The fact that longan, a cold-sensitive species, showed low expression levels for these three genes, suggests a possible strategy for genetic improvement of cold tolerance in this crop. Cold storage of apples is often used to extend post-harvest storage; however, it leads to superficial scald development, which is a major physiological disorder characterized by necrosis of the hypodermal cortical tissue. Karagiannis et al. applied a multiomics systems approach and created regulatory module networks to compare scald-affected and healthy apple phenotypes. Individual and combinatorial treatments with ozone , which induced scald symptoms, and 1-methylcyclopropene , which reversed O3-stimulated scald effect, were used to identify pathways and gene-to-protein-to-metabolite networks involved in scald prevention and sensitivity. Importantly, 1-MCP-induced scald tolerance correlated with the expression of genes involved in photosynthesis, stress responses, flavonoid biosynthesis, and ethylene signaling in apple peel and key TFs that may control some of these processes. This study represents an important contribution for future functional studies to develop improved apple cultivars to superficial scald. The acquisition of cold tolerance under conditions of varying light quality is essential for plants growing in regions with seasonal variation in both temperature and light . Photoinhibition, i.e., the downregulation of the electron transport chain, reduces plant productivity, but safeguards the photosynthetic apparatus during cold and light stress . Wang et al. investigated the role of light quality, specifically, low red to far-red ratios , nursery pots on photoprotection during cold stress in tomato. They showed that L-R/FR activated two pathways associated with cyclic electron flow : the PGR5/PGRL1A- and NDHdependent complexes, respectively.
These CEF complexes help to reduce cold-induced photo damage of the photosynthetic machinery by accelerating the thermal dissipation of excess energy, enhancing ROS scavenging, and reducing the hyperreduction of the electron transport chain. This work therefore provides a better understanding of the mechanistic relationship between varying light quality and low temperature in plant photosynthetic performance in temperate climates when seasonal variation induces these conditions.Spring frosts cause important economic losses in many fruit-producing areas of the world, and there is interest in identifying feasible approaches to mitigate these risks. Ethylene controls fruit ripening in climacteric species but it also plays an important role in plant stress responses . Published literature on the use of ethylene or ethylene-based compounds for protecting temperate fruit orchards against frost damage was reviewed . Experimental evidence of ethylene modulation of bud dormancy and blooming were presented and discussed. It was suggested that ethylene-delayed bloom and the associated frost protection may result from either the slowing down of floral bud responsiveness to seasonal temperature changes, an antagonistic interaction with other hormones such as abscisic acid or gibberellins, plant sensing of exogenous ethylene as a stress signal leading to longer dormancy, or ethylene-enhanced ROS accumulation resulting in extended bus dormancy. Because chilling stress in plants often leads to ROS accumulation, the questions arises whether improving the antioxidant capacity of tissues by the exogenous application of antioxidant treatments may help improve tolerance to cold as well as to other types of abiotic stress.
To this purpose, Tang et al. treated low bush blueberry seedlings with hydrogen sulfide , and found that treated plantlets performed better under low temperatures than the untreated controls, as shown by the alleviation of membrane peroxidation, the reduction of chlorophyll and carotenoid degradation, and the lessening of photosystem I and II photoinhibition. Conversely, the application of hypotaurine, a H2S scavenger, aggravated the oxidative symptoms under cold stress. Brassinolide is an important plant stress hormone shown to promote plant resistance to low-temperature environments. Zhang et al. investigated the effects of exogenous BR on the photosynthetic characteristics, leaf anatomical structure, and chloroplast ultrastructure of two species of tung tree seedlings under different temperature conditions. The results suggested that long-term low temperatures significantly reduced the photosynthetic efficiency of tung tree seedlings, affecting the formation of the internal structure of plant leaves and destroying the integrity and function of the chloroplast. To prevent this, external application of BR to tung tree seedlings could enhance the photosynthetic potential of tung trees by maintaining the stability of the leaf structure and morphology and alleviating the damage caused by cold injury. In summary, the papers in this collection illustrated the breadth of research aimed at understanding chilling responses in horticultural crops, but more importantly provided new insights that will further our future basic and applied research in this area.The human oral microbiome has been the subject of modern research for a considerably long time, from W. D. Miller’s works in the late 19th century to the exponentially increasing literature starting around 2002. The earliest published work in this area in the PubMed database goes back to 1958, and this work cites earlier works on the antibacterial characteristics of human saliva.
In addition to the anatomical components of the oral cavity and their respective roles in human health, the presence and activity of microbes in the oral cavity make this part of the body particularly interesting and difficult to study, as the relatively open nature and frequent use of the oral cavity make it a ready repository for microbes as well as a direct gateway into the body, especially into the upper respiratory system. In the last thirty years, research has begun to reveal the prevalence and diverse roles of the microbes, including strong correlations between some periodontal and respiratory diseases and bacterial members from certain genera such as Porphyromonas and Streptococcus. Despite the progress made in this field, details of the interactions among the microbes and between microbes and the host body are complex and remain difficult to elucidate. While many microbes in the oral cavity are commensal organisms that benefit from the host without harming the host, some microbes are pathogenic or opportunistically pathogenic organisms that take advantage of the weakening of the host immune system. Furthermore, the disruption of oral health can have broad and lasting consequences for the entire body. For example, even slight dental discomfort drastically decreases the range of nutrient intake and efficiency of food processing. On the microbial level, it is hardly surprising that dysbiosis of the oral microbial community can be readily linked to systemic diseases. On the other hand, this link can enable noninvasive as-sessments of the body by examination of salivary markers that are bacterial in origin. Clearly, the intricate and intimate relationships between host health and oral microbial communities, or the oral microbiome, merit deep delving, as the oral microbiome not only can act as an agent in but also serve as an indicator of human health and disease.Earlier research on the human oral microbiome faced considerable challenges due to difficulties in reliable cultivation of fastidious organisms and accurate identification of microbes. Though the effort to isolate and cultivate more oral bacterial phenotypes has been ongoing, cultivation of organisms remains a substantial obstacle. As of 2017, 32% of the 700 prokaryotic species identified in the oral cavity are known as uncultivated phenotypes. Part of the difficulty in cultivability stems from low prevalence and/or slow growth; other parts of uncultivability come from the nutritional and environmental requirements of certain strains that cannot be supplied in vitro, including resources supplied by other species and by the host system. For instance, the attachment of P. gingivalis to most strains of streptococcal cells except for the S. sanguis Challis strain is inhibited by saliva, and it extremely difficult to mimic the in vivo conditions of host salivary defense in laboratory cocultures of P. gingivalis and S. sanguis Challis strain. Another example of community-dependence is the parasitic relationship between certain microbial species, such as obligate epibiontic members of the candidate phylum TM7 parasitically preying upon an Actinomyces odontolyticus strain, which would make isolation of TM7 members impossible without the successful isolation, characterization, large pots plastic and cultivation of that particular Actinomyces odontolyticus strain. These commensal, symbiotic, or parasitic interactions among oral microbes are, in large part, stimulated by the physical proximity and biological efficiency conferred by biofilm structures in the oral microbiome, so failure to form biofilms in vitro or even thelack of a suitable surface for biofilm formation would severely interfere with the success of laboratory cultivations of more dependent members. For instance, dental plaque biofilms can exhibit a redox potential of -127mV during development at the enamel surface. This potential could be the key to the survival and proliferation of certain members of the dental plaque microbiome, but would not be possible without a proper biofilm structure.
The challenge of creating or replicating suitable environments in vitro for isolation and cultivation of bacteria is an active part of oral microbiology research, because comprehensive characterization of bacteria, including their virulence, relies heavily upon pure cultures. Ongoing efforts to develop and adapt cultivation methods for uncultivable microbes include simulating the natural environment of the oral microbiomes, supplementing with specific compounds, extending cultivation times, using assistive strains, and depleting environmental factors that contribute to the growth of other microbes. Other techniques applied to isolate and cultivate various species in other microbial communities include dilution to a smaller number of or a single cell, conditioning cultures with spent culture supernatants or cell-free extracts of helper strains, and adding signaling molecules to the cultures. These particular techniques, as-yet-untried in the cultivation of oral bacteria, are worthy of investigation for the purposes of not only isolating and cultivating yet uncultivable members of the oral community, but also enriching and expanding existing in vitro oral microbiome models. The other major challenge of oral microbiome research arises not from biological barriers to growing bacteria in vitro, but in the lack of proper methodology to distinguish among phenotypically identical but genotypically distinct species or strains. Many different methods have been used to achieve this goal. Visual identification with microscopy was widely used for some years. Confocal laser scanning microscopy has been used to identify streptococci bacteria in the early stages of dental plaque formation, determine viability, and study structure formation. Fluorescent microscopy with target-specific oligonucleotide probes has also been a common method of identificationand visualization. Non-visual methods include polymerase chain reaction to amplify 16S rRNA , PCR-based denaturing gradient gel electrophoresis, and DNA-DNA hybridization. These nucleic-acid-based techniques formed the earlier foundations of microbiome research, including the study of physical structures of the resident oral microbiome. However, they suffered from limitations such as lack of penetrative powers into the cell, incorrect hybridization of probes to targets, and PCR bias. In more recent years, the advent of high-throughput sequencing has allowed for more facile and in-depth studies of both the memberships and the abundances of the oral microbiome. In particular, the identification of microorganisms by genetic marker sequencing, especially the variable regions of the bacterial 16S rRNA, has exploded in popularity because of its versatility, ease of operation, and cost efficiency. High-throughput sequencing has deep roots in PCR, DNA microarrays, and fluorescence detection. The four most popular commercial HTS technologies rely on DNA synthesis and nucleic acid adapter ligation onto the template, and PCR lies at the heart of three of the four technologies. The Oxford Nanopore technology takes advantage of a porous bilayer through which a motor protein threads the growing DNA sequence. As the synthesized sequence proceeds through the bilayer, the base being added onto the growing strand is measured by the change in the current. This technique allows both strands of the template DNA to be sequenced, and can be done without PCR amplification. The sequencing technology from Pacific Biosciences relies on ligating hairpin adapters onto the template and using strand-displacing polymerase to sequence the template multiple times. Detection on the PacBio platform is achieved by reading signals from phosphate-labeled nucleotides in a zero-mode waveguide well. The third HTS platform comes from Life Technologies/ThermoFisher and relies on the concept of pyrosequencing. The beads are placed into microwells, and the immobilized DNA is amplified by emulsion-PCR. The nucleotide being incorporated is detected by measuring pH changes from hydrogen ion release, achieved by Ion Torrent’s semiconductor technology. These pH changes are converted to a voltage signal proportional to the number of bases incorporated. These three platforms have different applications, but are generally sub-optimal for fast identification of microorganisms. The average read lengths of the first two platforms are in the thousands of base pairs, far longer than the sufficient lengths for microbial identification, and the sequencing depth per run is limited to under 100,000 reads, which is too low for microbial communities with a large number of members.