Search

Polyphenolic compounds produced by plants can chelate iron, reducing its bioavailability to plant-associated bacteria. In response to limited iron levels, most bacteria produce siderophores to acquire needed iron quantities. The amount of phenolic compounds detected in methanolic washings of leaves of different plant species varied greatly, being nearly sevenfold higher in Viburnum tinus than in Phaseolus vulgaris. In species with high levels of total phenolics (e.g. Pelargonium hortorum), tannin concentration of leaf washings was also high and accounted for up to 85% of total phenolics. Both stimulation of production of the siderophore pyoverdine in Pseudomonas syringae strain B728a and inhibition of growth of an isogenic mutant I-1, deficient in pyoverdine production were associated with plants harbouring high levels of leaf surface phenolics. Levels of tannic acid sufficient to inhibit growth of the pyoverdine mutant in culture in an iron-reversible fashion were similar to tannin levels found on leaves of plants such as P. hortorum. Additionally, the amount of pyoverdines produced by P. syringae and quantified in leaf washings from a variety of plants was directly related to the concentration of tannins released from the leaf, indicating that tannins were responsible for sequestering iron. Phenolic compounds, principally tannins, may thus play an important role in plant–microbe interactions.

The effect of essential oils and individual monoterpenoids on soil-borne fungi, in pure and mixed cultures, in growth media and in the soil environment, was investigated. Essential oils were extracted from lavender (Lavandula stoechas), oregano (Origanum vulgare subsp. hirtum), sage (Salvia fruticosa) and spearmint (Mentha spicata). The monoterpenoids tested were fenchone, carvacrol, 1,8-cineole, carvone, α-pinene and terpinen-4-ol. Their effect was examined on growth and sporulation of Aspergillus terreus, Fusarium oxysporum, Penicillium expansum and Verticillium dahliae isolated from an organic cultivation of tomato. All tested essential oils and individual monoterpenoids inhibited mycelial growth in all fungi and conidial production in most fungi. The strongest inhibitory activity on mycelial growth was exhibited by oregano and spearmint oils and by carvacrol and carvone, respectively their main constituents. The inhibitory activity was clearly fungistatic in A. terreus and F. oxysporum but fungicidal in V. dahliae. On sporulation, clearly stimulatory effects were observed alongside inhibitory ones. Conidial production was always promoted by α-pinene in P. expansum and by sage oil in F. oxysporum. At certain dosages it was promoted by cineole and carvone in F. oxysporum, and by lavender oil in A. terreus and V. dahliae. Experiments with carvone and carvacrol against mixed fungal cultures in a soil environment showed that V. dahliae was the most sensitive and A. terreus the most tolerant of the four fungi. Our results demonstrate strong but divergent effects and selectivity of action of the lower terpenoids on fungal strains that can become serious pests of tomato. Of special importance is the complete inhibition of growth and conidial production of V. dahliae, a pathogen otherwise very resistant to chemical control.

We investigated the effect of inorganic salt concentration on the in vitro organogenesis of spearmint (Mentha spicata L.) by using Murashige and Skoog culture medium of different strength (full, half- and quarter strength). The highest number of shoots and roots induced per explant (3.5 and 10, respectively), as well as the maximum average shoot length (16 cm) was observed on half strength medium. The maximum leaf number (35 per explant) and average root length (7 cm) were associated with regeneration on full medium. The highest average phenolic content was observed on shoot extracts (7.20 mg/g f.w.) and root extracts (5.93 mg/g f.w.) cultured onto quarter and half strength medium, respectively. The strength of the culture medium was inversely correlated with the antioxidant activity of plant extracts. Therefore, for spearmint organogenesis, half strength MS medium offers a compromise between optimum growth in vitro and antioxidant phenolic accumulation.

We studied the epiphytic bacterial communities of the summer leaves of eight perennial species naturally occurring in a Mediterranean ecosystem. The species differ in essential-oil content (from rich in essential oil to non-producers) and composition, and also in life form (from herbaceous species to tall shrubs). We compared the epiphytic bacterial communities on the basis of (i) their abundance, (ii) their metabolic profile (derived by use of the BIOLOG Ecoplate system) and (iii) richness and diversity of substrates that they use, as a measure of functional diversity. Among all species, the aromatic Melissa officinalis was the most abundantly colonized. The bacterial communities on the leaves of the aromatic Myrtus communis, Calamintha nepeta and Melissa officinalis, and also of Cistus incanus catabolized all 31 substrates offered; those on the evergreen-sclerophyllous species, Arbutus unedo and Quercus coccifera, catabolized only 14 and 17 substrates, respectively. Carbohydrates were consistently used abundantly by all communities, whereas carboxylic acids were most variably used. On average, the group of aromatic plants scored higher regarding bacterial abundance, and richness and diversity of substrates used by the bacterial communities on their leaves; the lowest values for both substrate-use indices were recorded in A. unedo. Bacterial abundance or richness or diversity of substrates used did not vary with leaf oil content. Abundance was positively correlated with both substrate-use indices. Results support claims that the antimicrobial effects of essential oils are not exerted so much under natural conditions as reports based on biassays with pathogens usually show. Although essential oils play a part in the microbial colonization of the phyllosphere, it is not likely that inhibition of phyllosphere bacteria is essential oils’ primary role, at least in the Mediterranean environment.

The scope of this work was to examine whether leaf constitutive secondary metabolites play a role in determining bacterial colonization of the phyllosphere. To this aim, we surveyed nineteen native or cultivated plant species that share a common bacterial pool in a North Mediterranean area, and estimated the size of total and ice nucleation active (INA) bacterial populations on their leaves. Large differences in the colonization of their phyllosphere were found; the population size of epiphytic bacteria ranged from 7.5 × 102 to 1 × 106 CFU/g fresh weight, in eucalypt and celery, respectively. Species native in Mediterranean-type climate areas, particularly those belonging to the group of aromatic plants, are characterized by scarce presence of INA bacteria. The antibacterial activity of essential oils, surface phenolics and leaf tissue extracts was also estimated against the INA strains P. syringae and E. herbicola, isolated from two of these plant species. E. herbicola proved more sensitive than P. syringae. Of the species examined, oregano [Origanum vulgare L. subsp. hirtum (Link.) Ietswaart], an aromatic plant, had the highest antimicrobial activity, whereas six species showed no activity at all. Further experiments were performed with oregano and bean (Phaseolus vulgaris L.) that represent two extremes in their secondary metabolite content. Both plants were inoculated with P. syringae. By the end of incubation, the bacterial population on bean plants was about 100 times higher than that on oregano leaves. Scanning electron micrographs showed that bacterial growth on oregano leaves was confined to sites away from glandular hairs. Results from the bacterial colonization survey together with those from the toxicity tests showed that all species rich in antibacterial secondary metabolites harbored low leaf bacterial populations. These results provide substantial evidence that leaf secondary metabolites function as constitutive defense chemicals against microbial invasions. However, the fact that species with non- or moderately active leaf secondary metabolites are not always highly colonized suggests mediation of other unknown factors, the contribution of which requires further investigation.

We studied the structure and diversity of the phyllosphere bacterial community of a Mediterranean ecosystem, in summer, the most stressful season in this environment. To this aim, we selected nine dominant perennial species, namely Arbutus unedo, Cistus incanus, Lavandula stoechas, Myrtus communis, Phillyrea latifolia, Pistacia lentiscus, Quercus coccifera (woody), Calamintha nepeta, and Melissa officinalis (herbaceous). We also examined the extent to which airborne bacteria resemble the epiphytic ones. Genotype composition of the leaf and airborne bacteria was analysed by using denaturing gradient gel electrophoresis profiling of a 16S rDNA gene fragment; 75 bands were cloned and sequenced corresponding to 28 taxa. Of these, two were found both in the air and the phyllosphere, eight only in the air, and the remaining 18 only in the phyllosphere. Only four taxa were found on leaves of all nine plant species. Cluster analysis showed highest similarity for the five evergreen sclerophyllous species. Aromatic plants were not grouped all together: the representatives of Lamiaceae, bearing both glandular and non-glandular trichomes, formed a separate group, whereas the aromatic and evergreen sclerophyllous M. communis was grouped with the other species of the same habit. The epiphytic communities that were the richest in bacterial taxa were those of C. nepeta and M. officinalis (Lamiaceae). Our results highlight the remarkable presence of lactic acid bacteria in the phyllosphere under the harsh conditions of the Mediterranean summer, the profound dissimilarity in the structure of bacterial communities in phyllosphere and air, and the remarkable differences of leaf microbial communities on neighbouring plants subjected to similar microbial inocula; they also point to the importance of the leaf glandular trichome in determining colonization patterns.

freezing behaviour and relative importance of heterogeneous ice nuclei in affecting supercooling of Citrus sinensis fruits were studied. The size of an ice nucleation active (INA) Pseudomonas syringae pool inhabiting fruits was positively correlated with the nucleation temperature (NT) of the plant tissue, with a mean of 3-69 log colony forming units (CFU) fruit"1 at NT~$> —2-5 °C. The INA bacterial pool was responsible for «23% of the nucleation events occurring at this temperature, and «29% were attributed to an additional nucleating source. The latter was sensitive to bacterial ice nucleation inhibitors, it occupied a different microniche from that of P. syringae nuclei, yet was neither a fungus nor any of the bacterial strains ever reported as active. Treatment with an ice nucleation inactive bacterium antagonistic to INA bacteria, a lentil lectin, a protease, and guanidine reduced mean nucleation temperatures (MNT) of fruits to ^ —2-58, —2-66, —4-21, and —4-52 °C, respectively, compared to a MNT^; — 1-67 CC for the controls. Thus, the citrus-associated nucleator apparently contained active proteinaceous components but was void of carbohydrate-like groups reportedly encountered at or near the bacterial ice nucleating site. Despite the different origins of citrus nuclei, bacterial nucleation inhibitors reduced nucleation in the field by 50% at NT^ —2-5 °C, an effect probably exerted through the proteinaceous site possessed by both nuclei.

Elm and black locust samples were collected in the city of Thessaloniki, Greece from a number of locations with different pollutant concentrations and the hormonal content was determined using radioimmunoassay tests. In both tree species, a considerable decrease in endogenous cytokinins was detected in symptomless leaves from the polluted city center, suggesting latent senescence and thus latent injury. Similarly, cytokinin content was found to be lower in seedlings exposed to SO2‐O3 fumigation than in controls. Thus cytokinin levels may serve as useful indicators of the onset of pollution stress and/or of the degree of stress experienced by urban trees. Several environmental stresses were reported to promote an ABA increase. An increased ABA content in black locust samples from the polluted city center was also detected. This observation did not hold in elm, where comparable ABA levels were detected in trees in the city center, the suburbs, and in controls grown in filtered air. Hence ABA content cannot ...

The frost sensitivity of Citrus sinensis in relation to the presence of biogenic ice nuclei was studied. In commercially managed citrus groves the ice nucleation active (INA) bacterium Pseudomonas syringae reached 6 × 104 colony forming units (CFU) leaf−1, a population sufficiently high to catalyze ice formation. However, a transient loss of bacterial nucleation activity was noticeable at subzero field temperatures, followed by resumption as temperatures rose. This loss was apparently due to a temporary transition of INA to ice nucleation inactive (INI) bacteria. Field application of Bordeaux mixture, copper hydroxide, streptomycin, and 2-hydroxypropylmethanethiolsulfonate (HPMTS), resulted in reduction of INA bacterial populations to detectability (≤ 102 CFU leaf−1) limits. However, the corresponding reduction in ice nucleation events in treated samples as compared to controls at nucleation temperature ≥−3°C was not as dramatic. It ranged from approximately 7% in samples treated with the bactericide HPMTS, to 35% in samples treated with chemicals possessing combined bactericidal - fungicidal action (coppers). Since a quantitative relationship exists between ice nucleation events on individual leaves and the INA bacterial populations harbored by these leaves, these results suggest the co-existence of a bacterial and a proteinaceous, yet non-bacterial ice nucleating source in citrus, both active at ≥−3°C.

Effects of water stress on phenology, growth, stomatal activity and water status were assessed from April to November 1996 in 2-year-old seedlings of Quercus frainetto Ten. (Quercus conferta Kit.), Quercus pubescens Willd., Quercus macrolepis Kotschy (Quercus aegilops auct.) and Quercus ilex L. growing in containers in northern Greece. All four species developed more than 50% of their total leaf area before the beginning of June--an adaptation to arid climates. Well-irrigated plants tended to develop greater individual leaf area, number of leaves per plant, total plant leaf area, height and root:shoot ratios than water-stressed plants, but the difference between treatments was not significant for any parameter in any species. Quercus macrolepis appeared to be the most drought-tolerant of the four species. It maintained the highest number of leaves of the smallest size and increased the proportion of fine roots during drought. In all species, drought caused significant decreases in stomatal conductance and predawn and midday water potentials from mid-July until the end of August, when the lowest soil water content and highest mean daily air temperatures and midday leaf temperatures occurred; however, the responses were species-specific. Among the four species, Quercus macrolepis sustained the highest stomatal conductance despite very low water potentials, thus overcoming drought by means of desiccation tolerance. Quercus ilex decreased stomatal conductance even before severe water stress occurred, thereby avoiding desication during drought. Quercus pubescens had the highest water potential despite a high stomatal conductance, indicating that its leaf water status was independent of stomatal activity. Quercus frainetto was the least drought-resistant of the four species. During drought it developed very low water potentials despite markedly reduced stomatal aperture.

A strong net downward flux of bacteria was measured in a soybean field during two different rain events. The ice nucleation-activity and phytopathogenic host range of strains of Pseudomonas syringae isolated from rainwater and aerosols present during rain differed from those of the bacteria present on the soybeans. Bacteria that produce fluorescent pigments on an iron-deficient medium were 3-8%, 5-9%, and 81% of those isolated from aerosols during rain, from rain, and from the soybean leaves, respectively (...)

The effects of SO2 (2 ppm for 6 h), O3 (0.9 ppm for 5 h), and SO2–O3 mixtures (2 ppm SO2 and 0.9 ppm O3 for 5 h followed by SO2 for 1 h) were studied on injury and growth of 4-month-old, actively growing and quiescent Ulmus americana L. seedlings. In actively growing seedlings visible injury to leaves from SO2–O3 mixtures was evident within 24 h; from O3 within 36 to 48 h; and from SO2 within 48 h after fumigation ceased. Sulfur dioxide and SO2–O3 treatments reduced expansion of new leaves ( 1 cm long at time of fumigation) was markedly inhibited by all treatments; it was inhibited most by SO2–O3, an intermediate amount by SO2, and least by O3. Numbers of emerging leaves were significantly reduced by SO2, and by SO2–O3, but only at the end of the 1st week. After 5 weeks, stem dry weights were reduced by O3 and SO2–O3, and root dry weights were reduced by SO2 and SO2–O3. In quiescent plants pollution injury to leaves was similar to that in actively growing plants. However, all fumigation treatments induced severe defoliation in quiescent plants.

The effects of SO2 (2 ppm for 6 h), O3 (0.9 ppm for 5 h), and SO2–O3 mixtures (2 ppm SO2 and 0.9 ppm O3 for 5 h followed by SO2 for 1 h) were studied on metabolite contents of 4-month-old, actively growing and quiescent Ulmus americana L. seedlings. Effects on metabolite contents in leaves, stems, and roots were examined 24 h, 1 week, and 5 weeks after exposure to pollutants for actively growing seedlings, and 24 h and 1 week for quiescent seedlings. Fumigations caused significant reduction in total nonstructural carbohydrate and protein contents of actively growing seedlings within 24 h after fumigation. By the end of 5 weeks, however, carbohydrate and protein contents of new ( 1 cm long) of actively growing seedlings persisted throughout the 5-week period. Changes in lipid contents of actively growing seedlings were small and in most cases not significant. Exposure to pollutants decreased lipid contents of quiescent seedlings (up to 28%); this decrease was much lower than reduction in carbohydrates or proteins (up to approximately 65%). Metabolite contents of both actively growing and quiescent seedlings were reduced most by SO2–O3, an intermediate amount by SO2, and least by O3 treatment.