Nickel hyperaccumulation by Streptanthus polygaloides protects against the folivore Plutella xylostella (Lepidoptera: Plutellidae)

We determined the effectiveness of Ni as an elemental defence of Streptanthus polygaloides (Brassicaceae) against a crucifer specialist folivore, diamondback moth (DBM), Plutella xylostella. An oviposition experiment used arrays of S. polygaloides grown on Ni-amended (high-Ni) soil interspersed with plants grown on unamended (low-Ni) soil and eggs were allowed to hatch and larvae fed freely among plants in the arrays. We also explored oviposition preference by allowing moths to oviposit on foil sheets coated with high- or low-Ni plant extract. This was followed by an experiment using low-Ni plant extract to which varying amounts of Ni had been added and an experiment using sheets coated with sinigrin (allyl glucosinolate) as an oviposition stimulant. Diamondback moths laid 2.5-fold more eggs on low-Ni plants than on high-Ni plants and larval feeding was greater on low-Ni plants. High-Ni plants grew twice as tall, produced more leaves, and produced almost 3.5-fold more flowers. Low-Ni plants contained more allyl glucosinolate than high-Ni plants and moths preferred to oviposit on foil sheets dipped in low-Ni plant extract. Moths showed no preference when Ni concentration of low-Ni extract was varied and overwhelmingly preferred sinigrin coated sheets. We conclude that Ni hyperaccumulation is an effective elemental defence against this herbivore, increasing plant fitness through a combination of toxicity to DBM larvae and decreased oviposition by adults.     

Nickel hyperaccumulation by Thlaspi montanum var. montanum (Brassicaceae): a constitutive trait

Adaptations to particular stresses may occur only in populations experiencing those stresses or may be widespread within a species. Nickel hyperaccumulation is viewed as an adaptation to high-Ni (serpentine) soils, but few studies have determined if hyperaccumulation ability is restricted to populations from high-Ni soils or if it is a constitutive trait found in populations on both high- and low-Ni soils. We compared mineral element concentrations of Thlaspi montanum var. montanum plants grown on normal and high-Ni greenhouse soils to address this question. Seed sources were from four populations (two serpentine, two non-serpentine) in Oregon and northern California, USA. Plants from all populations were able to hyperaccumulate Ni, showing Ni hyperaccumulation to be a constitutive trait in this species. Populations differed in their ability to extract some elements (e.g., Ca, Mg, P) from greenhouse soils. We noted a negative correlation between tissue concentrations of Ni and Zn. We suggest that the ability to hyperaccumulate Ni has adaptive value to populations growing on non- serpentine soil. This adaptive value may be a consequence of metal-based plant defense against herbivores/pathogens, metal- based interference against neighboring plant species, or an efficient nutrient scavenging system. We suggest that the Ni hyperaccumulation ability of T. montanum var. montanum may be an inadvertent consequence of an efficient nutrient (possibly Zn or Ca) uptake system.     

Degradation of Alyssum murale biomass in soil

The Ni-hyperaccumulating plant Alyssum murale accumulates exceptionally high concentrations of nickel in its aboveground biomass. The reasons for hyperaccumulation remain unproven; however, it has been proposed that elemental alelopathy might be important. High-Ni leaves shed by the plant may create a "toxic zone" around the plant where germination or growth of competing plants is inhibited. The efficacy of this argument will partially depend upon the rate at which leaves degrade in soil and free metals are released, and the subsequent rate at which metals are bound to soil constituents. To test the degradation of biomass of hyperaccumulators, A. murale was grown on both high- and low-Ni soils to achieve high- (12.0 g Ni/kg) and low- (0.445 g Ni/kg) Ni biomass. Shredded leaf and stem biomass were added to a serpentine soil from Oregon that was originally used to grow high-Ni biomass and a low-Ni control soil from Maryland. Biomass Ni was readily soluble and extractable, suggesting near immediate release as biomass was added to soil Extractable nickel in soil amended with biomass declined rapidly over time due to Ni binding in soil These results suggest that Ni released from biomass of Ni hyperaccumulators may significantly affect their immediate niche only for short periods of time soon after leaf fall, but repeated application may create high Ni levels under and around hyperaccumulators.     

The defensive role of Ni hyperaccumulation by plants: a field experiment

Hyperaccumulation of Ni by plants is hypothesized to function as an elemental defense against herbivores and pathogens. Laboratory experiments have documented toxic effects to herbivores consuming high-Ni plant tissues, but this paper reports the first experiment to examine the defensive effectiveness of Ni hyperaccumulation under field conditions. The experiment was conducted at an ultramafic soil site naturally inhabited by the Ni hyperaccumulator Streptanthus polygaloides (Brassicaceae). Experimental treatments examined the response of herbivores to hyperaccumulated Ni, using exclosure and insecticide treatments to divide herbivores into groups based primarily upon herbivore size. Three soils (Ni-amended greenhouse soil, unamended greenhouse soil, ultramafic soil), three exclosure treatments (exclosure, control exclosure, no exclosure), and a systemic insecticide treatment were combined in a fractional factorial experimental design. Streptanthus polygaloides plants were grown in a greenhouse for 2 mo, transplanted into the field by inserting potted plants into holes dug on the experimental site, and periodically examined for herbivore damage during a 41-d period. Initial surveys showed greater amounts of insect damage to plants with low tissue Ni levels, confirming the defensive effect of Ni against some insect herbivores, but large herbivores (probably vertebrates) later consumed entire plants regardless of plant Ni status. We concluded that Ni was not an effective defense against these large herbivores, probably because their diets mix high-Ni S. polygaloides foliage with that of associated non-hyperaccumulating species. We suggest that such dietary dilution is one mechanism whereby some herbivores can circumvent elemental plant defenses.     

Infection Biology of Bacterial Wilt of Forage Grasses

Infection of perennial ryegrass (Lolium perenne L.), Italian ryegrass (Loliummultiflorum L.), and timothy (Phleum pratense L.) by Xanthomonas campestris pv. graminis and Xanthomonas campestris pv. phlei occurred mainly via wounds rather than natural openings. Nevertheless, bacteria were detected by isolation and immunofluorescence in plants sprayed with the pathogen without prior wounding and in plants in which intact ears had been dipped in inocula. High concentrations of bacteria were observed around the stomata of perennial ryegrass and timothy by scanning electron microscopy 48 h after inoculation. Perennial ryegrass and Italian ryegrass and individual plants of ryegrass and timothy differedin susceptibility to the pathogens.     

Dynamics of Ni-based defence and organic defences in the Ni hyperaccumulator, Streptanthus polygaloides (Brassicaceae)

Plants use chemical defences to reduce damage from herbivores and the effectiveness of these defences can be altered by biotic and abiotic factors, such as herbivory and soil resource availability. Streptanthus polygaloides , a nickel (Ni) hyperaccumulator, possesses both Ni-based defences and organic defences (glucosinolates), but the extent to which these defences interact and respond to environmental conditions is unknown. S. polygaloides plants were grown on high-Ni and low-Ni soil and concentrations of Ni and glucosinolates were compared with those of the congeneric non-hyperaccumulator, S. insignus spp. insignus , grown under the same conditions. Ni contents were highest (4000 μg g⁻¹ dry tissue) in S. polygaloides plants grown on high-Ni soil. Glucosinolate content was significantly higher in S. insignus than in S. polygaloides suggesting that plants defended by Ni produce a lower concentration of organic defences. In a separate experiment, high-Ni S. polygaloides plants were exposed to simulated herbivory or live folivores to determine the inducibility of Ni-based and organic defences. Contents of Ni were not affected by either herbivory treatment, whereas glucosinolate concentrations were >30% higher in damaged plants. We concluded that the Ni-based defence of S. polygaloides is not induced by herbivory.     

Natural variation of nickel,zinc and cadmium (hyper)accumulation in facultative serpentinophytes Noccaea kovatsii and N. praecox

Aims

Data on the variability of hyperaccumulation potential of the facultative serpentinophytes Noccaea kovatsii and N. praecox on different geological substrates are scarce. The aim of this study was to assess the accumulation potential of these two species from ultramafic and non-ultramafic substrates, with special emphasis on the hyperaccumulation of Ni, Zn and Cd.

Methods

Samples of plants and corresponding soils were collected from 16 sites covering a wide range of geochemistry. Elemental concentrations were determined in the roots, shoots and the rhizosphere soils. The pH, particle size distribution and content of organic matter were also determined in the soil samples.

Results

All ultramafic accessions of both species hyperaccumulated Ni with high intraspecific variability. Only one accession of N. kovatsii from a schist soil hyperaccumulated Zn, with also a high Cd accumulation. Accumulation and translocation of Ni in both species were much higher in the ultramafic accessions, whereas Zn accumulation and translocation was found in both ultramafic and non-ultramafic accessions.

Conclusions

Ni accumulation and translocation was restricted to ultramafic populations of both species, whereas it is a species-wide trait for Zn. This study provides new and comprehensive information on the natural variation of hyperaccumulation of Ni, Zn and Cd in N. kovatsii and N. praecox.

     

Effectiveness of alkyl dimethyl benzyl ammonium chloride in reducing the population of Xanthomonas campestris pv. vesicatoria and Pseudomonas syringae pv. syringae in tomatoes,beans, and peppers

Abstract

Microbial contamination of fruits and vegetables during growth, processing, and post-harvest is a serious problem in agricultural sectors. A study was undertaken to investigate the efficacy of alkyl dimethyl benzyl ammonium chloride (ADBAC) in reducing the population of Xanthomonas campestris pv. vesicatoria, and Pseudomonas syringae pv. syringae on tomatoes, beans, and peppers. Tomatoes, beans, and peppers were inoculated by dipping in bacteria for 15 min then fruits were dried for 2 hour at ambient temperature before they were treated with 0.1, 1, 10, 100, and 1000 ppm of ADBAC. Treatments with 10, 100, and 1000 ppm ADBAC caused an 8-log CFU/ml reduction of X. campestris pv. vesicatoria on surfaces of tomatoes. Treatments with 100 and 1000 ppm ADBAC caused an 8-log CFU/ml reduction of P. syringae pv. syringae and X. campestris pv. vesicatoria on surfaces of tomatoes and peppers, respectively. However, treatment of surfaces of beans with 1000 ppm of ADBAC caused an 8-log CFU/ml reduction of P. syringae pv. syringae. Overall, a 50% reduction on population counts of both pathogens was achieved with 100 and 1000 ppm ADBAC. No X. campestris pv vesicatoria, P. syringae pv. syringae, or other bacteria were detected on the control fruits inoculated with sterile distilled water. This study's findings suggest that ADBAC has good bactericidal and sanitizing activities and could potentially be useful as a new sanitizer for food safety.     

Nickel hyperaccumulation as an elemental defense of Streptanthus polygaloides (Brassicaceae): influence of herbivore feeding mode

No study of a single nickel (Ni) hyperaccumulator species has investigated the impact of hyperaccumulation on herbivores representing a variety of feeding modes. Streptanthus polygaloides plants were grown on high- or low-Ni soils and a series of no-choice and choice feeding experiments was conducted using eight arthropod herbivores. Herbivores used were two leaf-chewing folivores (the grasshopper Melanoplus femurrubrum and the lepidopteran Evergestis rimosalis), a dipteran rhizovore (the cabbage maggot Delia radicum), a xylem-feeder (the spittlebug Philaenus spumarius), two phloem-feeders (the aphid, Lipaphis erysimi and the spidermite Trialeurodes vaporariorum) and two cell-disruptors (the bug Lygus lineolaris and the whitefly Tetranychus urticae). Hyperaccumulated Ni significantly decreased survival of the leaf-chewers and rhizovore, and significantly reduced population growth of the whitefly cell-disruptor. However, vascular tissue-feeding insects were unaffected by hyperaccumulated Ni, as was the bug cell-disruptor. We conclude that Ni can defend against tissue-chewing herbivores but is ineffective against vascular tissue-feeding herbivores. The effects of Ni on cell-disruptors varies, as a result of either variation of insect Ni sensitivity or the location of Ni in S. polygaloides cells and tissues.     

Bacterial Streak Disease of Maize (Zea mays L.) in South Africa

Bacterial streak disease of maize is currently causing some concern among breeders in South Africa. The causal organism of this previously undescribed disease was successfully isolated and its pathogenicity established using KoCH's postulates. Standard physiological and biochemical tests used to identify phytopathogenic bacteria indicated that the bacterium is a Xanthomonas campestris pathovar. Comparisons between this organism and other recognized X. campestris pathovars of the Poaceae indicated that apart from some minor differences the maize streak pathogen is physiologically similar to X. campestris pv. holcicola. However, in repeated reciprocal inoculation experiments all attempts to induce disease symptoms in sorghum with the maize streak pathogen were unsuccessful. Conversely, X. campestris pv. holcicola did produce symptoms in maize leaves. In all the maize cultivars tested the symptoms produced by the maize streak pathogen were, however, always considerably more severe than those caused by X. campestris pv. holcicola. Notwithstanding its physiological similarity to X. campestris pv. holicola it would appear that on the grounds of host specificity the maize streak pathogen warrants new pathovar status. The name X. campestris pv. zeae is proposed.     

Inhibition of Seed Germination and Root Development Caused by Xanthomonas campestris pv. vesicatoria in Pepper and Tomato

Inoculation of pepper seeds with the leaf pathogen Xanthomonas campestris pv. vesicatoria inhibited pepper germination. The inhibitory effect, which was stronger in non-sterilized light textured soils, decreased with time, and after 20, days or more, there was no difference between inoculated and non-inoculated seeds. Inhibitory substance(s) within the cytoplasmatic fraction of pathogen cells inhibited the germination of non-host tomato seeds. No relationship between pathogenicity to pepper leaves and inhibition of pepper seed germination was detected. The inhibitory substance(s) was found in two out of four X. campestris pv. vesicatoria strains. Heat-killed bacteria suppressed growth of pepper but not tomato seedlings. It is, therefore, suggested that the inhibition of seed germination and the decrease in root development are different modes of X. campestris pv. vesicatoria pathogenesis toward pepper plants.     

核桃黑斑病拮抗放线菌WMF106的筛选、鉴定及防效

【背景】核桃黑斑病是由2种病原菌引起的细菌性病害,目前缺乏有效的生物防治方法。【目的】从核桃树根际土壤中筛选对核桃黑斑病病原菌具有拮抗效果的放线菌菌株,为该病害生防菌剂的开发提供基础。【方法】采用稀释涂布法分离放线菌,并以病原菌野油菜黄单胞菌(Xanthomonas campestris pv. campestris)和成团泛菌(Pantoea agglomerans)作为指示菌,利用平板对峙法和改良牛津杯法筛选具有高拮抗活性的菌株,通过形态学特征、生理生化特性和16SrRNA基因序列分析确定其分类地位,并测定其无菌发酵液的抗菌谱和室内防效。【结果】筛选到一株对野油菜黄单胞菌和成团泛菌均有较强拮抗作用的放线菌菌株WMF106,该菌株对2种病原菌的抑菌圈直径分别为2.38 cm和1.82 cm,无菌发酵液对2种病原菌的抑菌圈直径分别为1.75 cm和1.55 cm。根据菌株形态学、生理生化特性及16SrRNA基因序列分析,将菌株WMF106鉴定为暗蓝色链霉菌(Streptomyces caeruleatus)。该菌株对尖孢镰刀菌、腐皮镰孢菌、辣椒刺盘孢菌、灰葡萄孢菌、胶孢炭疽菌5种植物病原菌及大肠杆菌、金黄色葡萄球菌、铜绿假单胞菌、白色念珠菌、黑曲霉5种指示菌均有抑制作用,抗菌性能广谱高效,其无菌发酵液原液对离体叶片上由野油菜黄单胞菌和成团泛菌造成的核桃黑斑病防效分别为77.44%和58.33%。【结论】菌株WMF106可作为防治核桃黑斑病的生防材料,具有良好的开发价值和应用前景。     

Defence-related enzymatic response in cabbage to Xanthomonas campestris pv. campestris

Black rot of cabbage caused by Xanthomonas campestris pv. campestris is one of the most important diseases of crucifers worldwide. Expression of defence-related enzymes in cabbage in response to X. campestris pv. campestris was investigated in the current experiment. Among the defence-related enzymes (phynylalanine ammonia lyase, peroxidase, polyphenol oxidase, superoxide dismutase [SOD] and chitinase) and quantity of phenolic compounds studied in the present investigation, phenylalanine ammonia lyase (PAL), the key enzyme in the phenylpropanoid pathway was the first enzyme suppressed at three days after inoculation in X. campestris pv. campestris-cabbage system. Correlation analysis indicated that PAL and phenolic compounds are the two most important compounds determining the susceptibility of cabbage to X. campestris pv. campestris. Induction of peroxidase isoform-1 (Rf value: 0.059) and SOD isoform-1 (Rf value: 0.179) three days after pathogen inoculation implicated the role of these isozymes in susceptible cabbage – X. campestris pv. campestris interaction. This study demonstrates the susceptibility of cabbage to X. campestris pv. campestris is a result of declination of PAL and phenolic contents at biochemical level as a manifestation of increase in bacterial population at the cellular level within the host tissues.     

A dual role for proline iminopeptidase in the regulation of bacterial motility and host immunity

During plant–pathogen interactions, pathogenic bacteria have evolved multiple strategies to cope with the sophisticated defence systems of host plants. Proline iminopeptidase (PIP) is essential to Xanthomonas campestris pv. campestris (Xcc) virulence, and is conserved in many plant‐associated bacteria, but its pathogenic mechanism remains unclear. In this study, we found that disruption of pip in Xcc enhanced its flagella‐mediated bacterial motility by decreasing intracellular bis‐(3′,5′)‐cyclic dimeric guanosine monophosphate (c‐di‐GMP) levels, whereas overexpression of pip in Xcc restricted its bacterial motility by elevating c‐di‐GMP levels. We also found that PIP is a type III secretion system‐dependent effector capable of eliciting a hypersensitive response in non‐host, but not host plants. When we transformed pip into the host plant Arabidopsis, higher bacterial titres were observed in pip‐overexpressing plants relative to wild‐type plants after Xcc inoculation. The repressive function of PIP on plant immunity was dependent on PIP's enzymatic activity and acted through interference with the salicylic acid (SA) biosynthetic and regulatory genes. Thus, PIP simultaneously regulates two distinct regulatory networks during plant–microbe interactions, i.e. it affects intracellular c‐di‐GMP levels to coordinate bacterial behaviour, such as motility, and functions as a type III effector translocated into plant cells to suppress plant immunity. Both processes provide bacteria with the regulatory potential to rapidly adapt to complex environments, to utilize limited resources for growth and survival in a cost‐efficient manner and to improve the chances of bacterial survival by helping pathogens to inhabit the internal tissues of host plants.     

Nutritional Similarity between Leaf-Associated Nonpathogenic Bacteria and the Pathogen Is Not Predictive of Efficacy in Biological Control of Bacterial Spot of Tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log₁₀ of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

Transgenic tobacco resistant to a bacterial disease by the detoxification of a pathogenic toxin

Summary Some plant pathogens produce toxins which cause disease in infected plants. One of the pathogenic toxins, tabtoxin, is produced by Pseudomonas syringae pv. tabaci, which causes wildfire of tobacco. A tabtoxin resistance gene (ttr) coding for an acetyltransferase isolated from Pseudomonas syringae pv. tabaci was fused to the 35S promoter of the cauliflower mosaic virus (CaMV) to construct a chimeric gene for introduction into tobacco cells by Agrobacterium-mediated transformation. The transgenic tobacco plants showed high specific-expression of the ttr gene and no chlorotic symptoms caused by tabtoxin treatment or with infection by Pseudomonas syringae pv. tabaci. These results demonstrate a successful approach to obtain disease-resistant plants by detoxification of the pathogenic toxins which play an important role in pathogenesis.     

Xanthomonas campestris pv.parthenii pathovar nov. incitant of leaf blight of parthenium

A new bacterial leaf blight disease of parthenium (Parthenium hysterophorus L.) is described for the first time. The disease-causing bacterium was isolated and its morphological, physiological and biochemical characters were determined. The pathogenicity of bacterium is apparently limited only to parthenium. The pathogen was identified asXanthomonas campestris pv.parthenii pathovar nov. on the basis of morphological, physiological, biochemical and pathogenic characteristics.     

Defense response of a pepper cultivar cv. Sy-2 is induced at temperatures below 24°C

Temperature is one of the most important environmental factors that influence plant growth and development. Recent studies imply that plants show various responses to non-extreme ambient temperatures. Previously, we have found that a pepper cultivar cv. Sy-2 (Capsicum chinense) shows developmental defects at temperatures below 24°C. In this study, to gain new insights into the temperature sensitivity of cv. Sy-2, temperature-sensitive genes were screened using microarray techniques. At restrictive temperature of 20°C, almost one-fourth of the 411 up-regulated genes were defense related or predicted to be defense related. Further expression analyses of several defense-related genes showed that defense-related genes in cv. Sy-2 were constitutively expressed at temperatures below 24°C. Moreover, accumulation of high level of salicylic acid (SA) in cv. Sy-2 grown at 20°C suggests that the defense response is activated in the absence of pathogens. To confirm that the defense response is induced in cv. Sy-2 below 24°C, we evaluated the resistance to biotrophic bacterial pathogen Xanthomonas campestris pv. vesicatoria and necrotrophic fungal pathogen Cercospora capsici. Cv. Sy-2 showed enhanced resistance to X. campestris pv. vesicatoria, but not to C. capsici.