Effect of leaf surface waxes on leaf colonization by Pantoea agglomerans and Clavibacter michiganensis

Application of Acinetobacter indicus to promote cigarette smoke particulate matter phytoremediation: removal efficiency and plant-microbe interactions

Particulate matter (PM) is one of the most hazardous atmospheric pollutants. Several plant species show high potential to reduce air pollutants and are widely used as green belts to provide clean outdoor spaces for human well-being. However, high PM concentrations cause physiological changes and stress in plants. In this study, 11 species of Thai native perennial plants were exposed to PM generated from tobacco smoke. Wrightia religiosa (Teijsm. & Binn.) Benth. ex Kurz, Bauhinia purpurea DC. ex Walp. and Tectona grandis L.f. reduced PM effectively (which is in the typical range of 43.95 to 52.97%) compared to other plant species. In addition, the responses of perennial plants under PM stress at the proteomic level were also evaluated. Proteomic analysis of these three plant species showed that plants respond negatively to high PM concentrations, such as reducing several photosynthetic-related proteins and increasing plant stress response proteins. To improve PM phytoremediation efficiency and reduce plant stress from PM, perennial plant-microbe interactions were investigated. W. religiosa was inoculated with Acinetobacter indicus PS1, and high biosurfactant-producing strains clearly showed a higher PM removal efficiency than non-inoculated plants (9.48, 9.5 and 12.6% for PM1.0, PM2.5 and PM10, respectively). Inoculating W. religiosa with A. indicus PS1 maintained chlorophyll a and b concentrations. Moreover, the malondialdehyde (MDA) concentration of W. religiosa inoculated with A. indicus PS1 was lower than that of non-inoculated W. religiosa. The leaf wax content (µg/cm2) and biosurfactant (µg/cm2) of W. religiosa inoculated with A. indicus PS1 were also higher than those of non-inoculated W. religiosa. This study clearly showed that inoculating plants with A. indicus PS1 can help plants remediate PM and improve their PM stress response.

Project ChemicalBlooms: Collaborating with citizen scientists to survey the chemical diversity and phylogenetic distribution of plant epicuticular wax blooms

Plants use chemistry to overcome diverse challenges. A particularly striking chemical trait that some plants possess is the ability to synthesize massive amounts of epicuticular wax that accumulates on the plant's surfaces as a white coating visible to the naked eye. The ability to synthesize basic wax molecules appears to be shared among virtually all land plants, and our knowledge of ubiquitous wax compound synthesis is reasonably advanced. However, the ability to synthesize thick layers of visible epicuticular crystals ("wax blooms") is restricted to specific lineages, and our knowledge of how wax blooms differ from ubiquitous wax layers is less developed. Here, we recruited the help of citizen scientists and middle school students to survey the wax bloom chemistry of 78 species spanning dicot, monocot, and gymnosperm lineages. Using gas chromatography-mass spectrometry, we found that the major wax classes reported from bulk wax mixtures can be present in wax bloom crystals, with fatty acids, fatty alcohols, and alkanes being present in many species' bloom crystals. In contrast, other compounds including aldehydes, ketones, secondary alcohols, and triterpenoids were present in only a few species' wax bloom crystals. By mapping the 78 wax bloom chemical profiles onto a phylogeny and using phylogenetic comparative analyses, we found that secondary alcohol and triterpenoid-rich wax blooms were present in lineage-specific patterns that would not be expected to arise by chance. That finding is consistent with reports that secondary alcohol biosynthesis enzymes are found only in certain lineages but was a surprise for triterpenoids, which are intracellular components in virtually all plant lineages. Thus, our data suggest that a lineage-specific mechanism other than biosynthesis exists that enables select species to generate triterpenoid-rich surface wax crystals. Overall, our study outlines a general mode in which research scientists can collaborate with citizen scientists as well as middle and high school classrooms not only to enhance data collection and generate testable hypotheses but also to directly involve classrooms in the scientific process and inspire future STEM workers.

Plant cultivar determined bacterial community and potential risk of antibiotic resistance gene spread in the phyllosphere

The global increased antibiotic resistance level in pathogenic microbes has posed a significant threat to human health. Fresh vegetables have been recognized to be an important vehicle of antibiotic resistance genes (ARGs) from environments to human beings. Phyllosphere ARGs have been indicated to be changed with plant species, yet the influence of plant cultivar on the phyllospheric resistome is still unclear. Here, we detected the ARGs and bacterial communities in the phyllosphere of two cultivars of cilantros and their corresponding soils using high-throughput quantitative PCR technique and bacterial 16S rRNA gene-based high-throughput sequencing, respectively. We further identified the potential bacterial pathogens and analyzed the effects of plant cultivar on ARGs, mobile genetic elements (MGEs), microbiome and potential bacterial pathogens. The results showed that the cultivars did not affect the ARG abundance and composition, but significantly shaped the abundance of MGEs and the composition structure of bacteria in the phyllosphere. The relative abundance of potential bacterial pathogens was significantly higher in the phyllosphere than that in soils. Mantel test showed that the ARG patterns were significantly correlated to the patterns of potential bacterial pathogens. Our results suggested that the horizontal gene transfer of ARGs in the phyllosphere might be different between the two cultivars of cilantro and highlighted the higher risk of phyllospheric microorganisms compared with those in soils. These findings extend our knowledge on the vegetable microbiomes, ARGs, and potential pathogens, suggesting more agricultural and hygiene protocols are needed to control the risk of foodborne ARGs.

Effects of Domestication on Plant-Microbiome Interactions

Through the process of domestication, selection is targeted on a limited number of plant traits that are typically associated with yield. As an unintended consequence, domesticated plants often perform poorly compared to their wild progenitors for a multitude of traits that were not under selection during domestication, including abiotic and biotic stress tolerance. Over the past decade, advances in sequencing technology have allowed for the rigorous characterization of host-associated microbial communities, termed the microbiome. It is now clear that nearly every conceivable plant interaction with the environment is mediated by interactions with the microbiome. For this reason, plant-microbiome interactions are an area of great promise for plant breeding and crop improvement. Here, we review the literature to assess the potential impact that domestication has had on plant-microbiome interactions and the current understanding of the genetic basis of microbiome variation to inform plant breeding efforts. Overall, we find limited evidence that domestication impacts the diversity of microbiomes, but domestication is often associated with shifts in the abundance and composition of microbial communities, including taxa of known functional significance. Moreover, genome-wide association studies and mutant analysis have not revealed a consistent set of core candidate genes or genetic pathways that confer variation in microbiomes across systems. However, such studies do implicate a consistent role for plant immunity, root traits, root and leaf exudates and cell wall integrity as key traits that control microbiome colonization and assembly. Therefore, selection on these key traits may pose the most immediate promise for enhancing plant-microbiome interactions through breeding.

Epicuticular wax of sweet sorghum influenced the microbial community and fermentation quality of silage

Epicuticular wax, as secondary metabolites (hydrophobic compounds) covering plant surface, plays important roles in protecting plants from abiotic and biotic stresses. However, whether these compounds will influence fermentation process of silage is still not clear. In this study, two sweet sorghum cultivars with varying epicuticular wax on sheath (bloom), Yajin 2 (YJ, less bloom), and Jintian (JT, dense bloom), were harvested at flowering and maturing stages, and ensiled with or without bloom, aiming to evaluate the effects of bloom on fermentation quality, feed nutrition and microbial community. The bloom was collected manually with de-waxed cotton and extracted with chloroform. The results showed that the bloom reduced the concentrations of water-soluble carbohydrate and crude protein of the two cultivars at both stages, reduced lactic acid (LA) for YJ at both stages and for JT at flowering stage, and increased LA for JT at mature stage. The α-diversity of bacterial communities of the silage fermentation with bloom was significantly lower than that without bloom. Bloom increased the abundance of Lactobacillus, reduced that of Bacillus and Weissella, and significant correlations were observed between fermentative qualities and bacterial abundances. However, decreased diversity of bacterial community and the contents of LA implied that shifts in bacterial community might exert negative effects on silage fermentation. Our results suggest that bloom wax could alter the microbial community composition of ensiled sweet sorghums, which thus influence the fermentation qualities.

Interaction of surfactants with barley leaf surfaces: time-dependent recovery of contact angles is due to foliar uptake of surfactants

Time-dependent contact angle measurements of pure water on barley leaf surfaces allow quantifying the kinetics of surfactant diffusion into the leaf. Barley leaf surfaces were sprayed with three different aqueous concentrations (0.1, 1.0 and 10%) of a monodisperse (tetraethylene glycol monododecyl ether) and a polydisperse alcohol ethoxylate (BrijL4). After 10 min, the surfactant solutions on the leaf surfaces were dry leading to surfactant coverages of 1, 10 and 63 µg cm^-2, respectively. The highest surfactant coverage (63 µg cm^-2) affected leaf physiology (photosynthesis and water loss) rapidly and irreversibly and leaves were dying within 2-6 h. These effects on leaf physiology did not occur with the lower surfactant coverages (1 and 10 µg cm^-2). Directly after spraying of 0.1 and 1.0% surfactant solution and complete drying (10 min), leaf surfaces were fully wettable for pure water and contact angles were 0°. Within 60 min (0.1% surfactant) and 6 h (1.0% surfactant), leaf surfaces were non-wettable again and contact angles of pure water were identical to control leaves. Scanning electron microscopy investigations directly performed after surfactant spraying and drying indicated that leaf surface wax crystallites were partially or fully covered by surfactants. Wax platelets with unaltered microstructure were fully visible again within 2 to 6 h after treatment with 0.1% surfactant solutions. Gas chromatographic analysis showed that surfactant amounts on leaf surfaces continuously disappeared over time. Our results indicate that surfactants, applied at realistic coverages between 1 and 10 µg cm^-2 to barley leaf surfaces, leading to total wetting (contact angles of 0°) of leaf surfaces, are rapidly taken up by the leaves. As a consequence, leaf surface non-wettability is fully reappearing. An irreversible damage of the leaf surface fine structure leading to enhanced wetting and increased foliar transpiration seems highly unlikely at low surfactant coverages of 1 µg cm^-2.

Dining in Blue Light Impairs the Appetite of Some Leaf Epiphytes

Background: The phyllosphere is subjected to fluctuating abiotic conditions. This study examined the phenotypic plasticity (PP) of four selected non-phototrophic phyllosphere bacteria [control strain: Pseudomonas sp. DR 5-09; Pseudomonas agarici, Bacillus thuringiensis serovar israeliensis (Bti), and Streptomyces griseoviridis (SG)] regarding their respiration patterns and surfactant activity as affected by light spectrum and nutrient supply.

Methods: The PP of the strains was examined under four light regimes [darkness (control); monochromatic light-emitting diodes (LED) at 460 nm (blue) and 660 nm (red); continuously polychromatic white LEDs], in the presence of 379 substrates and conditions.

Results: Light treatment affected the studied bacterial strains regarding substrate utilization (Pseudomonas strains > SG > Bti). Blue LEDs provoked the most pronounced impact on the phenotypic reaction norms of the Pseudomonas strains and Bti. The two Gram-positive strains Bti and SG, respectively, revealed inconsistent biosurfactant formation in all cases. Biosurfactant formation by both Pseudomonas strains was supported by most substrates incubated in darkness, and blue LED exposure altered the surface activity profoundly. Blue and white LEDs enhanced biofilm formation in PA in highly utilized C-sources. Putative blue light receptor proteins were found in both Pseudomonas strains, showing 91% similarity with the sequence from NCBI accession number WP_064119393.

Conclusion: Light quality–nutrient interactions affect biosurfactant activity and biofilm formation of some non-phototrophic phyllosphere bacteria and are, thus, crucial for dynamics of the phyllosphere microbiome.

Caterpillar gut and host plant phylloplane mycobiomes differ: a new perspective on fungal involvement in insect guts

Compared with the highly diverse microbiota of leaves, herbivorous insects exhibit impoverished gut microbial communities. Research to date has focused on the bacterial component of these gut microbiomes, neglecting the fungal component. As caterpillar gut bacterial microbiomes are derived strongly from their diet, we hypothesized that their mycobiomes would reflect the host leaf mycobiomes. Using the ITS2 rDNA and V5-V6 16S rRNA gene regions for DNA metabarcoding of caterpillar gut and host leaf sample pairs we compared their mycobiome genus diversity and compositions and identified genera associated with caterpillar guts. Leaves and caterpillar guts harbored different mycobiomes with quite low qualitative similarity (Jaccard index = 38.03%). The fungal genera most significantly associated with the caterpillar gut included Penicillium, Mucor and unidentified Saccharomycetales, whereas leaf-associated genera included Holtermanniella, Gibberella (teleomorph of Fusarium) and Seimatosporium. Although caterpillar gut and leaf mycobiomes had similar genus richness overall, this indicator was not correlated for individual duplets. Moreover, as more samples entered the analysis, mycobiome richness increased more rapidly in caterpillar guts than in leaves. The results suggest that the mycobiota of the caterpillar gut differs from that of their feeding substrate; further, the mycobiomes appear to be richer than the well-studied bacterial microbiotas.

Adhesion and removal of E. coli K12 as affected by leafy green produce epicuticular wax composition, surface roughness, produce and bacterial surface hydrophobicity, and sanitizers

Contaminated leafy vegetables have been associated with high-profile outbreaks causing severe illnesses. A good understanding of the interactions between human pathogen and produce is important for developing improved food safety control strategies. Currently, the role played by produce surface physiochemical characteristics in such interactions is not well-understood. This work was performed to examine the effects of produce physiochemical characteristics, including surface roughness, epicuticular wax composition, and produce and bacteria surface hydrophobicity on attachment and removal of vegetative bacteria. Escherichia coli K12 was used as a model microorganism to evaluate attachment to and removal from five leafy green vegetables after washing with selected sanitizers. A detailed epicuticular wax component analysis was conducted and the changes of wax composition after sanitation were also evaluated. The results showed that E. coli K12 removal is positively correlated with alkanes, ketones, and total wax content on leaf surfaces. Vegetables with high surface wax content had less rough leaf surfaces and more bacterial removal than the low wax produce. Produce surface roughness positively correlated to E. coli K12 adhesion and negatively correlated to removal. The cells preferentially attached to cut vegetable surfaces, with up to 1.49 times more attachment than on leaf adaxial surfaces.

Different Responses in Mandarin Cultivars Uncover a Role of Cuticular Waxes in the Resistance to Citrus Canker

'Okitsu' is a mandarin cultivar showing substantial resistance to X. citri subsp. citri (X. citri). We have previously shown that this cultivar has significantly lower canker incidence and severity than 'Clemenules', particularly during early stages of leaf development in the field. This differential response is only seen when the leaves are inoculated by spraying, suggesting that leaf surface contributes to resistance. In this work, we have studied structural and chemical properties of leaf surface barriers of both cultivars. Ultrastructural analysis showed a thicker cuticle covering epidermal surface and guard cells in young 'Okitsu' leaves than in 'Clemenules'. This thicker cuticle was associated with a smaller stomatal aperture and reduced cuticle permeability. These findings correlated with an accumulation of cuticular wax components, including primary alcohols, alkanes, and fatty acids. None of these differences were observed in mature leaves, where both cultivars are equally resistant to the bacterium. Remarkably, mechanical alteration of cuticular thickness of young 'Okitsu' leaves allows canker development. Furthermore, cuticular waxes extracted from young 'Okitsu' leaves have higher antibacterial activity against X. citri than 'Clemenules'. Taken together, these data suggest that a faster development of epicuticular waxes in 'Okitsu' leaves play a central role in its resistance to X. citri.

Microbial colonization on the leaf surfaces of different genotypes of Napier grass

To address correlations between population sizes of microbes on the leaf surfaces and leaf morphological and physicochemical properties, various leaf morphological and physicochemical features as possible predictors of microbial population sizes on the leaf surfaces of four Napier grass cultivars were assessed. Results indicated microbes except for lactic acid bacteria (LAB) preferred to colonize the leaf surfaces bearing trichomes, and their population sizes were significantly correlated with trichomes, especially for yeasts. The population sizes of microbes were positively correlated with soluble sugar content (p < 0.05). Furthermore, no significant correlation was found between population sizes of microbes and wax content, except for yeasts. The multivariate regression trees (MRT) analysis showed different genotypes of leaf-microbe system could be characterized by four-leaf attributes with soluble sugar of leaf tissues being the primary explanatory attribute. Leaves with soluble sugar content below 9.72 mg g^-1 fresh weight (FW) were rarely colonized. For leaves with soluble sugar content above 9.72 mg g^-1 FW, water content was the next explanatory leaf attribute, followed by wax content on the leaf surfaces. Leaves with higher water content (> 73%) were more colonized, and small microbial population was associated with higher wax content (> 10.66 mg g^-1 dry matter). In conclusion, leaf chemical attributes have a higher contribution than morphological structure properties in determining population sizes of microbes on the leaf surfaces. The exuded soluble sugar and protein promote the development of microbial populations. For different genotypes of leaf-microbe system, the relationship between microbial abundance on their leaf surfaces and leaf morphological structure or physicochemical properties may be predicted by the MRT. Population sizes of microbes are primarily influenced by soluble sugar content under the water-rich conditions.

Formation of Escherichia coli O157:H7 Persister Cells in the Lettuce Phyllosphere and Application of Differential Equation Models To Predict Their Prevalence on Lettuce Plants in the Field

Escherichia coli O157:H7 (EcO157) infections have been recurrently associated with produce. The physiological state of EcO157 cells surviving the many stresses encountered on plants is poorly understood. EcO157 populations on plants in the field generally follow a biphasic decay in which small subpopulations survive over longer periods of time. We hypothesized that these subpopulations include persister cells, known as cells in a transient dormant state that arise through phenotypic variation in a clonal population. Using three experimental regimes (with growing, stationary at carrying capacity, and decaying populations), we measured the persister cell fractions in culturable EcO157 populations after inoculation onto lettuce plants in the laboratory. The greatest average persister cell fractions on the leaves within each regime were 0.015, 0.095, and 0.221%, respectively. The declining EcO157 populations on plants incubated under dry conditions showed the largest increase in the persister fraction (46.9-fold). Differential equation models were built to describe the average temporal dynamics of EcO157 normal and persister cell populations after inoculation onto plants maintained under low relative humidity, resulting in switch rates from a normal cell to a persister cell of 7.7 × 10-6 to 2.8 × 10-5 h-1 Applying our model equations from the decay regime, we estimated model parameters for four published field trials of EcO157 survival on lettuce and obtained switch rates similar to those obtained in our study. Hence, our model has relevance to the survival of this human pathogen on lettuce plants in the field. Given the low metabolic state of persister cells, which may protect them from sanitization treatments, these cells are important to consider in the microbial decontamination of produce.IMPORTANCE Despite causing outbreaks of foodborne illness linked to lettuce consumption, E. coli O157:H7 (EcO157) declines rapidly when applied onto plants in the field, and few cells survive over prolonged periods of time. We hypothesized that these cells are persisters, which are in a dormant state and which arise naturally in bacterial populations. When lettuce plants were inoculated with EcO157 in the laboratory, the greatest persister fraction in the population was observed during population decline on dry leaf surfaces. Using mathematical modeling, we calculated the switch rate from an EcO157 normal to persister cell on dry lettuce plants based on our laboratory data. The model was applied to published studies in which lettuce was inoculated with EcO157 in the field, and switch rates similar to those obtained in our study were obtained. Our results contribute important new knowledge about the physiology of this virulent pathogen on plants to be considered to enhance produce safety.

Evaluation of the Foliar Damage That Threatens a Millennial-Age Tree, Araucaria araucana (Molina) K. Koch, Using Leaf Waxes

A. araucana is an endemic species of the temperate forests from Chile and Argentina; protected in both countries and categorized as in danger of extinction. Individuals of this species have begun to show foliar damage (i.e., discoloration) in branches and upper parts. The discoloration begins from the base to the top and from the trunk to the branches with necrotic rings appearing; in some cases causing death; and is currently attributed to an as yet unknown disease. This study focuses on the first protective layer of plants against environmental stress and pathogens; known as leaf waxes. The abundance and distribution of three classes of leaf waxes (long chain fatty acids; alkanes and alcohols) were measured in healthy individuals of A. araucana from different sites and individuals that present foliar damage (sick individuals). In the case of sick individuals; their leaf waxes were measured considering the level of leaf damage; that is; leaves without; medium and full foliar damage. The most abundant class of leaf wax in both sick and healthy individuals was fatty acids; followed by alkanes and then alcohols; with common dominant chains; C28 fatty acid; C29 alkane and C24 alcohol. Sick individuals have higher abundances of alkanes and alcohols than healthy individuals. The leaves of sick individuals have lower values of distribution indices (the carbon preference index of fatty acids and average chain length of alkanes) as foliar damage increases that are interpreted as a reduction of in vivo biosynthesis of waxes. This is the first evidence of A. araucana response to a still unknown disease that is killing individuals of this endemic species.

Disordered wax platelets on Tradescantia pallida leaves create golden shine

Disordered arrangement of wax platelets on Tradescantia leaves increase long wavelength reflectance, contrary to the commonly observed UV-protection mechanism.

Breeding Crops for Enhanced Food Safety

An increasing global population demands a continuous supply of nutritious and safe food. Edible products can be contaminated with biological (e.g., bacteria, virus, protozoa), chemical (e.g., heavy metals, mycotoxins), and physical hazards during production, storage, transport, processing, and/or meal preparation. The substantial impact of foodborne disease outbreaks on public health and the economy has led to multidisciplinary research aimed to understand the biology underlying the different contamination processes and how to mitigate food hazards. Here we review the knowledge, opportunities, and challenges of plant breeding as a tool to enhance the food safety of plant-based food products. First, we discuss the significant effect of plant genotypic and phenotypic variation in the contamination of plants by heavy metals, mycotoxin-producing fungi, and human pathogenic bacteria. In addition, we discuss the various factors (i.e., temperature, relative humidity, soil, microbiota, cultural practices, and plant developmental stage) that can influence the interaction between plant genetic diversity and contaminant. This exposes the necessity of a multidisciplinary approach to understand plant genotype × environment × microbe × management interactions. Moreover, we show that the numerous possibilities of crop/hazard combinations make the definition and identification of high-risk pairs, such as Salmonella-tomato and Escherichia coli-lettuce, imperative for breeding programs geared toward improving microbial safety of produce. Finally, we discuss research on developing effective assays and approaches for selecting desirable breeding germplasm. Overall, it is recognized that although breeding programs for some human pathogen/toxin systems are ongoing (e.g., Fusarium in wheat), it would be premature to start breeding when targets and testing systems are not well defined. Nevertheless, current research is paving the way toward this goal and this review highlights advances in the field and critical points for the success of this initiative that were discussed during the Breeding Crops for Enhanced Food Safety workshop held 5-6 June 2019 at University of California, Davis.

Light spectrum modifies the utilization pattern of energy sources in Pseudomonas sp. DR 5-09

Despite the overruling impact of light in the phyllosphere, little is known regarding the influence of light spectra on non-phototrophic bacteria colonizing the leaf surface. We developed an in vitro method to study phenotypic profile responses of bacterial pure cultures to different bands of the visible light spectrum using monochromatic (blue: 460 nm; red: 660 nm) and polychromatic (white: 350–990 nm) LEDs, by modification and optimization of a protocol for the Phenotype MicroArray^™ technique (Biolog Inc., CA, USA). The new protocol revealed high reproducibility of substrate utilization under all conditions tested. Challenging the non-phototrophic bacterium Pseudomonas sp. DR 5–09 with white, blue, and red light demonstrated that all light treatments affected the respiratory profile differently, with blue LED having the most decisive impact on substrate utilization by impairing respiration of 140 substrates. The respiratory activity was decreased on 23 and 42 substrates under red and white LEDs, respectively, while utilization of one, 16, and 20 substrates increased in the presence of red, blue, and white LEDs, respectively. Interestingly, on four substrates contrasting utilization patterns were found when the bacterium was exposed to different light spectra. Although non-phototrophic bacteria do not rely directly on light as an energy source, Pseudomonas sp. DR 5–09 changed its respiratory activity on various substrates differently when exposed to different lights. Thus, ability to sense and distinguish between different wavelengths even within the visible light spectrum must exist, and leads to differential regulation of substrate usage. With these results, we hypothesize that different light spectra might be a hitherto neglected key stimulus for changes in microbial lifestyle and habits of substrate usage by non-phototrophic phyllospheric microbiota, and thus might essentially stratify leaf microbiota composition and diversity.

The intimate talk between plants and microorganisms at the leaf surface

The plant epidermis or cuticle is constantly exposed to external and internal environmental factors, including an enriched and diverse community of bacteria, yeast, fungi, viruses, and mites. It is not only where the plant has its first physical barrier, but also where organisms can be recognized and potentially where the plant defense responses can be triggered. The plant cuticle is a polymeric composite formed by an array of structurally and chemically heterogeneous compounds, including cutin and wax. A few studies have shown that cuticular components are essential and important drivers of the structure and size of the bacterial community. On the other hand, cuticular components are also important for both pathogens and plants, to initiate the pre-invasion and infection process and to activate the innate immune response, respectively. In this review, we explore current knowledge on the role of the cuticle during the intimate interactions between plants and microorganisms, in particular pathogenic and non-pathogenic bacteria and fungi. Finally, we propose new perspectives on the potential use of this information for agriculture.

Impact of Combined Abiotic and Biotic Stresses on Plant Growth and Avenues for Crop Improvement by Exploiting Physio-morphological Traits

Global warming leads to the concurrence of a number of abiotic and biotic stresses, thus affecting agricultural productivity. Occurrence of abiotic stresses can alter plant-pest interactions by enhancing host plant susceptibility to pathogenic organisms, insects, and by reducing competitive ability with weeds. On the contrary, some pests may alter plant response to abiotic stress factors. Therefore, systematic studies are pivotal to understand the effect of concurrent abiotic and biotic stress conditions on crop productivity. However, to date, a collective database on the occurrence of various stress combinations in agriculturally prominent areas is not available. This review attempts to assemble published information on this topic, with a particular focus on the impact of combined drought and pathogen stresses on crop productivity. In doing so, this review highlights some agriculturally important morpho-physiological traits that can be utilized to identify genotypes with combined stress tolerance. In addition, this review outlines potential role of recent genomic tools in deciphering combined stress tolerance in plants. This review will, therefore, be helpful for agronomists and field pathologists in assessing the impact of the interactions between drought and plant-pathogens on crop performance. Further, the review will be helpful for physiologists and molecular biologists to design agronomically relevant strategies for the development of broad spectrum stress tolerant crops.

Impact of Combined Abiotic and Biotic Stresses on Plant Growth and Avenues for Crop Improvement by Exploiting Physio-morphological Traits

Global warming leads to the concurrence of a number of abiotic and biotic stresses, thus affecting agricultural productivity. Occurrence of abiotic stresses can alter plant-pest interactions by enhancing host plant susceptibility to pathogenic organisms, insects, and by reducing competitive ability with weeds. On the contrary, some pests may alter plant response to abiotic stress factors. Therefore, systematic studies are pivotal to understand the effect of concurrent abiotic and biotic stress conditions on crop productivity. However, to date, a collective database on the occurrence of various stress combinations in agriculturally prominent areas is not available. This review attempts to assemble published information on this topic, with a particular focus on the impact of combined drought and pathogen stresses on crop productivity. In doing so, this review highlights some agriculturally important morpho-physiological traits that can be utilized to identify genotypes with combined stress tolerance. In addition, this review outlines potential role of recent genomic tools in deciphering combined stress tolerance in plants. This review will, therefore, be helpful for agronomists and field pathologists in assessing the impact of the interactions between drought and plant-pathogens on crop performance. Further, the review will be helpful for physiologists and molecular biologists to design agronomically relevant strategies for the development of broad spectrum stress tolerant crops.

Infection of Maize by Clavibacter michiganensis subsp. nebraskensis Does Not Require Severe Wounding

Goss's bacterial wilt and leaf blight of maize is caused by Clavibacter michiganensis subsp. nebraskensis. Infested residue is the primary source of inoculum and infection occurs via wounds caused by sand blasting, hail, or wind damage. The pathogen survives as an epiphyte on maize leaves and, because the disease has been observed on plants in the field with no obvious wounding, we wondered whether infection by epiphytic C. michiganensis subsp. nebraskensis and disease development could occur in the absence of severe wounding. Consequently, greenhouse experiments were done to evaluate disease development in the absence of wounding in ambient and increased humidity conditions. Maize plants at the V4 to V5 crop development stage were spray inoculated with a suspension of C. michiganensis subsp. nebraskensis (10⁸ cells ml^-1). Leaf blight incidence was assessed on whole plants and individual leaves; epiphytic populations of C. michiganensis subsp. nebraskensis were monitored by dilution plating of leaf washes; and epiphytic C. michiganensis subsp. nebraskensis colonization was visualized using scanning electron microscopy (SEM). Goss's leaf blight symptoms were observed on nonwounded plants in ambient (37.0% plant incidence) and increased humidity conditions (60.0% plant incidence). Populations of epiphytic C. michiganensis subsp. nebraskensis survived and increased on maize leaves, particularly at increased humidity. We observed C. michiganensis subsp. nebraskensis colonizing maize leaves in localized sites that included epidermal junctions, cuticle depressions, in and around stomata, and at the base of trichomes. Single cells and aggregates of C. michiganensis subsp. nebraskensis were observed within substomatal chambers using SEM. These data indicate that severe wounding is not necessary for C. michiganensis subsp. nebraskensis infection of maize, and stomata or trichomes may serve as entry points for the bacterium.

Draft genome of the Arabidopsis thaliana phyllosphere bacterium, Williamsia sp. ARP1

The Gram-positive actinomycete Williamsia sp. ARP1 was originally isolated from the Arabidopsis thaliana phyllosphere. Here we describe the general physiological features of this microorganism together with the draft genome sequence and annotation. The 4,745,080 bp long genome contains 4434 protein-coding genes and 70 RNA genes. To our knowledge, this is only the second reported genome from the genus Williamsia and the first sequenced strain from the phyllosphere. The presented genomic information is interpreted in the context of an adaptation to the phyllosphere habitat.

Chemical basis of unwettability in Liacaridae (Acari, Oribatida): specific variations of a cuticular acid/ester-based system

Oribatid mites of the family Liacaridae comprise a large number of species with smooth and shiny body surfaces that display extraordinary anti-wetting properties. The principle of liacarid unwettability is not related to micro-structured surfaces as present in many Oribatida ("Lotus effect") but the formation of raincoat-like lipid layers covering the epicuticle. We here conducted a comparative study on the chemistry of cuticular lipid layers in a selection of Liacaridae, including representatives of all major Central European genera, Liacarus, Dorycranosus, Adoristes, and Xenillus. Cuticular lipids of unwettable individuals were removed from mite bodies by hexane extraction, and were analyzed by GC-MS. Basically, two chemically distinguishable systems were found. Type I: cuticular lipids of Liacarus subterraneus, L. coracinus, L. nitens, Dorycranosus curtipilis, and Xenillus tegeocranus contained different carboxylic acids (C8-, C10-, C10:1-, C10:2-acids) and their corresponding di-glycerides in species-specific combinations. Type II: Adoristes ovatus exhibited a system of cuticular lipids composed of esters of pentanoic- and heptanoic acids with C14-, C15-, C16- and C17-alcohols. Interestingly, the chemistry of surface lipids did not reflect the morphology of the cuticle in the species investigated. Smooth and shiny cuticles, though exhibiting a specific pattern of round or slit-like pores, were found in representatives of Liacarus, Dorycranosus (all of which exhibiting cuticular chemistry of type I) and Adoristes (exhibiting cuticular chemistry of type II). Xenillus, possessing a rough, cerotegumental cement layer-covered surface, showed type I-chemistry. The acid-esters systems herein investigated are considered characteristic for the cuticular chemistry of Liacaridae or a lineage of these, and provide first insights into the comparative chemistry of the inner (=lipid) layer of the oribatid cerotegument.

Surface barriers of mandarin 'okitsu' leaves make a major contribution to canker disease resistance

Field evaluations have shown that Satsuma mandarin (Citrus unshiu) 'Okitsu' is one of the mandarin cultivars that shows substantial resistance to Xanthomonas citri subsp. citri (X. citri), the causal agent of citrus bacterial canker disease. However, the mechanisms underlying this resistance are not well understood. In this study, we have shown that 'Okitsu' leaves are nevertheless susceptible to X. citri infection during a period of their development; however, this period is shorter than that seen in the susceptible mandarin 'Clemenules' (C. clementina). Under controlled growth conditions, the resistance of 'Okitsu' to X. citri was associated with the age of the leaf and was evident in spray-inoculated plants but not in those inoculated by infiltration. Furthermore, X. citri showed reduced attachment and biofilm formation in 'Okitsu' leaves compared with 'Clemenules'. Taken together, our data suggest that structural features of the 'Okitsu' leaf surface, such as the physical properties of the cuticle, are involved in the resistance to X. citri.

A synthetic community approach reveals plant genotypes affecting the phyllosphere microbiota

The identity of plant host genetic factors controlling the composition of the plant microbiota and the extent to which plant genes affect associated microbial populations is currently unknown. Here, we use a candidate gene approach to investigate host effects on the phyllosphere community composition and abundance. To reduce the environmental factors that might mask genetic factors, the model plant Arabidopsis thaliana was used in a gnotobiotic system and inoculated with a reduced complexity synthetic bacterial community composed of seven strains representing the most abundant phyla in the phyllosphere. From a panel of 55 plant mutants with alterations in the surface structure, cell wall, defense signaling, secondary metabolism, and pathogen recognition, a small number of single host mutations displayed an altered microbiota composition and/or abundance. Host alleles that resulted in the strongest perturbation of the microbiota relative to the wild-type were lacs2 and pec1. These mutants affect cuticle formation and led to changes in community composition and an increased bacterial abundance relative to the wild-type plants, suggesting that different bacteria can benefit from a modified cuticle to different extents. Moreover, we identified ein2, which is involved in ethylene signaling, as a host factor modulating the community's composition. Finally, we found that different Arabidopsis accessions exhibited different communities, indicating that plant host genetic factors shape the associated microbiota, thus harboring significant potential for the identification of novel plant factors affecting the microbiota of the communities.

The leaves of plants are inhabited by a diverse community of microorganisms. These leaf inhabitants influence their hosts with respect to growth and resistance to abiotic and biotic stresses. Recent studies revealed that the bacterial communities associated with leaves undergo selection, resulting in conserved microbial communities. However, the factors that are involved in structuring of bacterial communities are not well understood. In order to uncover host genetic factors that determine the community composition and to exclude confounding environmental effects, we inoculated Arabidopsis thaliana with a synthetic bacterial community under controlled conditions We screened a panel of Arabidopsis mutants defective in various traits for alterations in community structure and abundance and were able to show that cuticle synthesis and ethylene perception affect the bacterial community. In addition, we identified plant ecotypes with drastic differences in the community composition. Our system can thus be used to identify additional host genes and to broaden insights into plant microbe interactions, potentially providing a basis for applied plant protection through the identification of traits that enhance growth of plant probiotic bacteria.

Quantitative evaluation of the host-colonizing capabilities of the enteric bacterium Pantoea using plant and insect hosts

The genus Pantoea is a highly diverse group comprising free-living, and both pathogenic and non-pathogenic host-associating species. Pathogenic isolates have been found to infect insects, plants and humans, yet it is unclear whether these isolates have similar pathogenic potential to the free-living environmental populations. Using MLSA of six housekeeping genes, we evaluated the phylogenetic relationships among 115 environmental and clinical (human) isolates representing 11 Pantoea species. An overlay of the location of isolation onto the resulting tree revealed that clinical and environmental isolates are interspersed, and do not form distinctive groups. We then conducted quantitative growth assays of our isolates using maize, onion and fruit flies as hosts. Notably, most clinical isolates were able to grow in both plant hosts often comparably or even better than the environmental isolates. There were no obvious growth or host colonization patterns that could distinguish those isolates with clinical potential. Growth of an isolate in one host could not be predicted based on its performance in another host, nor could host growth be predicted by phylogeny or source of isolation. This work demonstrates that the host-colonizing capabilities of all Pantoea species groups is unpredictable, indicating a broader host range and pathogenic potential than currently assumed.

Distinct phyllosphere bacterial communities on Arabidopsis wax mutant leaves

The phyllosphere of plants is inhabited by diverse microorganisms, however, the factors shaping their community composition are not fully elucidated. The plant cuticle represents the initial contact surface between microorganisms and the plant. We thus aimed to investigate whether mutations in the cuticular wax biosynthesis would affect the diversity of the phyllosphere microbiota. A set of four Arabidopsis thaliana eceriferum mutants (cer1, cer6, cer9, cer16) and their respective wild type (Landsberg erecta) were subjected to an outdoor growth period and analysed towards this purpose. The chemical distinctness of the mutant wax phenotypes was confirmed by gas chromatographic measurements. Next generation amplicon pyrosequencing of the bacterial communities showed distinct community patterns. This observation was supported by denaturing gradient gel electrophoresis experiments. Microbial community analyses revealed bacterial phylotypes that were ubiquitously present on all plant lines (termed “core” community) while others were positively or negatively affected by the wax mutant phenotype (termed “plant line-specific“ community). We conclude from this study that plant cuticular wax composition can affect the community composition of phyllosphere bacteria.

Arabidopsis cuticular waxes: advances in synthesis, export and regulation

Cuticular waxes and cutin form the cuticle, a hydrophobic layer covering the aerial surfaces of land plants and acting as a protective barrier against environmental stresses. Very-long-chain fatty acid derived compounds that compose the cuticular waxes are produced in the endoplasmic reticulum of epidermal cells before being exported to the environmental face of the epidermis. Twenty years of genetic studies on Arabidopsis thaliana have led to the molecular characterization of enzymes catalyzing major steps in fatty acid elongation and wax biosynthesis. Although transporters required for wax export from the plasma membrane have been identified, intracellular and extracellular traffic remains largely unknown. In accordance with its major function in producing an active waterproof barrier, wax metabolism is up-regulated at the transcriptional level in response to water deficiency. However its developmental regulation is still poorly described. Here, we discuss the present knowledge of wax functions, biosynthesis and transport as well as the regulation of these processes.

Interrelationships of food safety and plant pathology: the life cycle of human pathogens on plants

Bacterial food-borne pathogens use plants as vectors between animal hosts, all the while following the life cycle script of plant-associated bacteria. Similar to phytobacteria, Salmonella, pathogenic Escherichia coli, and cross-domain pathogens have a foothold in agricultural production areas. The commonality of environmental contamination translates to contact with plants. Because of the chronic absence of kill steps against human pathogens for fresh produce, arrival on plants leads to persistence and the risk of human illness. Significant research progress is revealing mechanisms used by human pathogens to colonize plants and important biological interactions between and among bacteria in planta. These findings articulate the difficulty of eliminating or reducing the pathogen from plants. The plant itself may be an untapped key to clean produce. This review highlights the life of human pathogens outside an animal host, focusing on the role of plants, and illustrates areas that are ripe for future investigation.

Phyllosphere bacterial communities of trichome-bearing and trichomeless Arabidopsis thaliana leaves

This study aimed to investigate whether the presence of trichomes as conspicuous physical attributes of the leaf surface affects the microbial community composition on Arabidopsis thaliana leaves. The A. thaliana ecotype Col-0 and its trichomeless gl1 mutant were grown in growth cabinets under climate-controlled conditions. The gl1 mutant showed a similar wax composition as the Col-0 wild type with slightly reduced amounts of C(29), C(31) and C(33) alkanes by GC/MS and GC/FID analyses. 120 bacterial isolates representing 39 bacterial genera were obtained from A. thaliana Col-0 leaf surfaces. Phylogenetic analysis of nearly full-length 16S rRNA sequences from 29 selected isolates confirmed their affiliation to the Proteobacteria (Alpha-, Beta-, Gamma-), Actinobacteria, Bacteroidetes and Firmicutes. The bacterial diversity on A. thaliana ecotype Col-0 and its gl1 mutant, devoid of trichomes, were further compared by denaturing gradient gel electrophoresis (DGGE). Banding patterns and sequencing of representative DGGE bands revealed the presence of phylotypes related to Sphingomonas (Alphaproteobacteria), Methylophilus (Betaproteobacteria) and Dyadobacter (Bacteroidetes) which are common phyllosphere inhabitants. Furthermore, wildtype and trichomeless mutant plants were exposed to outdoor conditions for 4-5 weeks. The DGGE gels showed only minor differences between the two plant lines, thus suggesting that trichomes per se do not affect bacterial diversity on Arabidopsis leaves under the experimental conditions tested.

Micro-scale water potential gradients visualized in soil around plant root tips using microbiosensors

Water availability and movement in soil are critical determinants of resource availability to, and interactions among, members of the soil community. However, it has been impossible to observe gradients in soil water potential empirically at millimetre spatial scales. Here we describe progress towards that goal using output from two microbial biosensors, Pantoea agglomerans BRT98/pPProGreen and Pseudomonas putida KT2442/pPProGreen, engineered with a reporter system based on the osmotically sensitive proU promoter from Escherichia coli. The proU-GFP construct in both microbiosensors produced green fluorescent protein (GFP) as a function total water potential in nonsterile soil. Controlled experiments in liquid culture showed that dramatically different microbiosensor growth rates (resulting from exposure to different salts as osmolytes) did not alter the GFP output as a function of water potential in either sensor, but P. agglomerans' GFP levels at a given water potential were strongly influenced by the type of carbon (energy) source available to the microbes. In non-sterile rhizosphere soil along Zea mays L. roots, though GFP expression was quite variable, microbiosensors reported statistically significantly more negative soil water potentials as a function of axial distance from root tips, reflecting the gradient in soil water potential hypothesized to develop during transpiration.

Identification of quantitative trait loci for resistance to shoot fly in sorghum [Sorghum bicolor (L.) Moench]

The shoot fly is one of the most destructive insect pests of sorghum at the seedling stage. Deployment of cultivars with improved shoot fly resistance would be facilitated by the use of molecular markers linked to QTL. The objective of this study was to dissect the genetic basis of resistance into QTL, using replicated phenotypic data sets obtained from four test environments, and a 162 microsatellite marker-based linkage map constructed using 168 RILs of the cross 296B (susceptible) x IS18551 (resistant). Considering five component traits and four environments, a total of 29 QTL were detected by multiple QTL mapping (MQM) viz., four each for leaf glossiness and seedling vigor, seven for oviposition, six for deadhearts, two for adaxial trichome density and six for abaxial trichome density. The LOD and R (2) (%) values of QTL ranged from 2.6 to 15.0 and 5.0 to 33%, respectively. For most of the QTL, IS18551 contributed resistance alleles; however, at six QTL, alleles from 296B also contributed to resistance. QTL of the related component traits were co-localized, suggesting pleiotropy or tight linkage of genes. The new morphological marker Trit for trichome type was associated with the major QTL for component traits of resistance. Interestingly, QTL identified in this study correspond to QTL/genes for insect resistance at the syntenic maize genomic regions, suggesting the conservation of insect resistance loci between these crops. For majority of the QTL, possible candidate genes lie within or very near the ascribed confidence intervals in sorghum. Finally, the QTL identified in the study should provide a foundation for marker-assisted selection (MAS) programs for improving shoot fly resistance in sorghum.

Development of a catabolically significant genetic probe for polycyclic aromatic hydrocarbon-degrading Mycobacteria in soil

A gene probe for the detection of polycyclic aromatic hydrocarbon (PAH) induced nidB and nidA dioxygenase genes has been designed from Mycobacteria JLS, KMS, and MCS. The probe detects a catabolic gene involved in the initial steps of PAH biodegradation in mycobacteria. The gene probe is comprised of three PCR primer sets designed to detect the genes that code for two subunits of the PAH induced dioxygenase enzyme within PAH-degrading mycobacteria. The probe was built by combining three primer sets with a DNA extraction procedure that was designed to lyse the gram-positive mycobacteria cells while in the soil matrix and remove PCR inhibitors. The probe was tested on PAH contaminated soils undergoing bioremediation through landfarming and uncontaminated soils from the same site. The PAH gene probe results demonstrate that the dioxygenase genes can be detected in soils. Sequencing the nidA and nidB PCR products verified that the genes were detected in soil. Comparisons of the sequences obtained from the soil probe to seven known nid gene sequences from various PAH-degrading mycobacteria showed between 97 and 99% nucleotide matches with the nidB gene and 95 and 99% matches with the nidA gene.

WIN1, a transcriptional activator of epidermal wax accumulation in Arabidopsis

Epicuticular wax forms a layer of hydrophobic material on plant aerial organs, which constitutes a protective barrier between the plant and its environment. We report here the identification of WIN1, an Arabidopsis thaliana ethylene response factor-type transcription factor, which can activate wax deposition in overexpressing plants. We constitutively expressed WIN1 in transgenic Arabidopsis plants, and found that leaf epidermal wax accumulation was up to 4.5-fold higher in these plants than in control plants. A significant increase was also found in stems. Interestingly, approximately 50% of the additional wax could only be released by complete lipid extractions, suggesting that not all of the wax is superficial. Gene expression analysis indicated that a number of genes, such as CER1, KCS1, and CER2, which are known to be involved in wax biosynthesis, were induced in WIN1 overexpressors. This observation indicates that induction of wax accumulation in transgenic plants is probably mediated through an increase in the expression of genes encoding enzymes of the wax biosynthesis pathway.