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Root system architecture (RSA) plays a central role in water and nutrient acquisition in plants. Plasticity and genetic variation in RSA can be used as an adaptive strategy to optimize plant performance under variable environments. We quantified phenotypic variation for seedling RSA among 44 diverse pea (Pisum sativumL.) genotypes, including breeding lines and germplasm accessions, grown under controlled conditions for 14 days using two‐dimensional hydroponic root imaging. Root image analysis revealed significant genotypic variability among the lines for all root traits, namely root length (RL), root diameter (RD), root volume, root surface area, number of tips, network width (NW), network depth (ND), and network convex area. Significant positive correlations were observed among the evaluated root traits, ranging from 0.5 to 0.9. Pea lines were ranked based on estimated means for root traits, with lines E20, F1, and F8 showing high rankings, while E4 and F5 received low rankings for most traits. To associate root traits with nitrogen (N) fixation and field agronomic performance, we performed redundancy analysis (RDA). The quantified root traits accounted for significant variation in the agronomic traits (R2= ∼30%, p< 0.001). RDA showed a positive association between lodging susceptibility and root system NW and between plant height and root system ND. RD was positively associated with grain yield and N fixation. N fixation was positively associated with the number of lateral roots. The findings of this study indicate that variation for seedling root traits in pea could aid selection for N fixation and other important agronomic traits. Image analysis of pea seedling roots revealed significant genotypic variability for key root traits.Redundancy analysis showed associations between several key agronomic traits of pea and root seedling traits.Assessing variation in seedling root traits could aid pea breeding.

Root system architecture (RSA) plays a pivotal role in efficient uptake of essential nutrients, such as phosphorous (P), nitrogen (N), and water. In soils with heterogeneous nutrient distribution, root plasticity can optimize acquisition and plant growth. Here, we present evidence that a constitutive RSA can confer benefits for sorghum grown under both sufficient and limiting growth conditions. Our studies, using P efficient SC103 and inefficient BTx635 sorghum cultivars, identified significant differences in root traits, with SC103 developing a larger root system with more and longer lateral roots, and enhanced shoot biomass, under both nutrient sufficient and deficient conditions. In addition to this constitutive attribute, under P deficiency, both cultivars exhibited an initial increase in lateral root development; however, SC103 still maintained the larger root biomass. Although N deficiency and drought stress inhibited both root and shoot growth, for both sorghum cultivars, SC103 again maintained the better performance. These findings reveal that SC103, a P efficient sorghum cultivar, also exhibited enhanced growth performance under N deficiency and drought. Our results provide evidence that this constitutive nature of RSA can provide an avenue for breeding nutrient- and drought-resilient crops.

Phytoextraction field experiments were conducted on soil contaminated with 0.39 to 8.7 Bq/g of 137Cs to determine the capacity of five plant species to accumulate 137Cs and the effects of three soil treatments on uptake. The plants tested were redroot pigweed (Amaranthus retroflexusL. var. aureus); a mixture of redroot pigweed and spreading pigweed (A. graecizansL.); purple amaranth (A. cruteusL.) × Powell's amaranth (A. powelliiS. Watson), referred to here as the amaranth hybrid; Indian mustard [Brassica juncea(L.) Czern.]; and cabbage (Brassica oleraceaL. var. capitata). For control plants, the concentration ratios (CR) of 137Cs were greatest for redroot pigweed and the amaranth hybrid, with average CR values of 1.0 ± 0.24 and 0.95 ± 0.14, respectively. The lowest value was for Indian mustard at 0.36 ± 0.10. The soil treatments included (i) application of NH4NO3solution to the soil after plants had matured, (ii) addition of composted manure to increase organic matter content of the soil, (iii) combination of the manure and ammonium solution treatments, and (iv) controls. The ammonium solution gave little overall increase in accumulation of 137Cs. The use of composted manure also had little influence, but the combination of the composted manure with application of ammonium solutions had a distinctly negative effect on plant uptake of 137Cs. On average the fraction of 137Cs taken up from the soil was reduced by 57.4 ± 1.2% compared with controls. This was the result of release of competing ions, primarily Ca, from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant 137Cs content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities 137Cs can be released from the shoots of plants as a result of sudden changes in soil solution chemistry.

Mineral nutrient deficiencies constitute major limitations for plant growth on agricultural soils around the world. To identify genes that possibly play roles in plant mineral nutrition, we recently generated a high-density array consisting of 1,280 genes from tomato (Lycopersicon esculentum) roots for expression profiling in nitrogen (N) nutrition. In the current study, we used the same array to search for genes induced by phosphorus (P), potassium (K(+)), and iron (Fe) deficiencies. RNA gel-blot analysis was conducted to study the time-dependent kinetics for expression of these genes in response to withholding P, K, or Fe. Genes previously not associated with P, K, and Fe nutrition were identified, such as transcription factor, mitogen-activated protein (MAP) kinase, MAP kinase kinase, and 14-3-3 proteins. Many of these genes were induced within 1 h after withholding the specific nutrient from roots of intact plants; thus, RNA gel-blot analysis was repeated for specific genes (transcription factor and MAP kinase) in roots of decapitated plants to investigate the tissue-specific location of the signal triggering gene induction. Both genes were induced just as rapidly in decapitated plants, suggesting that the rapid response to the absence of P, K, or Fe in the root-bathing medium is triggered either by a root-localized signal or because of root sensing of the mineral environment surrounding the roots. We also show that expression of Pi, K, and Fe transporter genes were up-regulated by all three treatments, suggesting coordination and coregulation of the uptake of these three essential mineral nutrients.

Al-induced release of Al-chelating ligands (primarily organic acids) into the rhizosphere from the root apex has been identified as a major Al tolerance mechanism in a number of plant species. In the present study, we conducted physiological investigations to study the spatial and temporal characteristics of Al-activated root organic acid exudation, as well as changes in root organic acid content and Al accumulation, in an Al-tolerant maize (Zea mays) single cross (SLP 181/71 x Cateto Colombia 96/71). These investigations were integrated with biophysical studies using the patch-clamp technique to examine Al-activated anion channel activity in protoplasts isolated from different regions of the maize root. Exposure to Al nearly instantaneously activated a concentration-dependent citrate release, which saturated at rates close to 0.5 nmol citrate h(-1) root(-1), with the half-maximal rates of citrate release occurring at about 20 microM Al(3+) activity. Comparison of citrate exudation rates between decapped and capped roots indicated the root cap does not play a major role in perceiving the Al signal or in the exudation process. Spatial analysis indicated that the predominant citrate exudation is not confined to the root apex, but could be found as far as 5 cm beyond the root cap, involving cortex and stelar cells. Patch clamp recordings obtained in whole-cell and outside-out patches confirmed the presence of an Al-inducible plasma membrane anion channel in protoplasts isolated from stelar or cortical tissues. The unitary conductance of this channel was 23 to 55 pS. Our results suggest that this transporter mediates the Al-induced citrate release observed in the intact tissue. In addition to the rapid Al activation of citrate release, a slower, Al-inducible increase in root citrate content was also observed. These findings led us to speculate that in addition to the Al exclusion mechanism based on root citrate exudation, a second internal Al tolerance mechanism may be operating based on Al-inducible changes in organic acid synthesis and compartmentation. We discuss our findings in terms of recent genetic studies of Al tolerance in maize, which suggest that Al tolerance in maize is a complex trait.

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexusL.), Indian mustard [Brassica juncea(L.) Czern.], and tepary bean (Phaseolus acutifoliusA. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.

The existence of metal hyperaccumulator species demonstrates that plants have the genetic potential to remove toxic metals from contaminated soil. Possibly, one of the best-known hyperaccumulators is Thlaspi caerulescens. This species has been shown to accumulate very high Zn concentrations without manifesting any sign of toxicity. Thus, T. caerulescens represents an excellent experimental system for studying metal hyperaccumulation in plants as it relates to phytoremediation. In this article, we review the results of an investigation into the physiology, biochemistry, and molecular regulation of Zn transport and accumulation in T. caerulescens compared with a nonaccumulator relative T. arvense. Physiological studies focused on the use of 65Zn radiotracer flux techniques to characterize zinc transport and compartmentation in the root, and translocation to the shoot. Transport studies indicated that a number of Zn transport sites were stimulated in T. caerulescens, contributing to the hyperaccumulation trait. Thus, Zn influx into root and leaf cells, and Zn loading into the xylem was greater in T. caerulescens compared with the nonaccumulator T. arvense. The 4.5-fold stimulation of Zn influx into the roots of T. caerulescens was hypothesized to be due to an overexpression of Zn transporters in this species. Additionally, compartmental analysis (radiotracer wash out or efflux techniques) was used to show that Zn was sequestered in the root vacuole of T. arvense inhibiting Zn translocation to the shoot in this nonaccumulator species. Molecular studies focused on the cloning and characterization of Zn transport genes in T. caerulescens. Functional complementation of a yeast Zn transport-defective mutant with a T. caerulescens cDNA library constructed in a yeast expression vector resulted in the cloning of a Zn transport cDNA, ZNT1. Expression of ZNT1 in yeast allowed for a physiological characterization of this transporter. ZNT1 was shown to encode a high-affinity Zn transporter that can also mediate low-affinity Cd transport. Biochemical analyses indicated that enhanced Zn transport in T. caerulescens results from a constitutively high expression of ZNT1 in roots and shoots. These results suggest that overexpression of ZNT1 may be linked to an alteration of the Zn tolerance mechanism in this species.

The role of acidification and chelating agents in the solubilization of uranium (U) from contaminated soil was examined in a series of experiments. Soil acidification and the addition of chelating agents were the two methods compared initially. The results indicated that the addition of citric acid solubilized more U than acidification or the other amendments tested. This increase in U solubility was, however, transitory. A subsequent experiment indicated that citrate concentration had a more dramatic effect on U solubility than did acidification. The greatest soluble U concentration during this experiment (775 mg kg−1soil, or ∼85% of the total U) was observed after 24 h in the presence of 20 millimoles citrate kg−1soil at pH 5. The persistence of U solubility over the 96‐h experimental period was primarily a function of pH and citrate degradation. In a separate experiment, in which citric acid rather than citrate was added to contaminated soil, the soluble U concentrations observed were generally lower than those observed in the presence of citrate. Citric acid decreased soil pH to values ≤3.6, and solubilized higher concentrations of Al and Fe than observed in the presence of citrate. Since the maximum solubilization of U was observed at pH 5, the implication of these results is that a combined approach, using both soil acidification and citric acid addition, may be necessary to maximize the phytoextraction of U from soils with a pH > 6.0.

The trivalent cation aluminum can cause chronic cytotoxicity in plants, animals and microorganisms. It has been suggested that Al interaction with cell membranes and enzyme metal binding sites may be involved in Al cytotoxicity. In this study, the binding of Al to microsomes and liposomes was found to be lipid dependent with the signal transduction element phosphatidylinositol‐4,5‐bisphosphate having the highest affinity for Al with an Al:lipid stoichiometry of 1:1. Al binding was only reduced in the presence of high concentrations of Ca2+(>1 mM). Both citrate and, to a lesser extent, malate were capable of preventing Al lipid binding, which is consistent with the involvement of these organic acids in a recently described Al detoxification mechanism in plants. The effects of AlCl3, Al‐citrate and ZnSO4on metal‐dependent enzyme activities (enolase, pyruvate kinase, H+‐ATPase, myosin, Calpain, proteinase K, phospholipase A2and arginase) was assayed in vitro. While Zn2+was capable of inhibiting all the enzymes except the H+‐ATPase, AlCl3and Al‐citrate had minimal effects except for with phospholipase A2where an interaction with AlCl3occurred. However, this could be negated by the addition of citrate. The results indicate that, contrary to current hypotheses, the toxic mode of Al is not through an interaction with enzymatic catalytic metal binding sites but may be through the interaction with specific membrane lipids.

The toxicity of Zn and Cu in three species from the genus Brassicawas examined to determine if these plants showed sufficient tolerance and metal accumulation to be used to phytoremediate a site contaminated with these two heavy metals. Hydroponically grown 12‐d‐old plants of Brassica juncea, B. rapa, and B. napuswere grown for an additional 14 d in the presence of either elevated Zn (6.5 mg L−1), Cu (0.32 mg L−1), or Zn+Cu to quantify the toxic effects of these metals on several different growth parameters. With few exceptions, both root and shoot dry weight for all three species decreased significantly in the presence of heavy metals. Cu inhibited lateral root elongation in B. rapa, B. napus, and, to a lesser extent, B. juncea, while Zn tended to decrease only lateral root diameter. Both metals reduced shoot Fe and Mn concentrations in all three Brassica spp. to levels associated with Fe and Mn deficiencies. These deficiencies, however, did not correlate with observed patterns of leaf chlorosis. Nonetheless, heavy metal‐induced inhibition of Fe and Mn accumulation may have been a significant factor in reducing plant growth. In terms of heavy metal removal, the Brassicaspp. were more effective at removing Zn from the nutrient solution than Cu. The extent of Zn and Cu removal was reduced in the presence of both metals, as compared to the single heavy metal treatments. The implications of these results for phytoremediation are discussed.

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 µM Al3+, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al3+ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.

The selectivity of heavy metal ion‐selective electrodes has often been inadequately characterized in the past. Only recently, alternative methods have become available to assess the response toward highly discriminated ion solutions. In this article, the selectivity of vibrating liquid membrane cadmium‐selective microelectrodes was characterized using the separate solution method, the fixed interference method, the matched potential method, and Bakker's method which demands one to condition the electrode membrane in discriminated ion solutions prior to measurement. In this study, the matched potential method gave selectivity values for the two least discriminated ions, lead(II) and copper(II), that were comparable to the ones obtained with SSM. Bakker's method, on the other hand, was the only method that reveals the extremely high ion selectivity over all other discriminated ions. This study shows that carrier‐based cadmium microelectrodes possess sufficient selectivity for many in situ applications, and that experimental biases may sometimes lead to experimental selectivity coefficient variations of about 12 orders of magnitude depending on the chosen method.

Abstract.: Aluminum inhibition of root growth is a major world agricultural problem where the cause of toxicity has been linked to changes in cellular calcium homeostasis. Therefore, the effect of aluminum ions (Al) on changes in cytoplasmic free calcium concentration ([Ca2+]c) was followed in root hairs of wild-type, Al-sensitive and Al-resistant mutants of Arabidopsis thaliana (L.) Heynh. Generally, Al exposure resulted in prolonged elevations in tip-localized [Ca2+]c in both wild-type and Al-sensitive root hairs. However, these Al-induced increases in [Ca2+]c were not tightly correlated with growth inhibition, occurring up to 15 min after Al had induced growth to stop. Also, in 32% of root hairs examined growth stopped without a detectable change in [Ca2+]c. In contrast, Al-resistant mutants showed little growth inhibition in response to AlCl3 exposure and in no case was a change in [Ca2+]c observed. Of the other externally applied stresses tested (oxidative and mechanical stress), both were found to inhibit root hair growth, but only oxidative stress (H2O2, 10 μM) caused a prolonged rise in [Ca2+]c similar to that induced by Al. Again this increase occurred after growth had been inhibited. The lack of a tight correlation between Al exposure, growth inhibition and altered [Ca2+]c dynamics suggests that although exposure of root hairs to toxic levels of Al causes an alteration in cellular Ca2+ homeostasis, this may not be a required event for Al toxicity. The elevation in [Ca2+]c induced by Al also strongly suggests that the phytotoxic action of Al in root hairs is not through blockage of Ca2+-permeable channels required for Ca2+ influx into the cytoplasm.

A field study was conducted to investigate the potential of three plant species for phytoremediation of a 137Cs‐contaminated site. Approximately 40‐fold more 137Cs was removed from the contaminated soil in shoots of red root pigweed (Amaranthus retroflexusL.) than in those of Indian mustard [Brassica juncea(L.) Czern] and tepary bean (Phaseolus acutifoliusA. Gray). The greater potential for 137Cs removal from the soil by A. retroflexuswas associated with both high concentration of 137Cs in shoots and high shoot biomass production. Approximately 3% of the total 137Cs was removed from the top 15 cm of the soil (which contained most of the soil radiocesium) in shoots of 3‐too‐old A. retroflexusplants. Soil leaching tests conducted with 0.1 and 0.5 MNH4NO3solutions eluted as much as 15 and 19%, respectively, of the soil 137Cs. Addition of NH4NO3to the soil, however, had no positive effect on 137Cs accumulation in shoots in any of the species investigated. It is proposed that either NH4NO3solution quickly percolated through the soil before interacting at specific 137Cs binding sites or radiocesium mobilized by NH4NO3application moved below the rhizosphere, becoming unavailable for root uptake. Further research is required to optimize the phytotransfer of the NH4NO3‐mobilized 137Cs. With two croppings of A. retroflexusper year and a sustained rate of extraction, phytoremediation of this 137Cs‐contaminated soil appears feasible in <15 yr.

Cultures of Anabaena cylindrica, grown on media containing 5 m? NH4Cl (which represses heterocyst formation), evolved hydrogen after a period of dark incubation under an argon atmosphere. This hydrogen production was not due to nitrogenase activity, which was nearly undetectable, but was due to a hydrogenase. Cultures grown on media with tungsten substituted for molybdenum had a high frequency of heterocysts (15%) and inactive nitrogenase after nitrogen starvation. The hydrogenase activity of these cultures was three-fold greater than the activity of non-heterocystous cultures. The effects of oxygen inhibition on hydrogen evolution by hetero-cystous cultures suggest that two pools of hydrogenase activity exist - an oxygen sensitive hydrogen evolution in vegetative cells and a relatively oxygen-resistent hydrogen evolution in heterocysts. In either case, inhibition by oxygen was reversible. Light had an inhibitory effect on net hydrogen evolution. Hydrogen production in vitro was much higher than in vivo, indicating that in vivo hydrogenase activity is limited by endogenous reductant supply.

Recently, ethylene was reported to be involved in the regulation of Fe(III)-chelate reducing capacity by cucumber (Cucuinis sativusL.) roots. Here, we studied the effect of two ethylene inhibitors, aminooxyacetic acid (AOA) and cobalt, on the Fe(III) reducing capacity in roots of mutant genotypes [E107pea [Pisum sativumL. (brz, brz)] and chloronervatomato (Lycopersicon esculentumL.) that exhibit high rates of Fe(III)-chelate reduction and excessive iron accumulation. The ethylene inhibitors, AOA and cobalt, markedly inhibited Fe(III)-chelate reducing capacity in roots of both genotypes. Over-expression of root Fe(III) reductase activity by both mutants appears to be related to ethylene. Possibly, both mutants are genetically defective in their ability to regulate root ethylene production. The large inhibitory effect of both ethylene inhibitors on Fe(III)-chelate reducing capacity in roots of the mutant tomato genotype, chloronerva, disputes the contention that the nicotianamine-Fe(II) complex is the repressior of the gene responsible for Fe(III)-chelate reductase activity, as previously suggested by others. However, since nicotianamine shares the same biosynthetic precursor as ethylene, i.e. S-adenosyl methionine, nicotianamine may affect Fe(III)-chelate reductase activity in dicot and non-grass monocot roots by influencing ethylene biosynthesis.

The magnitude and spatial localization of Ca2+, K+ and H+ fluxes in growing and non-growing Limnobium stoloniferum root hairs was determined using non-invasive, ion-selective vibrating microelectrodes. Both the spatial pattern and magnitude of the ionic flux was dependent on the particular ion in question. Both H+ and Ca2+ influx was localized almost exclusively to the tips of growing root hairs, suggesting that these fluxes may be involved in directing growth. Influx of K+ showed no distinct localization and uptake appeared uniform along the length of the root hair. Competitive inhibition of Ca2+ influx using a range of Mg+ concentrations indicated that the magnitude of the Ca2+ flux entering the root hair tip did not determine growth rate; however, the presence of Ca2+ on the external face of the membrane was implicit for root hair integrity. Aluminum proved to be a potent inhibitor of root hair growth. At an exogenous Al concentration of 20 µM a complete blockage of Ca2+ influx into root hair tips was observed, suggesting that Al blockage of Ca2+ influx could be involved in Al toxicity. However, at a lower Al concentration (2 µM), Ca2+ fluxes were unaffected while inhibition of growth was still observed along with a distinct swelling of the root hair tip. The swelling at the root hair tips was identical in appearance to that seen in the presence of microtubule inhibitors, suggesting that Al could influence a number of different sites at the plasma-membrane surface and within the cell. The possible role(s) of Ca2+ and H+ fluxes in directing tip growth are discussed.

Several mineral rhizotoxicities, including those induced by Al3+, H+, and Na+, can be relieved by elevated Ca2+ in the rooting medium. This leads to the hypothesis that the toxic cations displace Ca2+ from transport channels or surface ligands that must be occupied by Ca2+ in order for root elongation to occur. In this study with wheat (Triticum aestivum L.) seedlings, we have determined, in the case of Al3+, that (i) Ca2+, Mg2+, and Sr2+ are equally ameliorative, (ii) that root elongation does not increase as Ca2+ replaces Mg2+ or Sr2+ in the rooting media, and (iii) that rhizotoxicity is a function solely of Al3+ activity at the root-cell membrane surface as computed by a Gouy-Chapman-Stern model. The rhizotoxicity was indifferent to the computed membrane-surface Ca2+ activity. The rhizotoxicity induced by high levels of tris(ethylenediamine)cobaltic ion (TEC3+), in contrast to Al3+, was specifically relieved by Ca2+ at the membrane surface. The rhizotoxicity induced by H+ exhibited a weak specific response to Ca2+ at the membrane surface. We conclude that the Ca2+-displacement hypothesis fails in the case of Al3+ rhizotoxicity and that amelioration by cations (including monovalent cations) occurs because of decreased membrane-surface negativity and the consequent decrease in the membrane-surface activity of Al3+. However, TEC3+, but not Al3+, may be toxic because it inhibits Ca2+ uptake. The nature of the specific H+-Ca2+ interaction is uncertain.

The cation Al3+ is toxic to plants at micromolar concentrations and can severely inhibit root growth in solution experiments.Trivalent aluminum hydrolyzes in solution, and, apart from the Al3+ ion, which dominates speciation below pH 5.0, various mononuclear and polynuclear hydroxy-Al species can also occur (Kinraide 1991). Accumulating evidence suggests that Al3+ is the rhizotoxic species under the experimental conditions used in the present study (Kinraide 1991; Kinraide et al. 1992). The inhibition of Ca2+ uptake in roots by Al3+ has been proposed as a possible mechanism for Al3+ toxicity, and in this study the hypothesis was tested directly. Root growth and Ca2+ uptake were measured in 5-d-old seedlings of wheat (Triticum aestivum L. Thell) during exposure to Al3+ in a low-Ca2+ basal medium, and to Al3+ in the presence of added cations. Uptake of Ca2+ in whole roots and translocation to the shoot were measured using 45Ca2+, and localized measurements of net Ca2+ flux were also made at the root apex using the technique of microelectrode ion-flux estimation. Treatment with 2.64 µM AlCl3 in 226 µM CaCl2, at pH 4.5, severely inhibited root growth without affecting Ca2+ uptake. Addition of 30 mM Na2+, 3 mM Mg2+ or 50 µM tris(ethylenediamine)cobalt(III) to this Al3+ treatment restored root growth but significantly reduced Ca2+ uptake measured over the entire root system and at the root apex. The Al3+ and Ca2+ concentrations were adjusted so that the activities of the Al3+ and Ca2+ ions were constant in all solutions (1.5 µM and 200 µM, respectively). Root growth can be severely inhibited by Al3+ concentrations that do not affect Ca2+ uptake, while the addition of ameliorating cations depresses Ca2+ uptake. These results argue against the hypothesis that Al3+ inhibits root growth by reducing Ca2+ uptake.

We investigated the effects of Fe and Cu status of pea (Pisum sativum L.) seedlings on the regulation of the putative root plasma-membrane Fe(III)-chelate reductase that is involved in Fe(III)-chelate reduction and Fe2+ absorption in dicotyledons and nongraminaceous monocotyledons. Additionally, we investigated the ability of this reductase system to reduce Cu(II)-chelates as well as Fe(III)-chelates. Pea seedlings were grown in full nutrient solutions under control, -Fe, and -Cu conditions for up to 18 d. Iron(III) and Cu(II) reductase activity was visualized by placing roots in an agarose gel containing either Fe(III)-EDTA and the Fe(II) chelate, Na2bathophenanthrolinedisulfonic acid (BPDS), for Fe(III) reduction, or CuSO4, Na3citrate, and Na2-2,9-dimethyl-4,7-diphenyl-1, 10-phenanthrolinedisulfonic acid (BCDS) for Cu(II) reduction. Rates of root Fe(III) and Cu(II) reduction were determined via spectrophotometric assay of the Fe(II)-BPDS or the Cu(I)-BCDS chromophore. Reductase activity was induced or stimulated by either Fe deficiency or Cu depletion of the seedlings. Roots from both Fe-deficient and Cu-depleted plants were able to reduce exogenous Cu(II)-chelate as well as Fe(III)-chelate. When this reductase was induced by Fe deficiency, the accumulation of a number of mineral cations (i.e., Cu, Mn, Fe, Mg, and K) in leaves of pea seedlings was significantly increased. We suggest that, in addition to playing a critical role in Fe absorption, this plasma-membrane reductase system also plays a more general role in the regulation of cation absorption by root cells, possibly via the reduction of critical sulfhydryl groups in transport proteins involved in divalent-cation transport (divalent-cation channels?) across the root-cell plasmalemma.

An ion-selective vibrating-microelectrode system, which was originally used to measure extracellular Ca2+ gradients generated by Ca2+ currents, was used to study K+, H+ and Ca2+ transport in intact maize (Zea mays L.) roots and individual maize suspension cells. Comparisons were made between the vibrating ion-selective microelectrode, and a technique using stationary ion-selective microelectrodes to measure ionic gradients in the unstirred layer at the surface of plant roots. The vibrating-microelectrode system was shown to be a major improvement over stationary ion-selective microelectrodes, in terms of sensitivity and temporal resolution. With the vibrating ion microelectrode, it was easy to monitor K+ influxes into maize roots in a background K+ concentration of 10 mM or more, while stationary K+ electrodes were limited to measurements in a background K+ concentration of 0.3 mM or less. Also, with this system it was possible to conduct a detailed study of root Ca2+ transport, which was previously not possible because of the small fluxes involved. For example, we were able to investigate the effect of the excision of maize roots on Ca2+ influx. When an intact maize root was excised from the seedling at a position 3 cm from the site of measurement of Ca2+ transport, a rapid fourfold stimulation of Ca2+ influx was observed followed by dramatic oscillations in Ca2+ flux, oscillating between Ca2+ influx and efflux. These results clearly demonstrate that wound or perturbation responses of plant organs involve transient alterations in Ca2+ transport, which had previously been inferred by demonstrations of touch-induced changes in cytoplasmic calcium. The sensitivity of this system allows for the measurement of ion fluxes in individual plant cells. Using vibrating K+ and H+electrodes, it was possible to measure H+efflux and both K+ influx and efflux in individual maize suspension cells under different conditions. The availability of this technique will greatly improve our ability to study ion transport at the cellular level, in intact plant tissues and organs, and in specialized cells, such as root hairs or guard cells.

The effects of aluminum on the concentration-dependent kinetics of Ca2+ uptake were studied in two winter wheat (Triticum aestivum L.) cultivars, Al-tolerant Atlas 66 and Al-sensitive Scout 66. Seedlings were grown in 100 µM CaCl2 solution (pH 4.5) for 3 d. Subsequently, net Ca2+ fluxes in intact roots were measured using a highly sensitive technique, employing a vibrating Ca2+-selective microelectrode. The kinetics of Ca2+ uptake into cells of the root apex, for external Ca2+ concentrations from 20 to 300 µM, were found to be quite similar for both cultivars in the absence of external Al; Ca2+ transport could be described by Michaelis-Menten kinetics. When roots were exposed to solutions containing levels of Al that were toxic to Al-sensitive Scout 66 but not to Atlas 66 (5 to 20 µM total Al), a strong correlation was observed between Al toxicity and Al-induced inhibition of Ca2+ absorption by root apices. For Scout 66, exposure to Al immediately and dramatically inhibited Ca2+ uptake over the entire Ca2+ concentration range used for these experiments. Kinetic analyses of the Al-Ca interactions in Scout 66 roots were consistent with competitive inhibition of Ca2+ uptake by Al. For example, exposure of Scout 66 roots to increasing Al levels (from 0 to 10 µM) caused the Km for Ca2+ uptake to increase with each rise in Al concentration, from approx. 100 µM in the absence of Al to approx. 300 µM in the presence of 10 µM Al, while having no effect on the Vmax. The same Al exposures had little effect on the kinetics of Ca2+ uptake into roots of Atlas 66. The results of this study indicate that Al disruption of Ca2+ transport at the root apex may play an important role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars, and that differential Al tolerance may be associated with the ability of Ca2+-transport systems in cells of the root apex to resist disruption by potentially toxic levels of Al in the soil solution.

The development of plasma membrane‐associated iron(III) reductase activity was characterized in root systems of Pisum sativumduring the first 2 wk of growth, as plants were challenged with iron‐deficiency stress. Plants of a parental genotype (cv. Sparkle) and a functional iron‐deficiency mutant genotype (E107) were grown hydroponically with or without supplemental iron. Iron(III) reductase activity was visualized by placing the roots in an agarose matrix containing 0.2 idm Fe(III)‐ethylenediaminetetraacetic acid and 0.3 mM Na2‐bathophenanthrolinedisulfonic acid (BPDS). Red staining patterns, resulting from the formation of Fe(II)‐BPDS, were used to identify iron(III)‐reducing regions. Iron(III) reduction was extensive on roots of E107 as early as d 7, but not until d 11 for ‐Fe‐treated Sparkle. Roots of +Fe‐treated Sparkle showed limited regions of reductase activity throughout the period of study. For secondary lateral roots, iron(III) reduction was found for all growth types except + Fe‐treated Sparkle. Treating Sparkle plants alternately to a cycle of iron deficiency, iron sufficiency, and iron deficiency revealed that reductase activity at a given root zone could be alternatively present, absent, and again present. Our results suggest that for Pisumroots grown under the present conditions, iron‐deficiency stress induces the activation of iron(III) reductase capacity within 2 d.

Embryonic development represents an important reproductive phase of sexually reproducing plant species. The fusion of egg and sperm produces the plant zygote, a totipotent cell that, through cell division and cell identity specification in early embryogenesis, establishes the major cell lineages and tissues of the adult plant. The subsequent morphogenesis phase produces the full-sized embryo, while the late embryogenesis maturation process prepares the seed for dormancy and subsequent germination, ensuring continuation of the plant life cycle. In this review on embryogenesis, we compare the model eudicot Arabidopsis thalianawith monocot crops, focusing on genome activation, paternal and maternal regulation of early zygote development, and key organizers of patterning, such as auxin and WOX transcription factors. While the early stages of embryo development are apparently conserved among plant species, embryo maturation programs have diversified between eudicots and monocots. This diversification in crop species reflects the likely effects of domestication on seed quality traits that are determined during embryo maturation, and also assures seed germination in different environmental conditions. This review describes the most important features of embryonic development in plants, and the scope and applications of genomics in plant embryo studies.

Depletion of finite global rock phosphate (Pi) reserves will impose major limitations on future agricultural productivity and food security. Hence, modern breeding programmes seek to develop Pi-efficient crops with sustainable yields under reduced Pifertilizer inputs. In this regard, although the long-term responses of plants to Pistress are well documented, the early signalling events have yet to be elucidated. Here, we show plant tissue-specific responses to early Pistress at the transcription level and a predominant role of the plant vascular system in this process. Specifically, imposition of Pistress induces rapid and major changes in the mRNA population in the phloem translocation stream, and grafting studies have revealed that many hundreds of phloem-mobile mRNAs are delivered to specific sink tissues. We propose that the shoot vascular system acts as the site of root-derived Pistress perception, and the phloem serves to deliver a cascade of signals to various sinks, presumably to coordinate whole-plant Pihomeostasis.

The primary symptom of aluminium (Al) toxicity in higher plants is inhibition of root growth. In this study, we investigated the spatial sensitivity of maize (Zea mays L.) roots to Al. A divided-chamber technique indicated that only exposure of the terminal 10 to 15 mm of the root to Al resulted in inhibition of growth. Application of Al to all but this apical region of the root had little or no effect on growth for 24 h and caused minimal damage to the root tissue. Small agar blocks infused with Al were then applied to discrete areas of the apex of maize roots to determine which section (root cap, meristem or elongation zone) was more important to Al-induced inhibition of growth. The terminal 2�0 to 3�0 mm of root (root cap and meristem) must be exposed to Al for inhibition. Application of Al to the 3�0 mm of root proximal to this terminal zone (elongation zone) resulted in damage to the root tissue but no significant inhibition of growth. Therefore, the visible injuries incurred by roots during Al-stress are not associated directly with the inhibition of root growth. Furthermore, removal of the root cap had no effect on the Al-induced inhibition of root growth in solution experiments and argues against the root cap providing protection from Al stress or serving an essential role in the mechanism of toxicity. We suggest that the meristem is the primary site of Al-toxicity.