Burkholderia cepacia complex species: health hazards and biotechnological potential

The Burkholderia cepacia complex is a group of nine closely related bacterial species that have useful properties in the natural environment as plant pest antagonists, plant growth promoters and degradative agents of toxic substances. Because these species are human opportunistic pathogens, especially in cystic fibrosis patients, biotechnological applications that involve environmental releases have been severely restricted. Recent progress in understanding the taxonomy, epidemiology and ecology of the B. cepacia complex species has unravelled considerable variability in their pathogenicity and ecological properties, which has set the basis for a reassessment of the risk posed by individual species to human health.     

植物耐铝的生物化学与分子机理

某些耐铝植物在铝胁迫下分泌有机酸被认为是一个重要的抗性机制.从根系分泌出来的有机酸能与根际的Al3 结合,形成无毒性的螯合物,从而减轻了铝对根系的毒害.但是,铝诱导有机酸分泌的中间环节及调节机制至今仍不清楚.一些证据表明,铝能激活根尖细胞质膜内的阴离子通道,因而可以调节有机酸的分泌.近年来,人们开始注意一些信号分子如蛋白激酶、水杨酸等介导铝诱导有机酸的分泌,已经获得一些成果.同时,铝胁迫基因的分离和鉴定也为人们从分子水平上研究和认识铝胁迫下植物的抗性机制奠定了基础.     

Antioxidative enzymes and the Russian wheat aphid (Diuraphis noxia) resistance response in wheat (Triticum aestivum)

A crucial function of antioxidative enzymes is to remove excess reactive oxygen species (ROS), which can be toxic to plant cells. The effect of Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), infestation on the activities of antioxidative enzymes was investigated in the resistant (cv. Tugela DN) and the near-isogenic susceptible (cv. Tugela) wheat (Triticum aestivum L.). RWA infestation significantly induced the activity of superoxide dismutase, glutathione reductase and ascorbate peroxidase to higher levels in the resistant than in susceptible plants. These findings suggest the involvement of antioxidative enzymes in the RWA-wheat resistance response, which was accompanied by an early oxidative burst. The results are consistent with the role of ROS in the resistance response and the control of their levels to minimise toxic effects.     

Biological Activities of Analogues of Ser-Val-Gly-Glu (AS-I), a Toxin Pathogenic to Sunflower

The tetrapeptides Ser-Val-Glu-Gly, Hse-Val-Gly-Glu, Thr-Val-Gly-Glu, Val-Ser-Gly-Glu and Ser-Val-Glu-Gly, analogues of the AS-1 toxin which is pathogenic to sunflower, were examined for biological activity on sunflower and eight other plant species. These peptides were synthesized in solution or on a solid phase. Of the species studied by leaf inoculation, sunflower and zucchini were the most susceptible and cotton the most resistant to ail of the toxins tested. The effect of Ser-Val-Glu-Gly was similar to that reported for AS-I, whereas the tetrapeptide Thr-Val-Gly-Glu was the most toxic.     

The ecological significance of toxic nectar

Although plant-herbivore and plant-pollinator interactions have traditionally been studied separately, many traits are simultaneously under selection by both herbivores and pollinators. For example, secondary compounds commonly associated with herbivore defense have been found in the nectar of many plant species, and many plants produce nectar that is toxic or repellent to some floral visitors. Although secondary compounds in nectar and toxic nectar are geographically and phylogenetically widespread, their ecological significance is poorly understood. Several hypotheses have been proposed for the possible functions of toxic nectar, including encouraging specialist pollinators, deterring nectar robbers, preventing microbial degradation of nectar, and altering pollinator behavior. All of these hypotheses rest on the assumption that the benefits of toxic nectar must outweigh possible costs; however, to date no study has demonstrated that toxic nectar provides fitness benefits for any plant. Therefore, in addition to these adaptive hypotheses, we should also consider the hypothesis that toxic nectar provides no benefits or is tolerably detrimental to plants, and occurs due to previous selection pressures or pleiotropic constraints. For example, secondary compounds may be transported into nectar as a consequence of their presence in phloem, rather than due to direct selection for toxic nectar. Experimental approaches are necessary to understand the role of toxic nectar in plant-animal interactions.     

Proteases in pathogenesis and plant defence

Plant pathogens deliver a variety of virulence factors to host cells to suppress basal defence responses and create suitable environments for their propagation. Plants have in turn evolved disease resistance genes whose products detect the virulence factors as a signal of invasion and activate effective defence responses. Understanding how a virulence effector contributes to virulence on susceptible hosts but becomes an avirulence factor that triggers defence responses on resistance hosts has been a major focus in plant research. Recent studies have shown that a growing list of pathogen-encoded effectors functions as proteases that are secreted into plant cells to modify host proteins. In addition, several plant proteases have been found to function in activation of the defence mechanism. These findings reveal that post-translational modification of host proteins through proteolytic processing is a widely used mechanism in regulating the plant defence response.     

Identification of novel aphid-killing bacteria to protect plants

Aphids, including the peach-potato aphid, Myzus persicae, are major insect pests of agriculture and horticulture, and aphid control measures are limited. There is therefore an urgent need to develop alternative and more sustainable means of control. Recent studies have shown that environmental microbes have varying abilities to kill insects. We screened a range of environmental bacteria isolates for their abilities to kill target aphid species. Tests demonstrated the killing aptitude of these bacteria against six aphid genera (including Myzus persicae). No single bacterial strain was identified that was consistently toxic to insecticide-resistant aphid clones than susceptible clones, suggesting resistance to chemicals is not strongly correlated with bacterial challenge. Pseudomonas fluorescens PpR24 proved the most toxic to almost all aphid clones whilst exhibiting the ability to survive for over three weeks on three plant species at populations of 5–6 log CFU cm⁻² leaf. Application of PpR24 to plants immediately prior to introducing aphids onto the plants led to a 68%, 57% and 69% reduction in aphid populations, after 21 days, on Capsicum annuum, Arabidopsis thaliana and Beta vulgaris respectively. Together, these findings provide new insights into aphid susceptibility to bacterial infection with the aim of utilizing bacteria as effective biocontrol agents.     

A critical review of the developmental toxicity and teratogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin: recent advances toward understanding the mechanism

A specific teratogenic response is elicited in the mouse as a result of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin). The characteristic spectrum of structural malformations induced in mice following exposure to TCDD and structurally related congeners is highly reproducible and includes both hydronephrosis and cleft palate. In addition, prenatal exposure to TCDD has been shown to induce thymic hypoplasia. These three abnormalities occur at doses well below those producing maternal or embryo/fetal toxicity and are thus among the most sensitive indicators of dioxin toxicity. In all other laboratory species tested, TCDD causes maternal and embryo/fetal toxicity but does not induce a significant increase in the incidence of structural abnormalities even at toxic dose levels. Developmental toxicity occurs in a similar dose range across species; however, mice are particularly susceptible to development of TCDD-induced terata. Recent experiments using an organ culture were an attempt to address the issue of species and organ differences in sensitivity to TCDD. Human palatal shelves examined in this in vitro system were found to approximate the rat in terms of sensitivity for induction of cleft palate. Investigators have suggested that altered regulation of growth factors and their receptors may involve inappropriate proliferation and differentiation of target cells, ultimately producing TCDD-induced terata. Why the teratogenic effects of TCDD are so highly species and tissue specific, and which animal species most accurately predicts the response of the human embryo/fetus, at the levels of exposure experienced by humans, still remains to be clarified.     

Plant centromeres: structure and control

Recent work has led to a better understanding of the molecular components of plant centromeres. Conservation of at least some centromere protein constituents between plant and non-plant systems has been demonstrated. The identity and organization of plant centromeric DNA sequences are also beginning to yield to analysis. While there is little primary DNA sequence conservation among the characterized plant centromeres and their non-plant counterparts, some parallels in centromere genomic organisation can be seen across species. Finally, the emerging idea that centromere activity is controlled epigenetically finds support in an examination of the plant centromere literature.     

Economic plant species associated with prehistoric agriculture in the Maya lowlands

The issue of plant species used by the ancient Maya of the Yucatan region previous to A.D. 900–1,000 has involved a number of types of arguments, 5 of which are identified: ecological speculation, ethnobotany, plant relicts, linguistics/ iconography, and plant remains/fossils. Recent emphasis on uncovering and analyzing plant remains from Maya occupational and agricultural relics demonstrates that direct evidence from archaeological contexts can be obtained. This evidence, including fossil pollen, seeds, and stem and wood fragments, is used to evaluate various issues involving those species proposed to have been used by the Maya. The results support views dealing with the dominance of maize as a staple and the use of squash, agave, cotton, and tree species. Propositions concerning significance of ramón, cacao, root crops, and amaranth are not yet supported by direct evidence.     

Evolution of developmental mechanisms in plants

As our understanding of developmental mechanisms in flowering plant species has become more advanced, an appreciation of the need to understand how distinct plant morphologies are generated has grown. This has led to an awareness of the key morphological differences in distinct land plant groups and to an assessment of the major innovations that occurred during land plant evolution. Recent advances demonstrate how developmental toolkits have been recruited for related purposes in different land plant groups, but the limited number of examples highlights both the infancy of the field and the difficulty of working with non-flowering plants.     

The multidrug resistance-associated protein (MRP/ABCC) subfamily of ATP-binding cassette transporters in plants

In many different plant species, genes belonging to the multidrug resistance-associated protein (MRP, ABCC) subfamily of ABC transporters have been identified. Following the discovery of vacuolar transport systems for xenobiotic or plant-produced conjugated organic anions, plant MRPs were originally proposed to be primarily involved in the vacuolar sequestration of potentially toxic metabolites. Indeed, heterologous expression of different Arabidopsis MRPs in yeast demonstrates their activity as ATP-driven pumps for structurally diverse substrates. Recent analysis of protein-protein interactions and the characterization of knockout mutants in Arabidopsis suggests that apart from transport functions plant MRPs play additional roles including the control of plant transpiration through the stomata. Here, we review and discuss the diverse functions of plant MRP-type ABC transporters and present an organ-related and developmental analysis of the expression of Arabidopsis MRPs using the publicly available full-genome chip data.     

Most AAL toxin-sensitive Nicotiana species are resistant to the tomato fungal pathogen Alternaria alternata f. sp. lycopersici

The phytopathogenic fungus Alternaria alternata f. sp. lycopersici produces AAL toxins required to colonize susceptible tomato (Lycopersicon esculentum) plants. AAL toxins and fumonisins of the unrelated fungus Fusarium moniliforme are sphinganine-analog mycotoxins (SAMs), which are toxic for some plant species and mammalian cell lines. Insensitivity of tomato to SAMs is determined by the Alternaria stem canker gene 1 (Asc-1), and sensitivity is associated with a mutated Asc-1. We show that SAM-sensitive species occur at a low frequency in the Nicotiana genus and that candidate Asc-1 homologs are still present in those species. In Nicotiana spp., SAM-sensitivity and insensitivity also is mediated by a single codominant locus, suggesting that SAM-sensitive genotypes are host for A. alternata f. sp. lycopersici. Nicotiana umbratica plants homozygous for SAM-sensitivity are indeed susceptible to A. alternata f. sp. lycopersici. In contrast, SAM-sensitive genotypes of Nicotiana spegazzinii, Nicotiana acuminata var. acuminata, Nicotiana bonariensis, and Nicotiana langsdorffii are resistant to A. alternata f. sp. lycopersici infection concomitant with localized cell death. Additional (nonhost) resistance mechanisms to A. alternata f. sp. lycopersici that are not based on an insensitivity to SAMs are proposed to be present in Nicotiana species.     

蘑菇子实体内杀虫蛋白质的研究

许多蘑菇都对昆虫表现出毒性。为了证实与杀虫毒性有关的化合物,对14种蘑菇的毒性在水溶性、热敏性和可透析性等方面进行了研究。研究数据表明,蛋白质对大多数蘑菇子实体的杀虫活性起着重要作用,也许是一种可以用于植物抵抗害虫的基因源。在数种蛋白质中,凝集素和溶血素因不受蛋白酶的影响而成为良好的杀虫剂候选材料。     

Fungal phytotoxins: from basic studies to practical use (a review)

Recent materials are summarized, pertaining to classification of fungal phytotoxins, methods of their isolation, and assays for biological activity. Producers of phytotoxic substances have been characterized, and the chemical nature of phytotoxins has been subjected to analysis. The review gives consideration to the mechanisms of action of phytotoxins on susceptible plants and the mechanisms of plant resistance to such agents. Other matters discussed include prospects of utilizing basic knowledge of the nature and mechanisms of action of phytotoxins for developing means of plant protection against diseases and weeds and identifying or classifying fungi (chemosystematics).     

A review of the phytochemical support for the shifting defence hypothesis

Several theories have been developed to explain why invasive species are very successful and develop into pest species in their new area. The shifting defence hypothesis (SDH) argues that invasive plant species quickly evolve towards new defence levels in the invaded area because they lack their specialist herbivores but are still under attack by local (new) generalist herbivores. The SDH predicts that plants should increase their cheap, toxic defence compounds and lower their expensive digestibility reducing compounds. As a net result resources are saved that can be allocated to growth and reproduction giving these plants a competitive edge over the local plant species. We conducted a literature study to test whether toxic defence compounds in general are increased in the invaded area and if digestibility reducing compounds are lowered. We specifically studied the levels of pyrrolizidine alkaloids, a toxin which is known for its beneficial and detrimental impact against specialists and generalists, respectively. Digestibility reducers did not show a clear trend which might be due to the small number of studies and traits measured. The meta analysis showed that toxic compounds in general and pyrrolizidine alkaloid levels specifically, increased significantly in the invaded area, supporting the predictions of the SDH that a fast evolution takes place in the allocation towards defence.     

A review of fungal phytotoxins: from basic studies to practical use

Recent materials are summarized, pertaining to classification of fungal phytotoxins, methods of their isolation, and assays for biological activity. Producers of phytotoxic substances have been characterized, and the chemical nature of phytotoxins has been subjected to analysis. The review gives consideration to the mechanisms of action of phytotoxins on susceptible plants and the mechanisms of plant resistance to such agents. Other matters discussed include prospects of utilizing basic knowledge of the nature and mechanisms of action of phytotoxins for (1) developing means of plant protection against diseases and weeds and (2) identifying or classifying fungi (chemosystematics).     

Recent progress in the research of external Al detoxification in higher plants: a minireview

Aluminum (Al) is highly toxic to plant growth. The toxicity is characterized by rapid inhibition of root elongation. However, some plant species and cultivars have evolved some mechanisms for detoxifying Al both internally and externally. In this review, the recent progress made in the research of external detoxification of Al is described. Accumulating evidence has shown that organic acids play an important role in the detoxification of Al. Some plant species and cultivars respond to Al by secreting citrate, malate or oxalate from the roots. Recently, the anion channel of malate and citrate in the plasma membrane has been characterized and a gene encoding the malate channel has been cloned. The metabolism of organic acids seems to be poorly correlated with the Al-induced secretion of organic acid anions. A number of QTLs (quantitative trait loci) for Al resistance have been identified in rice, Arabidopsis, and other species. Transgenic plants with enhanced resistance to Al have also been reported, but introduction of multiple genes may be required to gain high Al resistance in future.     

Ferroxidase activity in a laccase-like multicopper oxidase from Liriodendron tulipifera.

Ferroxidase activity was detected in a laccase-like multicopper oxidase (LMCO) produced in transgenic tobacco cells expressing an LMCO cDNA (Ltlacc2.2) cloned from yellow-poplar (Liriodendron tulipifera). This marks the first report of ferroxidase activity associated with a plant laccase and suggests that some members of this plant enzyme family may have physiological functions based on activities other than their more widely recognized phenoloxidase activity. Recent work with LMCOs from bacteria, yeast and mammals has shown that metal oxidase activities in these enzymes can be important for their primary physiological functions, With respect to ferroxidase activity in certain plant LMCOs, it is proposed that the high levels of LMCO expression in plant vascular tissues may reflect the need for high-efficiency iron uptake pumps in tissues that undergo lignification during normal development. Such iron uptake pumps would function to minimize levels of free iron so that reactive oxygen species do not reach toxic levels when H2O2 is generated for peroxidase-mediated monolignol coupling during lignin deposition.     

The ecological impact of Phytophthora cinnamomi in the Stirling Range National Park,Western Australia

Abstract An assessment of the impact of the soil-borne pathogenic fungus, Phytophthora cinnamomi (Oomycetes, Peronosporales), on the plant communities of the Stirling Range National Park was carried out between December 1988 and April 1989. A total of 541 plant species were collected, with the Proteaceae and the Myrtaceae the two largest families in the study region. Of the 330 species assessed for susceptibility to P. cinnamomi, 118 (36%) were recorded as having at least some individuals in a population judged to have been killed by the fungus and 33 (10%) were highly sensitive to the pathogen (more than 80% of plants in a population killed). Several families had large numbers of susceptible species, while others were apparently unaffected by the pathogen. Notably, 85% of proteaceous species assessed were rated as susceptible to P. cinnamomi. Proteaceous elements had a mean projective foliage cover of 40% in healthy plant communities, but had a mean cover of only 10% at sites that had a long history of infestation with the fungus. In contrast, some species with low levels of susceptibility to the pathogen, such as some monocotyledons, were found to be more abundant at old-infested sites than at healthy sites. Growth form may also influence susceptibility, with herbaceous perennials, annuals and geophytes assessed in this survey apparently unaffected by the fungus whereas 48% of woody perennials surveyed were susceptible. Changes in the floristic structure of plant communities may influence the composition of associated animal communities. In particular, vertebrate flower visitors may be vulnerable since 59% of the species with vertebrate-pollinated flowers were found to be susceptible to the pathogen. This research highlights the serious ecological impact of P. cinnamomi on native plant communities and suggests that significant components of the flora and associated fauna of the southwest of Western Australia are endangered by this virulent pathogen.