QTL mapping of root hair and acid exudation traits and their relationship to phosphorus uptake in common bean

The relationship between root-hair growth, acid exudation and phosphorus (P) uptake as well as the quantitative trait loci (QTLs) associated with these traits were determined for a recombinant inbred line (RIL) population derived from the cross of two contrasting common bean (Phaseolus vulgaris L.) genotypes, DOR364 and G19833, which were grown in solution culture and under field conditions with low-P availability. In the solution-culture study, root-hair density, root-hair length, H⁺ exudation and total acid exudation were measured. Substantial genotypic variability was observed for these traits and their response to P availability. The P-efficient parent G19833 had greater root-hair density, longer root-hair length, and greater exudation of H⁺ and total acid than the P-inefficient genotype DOR364. These traits segregated continuously in the RIL population, with obvious tendency of trait transgression. Genetic analysis revealed that the root traits measured had various heritabilities, with h _b ² ranging from 43.24 to 86.70%. Using an integrated genetic map developed for the population, a total of 19 QTLs associated with root hair, acid exudation and P-uptake traits were detected on 8 linkage groups. P uptake in the field was positively correlated with total acid exudation, basal root-hair length, and basal root-hair density. Acid-exudation traits were intercorrelated, as were root-hair traits. Total acid exudation was positively correlated with basal root-hair density and length. Linkage analysis revealed that some of the root-trait QTLs were closely linked with QTLs for P uptake in the field. We propose that marker-assisted selection (MAS) might be a feasible alternative to conventional screening of phenotypic root traits.     

Effects of phosphorus availability and vesicular–arbuscular mycorrhizas on the carbon budget of common bean (Phaseolus vulgaris)

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean ( Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 μ m ) or moderate (10 μ m ) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below-ground respiration in low P plants was mainly a result of their increased root:shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low-P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low-P soils.     

Fractal geometry of root systems: Field observations of contrasting genotypes of common bean (Phaseolus vulgaris L.) grown under different phosphorus regimes

Root growth and architecture are important for phosphorus acquisition due to the relative immobility of P in the soil. Fractal geometry is a potential new approach to the analysis of root architecture. Substantial genetic variation in root growth and architecture has been observed in common bean. Common bean (Phaseolus vulgaris L.) genotypes with contrasting root architecture were grown under moderate and low P conditions in a field experiment. Linear and planar fractal dimension were measured by tracing root intercepts with vertical planes. Linear fractal dimension increased over time in efficient genotypes, but remained fairly constant over time in inefficient genotypes. Planar fractal dimension increased over time for all genotypes, but was higher in efficient than inefficient genotypes at the end of the experiment. Planar fractal dimension of medium P plants was found to correlate with shoot P content indicating fractal dimension to be a possible indicator for root P uptake. The increasing fractal dimension over time indicates that fractal analysis is a sensitive measure of root branching intensity. A less destructive method for acquisition of data that allows for continuous analysis of fractal geometry and thereby screening for more P efficient genotypes in the field is suggested. This method will allow the researcher to conduct fractal analysis and still complete field trials with final yield evaluation.     

菜豆根构型对低磷胁迫的适应性变化及基因型差异

利用特殊设计的营养袋纸培养和分层式磷控释砂培等根系生长系统结合计算机图像分析技术,以基根根长在生长介质各层的相对分布和基根平均生长角度为指标,定量测定菜豆（Ｐｈａｓｅｏｌｕｓ ｖｕｌｇａｒｉｓ Ｌ．）根构型在低磷胁迫下的适应性变化及其与磷效率的关系。结果表明,菜豆根构型对低磷胁迫具有适应性反应,在缺磷条件下基根向地性减弱,基根在生长介质表层相对分布增多、基根平均生长角度（与水平线夹角）变小,从而导     

Nitrogen fixation and plant growth of common bean (Phaseolus vulgaris L.) at different levels of phosphorus availability

The efficacy of the alumina system for differentiating between bean (Phaseolus vulgaris L.) genotypes for growth at different levels of phosphorus availability was determinated. In addition to response to P levels, comparisons were made between plants receiving N either from fertilizer or nitrogen fixation. When the cv. Carioca was provided with either 100 ppm of N or inoculated withRhizobium leguminosarum biovarphaseoli, differences in shoot dry weight and nodule number were related to P level. There was a greater proportion of green, ineffectivevs. red, active nodules at the low P concentration than at the higher P concentration. In a second experiment, two cvs., Puebla 152 and Carioca and the breeding line UW 24-21, either were inoculated with rhizobia or provided with 150ppm of N. Each genotype-nitrogen combination was grown at 8 levels of P. There was a positive effect of P level on shoot dry weight, nodule number and nodule mass. Root mass was affected less than nodule or shoot mass by the P level of the growth medium. Nodule mass, but not P concentration in the nodules, was affected by P level, whereas in the other plant tissues, P concentrations were lower at lower P levels in the media.     

Regulation of root hair density by phosphorus availability in Arabidopsis thaliana

We characterized the response of root hair density to phosphorus (P) availability in Arabidopsis thaliana. Arabidopsis plants were grown aseptically in growth media with varied phosphorus concentrations, ranging from 1 mmol m⁻³ to 2000 mmol m⁻³ phosphorus. Root hair density (number of root hairs per mm of root length) was analysed starting at 7 d of growth. Root hair density was highly regulated by phosphorus availability, increasing significantly in roots exposed to low-phosphorus availability. The initial root hairs produced by the radicle were not sensitive to phosphorus availability, but began to respond after 9 d of growth. Root hair density was about five times greater in low phosphorus (1 mmol m⁻³) than in high phosphorus (1000 mmol m⁻³) media. Root hair density decreased logarithmically in response to increasing phosphorus concentrations within that range. Root hair density also increased in response to deficiencies of several other nutrients, but not as strongly as to low phosphorus. Indoleacetic acid (IAA), the auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (CMPA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the ethylene synthesis inhibitor amino-oxyacetic acid (AOA) all increased root hair density under high phosphorus but had very little effect under low phosphorus. Low phosphorus significantly changed root anatomy, causing a 9% increase in root diameter, a 31% decrease in the cross-sectional area of individual trichoblasts, a 40% decrease in the cross-sectional area of individual atrichoblasts, and 45% more cortical cells in cross-section. The larger number of cortical cells and smaller epidermal cell size in low phosphorus roots increased the number of trichoblast files from eight to 12. Two-thirds of increased root hair density in low phosphorus roots was caused by increased likelihood of trichoblasts to form hairs, and 33% of the increase was accounted for by changes in low phosphorus root anatomy resulting in an increased number of trichoblast files. These results show that phosphorus availability can fundamentally alter root anatomy, leading to changes in root hair density, which are presumably important for phosphorus acquisition.     

Effect of phosphorus deficiency on growth angle of basal roots in Phaseolus vulgaris

Root architectural plasticity might be an important factor in the acquisition by plants of immobile nutrients such as phosphorus (P). In this study, we examined the effect of P availability on the orientation of basal roots with respect to gravity, and thereby on the growth angle of these roots of common bean (Phaseolus vulgaris L.). In one set of studies the growth angle of basal roots of bean seedlings was measured over time. Sixteen bean genotypes were examined; six showed a decrease in root orientation with respect to gravity in low P media, one increased orientation, and nine showed no difference within 5 d of basal root emergence. Bean taproots also showed decreased root orientation with respect to gravity in low P. Growth angle after 5 d was correlated with field performance of contrasting genotypes in low P tropical soils. Mineral deficiencies other than P did not cause changes in root angle. In a split pouch system that provided high or low P solution to different parts of the root system, the decrease in root angle in low P was found to be a response to global P availability, and not local to the portion of the root system in low P. Effects of P availability on root angle were associated with reduced shoot P concentration, but preceded effects on plant biomass accumulation and leaf area expansion. Results from growth pouches for genotype G 19833 were confirmed using a solid-phase buffered sand-culture system supplying P at three levels. Pea (Pisum sativum), soybean (Glycine max Williams), chickpea (Cicer arietinum), lima bean (Phaseolus lunatus), and lentil (Lens culinaris) were grown with and without P; soybean and pea also showed decreased basal root angles in low P.     

Responses of root architecture development to low phosphorus availability: a review

Background

Phosphorus (P) is an essential element for plant growth and development but it is often a limiting nutrient in soils. Hence, P acquisition from soil by plant roots is a subject of considerable interest in agriculture, ecology and plant root biology. Root architecture, with its shape and structured development, can be considered as an evolutionary response to scarcity of resources.

Scope

This review discusses the significance of root architecture development in response to low P availability and its beneficial effects on alleviation of P stress. It also focuses on recent progress in unravelling cellular, physiological and molecular mechanisms in root developmental adaptation to P starvation. The progress in a more detailed understanding of these mechanisms might be used for developing strategies that build upon the observed explorative behaviour of plant roots.

Conclusions

The role of root architecture in alleviation of P stress is well documented. However, this paper describes how plants adjust their root architecture to low-P conditions through inhibition of primary root growth, promotion of lateral root growth, enhancement of root hair development and cluster root formation, which all promote P acquisition by plants. The mechanisms for activating alterations in root architecture in response to P deprivation depend on changes in the localized P concentration, and transport of or sensitivity to growth regulators such as sugars, auxins, ethylene, cytokinins, nitric oxide (NO), reactive oxygen species (ROS) and abscisic acid (ABA). In the process, many genes are activated, which in turn trigger changes in molecular, physiological and cellular processes. As a result, root architecture is modified, allowing plants to adapt effectively to the low-P environment. This review provides a framework for understanding how P deficiency alters root architecture, with a focus on integrated physiological and molecular signalling.     

Influence of ethylene on adventitious root formation in mung bean hypocotyl cuttings

The relationship between ethylene and adventitious root formation in mung bean hypocotyl cuttings was studied.Ethephon, an ethylene-releasing compound, at 5 x 10 -5 M increased root number and root dry weight on hypo-cotyl cuttings. When ethephon was applied to hypocotyl at different times after excision, there were two effectivetimes for root production i.e. between 06 h and 18-24 h. These two time periods correspond to the induction phase and the late initiation phase of root development, respectively. After excision, three peaks of ethylene productionwere observed. The first peak commencing at 6 h started the sequence of reactions leading root formation, the second peak appearing at 12 h coincided with the beginning of the increase of the IAA level during primordia initiation, and the third peak showing at 48 h played a role in root differentiation and growth. Ethylene stimulated rooting by enhancing the increase in auxins. Thus it appears that the IAA-induced ethylene production may be a factor involved in the stimulation of adventitious root formation.     

Soil moisture and potassium affect the performance of symbiotic nitrogen fixation in faba bean and common bean

Potassium (K) is reported to improve plant's resistance against environmental stress. A frequently experienced stress for plants in the tropics is water shortage. It is not known if sufficient K supply would help plants to partially overcome the effects of water stress, especially that of symbiotic nitrogen fixation which is often rather low in the tropics when compared to that of temperate regions. Thus, the impact of three levels of fertilizer potassium (0.1, 0.8 and 3.0 mM K) on symbiotic nitrogen fixation was evaluated with two legumes under high (field capacity to 25% depletion) and low (less than 50% of field capacity) water regimes. Plants were grown in single pots in silica sand under controlled conditions with 1.5 mM N (¹⁵N enriched NH₄NO₃). The species were faba bean (Vicia faba L.), a temperate, amide producing legume and common bean (Phaseolus vulgaris L.), a tropical, ureide producing species. In both species, 0.1 mM K was insufficient for nodulation at both moisture regimes, although plant growth was observed. The supply of 0.8 or 3.0 mM K allowed nodulation and subsequent nitrogen fixation which appeared to be adequate for respective plant growth. High potassium supply had a positive effect on nitrogen fixation, on shoot and root growth and on water potential in both water regimes. Where nodulation occurred, variations caused by either K or water supply had no consequences on the percentage of nitrogen derived from the symbiosis. The present data indicate that K can apparently alleviate water shortage to a certain extent. Moreover it is shown that the symbiotic system in both faba bean and common bean is less tolerant to limiting K supply than plants themselves. However, as long as nodulation occurs, N assimilation from the symbiotic source is not selectively affected by K as opposed to N assimilation from fertilizer.     

The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model

We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r ² between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability. This revised version was published online in August 2006 with corrections to the Cover Date.     

沙地樟子松人工林土壤磷库及其有效性初步研究

对樟子松人工林土壤不同层次中的全磷、速效磷 ,各级无机磷、有机磷的含量及其相关性进行了初步分析。结果表明 ,樟子松人工林土壤中全磷和速效磷含量都很低 (全磷约为0 0 5～ 0 18mg·g-1,表层土壤速效磷为 1 5 0～ 2 2 4mg·kg-1) ,处于我国土壤磷含量的最低水平 ,且速效磷不到全磷的 0 5 % ,有机磷占全磷的 4 0 %～ 80 % ,非蓄闭态无机磷以Ca P为主 ,在各土层中均为Ca P >Fe P >Al P。表层土壤中各种形态磷含量都显著高于下面三层 ,是各种生物、物理、化学过程最活跃的区域 ,并且存在磷酸盐沿着土壤剖面的淋溶。速效磷含量与有机磷、Fe P、Al P显著相关。不同密度和林龄的磷素状况无显著差异。可见 ,影响樟子松人工林土壤磷素有效性的因素很复杂 ,在较短的时间尺度上不同形态无机磷的相互转化及其影响因素对整个系统的磷素循环起主要作用 ,Fe P、Al P可能是沙地樟子松人工林土壤速效磷的主要来源 ,而有机磷则是重要的潜在有效磷源。     

Effects of soil phosphorus availability,temperature and moisture on soil respiration in Eucalyptus pauciflora forest

Rates of soil respiration (CO₂ efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO₂ efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO₂ m^–2 hr^–1. Temperature and moisture are the main variables that cause variation in soil CO₂ efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO₂ efflux and exhibited a highly significant logarithmic relationship (r² = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO₂ efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO₂ efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO₂ efflux, and thus can be used to predict soil CO₂ efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha^–1 yr^–1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO₂ efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change.     

Effect of auxins and plant oligosaccharides on root formation and elongation growth of mung bean hypocotyls

The interaction of auxins – IAA, IBA or NAA – with galactoglucomannan oligosaccharides (GGMOs) on adventitious root formation and elongation growth of mung bean hypocotyl cuttings was studied. GGMOs induced adventitious roots in the absence of auxins; however, their effect was lower compared with IBA or NAA. On the other hand, in the presence of auxins, GGMOs inhibited adventitious root induction. Their effect depended on the concentration of oligosaccharides and the type of auxin used. The highest inhibition effect of GGMOs at a concentration of 10⁻⁸ M in the presence of IBA and NAA was observed. In the presence of IAA their inhibition was non-significant in regard to the concentration. The interaction of auxins with GGMOs resulted in the formation of adventitious roots on a shorter part of hypocotyls compared with the effect of auxins alone. However, roots were induced more extensively along the hypocotyls treated with GGMOs compared with the control. GGMOs inhibited the length of induced adventitious roots in the presence of IAA, while in combination with IBA or NAA they were ineffective. The elongation of hypocotyls induced by IAA or IBA was inhibited by GGMOs, too. However, in the presence of NAA or by endogenous growth they were without any significant effect on elongation growth. These findings suggest that GGMOs in certain concentrations might inhibit rooting and the elongation process dependant on auxin used.     

Immunological detection of bean common mosaic virus in French bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) leaves

Bean common mosaic potyvirus (BCMV) is an important seed borne pathogen of French bean. Differential inoculation with bean common mosaic virus at cotylodonary trifoliate leaf stage and pre-flowering stage of crop growth revealed that cotyledonary leaf infection favored maximum disease expression. Under immunosorbent electron microscopy (ISEM) the virus particles of filamentous structure having a diameter of 750 nm (l) and 15 nm (w) were observed. These particles gave positive precipitin tests with polyclonal antiserum of Potato virus Y.     

Utilizing the potential for increased nitrogen fixation in common bean

Many variables affect the amount of N₂ fixation that occurs in field-grown common beans (Phaseolus vulgaris L.). When environmental conditions are optimized, genetically superior plants that are nodulated with efficient rhizobia are able to fix enough N₂ to support grain yields of up to 2000 kg ha^–1. All of the required components are available for common bean to fulfill its potential as a nitrogen-fixing food crop. However, these components must be assembled and presented to growers in a usable package that is economically attractive.     

Construction and characterization of a common bean bacterial artificial chromosome library

We have constructed a common bean (Phaseolus vulgaris L.) bacterial artificial chromosome (BAC) library consisting of 33 792 clones and an estimated 3- to 5-fold coverage of the common bean genome. Leaf nuclei were used as the source for high-molecular-weight DNA, and an endonuclease/methylase competition assay was employed to partially cleave the DNA. The library was screened with a number of nuclear and mitochondrial probes. Each nuclear probe identified at least two BACs with an average insert size of ca. 100 kb. Only 26 clones were identified after hybridizing with mitochondrial probes, indicating contamination with organellar sequences is low. Numerous clones could be identified after screening the library with two repetitive probes flanking the nuclear fertility restorer Fr. Intriguingly, 12 clones appeared to hybridize to both markers, and restriction analysis of these clones revealed that they can be assembled into maximally four contigs, suggesting that these repetitive probes may be useful for the physical mapping of the Fr locus.     

Effects of plant species on phosphorus availability in a range of grassland soils

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using ³³P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot^–1) than ryegrass (1.1–15.6 mg pot^–1) from the soil except in the Temuka soil in which the level of available P (e.g., E_1min P_i, bicarbonate extractable P_i) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable P_i pool (E_{> 10m} P_i, reduced by 31.8% on the average) to the rapidly exchangeable P_i (E_1min-1d P_i, E_1d-10m P_i) pools in most soils. The values of R/r₁ (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties.