Essential Methods of Plant Sample Preparation for High-Resolution Scanning Electron Microscopy at Room Temperature

Comparison and development of scanning electron microscope techniques for delicate plant tissues

Cell deformation often occurs during sample preparation and imaging with scanning electron microscope (SEM), especially with delicate samples, which influences the accuracy of the results. Here we investigate the influence of several preparation methods on cell deformation, using water content and tissue hardness as indicators to classify "delicate" samples of plant species. The degree of deformation in samples resulting from five preparation methods was measured at the tissue and single-cell levels, revealing that a cryo- and methanol-fixation produced lower degrees of tissue dimension deformation and better preservation of cell shape for delicate samples, while for harder tissues, other preparation methods for a dehydrated specimen are also suitable. Stability and image quality of delicate samples could be improved with the application of a cryo-protectant combined with a lower cryo-stage temperature, e.g. - 30 °C. We show that the sample stability under the beam was improved by combining larger sample size and cryo-stage application. Furthermore, the influence of adaxial and abaxial tissue surfaces, the accelerating voltage, and sputter coating time on sample stability and image quality was evaluated. Our study is valuable for artifact reduction and easy application of SEM.

Synchronized Efficacy and Mechanism of Alkaline Fertilizer and Biocontrol Fungi for Fusariumoxysporum f. sp. cubense Tropical Race 4

The purpose of this study was to determine the effect and mechanism of alkaline fertilizer, bio-control fungi, and their synergistic application on control of Fusarium Tr4 incidence. Synchronized use of the alkaline fertilizer and biocontrol fungi eliminates rhizome browning and reduces the incidence rate of banana Fusarium wilt. The incidence of yellow leaves (ratio of yellow leaf to total leaf) and disease index in +Foc Tr4 CF treatment were the same (65%), while incidence of yellow leaves and disease index in +Foc Tr4 AFBCF were 31% and 33%, respectively. Under the stress of Foc Tr4 infection, the synergistic utilization of the alkaline fertilizer and biocontrol fungi would raise the activities of peroxidase, catalase and superoxide dismutase in banana roots. The root activity of banana was also increased. As a result, the banana height and stem diameter increments, shoot and root dry weight, accumulation of N, P and K in banana plants had been increased. The efficacy of the synergistic application of alkaline fertilizer and biocontrol fungi was not only reducing Foc Tr4 pathogen colonization and distribution in banana plants, but also preventing tylosis formation in vascular vessel effectively. Therefore, the normal transport of water and nutrients between underground and aboveground is ensured.

Hydrophobin CmHYD1 Is Involved in Conidiation, Infection and Primordium Formation, and Regulated by GATA Transcription Factor CmAreA in Edible Fungus, Cordyceps militaris

Hydrophobins are a family of small proteins exclusively secreted by fungi, and play a variety of roles in the life cycle. Cmhyd1, one of the hydrophobin class II members in Cordyceps militaris, has been shown to have a high transcript level during fruiting body development. Here, deletion of Cmhyd1 results in reduction in aerial mycelia, conidiation, hydrophobicity and infection ability, and complete inhibition of pigmentation and primordium differentiation. Cmhyd1 plays roles in conidiation and cuticle-bypassing infection by regulating the transcripts of frequency clock protein, Cmfrq, and velvet protein, Cmvosa, as well as primordium formation via the mitogen-activated protein kinase signaling pathway. Cmhyd1 also participates in stress response, including tolerance of mycelia to osmotic and oxidative stresses, and conidia to high or low temperatures. CmAreA, a transcription factor of nitrogen regulatory, is recruited to the promoter of Cmhyd1 and activates the transcription of Cmhyd1 with coactivator CmOTam using electrophoretic mobility shift assays and transient luciferase expression in tobacco. Furthermore, CmHYD1 is proved to regulate the transcription of Cmarea at different developmental stages via a positive feedback loop. These results reveal the diverse roles and regulation of Cmhyd1 in C. militaris, and provide insights into the developmental regulatory mechanism of mushrooms.

Ultrastructure and development of acanthocytes, specialized cells in Stropharia rugosoannulata, revealed by scanning electron microscopy (SEM) and cryo-SEM

Acanthocytes are special cells with a distinct spiky shape produced exclusively by the fungi of Stropharia and can be used to defend against nematodes. In the present study, the ultrastructure and development of acanthocytes were revealed by scanning electron microscopy (SEM) and cryo-SEM in S. rugosoannulata, a popular cultivated mushroom both in China and Europe. The acanthocytes were abundant on the surface of rhizomorph, casing soils, and vegetative mycelia of homokaryotic and heterokaryotic strains in S. rugosoannulata. The development of the acanthocyte was investigated with cryo-SEM, which has distinct advantage for observation of the ultrastructure of live, hydrated structures. Three distinct stages, including formation of lateral branch that was covered with patches, spiky structure formation, and maturation of acanthocytes, were identified and described. The irregular patches deposited on the surface of lateral branches and the holes in the spiky branches of the acanthocytes were reported for the first time. The environmental nitrogen level showed impact on acanthocyte production, but it seemed not to be the indispensable factor. Acid medium could delay the initiation of the acanthocyte formation but did not affect the overall morphology and structure, indicating that the central deposit of acanthocytes should be acid soluble. Acanthocytes of S. rugosoannulata have similar hydrophobicity to mycelia. The observation of ultrastructure and development process of acanthocytes provides insights into the ecological function and evolution of this special structure.