Identification of Volatile Organic Compounds Produced by Xenorhabdus indica Strain AB and Investigation of Their Antifungal Activities.

Xenorhabdus spp. are symbiotic bacteria associated with entomopathogenic nematodes to form a model complex that is used for the biological control of insect pests. These bacteria also produce secondary metabolites that have commercial potential in the pharmaceutical and agroforestry industries. Volatile organic compounds (VOCs) produced by the Xenorhabdus indica "strain AB" have been shown to have significant antifungal activity against Fusarium oxysporum f. sp. cucumerinum. Using gas chromatography-mass spectrometry, we identified 61 volatiles in the mixture of VOCs emitted by strain AB compared to a control strain, 6 of which were investigated for their antifungal activities. Of these, methyl anthranilate exhibited the highest mycelial growth suppression toward F. oxysporum, with a minimum inhibitory volume (MIV) of 50 µL/plate. Fluorescence assays, scanning electron microscopy, and measurements of the leakage of intracellular components revealed that the use of methyl anthranilate changed cell wall and cell membrane integrity as well as the permeability of the plasma membrane. Furthermore, methyl anthranilate treatment upregulated the transcription level of target genes related to redox reactions and the cell wall integrity pathway. The results suggest a novel mechanism used by Xenorhabdus spp. to overcome competitors during its life cycle and open up a new approach to using these bacteria in biological control. IMPORTANCE Fungal phytopathogens, particularly Fusarium oxysporum, are a major problem worldwide, especially in the postharvest of vital economic crops. Concerns about negative effects on the environment and human health have led to increasing restrictions on the use of chemical fungicides, and therefore, biological control agents are now being considered alternatives. It is in this context that we investigated the antifungal activity of VOCs produced by X. indica strain AB against F. oxysporum. We found that AB VOCs have a strong effect on the growth of the fungal phytopathogen. In addition, 85% of the identified volatile compounds were determined to be new compounds, opening up new lines of research to discover their properties, effects, and potential for pharmaceutical and agricultural applications. Antifungal assays proved that four of the six compounds with a high concentration in the GC-MS profile had a significant inhibitory effect on pathogen growth. Accordingly, this study opens up a new approach for the use of these bacteria in biocontrol.

Distribution and Assembly Processes of Soil Fungal Communities along an Altitudinal Gradient in Tibetan Plateau.

In soil ecosystems, fungi exhibit diverse biodiversity and play an essential role in soil biogeochemical cycling. Fungal diversity and assembly processes across soil strata along altitudinal gradients are still unclear. In this study, we investigated the structure and abundance of soil fungal communities among soil strata and elevational gradients on the Tibetan Plateau using Illumina MiSeq sequencing of internal transcribed spacer1 (ITS1). The contribution of neutral and niche ecological processes were quantified using a neutral community model and a null model-based methodology. Our results showed that fungal gene abundance increased along altitudinal gradients, while decreasing across soil strata. Along with altitudinal gradients, fungal α-diversity (richness) decreased from surface to deeper soil layers, while ß-diversity showed weak correlations with elevations. The neutral community model showed an excellent fit for neutral processes and the lowest migration rate (R2 = 0.75). The null model showed that stochastic processes dominate in all samples (95.55%), dispersal limitations were dominated at the surface layer and decreased significantly with soil strata, while undominated processes (ecological drift) show a contrary trend. The log-normal model and the null model (ßNTI) correlation analysis also neglect the role of niche-based processes. We conclude that stochastic dispersal limitations, together with ecological drifts, drive fungal communities.

Response of soil microbiome structure and its network profiles to four soil amendments in monocropping strawberry greenhouse.

With the constant surge of strawberry cultivation and human demand, widespread concern has been expressed about the severe soil and plant health problems caused by continuous strawberry cropping, particularly monocropping in greenhouses. Effective microorganisms (EM) and Bacillus subtilis (BS) have been extensively commercialized as biological control agents (BCAs) to promote plant growth and yield enhancement. However, their effects on soil microbes are obscure. To regulate the microbial community in continuous cropping strawberry soils, we developed four soil amendments based on these two BCAs by adding low and high contents of compost. The amplicon sequencing of bacterial and fungal ribosomal markers was applied to study the response of the soil microbiome structure. We noticed a sharp increase in bacterial diversity after adding EM-treated high compost and BS-treated low compost, while there was no significant change in fungal diversity among treatments. Through taxonomic classification and FUNGuild analysis, we found that the application of soil amendments resulted in a significant decline in the relative abundance of fungal plant pathogens (Rhizopus, Penicillium and Fusarium) in the soils; accordingly, the metabolic functions of a range of detrimental fungi were inhibited. Correlation analysis indicated that soil microbial community was indirectly driven by soil physicochemical properties. Co-occurrence networks revealed that soil amendments contributed to the connectivity of bacterial network, and EM-treated with high compost was the most complex and balanced. Collectively, EM-treated high compost and BS-treated low compost can well regulate the microbial community structure and thus maintain soil health.

The existence of C4-bundle-sheath-like photosynthesis in the mid-vein of C3 rice.

BACKGROUND: Recent studies have shown that C4-like photosynthetic pathways partly reside in photosynthetic cells surrounding the vascular system of C3 dicots. However, it is still unclear whether this is the case in C3 monocots, especially at the molecular level. RESULTS: In order to fill this gap, we investigated several characteristics required for C4 photosynthesis, including C4 pathway enzymes, cyclic/non-cyclic photophosphorylation rates, the levels and assembly state of photosynthetic machineries, in the mid-veins of C3 monocots rice with leaf laminae used as controls. The signature of photosystem photochemistry was also recorded via non-invasive chlorophyll a fluorescence and reflectance changes at 820 nm in vivo. Our results showed that rice mid-veins were photosynthetically active with higher levels of three C4 decarboxylases. Meanwhile, the linear electron transport chain was blocked in mid-veins due to the selective loss of dysfunctional photosystem II subunits. However, photosystem I was sufficient to support cyclic electron flow in mid-veins, reminiscent of the bundle sheath in C4 plants. CONCLUSIONS: The photosynthetic attributes required for C4 photosynthesis were identified for the first time in the monocotyledon model crop rice, suggesting that this is likely a general innate characteristic of C3 plants which might be preconditioned for the C4 pathway evolution. Understanding these attributes would provide a base for improved strategies for engineering C4 photosynthetic pathways into rice.

Optimization of electrooptic and pieozoelectric coupling effects in tetragonal relaxor-PT ferroelectric single crystals.

The electrooptic and piezoelectric coupling effects in tetragonal relaxor-based ferroelectric 0.62Pb(Mg1/3Nb2/3)O3-0.38PbTiO3 (PMN-0.38PT) and 0.88Pb(Zn1/3Nb2/3)O3-0.12PbTiO3 (PZN-0.12PT) single-domain crystals have been analyzed by the coordinate transformation. The orientation dependence of the electrooptic and half-wave voltage was calculated based on the full sets of refractive indices, electrooptic and piezoelectric coefficients. The optimum orientation cuts for achieving the best electrooptic coefficient and half-wave voltage were found. The lowset half-wave voltage is only 76 V for the PMN-0.38PT single-domain crystal. Compared to commonly used electrooptic crystal LiNbO3, tetragonal relaxor-PT ferroelectric single-domain crystals are much superior for optical modulation applications because of their much higher linear electrooptic coefficients and substantially lower half-wave voltages when the piezoelectric strain influence is considered.

Complete set of material constants of single domain (K, Na)(Nb, Ta)O3 single crystal and their orientation dependence.

A self-consistent complete set of dielectric, piezoelectric, and elastic constants for single domain Ta modified (K, Na)NbO3 (KNN) crystal was determined. This full set constant for single domain KNN-based crystals allowed the prediction of orientation dependence of the longitudinal dielectric, piezoelectric, elastic coefficients, and electromechanical coupling factors. The maximum piezoelectric and electromechanical properties were found to exist near [001] C . In addition, material constants of [001] C poled domain engineered single crystal with 4 mm symmetry were experimentally measured and compared with the calculated values. Based on this, extrinsic contribution to the piezoelectricity was estimated to be ∼20%.

Changes in chloroplast ultrastructure, fatty acid components of thylakoid membrane and chlorophyll a fluorescence transient in flag leaves of a super-high-yield hybrid rice and its parents during the reproductive stage.

In plants, it is well established that chloroplast is one of the early targgeted organelles to breakdown during leaves senescing. Here we applied a newly developed super-high-yield hybrid rice (Oryza sativa) LiangYouPeiJiu (LYPJ) and its parents lines to investigate changes in ultrastructure of chloroplasts, fatty acid composition of thylakoid membrane lipids and chlorophyll (Chl) a fluorescence transient in natural senescing leaves. We found that at full expansion of flag leaves in three lines, chloroplasts often showed oblong shapes with a typical membrane system of stroma and grana thylakoids, whereas their shapes had been changed from oblong to spherical during senescence. Our data showed that the initiation of senescence displayed accumulation of starch and an increase in the number and size of plastoglobuli with the damaged thylakoid membranes; subsequently, swollen thylakoid membranes in stroma and in grana with a significant increase in MDA content, and disorganization of thylakoid membrane system with significant changes in fatty acid composition of thylakoid membrane lipids were developed. Compared with its parents, the newly developed hybrid rice LYPJ had the highest capacity of carbohydrate transport from leaves (sources) to ears (sink), marked with the lowest accumulation of starch grains in the leaf chloroplasts, and the slowest senescing rate of chloroplast in overall leaf senescence process. Chl a fluorescence transients of three kinds of flag leaves were analyzed by so-called JIP-test. The results of analysis suggest that these findings inculding a high inherited activity of antioxidant enzymes and high photosynthetate transport to pretect chloroplast structure in the hybrid rice LYPJ have close relations to its super-high yield.