Leaf Colonization Research Articles

Low microbial diversity, yeast prevalence, and nematode-trapping fungal presence in fungal colonization and leaf microbiome of Serjania erecta

Medicinal plant microbiomes undergo selection due to secondary metabolite presence. Resident endophytic/epiphytic microorganisms directly influence plant’s bioactive compound synthesis. Hypothesizing low microbial diversity in Serjania erecta leaves, we assessed leaf colonization by epiphytic and endophytic fungi. Given its traditional medicinal importance, we estimated diversity in the endophytic fungal microbiome. Analyses included scanning electron microscopy (SEM), isolation of cultivable species, and metagenomics. Epiphytic fungi interacted with S. erecta leaf tissues, horizontally transmitted via stomata/trichome bases, expressing traits for nematode trapping. Cultivable endophytic fungi, known for phytopathogenic habits, didn’t induce dysbiosis symptoms. This study confirms low leaf microbiome diversity in S. erecta, with a tendency towards more fungal species, likely due to antibacterial secondary metabolite selection. The classification of Halicephalobus sp. sequence corroborated the presence of nematode eggs on the epidermal surface of S. erecta by SEM. In addition, we confirmed the presence of methanogenic archaea and a considerable number of methanotrophs of the genus Methylobacterium. The metagenomic study of endophytic fungi highlighted plant growth-promoting yeasts, mainly Malassezia, Leucosporidium, Meyerozyma, and Hannaella. Studying endophytic fungi and S. erecta microbiomes can elucidate their impact on beneficial bioactive compound production, on the other hand, it is possible that the bioactive compounds produced by this plant can recruit specific microorganisms, impacting the biological system.

Infection of Phytophthora palmivora Isolates on Arabidopsis thaliana.

Phytophthora palmivora, a hemibiotrophic oomycete, causes diseases in several economically important tropical crops, such as oil palm, which it is responsible for a devastating disease called bud rot (BR). Despite recent progress in understanding host resistance and virulence mechanisms, many aspects remain unknown in P. palmivora isolates from oil palm. Model pathosystems are useful for understanding the molecular interactions between pathogens and hosts. In this study, we utilized detached leaves and whole seedlings of Arabidopsis thaliana Col-0 to describe and evaluate the infection process of three P. palmivora isolates (CPPhZC-05, CPPhZC-04, CPPhZOC-01) that cause BR in oil palm. Two compatible isolates (CPPhZC-05 and CPPhZOC-01) induced aqueous lesions at 72 h post-inoculation (hpi), with microscopic visualization revealing zoospore encysting and appressorium penetration at 3 hpi, followed by sporangia generation at 72 hpi. In contrast, an incompatible isolate (CPPhZC-04) exhibited cysts that could not penetrate tissue, resulting in low leaf colonization. Gene expression of ten P. palmivora infection-related genes was quantified by RT-qPCR, revealing overexpression in compatible isolates, but not in the incompatible isolate. Additionally, key genes associated with salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) in Arabidopsis exhibited regulation during interaction with the three isolates. These findings demonstrate that P. palmivora can infect Arabidopsis Col-0, and variability is observed in the interaction between Arabidopsis-Col-0 and P. palmivora isolates. Establishing this pathosystem is expected to enhance our understanding of P. palmivora's pathology and physiology.

The adult plant resistance (APR) genes Yr18, Yr29 and Yr46 in spring wheat showed significant effect against important yellow rust races under North-West European field conditions

Yellow rust caused by Puccinia striiformis f. sp. tritici (Pst) is one of the most important wheat diseases. Adult plant resistance (APR) genes have gained the attention of breeders and scientists because they show higher durability compared to major race-specific genes. Here, we determined the effect of the APR genes Yr18, Yr29 and Yr46 in North-West European field conditions against three currently important Pst races. We used three pairs of sibling wheat lines developed at CIMMYT, which consisted of a line with the functional resistance gene and a sibling with its non-functional allele. All APR genes showed significant effects against the Pst races Warrior and Warrior (–), and a race of the highly aggressive strain PstS2. The effects of Yr18 and Yr46 were especially substantial in slowing down disease progress. This effect was apparent in both Denmark, where susceptible controls reached 100 percent disease severity, and in United Kingdom where disease pressure was lower. We further validated field results by quantifying fungal biomass in leaf samples and by micro-phenotyping of samples collected during early disease development. Microscopic image analyses using deep learning allowed us to quantify separately the APR effects on leaf colonization and pustule formation. Our results show that the three APR genes can be used in breeding yellow rust resistant varieties of spring wheat to be grown in North-West European conditions, and that deep learning image analysis can be an effective method to quantify effects of APR on colonisation and pustule formation.

Chloroplast protein StFC-II was manipulated by a Phytophthora effector to enhance host susceptibility.

Oomycete secretes a range of RxLR effectors into host cells to manipulate plant immunity by targeting proteins from several organelles. In this study, we report that chloroplast protein StFC-II is hijacked by a pathogen effector to enhance susceptibility. Phytophthora infestans RxLR effector Pi22922 is activated during the early stages of P. infestans colonization. Stable overexpression of Pi22922 in plants suppresses flg22-triggered reactive oxygen species (ROS) burst and enhances leaf colonization by P. infestans. A potato ferrochelatase 2 (FC-II, a nuclear-encoded chloroplast-targeted protein), a key enzyme for heme biosynthesis in chloroplast, was identified as a target of Pi22922 in the cytoplasm. The pathogenicity of Pi22922 in plants is partially dependent on FC-II. Overexpression of StFC-II decreases resistance of potato and Nicotiana benthamiana against P. infestans, and silencing of NbFC-II in N. benthamiana reduces P. infestans colonization. Overexpression of StFC-II increases heme content and reduces chlorophyll content and photosynthetic efficiency in potato leaves. Moreover, ROS accumulation both in chloroplast and cytoplasm is attenuated and defense-related genes are down-regulated in StFC-II overexpression transgenic potato and N. benthamiana leaves. Pi22922 inhibits E3 ubiquitin ligase StCHIP-mediated StFC-II degradation in the cytoplasm and promotes its accumulation in chloroplasts. In summary, this study characterizes a new mechanism that an oomycete RxLR effector suppresses host defenses by promoting StFC-II accumulation in chloroplasts, thereby compromising the host immunity and promoting susceptibility.

Effects of Leaf Species and Conditioning State of Fresh Leaves on Colonization by Stream and Pond Macroinvertebrates

Fresh, green leaves are increasing as resources in aquatic ecosystems due to more frequent severe spring and summer storms, but research on allochthonous resources typically uses senescent leaves. We examined macroinvertebrate colonization of green leaves of three native deciduous trees (red maple, red oak, and tulip poplar) over two weeks within both a stream and fishless pond. Leaf colonization varied depending on the taxa of leaves and colonizers, submersion time, and the ecosystem examined. Within the stream, the densities of isopods (Lirceus sp.) and snails (mostly the invasive Potamopyrgus antipodarum) did not vary significantly across leaf species. In contrast, mayflies (Tricorythodes sp.) in the stream colonized tulip poplar in greater numbers than red oak leaves, while higher densities of planarians (order Tricladida) occurred within red oak leaves. The numbers of mayflies and snails decreased significantly by the second week, but the densities of isopods and planaria within stream leaf packs were consistent. In contrast, within the pond, significantly more isopods (Caecidotea communis) were collected after the first than after the second week of submersion and in tulip poplar leaves. Clams (Sphaeriidae) in the pond, on the other hand, were more prevalent in the second week but did not discriminate among leaf species. While the number of leeches (mostly Helobdella stagnalis) was consistent across weeks, significantly fewer leeches resided within tulip poplar leaves than within red oak leaves. Our results suggest that there are no consistent colonization-preference rankings of species of fresh leaves across freshwater benthic macroinvertebrate taxa. Even within a functional feeding group (e.g., the two types of isopods and snails, all detritivore-herbivores), there were differences in colonization patterns. Therefore, increased allochthonous inputs of fresh leaf litter due to severe spring- and summer-time storms are likely to promote the populations of various taxa to different extents.

Hitching a Ride in the Phyllosphere: Surfactant Production of Pseudomonas spp. Causes Co-swarming of Pantoea eucalypti 299R

Here, we demonstrate the beneficial effect of surfactant-producing pseudomonads on Pantoea eucalypti 299R. We conducted a series of experiments in environments of increasing complexity. P. eucalypti 299R (Pe299R), and Pseudomonas sp. FF1 (Pff1) or Pe299R and surfactant-production deficient Pseudomonas sp. FF1::ΔviscB (Pff1ΔviscB) were co-inoculated in broth, on swarming agar plates, and on plants. In broth, there were no differences in the growth dynamics of Pe299R when growing in the presence of Pff1 or Pff1ΔviscB. By contrast, on swarming agar plates, Pe299R was able to co-swarm with Pff1 which led to a significant increase in Pe299R biomass compared to Pe299R growing with Pff1ΔviscB or in monoculture. Finally in planta, and using the single-cell bioreporter for reproductive success (CUSPER), we found a temporally distinct beneficial effect of Pff1 on co-inoculated Pe299R subpopulations that did not occur in the presence of Pff1ΔviscB. We tested three additional surfactant-producing pseudomonads and their respective surfactant knockout mutants on PE299R on swarming agar showing similar results. This led us to propose a model for the positive effect of surfactant production during leaf colonization. Our results indicate that co-motility might be common during leaf colonization and adds yet another facet to the already manyfold roles of surfactants.

Opportunistic pathogenicity observed for the endophytic fungus Diaporthe ueckerae on Gossypium hirsutum plants

AbstractCotton crops are routinely threatened by emerging fungal diseases. Fungal endophytes also can be considered latent phytopathogens. In this study we tested the hypothesis that an endophytic strain of Diaporthe, isolated from chlorotic leaves of cotton (Gossypium hirsutum), could trigger physiological effects of biotic stress in this oilseed plant. We also assessed the histopathological aspects of the mycelial interaction of the endophyte with the adaxial surface of G. hirsutum leaves. Thus, we studied the synthesis of photosynthetic pigments, pattern of gas exchange, and photochemistry of cotton plants subjected to inoculation with Diaporthe ueckerae via root and leaf at three different phenological stages (vegetative, reproductive, and maturation). Additionally, we histopathologically analyzed infected leaves using electron microscopy to study the process of leaf colonization by this endophytic fungus. We evidenced that D. ueckerae inoculation negatively affected the synthesis of photosynthetic pigments in plants at vegetative and reproductive stages. Moreover, inoculation also negatively affected the photosynthetic rate and carboxylation efficiency of these plants. We also found that the presence of the endophyte increased transpiration and decreased water use efficiency in the plants. Furthermore, foliar inoculation negatively affected stomatal conductance, whereas inoculation via leaf or root reduced the photochemical performance of cotton. We also observed that D. ueckerae colonizes the leaf tissues of G. hirsutum via glandular trichomes and forces penetration into the epidermis using appressoria, and the plant responds by closing the stomata. The observed physiological alterations are indicative of biotic stress, confirming the hypothesis that D. ueckerae may be an opportunistic phytopathogen for cotton plants.

Impact of Three Entomopathogenic Fungal Isolates on the Growth of Tomato Plants—Ectoapplication to Explore Their Effect on Tetranychus urticae

Tetranychus urticae is an important pest of tomato crops globally, affecting plant yield and growth. Beauveria bassiana and Metarhizium robertsii have the potential to control T. urticae. We investigated the influence of two B. bassiana (i.e., WG-12 and WG-19) isolates and one M. robertsii (WG-02) isolate when colonizing different plant organs (leaves, stems, and roots) and their influence on the growth of tomato plants, through foliar, root-dipping, and seed-soaking application techniques. We also examined the acaricidal activity of the three isolates against T. urticae (female adults), spraying tomato leaf discs with each isolate separately. After 28 days, WG-12 and WG-19 colonized 97 and 91% of the leaves after foliar inoculation, whereas WG-02 exhibited the lowest leaf colonization (76%). The height of the tomato plants, the root length, the number of leaves, and the weight of the biomass above and below the ground were enhanced significantly after inoculation with WG-02 vs. B. bassiana isolates and control. The complete mortality of T. urticae was caused by WG-12 and WG-02 after 10 days, whereas WG-19 killed 94% of the adults. For the effective management of T. urticae, we propose the application of the WG-02 isolate since it provides complete protection and promotes the growth of tomato plants.

Detritivore physiology and growth benefit from algal presence during microbial leaf colonization

AbstractIn headwater streams, riparian leaf litter is the primary food source for detritivores. While it is well known that aquatic fungi improve the nutritious quality of leaves, our understanding on whether and how benthic algae influence this process remains limited. Here, we hypothesized that the interplay between algae and fungi, termed “algal priming”, further enhances food quality. In a 40‐d microcosm experiment, we fed Gammarus fossarum of two size classes with Fagus sylvatica leaves of varying qualities: pure leaves (low quality), leaves colonized by fungi (intermediate quality), and leaves colonized by fungi in the presence of a diatom (high quality). Our results revealed that Gammarus' ingestion rates increased (55–164%) with food quality, spurring accelerated growth (4–14%), regardless of the size class. Furthermore, we observed a tendency for Gammarus' overall fatty acid (FA) quantity to rise with higher‐quality food (12–318%), with the FA profile exhibiting increased proportions of specific polyunsaturated FAs that are essential for detritivores. These observations can likely be attributed to leaf‐associated fungi, which are more readily assimilated than the leaves and are known as a source of FA. This enhancing effect by fungi was further amplified in the presence of diatoms, presumably through the positive effect of algal‐derived labile organic carbon, which supports fungal growth. Despite reduced autochthonous primary production in shaded headwater streams, the experimental findings from this study indicate a potential of enhanced secondary production and energy transfer to higher trophic levels within the aquatic ecosystem.

A computational model of Pseudomonas syringae metabolism unveils a role for branched-chain amino acids in Arabidopsis leaf colonization.

Bacterial pathogens adapt their metabolism to the plant environment to successfully colonize their hosts. In our efforts to uncover the metabolic pathways that contribute to the colonization of Arabidopsis thaliana leaves by Pseudomonas syringae pv tomato DC3000 (Pst DC3000), we created iPst19, an ensemble of 100 genome-scale network reconstructions of Pst DC3000 metabolism. We developed a novel approach for gene essentiality screens, leveraging the predictive power of iPst19 to identify core and ancillary condition-specific essential genes. Constraining the metabolic flux of iPst19 with Pst DC3000 gene expression data obtained from naïve-infected or pre-immunized-infected plants, revealed changes in bacterial metabolism imposed by plant immunity. Machine learning analysis revealed that among other amino acids, branched-chain amino acids (BCAAs) metabolism significantly contributed to the overall metabolic status of each gene-expression-contextualized iPst19 simulation. These predictions were tested and confirmed experimentally. Pst DC3000 growth and gene expression analysis showed that BCAAs suppress virulence gene expression in vitro without affecting bacterial growth. In planta, however, an excess of BCAAs suppress the expression of virulence genes at the early stages of infection and significantly impair the colonization of Arabidopsis leaves. Our findings suggesting that BCAAs catabolism is necessary to express virulence and colonize the host. Overall, this study provides valuable insights into how plant immunity impacts Pst DC3000 metabolism, and how bacterial metabolism impacts the expression of virulence.

Strain belonging to an emerging, virulent sublineage of ST131 Escherichia coli isolated in fresh spinach, suggesting that ST131 may be transmissible through agricultural products.

Food contamination with pathogenic Escherichia coli can cause severe disease. Here, we report the isolation of a multidrug resistant strain (A23EC) from fresh spinach. A23EC belongs to subclade C2 of ST131, a virulent clone of Extraintestinal Pathogenic E. coli (ExPEC). Most A23EC virulence factors are concentrated in three pathogenicity islands. These include PapGII, a fimbrial tip adhesin linked to increased virulence, and CsgA and CsgB, two adhesins known to facilitate spinach leaf colonization. A23EC also bears TnMB1860, a chromosomally-integrated transposon with the demonstrated potential to facilitate the evolution of carbapenem resistance among non-carbapenemase-producing enterobacterales. This transposon consists of two IS26-bound modular translocatable units (TUs). The first TU carries aac(6')-lb-cr, bla OXA-1, ΔcatB3, aac(3)-lle, and tmrB, and the second one harbors bla CXT-M-15. A23EC also bears a self-transmissible plasmid that can mediate conjugation at 20°C and that has a mosaic IncF [F(31,36):A(4,20):B1] and Col156 origin of replication. Comparing A23EC to 86 additional complete ST131 sequences, A23EC forms a monophyletic cluster with 17 other strains that share the following four genomic traits: (1) virotype E (papGII+); (2) presence of a PAI II536-like pathogenicity island with an additional cnf1 gene; (3) presence of chromosomal TnMB1860; and (4) frequent presence of an F(31,36):A(4,20):B1 plasmid. Sequences belonging to this cluster (which we named "C2b sublineage") are highly enriched in septicemia samples and their associated genetic markers align with recent reports of an emerging, virulent sublineage of the C2 subclade, suggesting significant pathogenic potential. This is the first report of a ST131 strain belonging to subclade C2 contaminating green leafy vegetables. The detection of this uropathogenic clone in fresh food is alarming. This work suggests that ST131 continues to evolve, gaining selective advantages and new routes of transmission. This highlights the pressing need for rigorous epidemiological surveillance of ExPEC in vegetables with One Health perspective.

Deciphering the underlying immune network of the potato defense response inhibition by Phytophthora infestans nuclear effector Pi07586 through transcriptome analysis.

Phytophthora infestans, a highly destructive plant oomycete pathogen, is responsible for causing late blight in potatoes worldwide. To successfully infect host cells and evade immunity, P. infestans secretes various effectors into host cells and exclusively targets the host nucleus. However, the precise mechanisms by which these effectors manipulate host gene expression and reprogram defenses remain poorly understood. In this study, we focused on a nuclear-targeted effector, Pi07586, which has been implicated in immune suppression. Quantitative real-time PCR (qRT-PCR) analysis showed Pi07586 was significant up-regulation during the early stages of infection. Agrobacterium-induced transient expression revealed that Pi07586 localized in the nucleus of leaf cells. Overexpression of Pi07586 resulted in increased leaf colonization by P. infestans. RNA-seq analysis revealed that Pi07586 effectively suppressed the expression of PR-1C-like and photosynthetic antenna protein genes. Furthermore, high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS) analysis indicated that Pi07586 overexpression led to a substantial decrease in abscisic acid (ABA), jasmonic acid (JA), and jasmonoyl-isoleucine (JA-Ile) levels, while not affecting salicylic acid (SA) and indole-3-acetic acid (IAA) production. These findings shed new light on the modulation of plant immunity by Pi07586 and enhance our understanding of the intricate relationship between P. infestans and host plants.

Effect of Different Light Wavelengths on Zymoseptoria tritici Development and Leaf Colonization in Bread Wheat

The wheat pathogen Zymoseptoria tritici can respond to light by modulating its gene expression. Because several virulence-related genes are differentially expressed in response to light, different wavelengths could have a crucial role in the Z. tritici–wheat interaction. To explore this opportunity, the aim of this study was to analyze the effect of blue (470 nm), red (627 nm), blue–red, and white light on the in vitro and in planta development of Z. tritici. The morphology (mycelium appearance, color) and phenotypic (mycelium growth) characteristics of a Z. tritici strain were evaluated after 14 days under the different light conditions in two independent experiments. In addition, bread wheat plants were artificially inoculated with Z. tritici and grown for 35 days under the same light treatments. The disease incidence, severity, and fungal DNA were analyzed in a single experiment. Statistical differences were determined by using an ANOVA. The obtained results showed that the different light wavelengths induced specific morphological changes in mycelial growth. The blue light significantly reduced colony growth, while the dark and red light favored fungal development (p < 0.05). The light quality also influenced host colonization, whereby the white and red light had stimulating and repressing effects, respectively (p < 0.05). This precursory study demonstrated the influence of light on Z. tritici colonization in bread wheat.

Phyllosphere symbiont promotes plant growth through ACC deaminase production

Plant growth promoting bacteria can confer resistance to various types of stress and increase agricultural yields. The mechanisms they employ are diverse. One of the most important genes associated with the increase in plant biomass and stress resistance is acdS, which encodes a 1-aminocyclopropane-1-carboxylate- or ACC-deaminase. The non-proteinogenic amino acid ACC is the precursor and means of long-distance transport of ethylene, a plant hormone associated with growth arrest. Expression of acdS reduces stress induced ethylene levels and the enzyme is abundant in rhizosphere colonizers. Whether ACC hydrolysis plays a role in the phyllosphere, both as assembly cue and in growth promotion, remains unclear. Here we show that Paraburkholderia dioscoreae Msb3, a yam phyllosphere symbiont, colonizes the tomato phyllosphere and promotes plant growth by action of its ACC deaminase. We found that acdS is required for improved plant growth but not for efficient leaf colonization. Strain Msb3 readily proliferates on the leaf surface of tomato, only occasionally spreading to the leaf endosphere through stomata. The strain can also colonize the soil or medium around the roots but only spreads into the root if the plant is wounded. Our results indicate that the degradation of ACC is not just an important trait of plant growth promoting rhizobacteria but also one of leaf dwelling phyllosphere bacteria. Manipulation of the leaf microbiota by means of spray inoculation may be more easily achieved than that of the soil. Therefore, the application of ACC deaminase containing bacteria to the phyllosphere may be a promising strategy to increasing plant stress resistance, pathogen control, and harvest yields.

Root colonization of industrial hemp (Cannabis sativa L.) by the endophytic fungi Metarhizium and Pochonia improves growth

Metarhizium is an endophytic, insect pathogenic fungus. Here we investigated the time-course of systemic and rhizospheric colonization by Metarhizium and its influence on the growth of industrial hemp (Cannabis sativa L.). Fungal colonization of hemp root, stem and leaf as well as rhizospheric soil, and agronomic parameters of hemp, were tested in four Metarhizium species with broad- (Metarhizium robertsii, Metarhizium anisopliae, Metarhizium brunneum) and narrow-insect (Metarhizium acridum) host ranges, as well as Pochonia chlamydosporia, a phylogenetically related endophytic fungus that is a nematode pathogen. Root and rhizosphere colonization were found in all Metarhizium species and Pochonia at 10, 20 and 30 days post-inoculation. Pochonia showed greatest root colonization compared to that of all Metarhizium species. Endophytic root colonization by Metarhizium was confirmed by confocal fluorescence microscopy using green fluorescent protein-expressing fungal strains. The genes, cat, mpl and EF1α were tested using a qPCR assay for Metarhizium and hemp. Root colonization by Metarhizium and Pochonia positively influenced shoot length, stem weight and root weight of hemp during the early stages of colonization. Our results suggest the potential for the application of Metarhizium and Pochonia as biocontrol and plant growth promoter agents during hemp colonization.

Disruptive Effects of Resveratrol on a Bacterial Pathogen of Beans.

Upon inoculation, common beans immune to Pseudomonas savastanoi pv phaseolicola race 5 (R5) accumulate resveratrol, a phytoalexin. How resveratrol acts upon on this bacterium is not known, although in animal pathogenic bacteria in vitro resveratrol reduces ATPase (ATP = adenosine triphosphate) activity, cellular motility, quorum sensing, and biofilm formation. In this study, mass spectrometry was used to monitor the effects of resveratrol on R5. R5 responded by producing multidrug efflux proteins to pump resveratrol out of cells. Changes in R5 enzyme abundances were consistent with a slowed tricarboxylic acid cycle, the consequence of which likely impeded ATP production by oxidative phosphorylation. There also were enzymatic shifts consistent with decreased amounts of flagellar proteins and decreased pools of purines. A motility assay confirmed a reduction in R5 flagellar movement in resveratrol, and mass spectrometry of metabolite extracts confirmed decreased pools of guanosine 5'-monophosphate and adenosine 5'-monophosphate. Mass spectrometry also detected the accumulation of a reactive aldehyde byproduct of resveratrol catabolism. Overall, the study reveals that resveratrol likely imparts its antibiotic activity during plant immunity by disturbing the bacterial tricarboxylic acid cycle, interfering with ATP biosynthesis at the electron transport chain, and by decreasing bacterial proteins needed for pathogenicity and leaf colonization. Mass spectrometry data files for this study can be retrieved from massive.ucsd.edu (MSV000090171 and MSV000090172).

The 'Candidatus Phytoplasma tritici' effector SWP12 degrades the transcription factor TaWRKY74 to suppress wheat resistance.

'Candidatus Phytoplasma tritici' ('Ca. P. tritici') is an insect-borne obligate pathogen that infects wheat (Triticum aestivum) causing wheat blue dwarf disease, and leads to yield losses. SWP12 is a potential effector secreted by 'Ca. P. tritici' that manipulates host processes to create an environment conducive to phytoplasma colonization, but the detailed mechanism of action remains to be investigated. In this study, the expression of SWP12 weakened the basal immunity of Nicotiana benthamiana and promoted leaf colonization by Phytophthora parasitica, Sclerotinia sclerotiorum, and tobacco mild green mosaic virus. Moreover, the expression of SWP12 in wheat plants promoted phytoplasma colonization. Triticum aestivum WRKY74 and N. benthamiana WRKY17 were identified as host targets of SWP12. The expression of TaWRKY74 triggered reactive oxygen species bursts, upregulated defense-related genes, and decreased TaCRR6 transcription, leading to reductions in NADH dehydrogenase complex (NDH) activity. Expression of TaWRKY74 in wheat increased plant resistance to 'Ca. P. tritici', and silencing of TaWRKY74 enhanced plant susceptibility, which indicates that TaWRKY74 is a positive regulator of wheat resistance to 'Ca. P. tritici'. We showed that SWP12 weakens plant resistance and promotes 'Ca. P. tritici' colonization by destabilizing TaWRKY74.

Effects of leaf quality and colonization time on the abundance of bacteria in an experimental design

Small rivers, henceforth streams, depend on organic matter (nutrients and energy) from riparian vegetation. The quality of such allochthonous debris is determinant for the transformation of organic matter compounds, where the bacterial community has a crucial role in the final decomposition of the substrate. During bacterial colonization, debris with higher concentration of nutrients (more palatable) is prioritized, which accelerates the process. This study investigated the effects of leaf palatability of two native trees on bacterial colonization (abundance) over time, through a laboratory experiment that lasted 60 days. Values of C, N, P, C:N, C:P, polyphenols, tannins, lignin, lignin:N and leaf toughness of both species were compared. Bacterial abundance was higher in species with higher nitrogen values, although they had higher leaf toughness and more polyphenols, which differs from studies indicating that high leaf toughness represents low nutritional quality. The colonization time did not influence bacterial abundance. Therefore, processes degrading riparian vegetation and reducing nutritional quality can affect local decomposition, decreasing bacterial abundance.

Arbuscular Mycorrhizal Fungi Inoculation Improves Flower Yield and Postharvest Quality Component of Gerbera Grown under Different Salinity Levels

High salinity levels of irrigated water and the accumulation of salt over time in the soil is a major concern worldwide, including in Jordan. The objective of this two-year study was to assess the influence of arbuscular mycorrhizal fungi (AMF) inoculation on the physiology, yield, and flower quality of gerbera (Gerbera jamesonii cvs. Beaudine and Palm Beach) under different salinity levels (0.0, 20.0 and 40.0 mM-NaCl). The study was arranged in a randomized complete block design with five replicates. During the experimental period (2018–2019), chlorophyll content index (SPAD), leaf gas exchange (photosynthesis, Pn; stomatal conductance, gs; transpiration, E), flower yield, flower quality (pedicel length and diameter, number of days to flowering, flower diameter, and vase life), root sporulation, and colonization were measured. Irrigation with saline water (20 and 40 mM-NaCl) significantly increased salt accumulation in soil. The mean soil electrical conductivity (EC) after two growing seasons for the 20 mM-NaCl treatment was 2.9 dS m−1 and 4.4 dS m−1 for the 40 mM. High salinity level (40 mM-NaCl) reduced root AMF sporulation by 53–62% and colonization by 12–25% across cultivars. Interestingly, root colonization was higher than 50% across salinity level and in both cultivars. Saline water at 40 mM-NaCl significantly reduced SPAD, Pn, gs, E, flower yield, and quality component, especially vase life. Interestingly, leaf chlorophyll content index from AMF-inoculated plants was significantly higher than uninoculated ones across cultivars at the second growing season. In addition, inoculation with AMF significantly increased yield in both ‘Beaudine’ (34–40%) and ‘Palm Beach’ (42–44%) cultivars and across the study period, 2018 to 2019. In addition, AMF increased vase life in ‘Beaudine’ by 19% to 28% and in ‘Palm Beach’ by 21% to 22%. Overall, our results revealed that gerbera growers can increase their flower yield and postharvest flower quality component (vase life) under saline conditions (soil EC < 4.4 dS m−1) by inoculating the seedlings with AMF.

Physiological and biochemical insights into induced resistance on tomato against septoria leaf spot by a phosphite combined with free amino acids

Septoria leaf spot (SLS), caused by Septoria lycopersici, is a destructive disease in tomato. This study investigated the use of phosphite and potassium combined with l-α-free amino acids (named induced resistance (IR) stimulus), to boost tomato defense responses against S. lycopersici infection. Plants were sprayed with water or with the IR stimulus and challenged or not with S. lycopersici. Mycelial growth of S. lycopersici and conidia germination were not affected by the IR stimulus in vitro. Reduction in SLS severity and less leaf tissue colonization by S. lycopersici (lower TEF-1α expression) occurred for IR stimulus-sprayed plants. The IR stimulus-sprayed plants showed lower concentrations of malondialdehyde, hydrogen peroxide, and superoxide anion radical while the concentrations of sucrose, fructose, and starch and activities of ascorbate peroxidase, catalase, glutathione reductase, and superoxide dismutase were great compared to water-sprayed plants in response to S. lycopersici infection. Less impairment on the photosynthetic apparatus not related to biochemical limitations, abrupt increase in transpiration rate, and higher concentrations of chlorophyll a+b and carotenoids occurred for IR stimulus-sprayed plants facing reduced SLS symptoms. Genes involved in host defense reactions (CHI3, CHI9, GLU, PAL3, POX3, PPOB, and PPOF) and those related to systemic acquired resistance (PAL3 and ICS) or induced systemic resistance ((ethylene production - ACO2, ACO3, ACO5, and ACO4) and jasmonic acid signaling pathway (LOX1.1, LOXB, LOXC, and PDF1.2)) were up-regulated for IR stimulus-sprayed and infected plants. These findings highlight the potential of using this IR stimulus for SLS management.