Root Caps and Rhizosphere

In this paper we discuss recent work on the physiological, molecular, and mechanical mechanisms that underlie the capacity of root caps to modulate the properties of the rhizosphere and thereby foster plant growth and development. The root cap initially defines the rhizosphere by its direction of growth, which in turn occurs in response to gradients in soil conditions and gravity. The ability of the root cap to modulate its environment is largely a result of the release of exudates and border cells, and so provides a potential method to engineer the rhizosphere. Factors affecting the release of border cells from the outer surface of the root cap, and function of these cells and their exudates in the rhizosphere, are considered in detail. Release of border cells into the rhizosphere depends on soil matric potential and mechanical impedance, in addition to a host of other environmental conditions. There is good evidence of unidentified feedback signals between border cells and the root cap meristem, and some potential mechanisms are discussed. Root border cells play a significant mechanical role in decreasing frictional resistance to root penetration, and a conceptual model for this function is discussed. Root and border cell exudates influence specific interactions between plant hosts and soil organisms, including pathogenic fungi. The area of exudates and border cell function in soil is an exciting and developing one that awaits the production of appropriate mutant and transgenic lines for further study in the soil environment.     

Root Caps and Rhizosphere

In this paper we discuss recent work on the physiological, molecular, and mechanical mechanisms that underlie the capacity of root caps to modulate the properties of the rhizosphere and thereby foster plant growth and development. The root cap initially defines the rhizosphere by its direction of growth, which in turn occurs in response to gradients in soil conditions and gravity. The ability of the root cap to modulate its environment is largely a result of the release of exudates and border cells, and so provides a potential method to engineer the rhizosphere. Factors affecting the release of border cells from the outer surface of the root cap, and function of these cells and their exudates in the rhizosphere, are considered in detail. Release of border cells into the rhizosphere depends on soil matric potential and mechanical impedance, in addition to a host of other environmental conditions. There is good evidence of unidentified feedback signals between border cells and the root cap meristem, and some potential mechanisms are discussed. Root border cells play a significant mechanical role in decreasing frictional resistance to root penetration, and a conceptual model for this function is discussed. Root and border cell exudates influence specific interactions between plant hosts and soil organisms, including pathogenic fungi. The area of exudates and border cell function in soil is an exciting and developing one that awaits the production of appropriate mutant and transgenic lines for further study in the soil environment.     

根缘细胞的发生和生物学作用

植物根尖每天都要代谢大量的根缘细胞,这些根缘细胞能改变土壤的生化特性,平衡根际环境,保护植物的根尖免受外界胁迫如铝毒、线虫和病菌的侵染等伤害。通过对根缘细胞数量和特性的调节可协调植物与外界环境的互作。本文对根缘细胞的发生、根尖的防御机制及发生的内在原因等作了综述。     

根缘细胞的发生和生物学作用

植物根尖每天都要代谢大量的根缘细胞 ,这些根缘细胞能改变土壤的生化特性 ,平衡根际环境 ,保护植物的根尖免受外界胁迫如铝毒、线虫和病菌的侵染等伤害。通过对根缘细胞数量和特性的调节可协调植物与外界环境的互作。本文对根缘细胞的发生、根尖的防御机制及发生的内在原因等作了综述。     

高等植物根边缘细胞的发育调控及其生物学功能

植物根边缘细胞是从根冠表皮游离出来并聚集在根尖周围的一群特殊细胞,以前曾称为根冠脱落细胞.最近的证据表明,绝大多数物种边缘细胞具有生物学活性,其发育是受内外信号调控.边缘细胞一旦从根表皮游离后,其代谢活性大大上升、基因表达明显不同于根冠细胞.最近,与边缘细胞发育早期和晚期相关的两个基因PsUGT1和RCPME1分别被克隆和鉴定.边缘细胞能特异性地合成、分泌一系列的化学物质,包括花色素苷、抗生素、特异性酶类及其他化学物质能抑制或促进根际周围的细菌、真菌、病毒、线虫等的生长以及中和根际周围一些有毒化学物质如铝毒.因此,边缘细胞在植物生长发育过程中起着多种生物学功能.     

高等植物根边缘细胞的发育调控及其生物学功能

植物根边缘细胞是从根冠表皮游离出来并聚集在根尖周围的一群特殊细胞 ,以前曾称为根冠脱落细胞。最近的证据表明 ,绝大多数物种边缘细胞具有生物学活性 ,其发育是受内外信号调控。边缘细胞一旦从根表皮游离后 ,其代谢活性大大上升、基因表达明显不同于根冠细胞。最近 ,与边缘细胞发育早期和晚期相关的两个基因PsUGT1和RCPME1分别被克隆和鉴定。边缘细胞能特异性地合成、分泌一系列的化学物质 ,包括花色素苷、抗生素、特异性酶类及其他化学物质能抑制或促进根际周围的细菌、真菌、病毒、线虫等的生长以及中和根际周围一些有毒化学物质如铝毒。因此 ,边缘细胞在植物生长发育过程中起着多种生物学功能     

The potential for nitrification and nitrate uptake in the rhizosphere of wetland plants: a modelling study

BACKGROUND AND AIMS: It has recently found that lowland rice grown hydroponically is exceptionally efficient in absorbing NO3-, raising the possibility that rice and other wetland plants growing in flooded soil may absorb significant amounts of NO3- formed by nitrification of NH4+ in the rhizosphere. This is important because (a) this NO3- is otherwise lost through denitrification in the soil bulk; and (b) plant growth and yield are generally improved when plants absorb their nitrogen as a mixture of NO3- and NH4+ compared with growth on either N source on its own. A mathematical model is developed here with which to assess the extent of NO3- absorption from the rhizosphere by wetland plants growing in flooded soil, considering the important plant and soil processes operating. METHODS: The model considers rates of O2 transport away from an individual root and simultaneous O2 consumption in microbial and non-microbial processes; transport of NH4+ towards the root and its consumption in nitrification and uptake at the root surface; and transport of NO3- formed from NH4+ towards the root and its consumption in denitrification and uptake by the root. The sensitivity of the model's predictions to its input parameters is tested over the range of conditions in which wetland plants grow. KEY RESULTS: The model calculations show that substantial quantities of NO3- can be produced in the rhizosphere of wetland plants through nitrification and taken up by the roots under field conditions. The rates of NO3- uptake can be comparable with those of NH4+. The model also shows that rates of denitrification and subsequent loss of N from the soil remain small even where NO3- production and uptake are considerable. CONCLUSIONS: Nitrate uptake by wetland plants may be far more important than thought hitherto. This has implications for managing wetland soils and water, as discussed in this paper.     

A model of water flow through plants incorporating shoot/root 'message' control of stomatal conductance

Abstract. A model of water flow from the soil into the plant, and from the plant to the atmosphere is described. There are three state variables in the model: the soil, root and shoot water contents. The flow rate of water from the soil to the root is calculated by dividing the gradient in water potential by a resistance, comprising the resistance from the bulk soil to the root surface, and that from the root surface to the root interior. The resistance in the soil depends on the soil hydraulic conductivity, which in turn depends on the soil water potential. The flow rate from the root to the shoot is given by the gradient in water potential divided by a resistance, which depends on the structural dry mass of the plant. Transpiration is described by the Penman-Monteith equation. The plant water characteristics can be modified to take account of osmotic and cell wall rigidity parameters. The model incorporates the concept of shoot/root ‘messages’ of water stress, which influence stomatal conductance. The message works through the generation of a hormone as the pressure potential in the shoot (mesophyll) or root falls. This hormone induces a shift of osmoticum from the guard cells to the surrounding mesophyll cells, which causes an increase (i.e. closer to zero) in the osmotic potential in these cells. This, in turn, causes a decrease in their pressure potential, and so reduces stomatal conductance. The model is used as a framework to address some of the issues that have recently been raised concerning the role of water potential in describing water flow through plants. We conclude that, with the hormone present, there is unlikely to be a unique relationship between stomatal conductance and shoot total water potential, since stomatal conductance depends on the pressure potential in the guard cells, which may differ from that in other cells. Nevertheless, this does not imply that water potential is not an important, and indeed fundamental, component for describing water flow through plants. Other aspects of water flow through plants are also considered, such as diurnal patterns of shoot, root and soil water potential components. It is seen that these may differ from the commonly held view that, as the soil dries down, they all attain the same values during the dark period, and which, as we show, is largely unsubstantiated either theoretically or experimentally.     

植物在CH4产生、氧化和排放中的作用

综合评述了植物对CH4产生、内源CH4氧化和CH4排放的影响．不同植物释放根系分泌物能力的不同是造成CH4产生量差异的主要原因。而植物不同生育期分泌分泌物能力的差异是造成季节性变化的关键．植物泌O2能力的高低和季节性变化通过影响内源CH4的氧化来改变CH4的排放数量．植物问通气组织数量和密度的差异及其随生育期的变化,通过影响对CH4的传输能力来改变CH4的排放量．因此,植物排放CH4的通量及其季节性变化规律是由植物根系分泌分泌物能力、分泌O2能力和传输CH4能力综合决定的．     

The production and release of living root cap border cells is a function of root apical meristem type in dicotyledonous angiosperm plants

BACKGROUND AND AIMS: The root apical meristems (RAM) of flowering plant roots are organized into recognizable pattern types. At present, there are no known ecological or physiological benefits to having one RAM organization type over another. Although there are phylogenetic distribution patterns in plant groups, the possible evolutionary advantages of different RAM organization patterns are not understood. Root caps of many flowering plant roots are known to release living border cells into the rhizosphere, where the cells are believed to have the capacity to alter conditions in the soil and to interact with soil micro-organisms. Consequently, high rates of border cell production may have the potential to benefit plant growth and development greatly, and to provide a selective advantage in certain soil environments. This study reports the use of several approaches to elucidate the anatomical and developmental relationships between RAM organization and border cell production. METHODS: RAM types from many species were compared with numbers of border cells released in those species. In addition, other species were grown, fixed and sectioned to verify their organization type and capacity to produce border cells. Root tips were examined microscopically to characterize their pattern and some were stained to determine the viability of root cap cells. KEY RESULTS: The first report of a correlation between RAM organization type and the production and release of border cells is provided: species exhibiting open RAM organization produce significantly more border cells than species exhibiting closed apical organization. Roots with closed apical organization release peripheral root cap cells in sheets or large groups of dead cells, whereas root caps with open organization release individual living border cells. CONCLUSIONS: This study, the first to document a relationship between RAM organization, root cap behaviour and a possible ecological benefit to the plant, may yield a framework to examine the evolutionary causes for the diversification of RAM organization types across taxa.     

植物化学通讯研究进展

 生物的信息传递是生命科学中引人入胜的研究领域之一,生物种间种内和个体内都存在着物理和化学等各种信息交流方式。植物种间种内是否通过物理信号进行通讯交流还是一个未知数,但邻近的同种或异种植物通过化学物质为媒介的通讯关系确是客观存在的。最近,愈来愈多的研究证明：许多陆生植物种可以合成并释放特定的次生物质,这些次生物质可以通过空气和土壤两种载体进行信息传递,尤其是在植物受到侵袭和寄生条件下。茉莉酮酸甲酯、水杨酸甲酯和乙烯等挥发性次生物质被确证为以空气为媒介进行植物种间和种内通讯的化学信号分子。植物根分泌的黄酮和氢醌等分子也可以经土壤媒介传递信息。由于在自然条件下植物根系分泌物的收集和活性信号分子的俘获及鉴定技术还未能突破,这增加了以土壤为媒介的植物种间和种内化学通讯关系研究的难度。但不论如何,植物的化学通讯是植物种间和种内交流的主要方式,植物间的化学通讯关系的研究还处于突破的前夜,这方面的任一研究成果都会引起世界性的关注。因此,破译植物种间和种内化学通讯密码具有重要的学术价值。     

Correlation of Pectolytic Enzyme Activity with the Programmed Release of Cells from Root Caps of Pea (Pisum sativum)

In many plant species, the daily release of hundreds to thousands of healthy cells from the root cap into the soil is a normal process, whose function is unknown. We studied the separation of the cells in pea (Pisum sativum) using an aeroponic system in which separated cells were retained on the root until they were washed off for counting. We found that cell separation is a developmentally regulated, temperature-sensitive process that appears to be regulated independently of root growth. No cells were released from very young roots. When plants were grown aeroponically, cell numbers increased with increasing root length to a mean of 3400 cells per root, at which point the release of new cells ceased. The process could be reset and synchronized by washing the root in water to remove shed cells. Cell separation from the root cap was correlated with pectolytic enzyme activity in root cap tissue. Because these cells that separate from the root cap ensheath the root as it grows and thus provide a cellular interface between the root surface and the soil, we propose to call the cells “root border cells.”     

植物根边缘细胞的抗逆性研究进展

综述了近几年来国内外有关植物根边缘细胞抗逆性方面的研究,重点概述植物根边缘细胞对生物与非生物胁迫的响应及其相应的抗性机理。在生物胁迫下,边缘细胞能吸引和固定病原根结线虫,排斥或约束致病性细菌,可作为真菌感染的假目标,减少或避免各种病原菌对根尖的伤害。在非生物胁迫下,边缘细胞通过分泌粘液、诱导ROS产生刺激细胞死亡以抵抗铝毒,并通过其数量的改变来调节高温、高浓度CO2等多种生理反应。最后在当前植物根边缘细胞研究的基础上,提出了今后的研究方向。     

化感胁迫诱导植物细胞损伤研究进展

化感胁迫(allelochemical stress)是指一种植物通过淋溶、挥发、根系分泌和残株腐解等途径释放化学物质,对另一种植物(包括微生物)产生直接或间接的伤害作用。有害化感物质对受体植物具有显著的细胞毒性,影响根边缘细胞的形成过程和活性,改变细胞壁和细胞膜的特性,破坏细胞内部结构,干扰细胞有丝分裂过程和基因表达模式;此外,化感胁迫往往伴随着氧化胁迫,受体植物细胞活性氧(ROS)水平升高,膜脂过氧化程度加剧,抗氧化系统被破坏,ROS影响与凋亡相关的信号调控过程,引起细胞大量死亡。因此,化感胁迫诱导的氧化胁迫可能是引起细胞凋亡的原因之一。阐明化感胁迫介导的氧化损伤和细胞损伤的相互关系以及根边缘细胞对化感胁迫的响应机制,是今后研究化感作用机制的一个方向。     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

Vertical gradient in soil temperature stimulates development and increases biomass accumulation in barley

We have detailed knowledge from controlled environment studies on the influence of root temperature on plant performance, growth and morphology. However, in all studies root temperature was kept spatially uniform, which motivated us to test whether a vertical gradient in soil temperature affected development and biomass production. Roots of barley seedlings were exposed to three uniform temperature treatments (10, 15 or 20°C) or to a vertical gradient (20-10°C from top to bottom). Substantial differences in plant performance, biomass production and root architecture occurred in the 30-day-old plants. Shoot and root biomass of plants exposed to vertical temperature gradient increased by 144 respectively, 297%, compared with plants grown at uniform root temperature of 20°C. Additionally the root system was concentrated in the upper 10cm of the soil substrate (98% of total root biomass) in contrast to plants grown at uniform soil temperature of 20°C (86% of total root biomass). N and C concentrations in plant roots grown in the gradient were significantly lower than under uniform growth conditions. These results are important for the transferability of 'normal' greenhouse experiments where generally soil temperature is not controlled or monitored and open a new path to better understand and experimentally assess root-shoot interactions.     

Revegetation of a lakeside barren area by the application of plant growth-promoting rhizobacteria

The growth stimulation of wild plants by several bacterial species showing plant growth-promoting capabilities was examined in a barren lakeside area at Lake Paro, Korea. Microbial numbers and activities in the field soil were monitored for 73 days after inoculation of the bacteria. The acridine orange direct counts for the total soil bacterial populations ranged between 2.0-2.3x10(9) cells/g soil and 1.4-1.8x10(9) cells/g soil in the inoculated and uninoculated soils, respectively. The numbers of Pseudomonas spp., which is known as a typical plant growth-promoting rhizobacteria, and the total microbial activity were higher in the inoculated soil compared to those in the uninoculated soil. The average shoot and root lengths of the wild plants grown in the inoculated soil were 17.3 cm and 12.4 cm, respectively, and longer than those of 11.4 cm and 8.5 cm in the uninoculated soil. The total dry weight of the harvested wild plants was also higher in the inoculated soil (42.0 g) compared to the uninoculated soil (35.1 g). The plant growth-promoting capabilities of the inoculated bacteria may be used for the rapid revegetation of barren or disturbed land, and as biofertilizer in agriculture.     

High and Low Throughput Screens with Root-knot Nematodes Meloidogyne spp.

Root-knot nematodes (genus Meloidogyne) are obligate plant parasites. They are extremely polyphagous and considered one of the most economically important plant parasitic nematodes. The microscopic second-stage juvenile (J2), molted once in the egg, is the infective stage. The J2s hatch from the eggs, move freely in the soil within a film of water, and locate root tips of suitable plant species. After penetrating the plant root, they migrate towards the vascular cylinder where they establish a feeding site and initiate feeding using their stylets. The multicellular feeding site is comprised of several enlarged multinuclear cells called ''giant cells'' which are formed from cells that underwent karyokinesis (repeated mitosis) without cytokinesis. Neighboring pericycle cells divide and enlarge in size giving rise to a typical gall or root knot, the characteristic symptom of root-knot nematode infection. Once feeding is initiated, J2s become sedentary and undergo three additional molts to become adults. The adult female lays 150-250 eggs in a gelatinous matrix on or below the surface of the root. From the eggs new infective J2s hatch and start a new cycle. The root-knot nematode life cycle is completed in 4-6 weeks at 26-28°C.Here we present the traditional protocol to infect plants, grown in pots, with root-knot nematodes and two methods for high-throughput assays. The first high-throughput method is used for plants with small seeds such as tomato while the second is for plants with large seeds such as cowpea and common bean. Large seeds support extended seedling growth with minimal nutrient supplement. The first high throughput assay utilizes seedlings grown in sand in trays while in the second assay plants are grown in pouches in the absence of soil. The seedling growth pouch is made of a 15.5 x 12.5cm paper wick, folded at the top to form a 2-cm-deep trough in which the seed or seedling is placed. The paper wick is contained inside a transparent plastic pouch. These growth pouches allow direct observation of nematode infection symptoms, galling of roots and egg mass production, under the surface of a transparent pouch. Both methods allow the use of the screened plants, after phenotyping, for crossing or seed production. An additional advantage of the use of growth pouches is the small space requirement because pouches are stored in plastic hanging folders arranged in racks.     

西洋参根残体对自身生长的双重作用

无论在自然生态环境还是在人工农田环境下,植株残体进入土壤后都会对土壤的物理化学性质以及后茬植物的生长产生重要影响。西洋参（Panax quinquefolium L.）为人参属多年生名贵药材,在栽培生产中存在严重的连作障碍问题。为了探明秋后残留在土壤中的须根降解产物对来年植株生长的影响,以及收获后残留在田间的根茬对连作西洋参生长的作用,本实验以3年生西洋参苗为研究对象,采用室内水培试验以及田间盆栽试验,通过添加西洋参根的粉碎物模拟根残体,测定其对西洋参生长的影响。水培试验中全营养液中分别添加0.02 mg/mL、0.1 mg/mL、0.5 mg/mL西洋参根粉碎物,处理后每隔5天测定植株叶片展开情况、株高、冠幅等生长指标。盆栽试验在土壤中添加0.1 mg/g根粉碎物,于栽种后1-2个月测定西洋参叶片展开情况、株高、冠幅等生长指标;水培及盆栽试验均于展叶期、现蕾期、结果期测定地上部及地下部生物量。采用高效液相色谱法（HPLC）测定根围土壤中8种酚酸类化合物的含量。试验结果表明,水培溶液中添加0.02-0.5 mg/mL根残体,可显著抑制西洋参自身地上部分生长,推迟展叶期,结果期生物量降低14.9%-45.0%;对地下部分的影响主要表现为在展叶期显著促进须根生长(p<0.05)。与水培试验相比,盆栽土壤中添加0.1 mg/g根残体同样导致西洋参展叶期推迟;不同的是处理组的地上、地下部及须根的平均生物量均高于对照。另外,添加根残体后盆栽西洋参根围土壤中丁香酸、香草醛、p-香豆酸、阿魏酸等酚酸类化感物质含量下降49.1%-81.4%,但作为逆境信号物质的水杨酸含量升高59.9%。以上结果可以初步确认根残体对西洋参早期生长具有自毒和促进的双重作用,表现为抑制地上部分生长,导致生物量显著下降;同时在生长早期促进须根生长;但在田间环境下,自毒作用可能受根残体降解速度以及土壤对降解产物吸附的影响有所减弱,使促进作用更为明显。     

Mechanisms for cellular transport and release of allelochemicals from plant roots into the rhizosphere

Allelochemicals and other metabolites released by plant roots play important roles in rhizosphere signalling, plant defence and responses to abiotic stresses. Plants use a variety of sequestration and transport mechanisms to move and export bioactive products safely into the rhizosphere. The use of mutants and molecular tools to study gene expression has revealed new information regarding the diverse group of transport proteins and conjugation processes employed by higher plants. Transport systems used for moving secondary products into and out of root cells are similar to those used elsewhere in the plant but are closely linked to soil environmental conditions and local root health. Root cells can rapidly generate and release large quantities of allelochemicals in response to stress or local rhizosphere conditions, so the production and transport of these compounds in cells are often closely linked. Plants need to manage the potentially toxic allelochemicals and metabolites they produce by sequestering them to the vacuole or other membrane-bound vesicles. These compartments provide secure storage areas and systems for safely moving bioactive chemicals throughout the cytosol. Release into the apoplast occurs either by exocytosis or through membrane-bound transport proteins. This review discusses the possible transport mechanisms involved in releasing specific root-produced allelochemicals by combining microscopic observations of the specialized root cells with the physical and chemical properties of the exudates.