The phyllosphere

Microbial influencers and cotton leaf curl disease (CLCuD) susceptibility: a network perspective

Biotic stresses, such as plant viruses, e.g., cotton leaf curl virus (CLCuV), can alter root-associated and leaf-associated microbial diversities in plants. There are complex ecological dynamics at play, with each microbe contributing to a multitude of biotic and abiotic interactions, thus deciding the stability of the plant's ecosystem in response to the disease. Deciphering these networks of interactions is a challenging task. The inferential research in microbiome is also at a nascent stage, often constrained by the underlying analytical assumptions and the limitations with respect to the depth of sequencing. There is also no real consensus on network-wide statistics to identify the influential microbial players in a network. Guided by the latest developments in network science, including recently published metrics such as Integrated View of Influence (IVI) and some other centrality measures, this study provides an exposé of the most influential nodes in the rhizospheric and phyllospheric microbial networks of the cotton leaf curl disease (CLCuD) susceptible, partially tolerant, and resistant cotton varieties. It is evident from our results that the CLCuD-resistant Gossypium arboreum possesses an equal share of keystone species, which helps it to withstand ecological pressures. In the resistant variety, the phyllosphere harbors the most influential nodes, whereas in the susceptible variety, they are present in the rhizosphere. Based on hubness score, spreading score, and IVI, the top 10 occurring keystone species in the FDH-228 (resistant) variety include Actinokineospora, Cohnella, Thermobacillus, Clostridium, Desulfofarcimen, and MDD-D21. Elusimicrobia, Clostridium-sensu-stricto_12, Candidatus woesebacteria, and Dyella were identified as the most influential nodes in the PFV-1 (partially tolerant) variety. In the PFV-2 (susceptible) variety, the keystone species were identified as Georginia, Nesterenkonia, Elusimicrobia MVP-88, Acetivibrio, Tepedisphaerales, Chelatococcus, Nitrosospira, and RCP2-54. This concept deciphers the diseased and healthy plant's response to viral disease, which may be microbially mediated.

Dynamic changes in fungal communities and functions in different air-curing stages of cigar tobacco leaves

Introduction

Air curing (AC) plays a crucial role in cigar tobacco leaf production. The AC environment is relatively mild, contributing to a diverse microbiome. Fungi are important components of the tobacco and environmental microbiota. However, our understanding of the composition and function of fungal communities in AC remains limited.

Methods

In this study, changes in the chemical constituents and fungal community composition of cigar tobacco leaves during AC were evaluated using flow analysis and high-throughput sequencing.

Results

The moisture, water-soluble sugar, starch, total nitrogen, and protein contents of tobacco leaves exhibited decreasing trends, whereas nicotine showed an initial increase, followed by a decline. As determined by high-throughput sequencing, fungal taxa differed among all stages of AC. Functional prediction showed that saprophytic fungi were the most prevalent type during the AC process and that the chemical composition of tobacco leaves is significantly correlated with saprophytic fungi.

Conclusion

This study provides a deeper understanding of the dynamic changes in fungal communities during the AC process in cigar tobacco leaves and offers theoretical guidance for the application of microorganisms during the AC process.

Applied microbiology of the phyllosphere

The phyllosphere, or plant leaf surface, represents a microbial ecosystem of considerable size, holding extraordinary biodiversity and enormous potential for the discovery of new products, tools, and applications in biotechnology, agriculture, medicine, and elsewhere. This mini-review highlights the applied microbiology of the phyllosphere as an original field of study concerning itself with the genes, gene products, natural compounds, and traits that underlie phyllosphere-specific adaptations and services that have commercial and economic value for current or future innovation. Examples include plant-growth-promoting and disease-suppressive phyllobacteria, probiotics and fermented foods that support human health, as well as microbials that remedy foliar contamination with airborne pollutants, residual pesticides, or plastics. Phyllosphere microbes promote plant biomass conversion into compost, renewable energy, animal feed, or fiber. They produce foodstuffs such as thickening agents and sugar substitutes, industrial-grade biosurfactants, novel antibiotics and cancer drugs, as well as enzymes used as food additives or freezing agents. Furthermore, new developments in DNA sequence-based profiling of leaf-associated microbial communities allow for surveillance approaches in the context of food safety and security, for example, to detect enteric human pathogens on leafy greens, predict plant disease outbreaks, and intercept plant pathogens and pests on internationally traded goods. KEY POINTS: • Applied phyllosphere microbiology concerns leaf-specific adaptations for economic value • Phyllobioprospecting searches the phyllosphere microbiome for product development • Phyllobiomonitoring tracks phyllosphere microbial profiles for early risk detection.

Unraveling two decades of phyllosphere endophytes: tracing research trends and insights through visualized knowledge maps, with emphasis on microbial interactions as emerging frontiers

Phyllosphere endophytes play a critical role in a myriad of biological functions, such as maintaining plant health and overall fitness. They play a determinative role in crop yield and quality by regulating vital processes, such as leaf functionality and longevity, seed mass, apical growth, flowering, and fruit development. This study conducted a comprehensive bibliometric analysis aiming to review the prevailing research trajectories in phyllosphere endophytes and harness both primary areas of interest and emerging challenges. A total of 156 research articles on phyllosphere endophytes, published between 2002 and 2022, were retrieved from the Web of Science Core Collection (WoSCC). A systematic analysis was conducted using CiteSpace to visualize the evolution of publication frequency, the collaboration network, the co-citation network, and keywords co-occurrence. The findings indicated that initially, there were few publications on the topic of phyllosphere endophytes. However, from 2011 onwards, there was a notable increase in the number of publications on phyllosphere endophytes, gaining worldwide attention. Among authors, Arnold, A Elizabeth is widely recognized as a leading author in this research area. In terms of countries, the USA and China hold the highest rankings. As for institutional ranking, the University of Arizona is the most prevalent and leading institute in this particular subject. Collaborative efforts among the authors and institutions tend to be confined to small groups, and a large-scale collaborative network needs to be established. This study identified the influential journals, literature, and hot research topics. These findings also highlight the interconnected nature of key themes, e.g., phyllosphere endophyte research revolves around the four pillars: diversity, fungal endophytes, growth, and endophytic fungi. This study provides an in-depth perspective on phyllosphere endophytes studies, revealing the identification of biodiversity and microbial interaction of phyllosphere endophytes as the principal research frontiers. These analytical findings not only elucidate the recent trajectory of phyllosphere endophyte research but also provide invaluable insights for similar studies and their potential applications on a global scale.

Needle bacterial community structure across the species range of limber pine

Bacteria on and inside leaves can influence forest tree health and resilience. The distribution and limits of a tree species' range can be influenced by various factors, with biological interactions among the most significant. We investigated the processes shaping the bacterial needle community across the species distribution of limber pine, a widespread Western conifer inhabiting a range of extreme habitats. We tested four hypotheses: (i) Needle community structure varies across sites, with site-specific factors more important to microbial assembly than host species selection; (ii) dispersal limitation structures foliar communities across the range of limber pine; (iii) the relative significance of dispersal and selection differs across sites in the tree species range; and (iv) needle age structures bacterial communities. We characterized needle communities from the needle surface and tissue of limber pine and co-occurring conifers across 16 sites in the limber pine distribution. Our findings confirmed that site characteristics shape the assembly of bacterial communities across the host species range and showed that these patterns are not driven by dispersal limitation. Furthermore, the strength of selection by the host varied by site, possibly due to differences in available microbes. Our study, by focusing on trees in their natural setting, reveals real needle bacterial dynamics in forests, which is key to understanding the balance between stochastic and deterministic processes in shaping forest tree-microbe interactions. Such understanding will be necessary to predict or manipulate these interactions to support forest ecosystem productivity or assist plant migration and adaptation in the face of global change.

Review: Research progress on seasonal succession of phyllosphere microorganisms

Phyllosphere microorganisms have recently attracted the attention of scientists studying plant microbiomes. The origin, diversity, functions, and interactions of phyllosphere microorganisms have been extensively explored. Many experiments have demonstrated seasonal cycles of phyllosphere microbes. However, a comprehensive comparison of these separate investigations to characterize seasonal trends in phyllosphere microbes of woody and herbaceous plants has not been conducted. In this review, we explored the dynamic changes of phyllosphere microorganisms in woody and non-woody plants with the passage of the season, sought to find the driving factors, summarized these texts, and thought about future research trends regarding the application of phyllosphere microorganisms in agricultural production. Seasonal trends in phyllosphere microorganisms of herbaceous and woody plants have similarities and differences, but extensive experimental validation is needed. Climate, insects, hosts, microbial interactions, and anthropogenic activities are the diverse factors that influence seasonal variation in phyllosphere microorganisms.

Conspecific versus heterospecific transmission shapes host specialization of the phyllosphere microbiome

In disease ecology, pathogen transmission among conspecific versus heterospecific hosts is known to shape pathogen specialization and virulence, but we do not yet know if similar effects occur at the microbiome level. We tested this idea by experimentally passaging leaf-associated microbiomes either within conspecific or across heterospecific plant hosts. Although conspecific transmission results in persistent host-filtering effects and more within-microbiome network connections, heterospecific transmission results in weaker host-filtering effects but higher levels of interconnectivity. When transplanted onto novel plants, heterospecific lines are less differentiated by host species than conspecific lines, suggesting a shift toward microbiome generalism. Finally, conspecific lines from tomato exhibit a competitive advantage on tomato hosts against those passaged on bean or pepper, suggesting microbiome-level host specialization. Overall, we find that transmission mode and previous host history shape microbiome diversity, with repeated conspecific transmission driving microbiome specialization and repeated heterospecific transmission promoting microbiome generalism.

Potential functions of the shared bacterial taxa in the citrus leaf midribs determine the symptoms of Huanglongbing

Instruction

Citrus is a globally important fruit tree whose microbiome plays a vital role in its growth, adaptability, and resistance to stress.

Methods

With the high throughput sequencing of 16S rRNA genes, this study focused on analyzing the bacterial community, especially in the leaf midribs, of healthy and Huanglongbing (HLB)-infected plants.

Results

We firstly identified the shared bacterial taxa in the midribs of both healthy and HLB-infected plants, and then analyzed their functions. Results showed that the shared bacterial taxa in midribs belonged to 62 genera, with approximately 1/3 of which modified in the infected samples. Furthermore, 366 metabolic pathways, 5851 proteins, and 1833 enzymes in the shared taxa were predicted. Among these, three metabolic pathways and one protein showed significant importance in HLB infection. With the random forest method, six genera were identified to be significantly important for HLB infection. Notably, four of these genera were also among the significantly different shared taxa. Further functional characterization of these four genera revealed that Pseudomonas and Erwinia likely contributed to plant defense against HLB, while Streptomyces might have implications for plant defense against HLB or the pathogenicity of Candidatus Liberibacter asiaticus (CLas).

Disccusion

Overall, our study highlights that the functions of the shared taxa in leaf midribs are distinguished between healthy and HLB-infected plants, and these microbiome-based findings can contribute to the management and protection of citrus crops against CLas.

Diversity and biogeography of plant phyllosphere bacteria are governed by latitude-dependent mechanisms

Predicting and managing the structure and function of plant microbiomes requires quantitative understanding of community assembly and predictive models of spatial distributions at broad geographic scales. Here, we quantified the relative contribution of abiotic and biotic factors to the assembly of phyllosphere bacterial communities, and developed spatial distribution models for keystone bacterial taxa along a latitudinal gradient, by analyzing 16S rRNA gene sequences from 1453 leaf samples taken from 329 plant species in China. We demonstrated a latitudinal gradient in phyllosphere bacterial diversity and community composition, which was mostly explained by climate and host plant factors. We found that host-related factors were increasingly important in explaining bacterial assembly at higher latitudes while nonhost factors including abiotic environments, spatial proximity and plant neighbors were more important at lower latitudes. We further showed that local plant-bacteria associations were interconnected by hub bacteria taxa to form metacommunity-level networks, and the spatial distribution of these hub taxa was controlled by hosts and spatial factors with varying importance across latitudes. For the first time, we documented a latitude-dependent importance in the driving factors of phyllosphere bacteria assembly and distribution, serving as a baseline for predicting future changes in plant phyllosphere microbiomes under global change and human activities.

Exploration of phyllosphere microbiomes in wheat varieties with differing aphid resistance

BACKGROUND

Leaf-associated microbes play an important role in plant development and response to exogenous stress. Insect herbivores are known to alter the phyllosphere microbiome. However, whether the host plant's defense against insects is related to the phyllosphere microbiome remains mostly elusive. Here, we investigated bacterial communities in the phyllosphere and endosphere of eight wheat cultivars with differing aphid resistance, grown in the same farmland.

RESULTS

The bacterial community in both the phyllosphere and endosphere showed significant differences among most wheat cultivars. The phyllosphere was connected to more complex and stable microbial networks than the endosphere in most wheat cultivars. Moreover, the genera Pantoea, Massilia, and Pseudomonas were found to play a major role in shaping the microbial community in the wheat phyllosphere. Additionally, wheat plants showed phenotype-specific associations with the genera Massilia and Pseudomonas. The abundance of the genus Exiguobacterium in the phyllosphere exhibited a significant negative correlation with the aphid hazard grade in the wheat plants.

CONCLUSION

Communities of leaf-associated microbes in wheat plants were mainly driven by the host genotype. Members of the genus Exiguobacterium may have adverse effects on wheat aphids. Our findings provide new clues supporting the development of aphid control strategies based on phyllosphere microbiome engineering.

Core community drives phyllosphere bacterial diversity and function in multiple ecosystems

The phyllosphere provides a habitat for a large sum of microorganisms which are modulated by numerous biotic and abiotic factors. While it is logical that host lineage must have some effect on the phyllosphere habitat, it is unclear if phyllospheres harbor similar microbial core communities across multiple ecosystems at the continental-scale. Here we collected 287 phyllosphere bacterial communities from seven ecosystems (including paddy field, dryland, urban area, protected agricultural land, forest, wetland, and grassland) in east-China to identify the regional core community and to characterize the importance of such communities in maintaining phyllosphere bacterial community structure and function. Despite significantly different bacterial richness and structure, the seven studied ecosystems contained a similar regional core community of 29 OTUs that comprised 44.9 % of the total bacterial abundance. The regional core community was less affected by environmental variables and less connected in the co-occurrence network compared with other non-core OTUs (the whole minus regional core community). Furthermore, the regional core community also had a large proportion (>50 %) of a constrained set of nutrient metabolism related functional potentials and less functional redundancy. This study suggests there is a robust regional core phyllosphere community regardless of ecosystem or spatial and environmental heterogeneity, and supports the argument that core communities are pivotal in maintaining microbial community structure and function.

Microbiome Interconnectedness throughout Environments with Major Consequences for Healthy People and a Healthy Planet

Microbiomes have highly important roles for ecosystem functioning and carry out key functions that support planetary health, including nutrient cycling, climate regulation, and water filtration. Microbiomes are also intimately associated with complex multicellular organisms such as humans, other animals, plants, and insects and perform crucial roles for the health of their hosts. Although we are starting to understand that microbiomes in different systems are interconnected, there is still a poor understanding of microbiome transfer and connectivity. In this review we show how microbiomes are connected within and transferred between different habitats and discuss the functional consequences of these connections. Microbiome transfer occurs between and within abiotic (e.g., air, soil, and water) and biotic environments, and can either be mediated through different vectors (e.g., insects or food) or direct interactions. Such transfer processes may also include the transmission of pathogens or antibiotic resistance genes. However, here, we highlight the fact that microbiome transmission can have positive effects on planetary and human health, where transmitted microorganisms potentially providing novel functions may be important for the adaptation of ecosystems.

Nigrospora oryzae Causing Leaf Spot Disease on Chrysanthemum × morifolium Ramat and Screening of Its Potential Antagonistic Bacteria

Chrysanthemum × morifolium Ramat. is a famous perennial herb with medicinal, edible, and ornamental purposes, but the occurrence of plant diseases can reduce its value. A serious disease that caused leaf spots in C. morifolium appeared in 2022 in Tongxiang City, Zhejiang Province, China. The C. morifolium leaves with brown spots were collected and used for pathogen isolation. By completing Koch's postulates, it was proven that the isolate had pathogenicity to infect C. morifolium. It was determined that the pathogen isolated from chrysanthemum leaves was Nigrospora oryzae, through morphology and a multilocus sequence analysis method using a combination of the internal transcribed spacer gene (ITS), beta-tubulin gene (TUB2), and translation elongation factor 1-alpha gene (TEF1-α). This is the first report of C. morifolium disease caused by N. oryzae in the world. Through dual culture assay on PDA plates, 12 strains of bacteria with antagonistic effects were selected from 231 strains from the C. morifolium phyllosphere, among which Bacillus siamensis D65 had the best inhibitory effect on N. oryzae growth. In addition, the components of a strain D65 fermentation broth were profiled by SPME-GC-Q-TOF analysis, providing a foundation for further application and research of biological control.

Leaf side determines the relative importance of dispersal versus host filtering in the phyllosphere microbiome

Leaves harbor distinct microbial communities that can have an important impact on plant health and microbial ecosystems worldwide. Nevertheless, the ecological processes that shape the composition of leaf microbial communities remain unclear, with previous studies reporting contradictory results regarding the importance of bacterial dispersal versus host selection. This discrepancy could be driven in part because leaf microbiome studies typically consider the upper and lower leaf surfaces as a single entity despite these habitats possessing considerable anatomical differences. We characterized the composition of bacterial phyllosphere communities from the upper and lower leaf surfaces across 24 plant species. Leaf surface pH and stomatal density were found to shape phyllosphere community composition, and the underside of leaves had lower richness and higher abundances of core community members than upper leaf surfaces. We found fewer endemic bacteria on the upper leaf surfaces, suggesting that dispersal is more important in shaping these communities, with host selection being a more important force in microbiome assembly on lower leaf surfaces. Our study illustrates how changing the scale in which we observe microbial communities can impact our ability to resolve and predict microbial community assembly patterns on leaf surfaces. IMPORTANCE Leaves can harbor hundreds of different bacterial species that form unique communities for every plant species. Bacterial communities on leaves are really important because they can, for example, protect their host against plant diseases. Usually, bacteria from the whole leaf are considered when trying to understand these communities; however, this study shows that the upper and lower sides of a leaf have a very different impact on how these communities are shaped. It seems that the bacteria on the lower leaf side are more closely associated with the plant host, and communities on the upper leaf side are more impacted by immigrating bacteria. This can be really important when we want to treat, for example, crops in the field with beneficial bacteria or when trying to understand host-microbe interactions on the leaves.

Mangroves leaves phyllosphere bacteria community and its ability to survive under pyrene stress during the acclimation process

Plants in general and mangroves in particular can harbor hyper-diverse microorganisms in their different compartments including the phyllosphere area. This study used the leaves of three mangrove species; black mangrove (Avicenia germinans), red mangrove (Rhizophora mangle) and mangrove apple (Sonneratia alba) in order to evaluate the phyllosphere epiphytic bacterial community on their leaves surface and assess the ability of some epiphytic bacteria to tolerate and survive under pyrene stress. Through the 16S rRNA genes sequencing, 380203, 405203 and 344863 OTUs were identified respectively in the leaves of mangroves apple, black and red mangroves. The identified OTUs was positively correlated with leaves-wax (p < 0.05, r² = 0.904), nitrogen (r² = 0.72), phosphorus content (r² = 0.62) and the water factor (r² = 0.93). It was however highly and negatively correlated with the canopy cover (r² = 0.93). The pyrene degradation rate in the mineral salt medium (MSM) containing pyrene as external stress was different in each mangrove species and varied depending on various factors. Therefore, through the succession culture in MSM, several bacteria strain belonging to Rhizobiales and Enterobacteres were found to be abundant in red mangroves. Bacteria belonging to Bacilliales and Sphingobacteriales were more abundant in mangroves apples and bacteria from Xanthomonadales and Sphingomonadales were more presents in back mangroves. The important finding was to reveal that the black mangrove at the non-submerged substrate, recorded the highest number of OTU, coinciding with its highest leaf's nitrogen and phosphorus content and most importantly, its highest rate of pyrene degradation. The general result of this study join previous research results and get place in the mangrove agenda, as part of a better understanding insight into the role of plant identity in driving the phyllosphere epiphytic microbial community structures in mangrove ecosystems.

Plant species shape the bacterial communities on the phyllosphere in a hyper-arid desert

Microorganisms are an important component of global biodiversity. However, they are vulnerable to hyper-arid climates in desert regions. Xerophytes are desert vegetation with unique biodiversity. However, little is known about the identities and communities of phyllosphere epiphytic microorganisms inhabiting the xerophyte leaf surface in the hot and dry environment. The diversity and community composition of phyllosphere epiphytes on different desert plants in Gansu, China, was investigated using the next-generation sequencing technique, revealing the diversity and community composition of the phyllosphere epiphytic bacteria associated with desert xerophytes. In addition, the ecological functions of the bacterial communities were investigated by combining the sequence classification information and prokaryotic taxonomic function annotation (FAPROTAX). This study determined the phyllosphere bacterial community composition, microbial interactions, and their functions. Despite harsh environments in the arid desert, we found that there are still diverse epiphytic bacteria on the leaves of desert plants. The bacterial communities mainly included Actinobacteria (52.79%), Firmicutes (31.62%), and Proteobacteria (12.20%). Further comparisons revealed different microbial communities, including Firmicutes at the phylum and Paenibacillaceae at the family level, in the phyllosphere among different plants, suggesting that the host plants had strong filter effects on bacteria. Co-occurrence network analysis revealed positive relationships were dominant among different bacterial taxa. The abundance of Actinobacteria and Proteobacteria was positively correlated, demonstrating their mutual relationship. On the other hand, the abundance of Firmicutes was negatively correlated, which suggested that they inhibit the growth of other bacterial taxa. FAPROTAX prediction revealed that chemoheterotrophy (accounting for 39.02% of the community) and aerobic chemoheterotrophy (37.01%) were the main functions of the leaf epiphytic bacteria on desert plants. This study improves our understanding of the community composition and ecological functions of plant-associated microbial communities inhabiting scattered niches in the desert ecosystem. In addition, the study provides insight into the biodiversity assessment in the desert region.

Seasonal activities of the phyllosphere microbiome of perennial crops

Understanding the interactions between plants and microorganisms can inform microbiome management to enhance crop productivity and resilience to stress. Here, we apply a genome-centric approach to identify ecologically important leaf microbiome members on replicated plots of field-grown switchgrass and miscanthus, and to quantify their activities over two growing seasons for switchgrass. We use metagenome and metatranscriptome sequencing and curate 40 medium- and high-quality metagenome-assembled-genomes (MAGs). We find that classes represented by these MAGs (Actinomycetia, Alpha- and Gamma- Proteobacteria, and Bacteroidota) are active in the late season, and upregulate transcripts for short-chain dehydrogenase, molybdopterin oxidoreductase, and polyketide cyclase. Stress-associated pathways are expressed for most MAGs, suggesting engagement with the host environment. We also detect seasonally activated biosynthetic pathways for terpenes and various non-ribosomal peptide pathways that are poorly annotated. Our findings support that leaf-associated bacterial populations are seasonally dynamic and responsive to host cues.

Distinctive Structure and Assembly of Phyllosphere Microbial Communities between Wild and Cultivated Rice

Wild rice has been demonstrated to possess enriched genetic diversity and multiple valuable traits involved in disease/pest resistance and abiotic stress tolerance, which provides a potential resource for sustainable agriculture. However, unlike the plant compartments such as rhizosphere, the structure and assembly of phyllosphere microbial communities of wild rice remain largely unexplored. Through amplicon sequencing, this study compared the phyllosphere bacterial and fungal communities of wild rice and its neighboring cultivated rice. The core phyllosphere microbial taxa of both wild and cultivated rice are dominated with Pantoea, Methylobacterium, Nigrospora, and Papiliotrema, which are potentially beneficial to rice growth and health. Compared to the cultivated rice, Methylobacterium, Sphingomonas, Phaeosphaeria, and Khuskia were significantly enriched in the wild rice phyllosphere. The potentially nitrogen-fixing Methylobacterium is the dominated wild-enriched microbe; Sphingomonas is the hub taxon of wild rice networks. In addition, the microbiota of wild rice was more governed by deterministic assembly with a more complicated and stable community network than the cultivated rice. Our study provides a list of the beneficial microbes in the wild rice phyllosphere and reveals the microbial divergence between wild rice and cultivated rice in the original habitats, which highlights the potential selective role of wild rice in recruiting specific microbiomes for enhancing crop performance and promoting sustainable food production. IMPORTANCE Plant microbiota are being considered a lever to increase the sustainability of food production under a changing climate. In particular, the microbiomes associated with ancestors of modern cultivars have the potential to support their domesticated cultivars. However, few efforts have been devoted to studying the biodiversity and functions of microbial communities in the native habitats of ancestors of modern crop species. This study provides a list of the beneficial microbes in the wild rice phyllosphere and explores the microbial interaction patterns and the functional profiles of wild rice. This information could be useful for the future utilization of the plant microbiome to enhance crop performance and sustainability, especially in the framework of sustainable agroecosystems.

Understanding the sugar beet holobiont for sustainable agriculture

The importance of crop-associated microbiomes for the health and field performance of plants has been demonstrated in the last decades. Sugar beet is the most important source of sucrose in temperate climates, and-as a root crop-yield heavily depends on genetics as well as on the soil and rhizosphere microbiomes. Bacteria, fungi, and archaea are found in all organs and life stages of the plant, and research on sugar beet microbiomes contributed to our understanding of the plant microbiome in general, especially of microbiome-based control strategies against phytopathogens. Attempts to make sugar beet cultivation more sustainable are increasing, raising the interest in biocontrol of plant pathogens and pests, biofertilization and -stimulation as well as microbiome-assisted breeding. This review first summarizes already achieved results on sugar beet-associated microbiomes and their unique traits, correlating to their physical, chemical, and biological peculiarities. Temporal and spatial microbiome dynamics during sugar beet ontogenesis are discussed, emphasizing the rhizosphere formation and highlighting knowledge gaps. Secondly, potential or already tested biocontrol agents and application strategies are discussed, providing an overview of how microbiome-based sugar beet farming could be performed in the future. Thus, this review is intended as a reference and baseline for further sugar beet-microbiome research, aiming to promote investigations in rhizosphere modulation-based biocontrol options.

Quercus ilex Phyllosphere Microbiome Environmental-Driven Structure and Composition Shifts in a Mediterranean Contex

The intra- and interdomain phyllosphere microbiome features of Quercus ilex L. in a Mediterranean context is reported. We hypothesized that the main driver of the phyllosphere microbiome might be the season and that atmospheric pollutants might have a co-effect. Hence, we investigated the composition of epiphytic bacteria and fungi of leaves sampled in urban and natural areas (in Southern Italy) in summer and winter, using microscopy and metagenomic analysis. To assess possible co-effects on the composition of the phyllosphere microbiome, concentrations of particulate matter and polycyclic aromatic hydrocarbons (PAHs) were determined from sampled leaves. We found that environmental factors had a significative influence on the phyllosphere biodiversity, altering the taxa relative abundances. Ascomycota and Firmicutes were higher in summer and in urban areas, whereas a significant increase in Proteobacteria was observed in the winter season, with higher abundance in natural areas. Network analysis suggested that OTUs belonging to Acidobacteria, Cytophagia, unkn. Firmicutes(p), Actinobacteria are keystone of the Q. ilex phyllosphere microbiome. In addition, 83 genes coding for 5 enzymes involved in PAH degradation pathways were identified. Given that the phyllosphere microbiome can be considered an extension of the ecosystem services offered by trees, our results can be exploited in the framework of Next-Generation Biomonitoring.

Genomic Characterization of Aureimonas altamirensis C2P003-A Specific Member of the Microbiome of Fraxinus excelsior Trees Tolerant to Ash Dieback

Some European ash trees show tolerance towards dieback caused by the invasive pathogen Hymenoscyphus fraxineus. The microbiome of these trees harbours a range of specific bacterial groups. One of these groups belonging to the species Aureimonas altamirensis was studied in detail by genome analysis and a plant inoculation trial. The strain group was shown to be phylogenetically distinct from clinical isolates by 16S rRNA analysis and phylogenomics. Genome analysis of a representative strain C2P003 resulted in a large number of unique gene sequences in comparison to other well-studied strains of the species. A functional analysis of the genome revealed features associated with the synthesis of exopolysaccharides, protein secretion and biofilm production as well as genes for stress adaptation, suggesting the ability of C2P003 to effectively colonize ash leaves. The inoculation of ash seedlings with C2P003 showed a significant positive effect on the plant health of the seedlings that were exposed to H. fraxineus infection. This effect was maintained over a period of three years and was accompanied by a significant shift in the bacterial microbiome composition one year after inoculation. Overall, the results indicate that C2P003 may suppress H. fraxineus in or on ash leaves via colonization resistance or indirectly by affecting the microbiome.

Pathways to engineering the phyllosphere microbiome for sustainable crop production

Current disease resistance breeding, which is largely dependent on the exploitation of resistance genes in host plants, faces the serious challenges of rapidly evolving phytopathogens. The phyllosphere is the largest biological surface on Earth and an untapped reservoir of functional microbiomes. The phyllosphere microbiome has the potential to defend against plant diseases. However, the mechanisms of how the microbiota assemble and function in the phyllosphere remain largely elusive, and this restricts the exploitation of the targeted beneficial microbes in the field. Here we review the endogenous and exogenous cues impacting microbiota assembly in the phyllosphere and how the phyllosphere microbiota in turn facilitate the disease resistance of host plants. We further construct a holistic framework by integrating of holo-omics, genetic manipulation, culture-dependent characterization and emerging artificial intelligence techniques, such as deep learning, to engineer the phyllosphere microbiome for sustainable crop production.

Ecology and functional potential of phyllosphere yeasts

The phyllosphere (i.e., the aerial parts of plants) harbors a rich microbial life, including bacteria, fungi, viruses, and yeasts. Current knowledge of yeasts stems primarily from industrial and medical research on Saccharomyces cerevisiae and Candida albicans, both of which can be found on plant tissues. For most other yeasts found in the phyllosphere, little is known about their ecology and functions. Here, we explore the diversity, dynamics, interactions, and genomics of yeasts associated with plant leaves and how tools and approaches developed for model yeasts can be adopted to disentangle the ecology and natural functions of phyllosphere yeasts. A first genomic survey exemplifies that we have only scratched the surface of the largely unexplored functional potential of phyllosphere yeasts.

The rhizosphere microbiome: Plant-microbial interactions for resource acquisition

While horticulture tools and methods have been extensively developed to improve the management of crops, systems to harness the rhizosphere microbiome to benefit plant crops are still in development. Plants and microbes have been coevolving for several millennia, conferring fitness advantages that expand the plant's own genetic potential. These beneficial associations allow the plants to cope with abiotic stresses such as nutrient deficiency across a wide range of soils and growing conditions. Plants achieve these benefits by selectively recruiting microbes using root exudates, positively impacting their nutrition, health and overall productivity. Advanced knowledge of the interplay between root exudates and microbiome alteration in response to plant nutrient status, and the underlying mechanisms there of, will allow the development of technologies to increase crop yield. This review summarizes current knowledge and perspectives on plant-microbial interactions for resource acquisition and discusses promising advances for manipulating rhizosphere microbiomes and root exudation.

Weather stressors correlate with Escherichia coli and Salmonella enterica persister formation rates in the phyllosphere: a mathematical modeling study

Enteric pathogens can enter a persister state in which they survive exposure to antibiotics and physicochemical stresses. Subpopulations of such phenotypic dormant variants have been detected in vivo and in planta in the laboratory, but their formation in the natural environment remains largely unexplored. We applied a mathematical model predicting the switch rate to persister cell in the phyllosphere to identify weather-related stressors associated with E. coli and S. enterica persister formation on plants based on their population dynamics in published field studies from the USA and Spain. Model outputs accurately depicted the bi-phasic decay of bacterial population sizes measured in the lettuce and spinach phyllosphere in these studies. Predicted E. coli persister switch rate on leaves was positively and negatively correlated with solar radiation intensity and wind velocity, respectively. Likewise, predicted S. enterica persister switch rate correlated positively with solar radiation intensity; however, a negative correlation was observed with air temperature, relative humidity, and dew point, factors involved in water deposition onto the phylloplane. These findings suggest that specific environmental factors may enrich for dormant bacterial cells on plants. Our model quantifiably links persister cell subpopulations in the plant habitat with broader physical conditions, spanning processes at different granular scales.

Investigating genetic diversity within the most abundant and prevalent non-pathogenic leaf-associated bacteria interacting with Arabidopsis thaliana in natural habitats

Microbiota modulates plant health and appears as a promising lever to develop innovative, sustainable and eco-friendly agro-ecosystems. Key patterns of microbiota assemblages in plants have been revealed by an extensive number of studies based on taxonomic profiling by metabarcoding. However, understanding the functionality of microbiota is still in its infancy and relies on reductionist approaches primarily based on the establishment of representative microbial collections. In Arabidopsis thaliana, most of these microbial collections include one strain per OTU isolated from a limited number of habitats, thereby neglecting the ecological potential of genetic diversity within microbial species. With this study, we aimed at estimating the extent of genetic variation between strains within the most abundant and prevalent leaf-associated non-pathogenic bacterial species in A. thaliana located south-west of France. By combining a culture-based collection approach consisting of the isolation of more than 7,000 bacterial colonies with an informative-driven approach, we isolated 35 pure strains from eight non-pathogenic bacterial species. We detected significant intra-specific genetic variation at the genomic level and for growth rate in synthetic media. In addition, significant host genetic variation was detected in response to most bacterial strains in in vitro conditions, albeit dependent on the developmental stage at which plants were inoculated, with the presence of both negative and positive responses on plant growth. Our study provides new genetic and genomic resources for a better understanding of the plant-microbe ecological interactions at the microbiota level. We also highlight the need of considering genetic variation in both non-pathogenic bacterial species and A. thaliana to decipher the genetic and molecular mechanisms involved in the ecologically relevant dialog between hosts and leaf microbiota.

Illuminating protist diversity in pitcher plants and bromeliad tanks

Many species of plants have evolved structures called phytotelmata that store water and trap detritus and prey. These structures house diverse communities of organisms, the inquiline microbiome, that aids breakdown of litter and prey. The invertebrate and bacterial food webs in these systems are well characterized, but less is known about microbial eukaryotic community dynamics. In this study we focus on microbes in the SAR clade (Stramenopila, Alveolata, Rhizaria) inhabiting phytotelmata. Using small subunit rDNA amplicon sequencing from repeated temporal and geographic samples of wild and cultivated plants across the Northeast U.S.A., we demonstrate that communities are variable within and between host plant type. Across habitats, communities from tropical bromeliads grown in a single room of a greenhouse were nearly as heterogeneous as wild pitcher plants spread across hundreds of kilometers. At the scale of pitcher plants in a single bog, analyses of samples from three time points suggest that seasonality is a major driver of protist community structure, with variable spring communities transitioning to more homogeneous communities that resemble the surrounding habitat. Our results indicate that protist communities in phytotelmata are variable, likely due to stochastic founder events and colonization/competition dynamics, leading to tremendous heterogeneity in inquiline microeukaryotic communities.

Bacteriophages Roam the Wheat Phyllosphere

The phyllosphere microbiome plays an important role in plant fitness. Recently, bacteriophages have been shown to play a role in shaping the bacterial community composition of the phyllosphere. However, no studies on the diversity and abundance of phyllosphere bacteriophage communities have been carried out until now. In this study, we extracted, sequenced, and characterized the dsDNA and ssDNA viral community from a phyllosphere for the first time. We sampled leaves from winter wheat (Triticum aestivum), where we identified a total of 876 virus operational taxonomic units (vOTUs), mostly predicted to be bacteriophages with a lytic lifestyle. Remarkably, 848 of these vOTUs corresponded to new viral species, and we estimated a minimum of 2.0 × 10⁶ viral particles per leaf. These results suggest that the wheat phyllosphere harbors a large and active community of novel bacterial viruses. Phylloviruses have potential applications as biocontrol agents against phytopathogenic bacteria or as microbiome modulators to increase plant growth-promoting bacteria.

Unlocking the Changes of Phyllosphere Fungal Communities of Fishscale Bamboo (Phyllachora heterocladae) under Rhombic-Spot Disease Stressed Conditions

As an important nonwood bioresource, fishscale bamboo (Phyllachora heterocladae Oliver) is widely distributed in the subtropical region of China. Rhombic-spot disease, caused by Neostagonosporella sichuanensis, is one of the most serious diseases that threatens fishscale bamboo health. However, there is limited knowledge about how rhombic-spot disease influences the diversity and structures of phyllosphere fungal communities. In this study, we investigated the phyllosphere fungal communities from stems, branches, and leaves of fishscale bamboo during a rhombic-spot disease outbreak using 18S rRNA sequencing. We found that only the phyllosphere fungal community from stems was significantly affected by pathogen invasion in terms of community richness, diversity, and structure. FUNGuild analysis revealed that the major classifications of phyllosphere fungi based on trophic modes in stems, branches, and leaves changed from symbiotroph-pathotroph, no obvious dominant trophic mode, and symbiotroph to saprotroph, saprotroph–pathotroph–symbiotroph, and saprotroph–symbiotroph, respectively, after pathogen invasion. The fungal community composition of the three tissues displayed significant differences at the genus level between healthy and diseased plants. The associations among fungal species in diseased samples showed more complex co-occurrence network structures than those of healthy samples. Taken together, our results highlight the importance of plant pathological conditions for the assembly of phyllosphere fungal communities in different tissues.

Cytokinin-microbiome interactions regulate developmental functions

BACKGROUND

The interaction of plants with the complex microbial networks that inhabit them is important for plant health. While the reliance of plants on their microbial inhabitants for defense against invading pathogens is well documented, the acquisition of data concerning the relationships between plant developmental stage or aging, and microbiome assembly, is still underway. The plant hormone cytokinin (CK) regulates various plant growth and developmental processes. Here, examining the relationships between plant development and microbiome assembly, we observed developmental-age dependent changes in the phyllopshere microbiome. We show that age-related shifts in microbiome content vary based on content of, or sensitivity to, CK.

RESULTS

We found a developmental age associated decline in microbial richness and diversity, accompanied by a decline in the presence of growth promoting and resistance inducing Bacilli in the phyllosphere. This decline was absent from CK-rich or CK-hypersensitive genotypes. Bacillus isolates we obtained from CK rich genotypes were found to alter the expression of developmental genes to support morphogenesis and alter the leaf developmental program when applied to seedlings, and enhance yield and agricultural productivity when applied to mature plants.

CONCLUSIONS

Our results support the notion that CK supports developmental functions in part via the bacterial community.

Plant neighborhood shapes diversity and reduces interspecific variation of the phyllosphere microbiome

Microbial communities associated with plant leaf surfaces (i.e., the phyllosphere) are increasingly recognized for their role in plant health. While accumulating evidence suggests a role for host filtering of its microbiota, far less is known about how community composition is shaped by dispersal, including from neighboring plants. We experimentally manipulated the local plant neighborhood within which tomato, pepper, or bean plants were grown in a 3-month field trial. Focal plants were grown in the presence of con- or hetero-specific neighbors (or no neighbors) in a fully factorial combination. At 30-day intervals, focal plants were harvested and replaced with a new age- and species-matched cohort while allowing neighborhood plants to continue growing. Bacterial community profiling revealed that the strength of host filtering effects (i.e., interspecific differences in composition) decreased over time. In contrast, the strength of neighborhood effects increased over time, suggesting dispersal from neighboring plants becomes more important as neighboring plant biomass increases. We next implemented a cross-inoculation study in the greenhouse using inoculum generated from the field plants to directly test host filtering of microbiomes while controlling for directionality and source of dispersal. This experiment further demonstrated that focal host species, the host from which the microbiome came, and in one case the donor hosts' neighbors, contribute to variation in phyllosphere bacterial composition. Overall, our results suggest that local dispersal is a key factor in phyllosphere assembly, and that demographic factors such as nearby neighbor identity and biomass or age are important determinants of phyllosphere microbiome diversity.

Fungal diversity within the phyllosphere of Pinus massoniana and the possible involvement of phyllospheric fungi in litter decomposition

Fungi play key roles in forest ecosystems and help to shape the forest's diverse functions. However, little is known about the diversity of phyllospheric fungi or their possible relationships with fungal communities residing in different micro-environments of Pinus massoniana forests. We investigated seven different sample types: mature needles (NM), dead needles (ND), needles falling as litter (L), fermenting needles (F), humus (H), top soil (0-20 cm) (TS), and secondary soil (20-40 cm) (SS). These seven fungal communities were examined and compared with ITS amplicons using a high-throughput sequencing technique. A total of 1213 fungal operational taxonomic units (OTUs) were obtained at a 97% sequence similarity level. Distinct fungal communities were associated with different sample types. A greater number of OTUs were present in both NM and F samples than those shared by both NM and TS samples, indicating that phyllospheric fungi may play crucial roles in litter decomposition. Sixty OTUs (the core microbiome) were found in all sample types, and they may probably play different ecological roles in different sample types. These findings extend our knowledge of the fungal diversity of the phyllosphere and its possible interactions with fungal communities found in distinct forest micro-habitats.

Home-field advantage of litter decomposition: from the phyllosphere to the soil

Plants often associate with specialized decomposer communities that increase plant litter breakdown, a phenomenon that is known as the 'home-field advantage' (HFA). Although the concept of HFA has long considered only the role of the soil microbial community, explicit consideration of the role of the microbial community on the foliage before litter fall (i.e. the phyllosphere community) may help us to better understand HFA. We investigated the occurrence of HFA in the presence vs absence of phyllosphere communities and found that HFA effects were smaller when phyllosphere communities were removed. We propose that priority effects and interactions between phyllosphere and soil organisms can help explain the positive effects of the phyllosphere at home, and suggest a path forward for further investigation.

The Genomics and Cell Biology of Host-Beneficial Intracellular Infections

Microbes gain access to eukaryotic cells as food for bacteria-grazing protists, for host protection by microbe-killing immune cells, or for microbial benefit when pathogens enter host cells to replicate. But microbes can also gain access to a host cell and become an important-often required-beneficial partner. The oldest beneficial microbial infections are the ancient eukaryotic organelles now called the mitochondrion and plastid. But numerous other host-beneficial intracellular infections occur throughout eukaryotes. Here I review the genomics and cell biology of these interactions with a focus on intracellular bacteria. The genomes of host-beneficial intracellular bacteria have features that span a previously unfilled gap between pathogens and organelles. Host cell adaptations to allow the intracellular persistence of beneficial bacteria are found along with evidence for the microbial manipulation of host cells, but the cellular mechanisms of beneficial bacterial infections are not well understood. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Phyllosphere microbiota: Community dynamics and its interaction with plant hosts

Plants are colonized by various microorganisms in natural environments. While many studies have demonstrated key roles of the rhizosphere microbiota in regulating biological processes such as nutrient acquisition and resistance against abiotic and biotic challenges, less is known about the role of the phyllosphere microbiota and how it is established and maintained. This review provides an update on current understanding of phyllosphere community assembly and the mechanisms by which plants and microbes establish the phyllosphere microbiota for plant health.