Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

Using hydrocultured pea plants, 109 bacterial strains (42 from shoots) were isolated from shoots, roots, and from the hydroculture medium. 58 different strains (26 from shoots) were able to produce IAA from tryptophan, 15 different strains (7 from shoots) were able lo destroy IAA. (Included are 13 strains possessing both properties.) As far as they could be identified, the IAA-producing and -destroying strains belong to Pseudomonas (by far dominating), Achromobacter, Alcaligenses, Bacillus, and Flavobacterium. The IAA-destroying activity strongly depends on the physiological state of the bacteria and the milieu conditions. Bacterial IAA production (but not IAA-degradation) is supposed to be important for the plant.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

More “diffusible” auxin is received from nonsterile than from sterile corn coleoptile tips. An artificial reinfection of sterile coleoptiles with epiphytic, IAA-producing bacteria strains does, a superinfection of nonsterile coleoptiles does not increase the auxin amount. The difference between sterile and nonsterile tips persists if diffusion from the coleoptile surface is excluded by covering the surface with a paraffin layer. The greater the distance from the apex, the higher becomes the superiority of nonsterile tips. An artificial bacterial contamination of the contact face between tip and receiver agar block, or addition of glucose and tryptophan to the agar block, do not influence the received auxin amount. Consequently the additional, bacteria-produced auxin delivered by the nonsterile tip is not produced at the cut surface or in the agar but is present in the tissues of the coleoptile tip.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

Epiphytic, IAA-producing bacteria strains were fed with ¹⁴C-tryptophan (Try). ¹⁴C-Try absorption and, after transfer to a Try-free medium, ¹⁴C-IAA output were stated. Using 4 different methods, the ¹⁴C-Try containing bacteria were applied to the tips of sterile corn coleoptiles and the ‘diffusible’ auxin collected at the coleoptile bases by means of agar blocks. ¹⁴C-IAA was detected in the agar blocks. Sterile coleoptiles the tips of which were wupplied with ¹⁴C-Try also deliver some ¹⁴C-IAA at their bases, but much less than both sterile coleoptiles supplied with ¹⁴C-Try-containing bacteria and nonsterile supplied with ¹⁴C-Try.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

Homogenates of epicotyls or roots of nonsterile pea plants incubated with tryptophan produce IAA within 1 to 4 hours, which was detected by means of the Avena curvature test and thin layer chromatography. Three results prove this short-term IAA production to be mainly caused by epiphytic bacteria: 1) Homogenates of sterile plant parts catalyze a conversion of tryptophan to IAA, a hundredfold lower. 2) Chloramphenicol or streptomycin very actively reduce the IAA gain obtained with nonsterile homogenates. 3) Washing solutions of nonsterile plant parts which do not contain plant enzymes but only epiphytic bacteria, produce IAA from tryptophan, too. IAA synthesis from tryptophan in vitro by enzymes of the pea plant occurs with lower intensity than hitherto known; possibly it is physiologically unimportant. It is discussed to what extent the hitherto existing research work about the IAA biogenesis in higher plants might be incriminated by disregarding tbe rôle of epiphytic bacteria.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

The auxin content (extractable and ‘diffusible’ auxin) of non-sterile corn plants is much more increased by a tryptophan application than the auxin content of sterile plants. This effect is independent of the mode of tryptophan application (spray or supply with the transpiration stream). The epiphytic bacteria settling the shoot surface are responsible for this effect, since in special experiments the rhizosphere was separated from the tryptophan treatment. Sterilized plants which were artificially reinfected with epiphytic IAA-producing bacteria strains behave like non-sterile plants. Non-sterile plants which were superinfected with these bacteria strains have a still higher capacity to convert tryptophan to auxin.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metaholism

During an ether extraction (20 h, 4°C), nonsterilc pea epicotyls deliver at least 5 times more auxin than sterile ones. A great part of the additional auxin originates from a bacterial auxin production during the extraction. Presence of chloramphenicol or streptomycin during the extraction lowers the auxin amount extractable from nonsterile epicotyls. In extraction conditions (i.e. covered with ether, 20 h, 4°C), epiphytic bacteria contained in homogenates of nonsterile plant parts produce IAA from added tryptophan. Furthermore, first results are presented underlining the fact that epiphytic bacteria already produce auxin at the surface of nonsterile plant parts (before an extraction).     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism VI. The Influence of the Epiphytic Bacteria on the Content of Extractable Auxin in the Plant

Sterile plants of maize, pea, and cucumber contain less auxin (extracted with methanol or ether) than nonsterile ones. The auxin content is restored within one day by reinfecting sterile plants (or only the shoots, with roots and culture medium remaining sterile) with epiphytic bacteria strains able to produce IAA or with soaking water of nonsterile seeds. Reinfection with bacteria, strains unable to produce IAA is ineffective. — The possibility of a bacterial auxin production during methanol extraction was excluded.     

Ecochemical Studies of Interrelationships between Epiphytic Bacteria and Host Plants via Secondary Metabolites

The plant surface, which is representative of the phylloplane and rhizoplane, is a characteristic habitat for microorganisms. In this review, the ecological roles of phytoepiphytic bacteria will be described. The phylloplane and rhizoplane, which are adjacent to the atmosphere and soil sphere respectively, accumulate topically and/or selectively release secondary metabolites that are specific to the plant genera and species which reside within these regions. Some epiphytes have abilities to decarboxylate xenobiotic phenolic acids that have accumulated in the plant tissues and surfaces as a majority of such secondary metabolites. In physicochemically stressed soil, rhizosphere microflora often remedy such microenvironments within the rhizosphere in order to assist in the survival of the host, and some of the microfloral compositions behave as if they were symbionts. Specifically, some Sphingomonas spp., which are frequently isolated from the rhizosphere of acidic soil-tolerant plants in tropical zones, make possible the development of a rhizo-biocomplex. In this review, the possibility of rhizosphere regulation utilizing such a rhizo-biocomplex is discussed.     

Aerial Dispersal of Epiphytic Bacteria over Bean Plants

Plant canopies are strong sources of bacterial aerosols during sunny days when the leaves are dry. Bacterial concentration, upward flux, and deposition onto exposed petri plates were measured over snap beans during three growing seasons. A net upward flux of bacteria occurred only during the warm part of sunny days, not at night when leaves were wet with dew or when a thermal inversion was present. Aerosol source strength was positively correlated with wind speed. Upward fluxes were higher on days after rain than on days when the soil was dry. Other unidentified sources of variability in source strength probably exist. Canopy-level deposition, apparently due to intermediate-scale transport of bacteria in fairly concentrated clouds, can occur in the early evening.     

Pathways of IAA Production from Tryptophan by Plants and by Their Epiphytic Bacteria: A Comparison

Metabolites of tryptophan were investigated using 2 systems: a bacterial (Peastem homogenates containing the epiphytic bacteria) and a plant system (pea stem sections under sterile conditions). The plant system produces: indolepyruvic acid (IPyA), indoleacetaldehyde (IAAld) indoleacetic acid (IAA), indoleethanol (tryptophol, IAAol), indolecarboxylie acid (ICA), indolecarboxaldehyde (ICAld). Bacteria produce additionally: indoleactic acid (ILA), tryptamine (TNH₂) and the unknown Xb and Yb, but IAAld was not detected. A nonacidic inhibitor extract from pea stems decreases the gain of IAA, IPyA, ILA, Yb. It increases the gain of IAAld, IAAol, TNH₂, Xb, and (only in the bacterial system) ICA and ICAld. Three sites of inhibitor action are suggested, namely the steps Try → IPyA, TNH₂→ IAAld, IAAld → IAA.     

Interactions between Diatoms and Bacteria

Summary: Diatoms and bacteria have cooccurred in common habitats for hundreds of millions of years, thus fostering specific associations and interactions with global biogeochemical consequences. Diatoms are responsible for one-fifth of the photosynthesis on Earth, while bacteria remineralize a large portion of this fixed carbon in the oceans. Through their coexistence, diatoms and bacteria cycle nutrients between oxidized and reduced states, impacting bioavailability and ultimately feeding higher trophic levels. Here we present an overview of how diatoms and bacteria interact and the implications of these interactions. We emphasize that heterotrophic bacteria in the oceans that are consistently associated with diatoms are confined to two phyla. These consistent bacterial associations result from encounter mechanisms that occur within a microscale environment surrounding a diatom cell. We review signaling mechanisms that occur in this microenvironment to pave the way for specific interactions. Finally, we discuss known interactions between diatoms and bacteria and exciting new directions and research opportunities in this field. Throughout the review, we emphasize new technological advances that will help in the discovery of new interactions. Deciphering the languages of diatoms and bacteria and how they interact will inform our understanding of the role these organisms have in shaping the ocean and how these interactions may change in future oceans.     

Auxin and Plant-Microbe Interactions

Microbial synthesis of the phytohormone auxin has been known for a long time. This property is best documented for bacteria that interact with plants because bacterial auxin can cause interference with the many plant developmental processes regulated by auxin. Auxin biosynthesis in bacteria can occur via multiple pathways as has been observed in plants. There is also increasing evidence that indole-3-acetic acid (IAA), the major naturally occurring auxin, is a signaling molecule in microorganisms because IAA affects gene expression in some microorganisms. Therefore, IAA can act as a reciprocal signaling molecule in microbe-plant interactions. Interest in microbial synthesis of auxin is also increasing in yet another recently discovered property of auxin in Arabidopsis. Down-regulation of auxin signaling is part of the plant defense system against phytopathogenic bacteria. Exogenous application of auxin, e.g., produced by the pathogen, enhances susceptibility to the bacterial pathogen.The phytohormone auxin (from the Greek “auxein,” meaning to grow) regulates a whole repertoire of plant developmental processes, as documented in previous articles on this topic. Perhaps less well known is the fact that some microorganisms also produce auxin (Costacurta and Vanderleyden 1995; Patten and Glick 1996). In their interaction with plants, these microorganisms can interfere with plant development by disturbing the auxin balance in plants. This is best documented for phytopathogenic bacteria like Agrobacterium spp. and Pseudomonas savastanoi pv. savastanoi, causing tumors and galls, respectively (Jameson 2000; Mole et al. 2007), and plant growth promoting rhizobacteria (PGPR) such as Azospirillum spp. that impact on plant root development (Persello-Cartieaux et al. 2003; Spaepen et al. 2007a). The term rhizobacteria refers to the fact that their numbers are highly enriched in the rhizosphere, i.e., the narrow band of soil that surrounds the root (Hiltner 1904; Smalla et al. 2006; van Loon 2007). Of more recent date is the observation that auxin (indole-3-acetic acid or IAA) is a signaling molecule in some microorganisms (Spaepen et al. 2007a). Bringing these data together, it follows that auxin can have a major impact in microorganism-plant interactions. This is the main theme addressed in this article. Finally, the recent finding that auxin signaling in plants is also part of the Arabidopsis defense response against a leaf pathogen (Navarro et al. 2006) is discussed in relation to bacterial IAA synthesis.     

Comparison of Diversity and Functions of Epiphytic Bacteria from Cultivated and Weed Plants in Agrocenoses

Diversity of epiphytic bacterial complexes from cultivated and weed plants was compared according to the frequency of predominance of bacterial taxa in these communities and using the principal component method. The presence of both common dominants on cultivated and weed plants and distinct differences in the composition of the epiphytic bacterial complexes of these plants was revealed. The representatives of chemolithotrophic bacteria capable of tetrathionate and thiosulfate assimilation were found only on weed plants. The antibiotic activity of bacteria isolated from weed plants was almost twice as high as that found for bacteria from cultivated plants. The obtained data indicate the positive effect of bacterial communities of weed plants, which make it possible to protect cultivated plants from phytopathogens.     

Allelopathic Bacteria and Their Impact on Higher Plants

The impact of allelopathic, nonpathogenic bacteria on plant growth in natural and agricultural ecosystems is discussed. In some natural ecosystems, evidence supports the view that in the vicinity of some allelopathically active perennials (e.g., Adenostoma fasciculatum, California), in addition to allelochemicals leached from the shrub's canopy, accumulation of phytotoxic bacteria or other allelopathic microorganisms amplify retardation of annuals. In agricultural ecosystems allelopathic bacteria may evolve in areas where a single crop is grown successively, and the resulting yield decline cannot be restored by application of minerals. Transfer of soils from areas where crop suppression had been recorded into an unaffected area induced crop retardation without readily apparent symptoms of plant disease. Susceptibility of higher plants to deleterious rhizobacteria is often manifested in sandy or so-called skeletal soils. Evaluation of phytotoxic activity under controlled conditions, as well as ways to apply allelopathic bacteria in the field, is approached. The allelopathic effect may occur directly through the release of allelochemicals by a bacterium that affects susceptible plant(s) or indirectly through the suppression of an essential symbiont. The process is affected by nutritional and other environmental conditions, some may control bacterial density and the rate of production of allelochemicals. Allelopathic nonpathogenic bacteria include a wide range of genera and secrete a diverse group of plant growth-mediating allelochemicals. Although a limited number of plant growth-promoting bacterial allelochemicals have been identified, a considerable number of highly diversified growth-inhibiting allelochemicals have been isolated and characterized. Some species may produce more than one allelochemical; for example, three different phyotoxins, geldanamycin, nigericin, and hydanthocidin, were isolated from Streptomyces hygroscopicus. Efforts to introduce naturally produced allelochemicals as plant growth-regulating agents in agriculture have yielded two commercial herbicides, phosphinothricin, a product of Streptomyces viridochromogenes, and bialaphos from S. hygroscopicus. Many species of allelopathic bacteria that affect growth of higher plants are not plant specific, but some do exhibit specificity; for example, dicotyledonous plants were more susceptible to Pseudomonas putida than were monocotyledons. Differential susceptibility of higher plants to allelopathic bacteria was noted also in much lower taxonomical categories, at the subspecies level, in different cultivars of wheat, or of lettuce. Therefore, when test plants are employed to evaluate bacterial allelopathy, final evaluation must include those species that are assumed to be suppressed in nature. The release of allelochemicals from plant residues in plots of ‘continuous crop cultivation’ or from allelopathic living plants may induce the development of specific allelopathic bacteria. Both the rate by which a bacterium gains from its allelopathic activity through utilizing plant excretions, and the reasons for the developing of allelopathic bacteria in such habitats, are important goals for further research.     

植物生长素与体细胞胚发生

简要介绍了植物生长素对体细胞胚发育的作用及其调控机制的研究进展。     

Timing of the Auxin Response in Coleoptiles and Its Implications Regarding Auxin Action

The timing of the auxin response was followed in oat and corn coleoptile tissue by a sensitive optical method in which the elongation of about a dozen coleoptile segments was recorded automatically. The response possesses a latent period of about 10 min at 23°C, which is extended by low concentrations of KCN or by reducing the temperature, but is not extended by pretreatments with actinomycin D, puromycin, or cycloheximide at concentrations that partially inhibit the elongation response. Analysis of the data indicates that auxin probably does not act on the elongation of these tissues by promoting the synthesis of informational RNA or of enzymatic protein. Not excluded is the possibility that auxin acts at the translational level to induce synthesis of a structural protein, such as cell wall protein or membrane protein. While the data do not provide direct support for this hypothesis, the speed with which cycloheximide inhibits elongation suggests that continual protein synthesis may be important in the mechanism of cell wall expansion.     

Relationship between Auxin and Amino Acid Metabolism of Tobacco Protoplast-Derived Cells

Single amino acids were found to be highly toxic to protoplast-derived cells of tobacco (Nicotiana tabacum cv Xanthi) cultured at low density in a culture medium containing a low naphthaleneacetic acid concentration (0.05 micromolar). The cytotoxicities of alanine, aspartic acid, asparagine, glutamic acid, glutamine, glycine, lysine, proline, and valine were reduced when the naphthaleneacetic acid concentration of the culture medium was increased to 1 micromolar. This selective modification of amino acid toxicity by naphthaleneacetic acid could not be correlated with modifications of uptake rates or incorporation of these amino acids into protein or amino acid-auxin conjugates. A mutant clone resistant to high naphthaleneacetic acid concentrations and affected in root morphogenesis did not display, at the cellular level, the naphthaleneacetic acidmediated modification of amino acid cytotoxicity.