cortex could occur with the increase of the IAA level. Expression of the DR5:GUS auxin reporter was also less effectively induced, and the AXR3 auxin repressor protein was less effectively eliminated in such root portions, suggesting that decreased auxin responsiveness may accompany the inhibition. Our study highlights a connection between auxin-regulated inhibition of parent root elongation and a decrease in lateral root initiation. This may be required to regulate the spacing of lateral roots and optimize root architecture to environmental demands.     

Evaluation in field conditions of a three-dimensional architectural model of the maize root system: Comparison of simulated and observed horizontal root maps

Most existing water and nutrient uptake models are based on the assumption that roots are evenly distributed in the soil volume. This assumption is not realistic for field conditions, and significantly alters water or nutrient uptake calculations. Therefore, development of models of root system growth that account for the spatial distribution of roots is necessary.The objective of this work was to test a three dimensional architectural model of the maize root system by comparing simulated horizontal root maps with observed root maps obtained from the field. The model was built using the current knowledge on maize root system morphogenesis and parameters obtained under field conditions. Simulated root maps (0.45 × 0.75 m) of horizontal cross sections at 3 depths and 3 dates were obtained by using the model for a plant population. Actual root maps were obtained in a deep, barrier-free clay-loamy soil by digging pits, preparing selected horizontal planes and recording root contacts on plastic sheets.Results showed that both the number of cross-sections of axile roots, and their spatial distribution characterized with the R-index value of Clark and Evans (1954), were correctly accounted for by the model at all dates and depths. The number of cross-sections of laterals was also correctly predicted. However, laterals were more clustered around axile roots on simulated root maps than on observed root maps. Although slight discrepancies appeared between simulated and observed root maps in this respect, it was concluded that the model correctly accounted for the general colonization pattern of the soil volume by roots under a maize crop.     

大气CO2浓度升高对植物根系的影响

植物长期生长在CO2浓度不断升高的环境中,其结构和功能都将受到影响,这种影响不仅表现在植物的地上部分,同时也表现在植物的地下部分(根系),尤其是细根的长度、直径、产量、周转以及根与枝的分配模式等方面。植物根系结构和功能的改变影响植物地上部分和生态系统物质循环中的碳动态及土壤中碳库的变化。目前有关大气CO2浓度升高对根系动态影响的研究报道主要包括大气CO2浓度升高对根系结构(直径、分枝、长度、数量等)和根系生理(周转率、产量、碳分配模式等)的影响2个方面。目前,该领域研究还存在一些不足,例如在CO2浓度升高条件下,对植物根系内部的调控机制,以及由其引起的物质循环和能量流动的动态变化的了解较少;至今没有令人信服的证据说明大气CO2浓度升高使根系周转升高还是降低。今后应加强研究在CO2浓度升高条件下根系的周转变化和光合产物分配模式变化,CO2浓度升高和外界环境因素的共同作用对根系的影响,以及采用不同研究方法和研究对象在不同立地条件下开展升高CO2浓度对根系影响的对比研究等。     

Intrinsic and environmental response pathways that regulate root system architecture

Root system development is an important agronomic trait. The right architecture in a given environment allows plants to survive periods of water of nutrient deficit, and compete effectively for resources. Root systems also provide an optimal system for studying developmental plasticity, a characteristic feature of plant growth. This review proposes a framework for describing the pathways regulating the development of complex structures such as root systems: intrinsic pathways determine the characteristic architecture of the root system in a given plant species, and define the limits for plasticity in that species. Response pathways co-ordinate environmental cues with development by modulating intrinsic pathways. The current literature describing the regulation of root system development is summarized here within this framework. Regulatory pathways are also organized based on their specific developmental effect in the root system. All the pathways affect lateral root formation, but some specifically target initiation of the lateral root, while others target the development and activation of the lateral root primordium, or the elongation of the lateral root. Finally, we discuss emerging approaches for understanding the regulation of root system architecture.     

植物根系固岸抗蚀作用研究进展

植物根系对保持滨岸区域土壤岸滩的稳定性具有重要作用。本文从根系固岸机制、根系固岸的评价方法、植物根系与固岸抗蚀关系等几个方面对植物根系的固岸抗蚀作用进行了综述。在这些方面目前已经取得了许多富有价值的成果,但相比之下,西方国家处于领先地位,中国在这些方面的研究还较少,目前仍处于起步阶段。建议未来应加强植物根系季节或年际变化规律与岸滩侵蚀季节或年际变化规律研究的结合,以及植物地上部分管理措施对地下部分特性影响的研究。     

Long‐distance abscisic acid signalling under different vertical soil moisture gradients depends on bulk root water potential and average soil water content in the root zone

To determine how root‐to‐shoot abscisic acid (ABA) signalling is regulated by vertical soil moisture gradients, root ABA concentration ([ABA]_root), the fraction of root water uptake from, and root water potential of different parts of the root zone, along with bulk root water potential, were measured to test various predictive models of root xylem ABA concentration [RX‐ABA]_sap. Beans (Phaseolus vulgaris L. cv. Nassau) were grown in soil columns and received different irrigation treatments (top and basal watering, and withholding water for varying lengths of time) to induce different vertical soil moisture gradients. Root water uptake was measured at four positions within the column by continuously recording volumetric soil water content (θ_v). Average θ_v was inversely related to bulk root water potential (Ψ_root). In turn, Ψ_root was correlated with both average [ABA]_root and [RX‐ABA]_sap. Despite large gradients in θ_v, [ABA]_root and root water potential was homogenous within the root zone. Consequently, unlike some split‐root studies, root water uptake fraction from layers with different soil moisture did not influence xylem sap (ABA). This suggests two different patterns of ABA signalling, depending on how soil moisture heterogeneity is distributed within the root zone, which might have implications for implementing water‐saving irrigation techniques.     

Plant injury due to oxygen deficiency in the root environment of soilless culture: A review

The deficiency of oxygen concentration in root environment linked to waterlogging conditions caused important injuries for plants. These effects could be reproduced by oxygen deficient nutrient solution. This bibliographical synthesis has been centered on experimental results obtained on plants cultivated in soilless culture. This review paper presents a methodology used to study oxygen depletion in a nutrient solution and to calculate root respiration rates. The main factors influencing root respiration are reviewed as well as the consequences of oxygen deficiency on roots and shoots functioning. This study would not be complete without some information on the main mechanisms of plant adaptation to oxygen deficiency.     

根际微生物调控植物根系构型研究进展

根系构型是最重要的植物形态特征之一,具有可塑性,既由遗传因素控制,又受到许多环境因子的调控。近年的大量研究表明,根际微生物能够调控植物的根系构型,进而影响植物的一系列生理与生态过程。综述丛枝菌根真菌(AMF)、根瘤菌、植物根际促生菌(PGPR)等重要根际微生物类群对植物根系构型的调控模式以及相应的调控机理,并对进一步的研究进行了展望,旨在为今后的相关研究和实际应用提供参考。     

Shoot-supplied ammonium targets the root auxin influx carrier AUX1 and inhibits lateral root emergence in Arabidopsis

Deposition of ammonium (NH₄⁺) from the atmosphere is a substantial environmental problem. While toxicity resulting from root exposure to NH₄⁺ is well studied, little is known about how shoot‐supplied ammonium (SSA) affects root growth. In this study, we show that SSA significantly affects lateral root (LR) development. We show that SSA inhibits lateral root primordium (LRP) emergence, but not LRP initiation, resulting in significantly impaired LR number. We show that the inhibition is independent of abscisic acid (ABA) signalling and sucrose uptake in shoots but relates to the auxin response in roots. Expression analyses of an auxin‐responsive reporter, DR5:GUS, and direct assays of auxin transport demonstrated that SSA inhibits root acropetal (rootward) auxin transport while not affecting basipetal (shootward) transport or auxin sensitivity of root cells. Mutant analyses indicated that the auxin influx carrier AUX1, but not the auxin efflux carriers PIN‐FORMED (PIN)1 or PIN2, is required for this inhibition of LRP emergence and the observed auxin response. We found that AUX1 expression was modulated by SSA in vascular tissues rather than LR cap cells in roots. Taken together, our results suggest that SSA inhibits LRP emergence in Arabidopsis by interfering with AUX1‐dependent auxin transport from shoot to root.     

Arabidopsis Lateral Root Development 3 is essential for early phloem development and function, and hence for normal root system development

We have identified a gene, Lateral Root Development 3 (LRD3), that is important for maintaining a balance between primary and lateral root growth. The lrd3 mutant has decreased primary root growth and increased lateral root growth. We determined that the LRD3 gene encodes a LIM-domain protein of unknown function. LRD3 is expressed only in the phloem companion cells, which suggested a role in phloem function. Indeed, while phloem loading and export from the shoot appear to be normal, delivery of phloem to the primary root tip is limited severely in young seedlings. Abnormalities in phloem morphology in these seedlings indicate that LRD3 is essential for correct early phloem development. There is a subsequent spontaneous recovery of normal phloem morphology, which is correlated tightly with increased phloem delivery and growth of the primary root. The LRD3 gene is one of very few genes described to affect phloem development, and the only one that is specific to early phloem development. Continuous growth on auxin also leads to recovery of phloem development and function in lrd3, which demonstrates that auxin plays a key role in early phloem development. The root system architecture and the pattern of phloem allocation in the lrd3 root system suggested that there may be regulated mechanisms for selectively supporting certain lateral roots when the primary root is compromised. Therefore, this study provides new insights into phloem-mediated resource allocation and its effects on plant root system architecture.     

滨岸不同植物配置模式的根系空间分布特征

崇明岛位于长江河口,是世界上最大的冲积岛。滨岸植物配置模式对防止侵蚀、坍塌等具有不同的作用。以崇明岛南岸4种不同的植物配置模式:芦苇(Phragmites australis)-海三棱藨草(Scirpus mariqueter)模式(PSM)、池杉(Taxodium ascendens)-芦苇-海三棱藨草模式(TAPSM)、杂交柳(Salix matsudana×alba)-芦苇-海三棱藨草模式(SPSM)及落羽杉(Taxodium distichum)-芦苇-海三棱藨草模式(TDPSM)为对象,对不同植物配置模式在低、中、高3个潮位根系空间分布进行了调查和分析。结果表明:(1)4种模式中0-40cm土层内平均总根长最大的为SPSM模式,其值为137.0cm/cm2,平均总根长最小的为TAPSM模式(91.4cm/cm2);在3种乔木增配模式中,草本植物根长占总根长比例达94.6%-98.1%。(2)除SPSM模式外,其他3种植物配置模式根长密度均随土层加深而减小,这3种模式根长密度最大的土层皆为0-10cm土层,分别为各自最底层根长密度的15.1倍(PSM)、4.9倍(TDPSM)和2.0倍(TAPSM);SPSM模式在10-20cm土层根长密度最大。(3)在所有4种模式中,直径Φ0.1mm的微细根对总根长密度的贡献均为最大,比例从74.7%到81.7%,其次为直径0.1mm≤Φ1mm的细根,直径Φ≥5mm的大根极少。(4)秩和检验显示,4种模式在低、中、高3个潮位根长密度的差异并不一致。根系能够提高土壤抗侵蚀能力,研究4种模式根系空间分布特征,可以为崇明岛滨岸植物配置,建设抗蚀护滩植被带提供科学依据。     

STARTS--a stable root transformation system for rapid functional analyses of proteins of the monocot model plant barley

Large data sets are generated from plants by the various 'omics platforms. Currently, a limiting step in data analysis is the assessment of protein function and its translation into a biological context. The lack of robust high-throughput transformation systems for monocotyledonous plants, to which the vast majority of crop plants belong, is a major restriction and impedes exploitation of novel traits in agriculture. Here we present a stable root transformation system for barley, termed STARTS, that allows assessment of gene function in root tissues within 6 weeks. The system is based on the finding that a callus, produced on root induction medium from the scutellum of the immature embryo, is able to regenerate roots from single transformed cells by concomitant suppression of shoot development. Using Agrobacterium tumefaciens-mediated transfer of genes involved in root development and pathogenesis, we show that those calli regenerate large amounts of uniformly transformed roots for in situ functional analysis of newly expressed proteins.