Deciphering core phyllomicrobiome assemblage on rice genotypes grown in contrasting agroclimatic zones: implications for phyllomicrobiome engineering against blast disease

Unraveling endophytic diversity in dioecious Siraitia grosvenorii: implications for mogroside production

Host and tissue-specificity of endophytes are important attributes that limit the endophyte application on multiple crops. Therefore, understanding the endophytic composition of the targeted crop is essential, especially for the dioecious plants where the male and female plants are different. Here, efforts were made to understand the endophytic bacterial composition of the dioecious Siraitia grosvenorii plant using 16S rRNA amplicon sequencing. The present study revealed the association of distinct endophytic bacterial communities with different parts of male and female plants. Roots of male and female plants had a higher bacterial diversity than other parts of plants, and the roots of male plants had more bacterial diversity than the roots of female plants. Endophytes belonging to the phylum Proteobacteria were abundant in all parts of male and female plants except male stems and fruit pulp, where the Firmicutes were most abundant. Class Gammaproteobacteria predominated in both male and female plants, with the genus Acinetobacter as the most dominant and part of the core microbiome of the plant (present in all parts of both, male and female plants). The presence of distinct taxa specific to male and female plants was also identified. Macrococcus, Facklamia, and Propionibacterium were the distinct genera found only in fruit pulp, the edible part of S. grosvenorii. Predictive functional analysis revealed the abundance of enzymes of secondary metabolite (especially mogroside) biosynthesis in the associated endophytic community with predominance in roots. The present study revealed bacterial endophytic communities of male and female S. grosvenorii plants that can be further explored for monk fruit cultivation, mogroside production, and early-stage identification of male and female plants. KEY POINTS: • Male and female Siraitia grosvenorii plants had distinct endophytic communities • The diversity of endophytic communities was specific to different parts of plants • S. grosvenorii-associated endophytes may be valuable for mogroside biosynthesis and monk fruit cultivation.

Harnessing biological nitrogen fixation in plant leaves

The importance of biological nitrogen fixation (BNF) in securing food production for the growing world population with minimal environmental cost has been increasingly acknowledged. Leaf surfaces are one of the biggest microbial habitats on Earth, harboring diverse free-living N2-fixers. These microbes inhabit the epiphytic and endophytic phyllosphere and contribute significantly to plant N supply and growth. Here, we summarize the contribution of phyllosphere-BNF to global N cycling, evaluate the diversity of leaf-associated N2-fixers across plant hosts and ecosystems, illustrate the ecological adaptation of N2-fixers to the phyllosphere, and identify the environmental factors driving BNF. Finally, we discuss potential BNF engineering strategies to improve the nitrogen uptake in plant leaves and thus sustainable food production.

Tillandsia landbeckii phyllosphere and laimosphere as refugia for bacterial life in a hyperarid desert environment

BACKGROUND

The lack of water is a major constraint for microbial life in hyperarid deserts. Consequently, the abundance and diversity of microorganisms in common habitats such as soil are strongly reduced, and colonization occurs primarily by specifically adapted microorganisms that thrive in particular refugia to escape the harsh conditions that prevail in these deserts. We suggest that plants provide another refugium for microbial life in hyperarid deserts. We studied the bacterial colonization of Tillandsia landbeckii (Bromeliaceae) plants, which occur in the hyperarid regions of the Atacama Desert in Chile, one of the driest and oldest deserts on Earth.

RESULTS

We detected clear differences between the bacterial communities being plant associated to those of the bare soil surface (PERMANOVA, R2 = 0.187, p = 0.001), indicating that Tillandsia plants host a specific bacterial community, not only dust-deposited cells. Moreover, the bacterial communities in the phyllosphere were distinct from those in the laimosphere, i.e., on buried shoots (R2 = 0.108, p = 0.001), indicating further habitat differentiation within plant individuals. The bacterial taxa detected in the phyllosphere are partly well-known phyllosphere colonizers, but in addition, some rather unusual taxa (subgroup2 Acidobacteriae, Acidiphilum) and insect endosymbionts (Wolbachia, "Candidatus Uzinura") were found. The laimosphere hosted phyllosphere-associated as well as soil-derived taxa. The phyllosphere bacterial communities showed biogeographic patterns across the desert (R2 = 0.331, p = 0.001). These patterns were different and even more pronounced in the laimosphere (R2 = 0.467, p = 0.001), indicating that different factors determine community assembly in the two plant compartments. Furthermore, the phyllosphere microbiota underwent temporal changes (R2 = 0.064, p = 0.001).

CONCLUSIONS

Our data demonstrate that T. landbeckii plants host specific bacterial communities in the phyllosphere as well as in the laimosphere. Therewith, these plants provide compartment-specific refugia for microbial life in hyperarid desert environments. The bacterial communities show biogeographic patterns and temporal variation, as known from other plant microbiomes, demonstrating environmental responsiveness and suggesting that bacteria inhabit these plants as viable microorganisms. Video Abstract.

Differentiated responses of the phyllosphere bacterial community of the yellowhorn tree to precipitation and temperature regimes across Northern China

Introduction

As an ephemeral and oligotrophic environment, the phyllosphere harbors many highly diverse microorganisms. Importantly, it is known that their colonization of plant leaf surfaces is considerably influenced by a few abiotic factors related to climatic conditions. Yet how the dynamics of phyllosphere bacterial community assembly are shaped by detailed climatological elements, such as various bioclimatic variables, remains poorly understood.

Methods

Using high-throughput 16S rRNA gene amplicon sequencing technology, we analyzed the bacterial communities inhabiting the leaf surfaces of an oilseed tree, yellowhorn (Xanthoceras sorbifolium), grown at four sites (Yinchuan, Otogqianqi, Tongliao, and Zhangwu) whose climatic status differs in northern China.

Results and Discussion

We found that the yellowhorn phyllosphere's bacterial community was generally dominated by four phyla: Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Nevertheless, bacterial community composition differed significantly among the four sampled site regions, indicating the possible impact of climatological factors upon the phyllosphere microbiome. Interestingly, we also noted that the α-diversities of phyllosphere microbiota showed strong positive or negative correlation with 13 bioclimatic factors (including 7 precipitation factors and 6 temperature factors). Furthermore, the relative abundances of 55 amplicon sequence variants (ASVs), including three ASVs representing two keystone taxa (the genera Curtobacterium and Streptomyces), exhibited significant yet contrary responses to the precipitation and temperature climatic variables. That pattern was consistent with all ASVs' trends of possessing opposite correlations to those two parameter classes. In addition, the total number of links and nodes, which conveys community network complexity, increased with rising values of most temperature variables. Besides that, remarkably positive relevance was found between average clustering coefficient and most precipitation variables. Altogether, these results suggest the yellowhorn phyllosphere bacterial community is capable of responding to variation in rainfall and temperature regimes in distinctive ways.

New Insights on Endophytic Microbacterium-Assisted Blast Disease Suppression and Growth Promotion in Rice: Revelation by Polyphasic Functional Characterization and Transcriptomics

Plant growth-promoting endophytic microbes have drawn the attention of researchers owing to their ability to confer fitness benefits in many plant species. Here, we report agriculturally beneficial traits of rice-leaf-adapted endophytic Microbacterium testaceum. Our polyphasic taxonomic investigations revealed its identity as M. testaceum. The bacterium displayed typical endophytism in rice leaves, indicated by the green fluorescence of GFP-tagged M. testaceum in confocal laser scanning microscopy. Furthermore, the bacterium showed mineral solubilization and production of IAA, ammonia, and hydrolytic enzymes. Tobacco leaf infiltration assay confirmed its non-pathogenic nature on plants. The bacterium showed antifungal activity on Magnaporthe oryzae, as exemplified by secreted and volatile organic metabolome-mediated mycelial growth inhibition. GC-MS analysis of the volatilome of M. testaceum indicated the abundance of antimicrobial compounds. Bacterization of rice seedlings showed phenotypic traits of MAMP-triggered immunity (MTI), over-expression of OsNPR1 and OsCERK, and the consequent blast suppressive activity. Strikingly, M. testaceum induced the transcriptional tradeoff between physiological growth and host defense pathways as indicated by up- and downregulated DEGs. Coupled with its plant probiotic features and the defense elicitation activity, the present study paves the way for developing Microbacterium testaceum-mediated bioformulation for sustainably managing rice blast disease.

Recent trends in nitrogen cycle and eco-efficient nitrogen management strategies in aerobic rice system

Rice (Oryza sativa L.) is considered as a staple food for more than half of the global population, and sustaining productivity under a scarcity of resources is challenging to meet the future food demands of the inflating global population. The aerobic rice system can be considered as a transformational replacement for traditional rice, but the widespread adaptation of this innovative approach has been challenged due to higher losses of nitrogen (N) and reduced N-use efficiency (NUE). For normal growth and developmental processes in crop plants, N is required in higher amounts. N is a mineral nutrient and an important constituent of amino acids, nucleic acids, and many photosynthetic metabolites, and hence is essential for normal plant growth and metabolism. Excessive application of N fertilizers improves aerobic rice growth and yield, but compromises economic and environmental sustainability. Irregular and uncontrolled use of N fertilizers have elevated several environmental issues linked to higher N losses in the form of nitrous oxide (N2O), ammonia (NH3), and nitrate (NO3 -), thereby threatening environmental sustainability due to higher warming potential, ozone depletion capacities, and abilities to eutrophicate the water resources. Hence, enhancing NUE in aerobic rice has become an urgent need for the development of a sustainable production system. This article was designed to investigate the major challenge of low NUE and evaluate recent advances in pathways of the N cycle under the aerobic rice system, and thereby suggest the agronomic management approaches to improve NUE. The major objective of this review is about optimizing the application of N inputs while sustaining rice productivity and ensuring environmental safety. This review elaborates that different soil conditions significantly shift the N dynamics via changes in major pathways of the N cycle and comprehensively reviews the facts why N losses are high under the aerobic rice system, which factors hinder in attaining high NUE, and how it can become an eco-efficient production system through agronomic managements. Moreover, it explores the interactive mechanisms of how proper management of N cycle pathways can be accomplished via optimized N fertilizer amendments. Meanwhile, this study suggests several agricultural and agronomic approaches, such as site-specific N management, integrated nutrient management (INM), and incorporation of N fertilizers with enhanced use efficiency that may interactively improve the NUE and thereby plant N uptake in the aerobic rice system. Additionally, resource conservation practices, such as plant residue management, green manuring, improved genetic breeding, and precision farming, are essential to enhance NUE. Deep insights into the recent advances in the pathways of the N cycle under the aerobic rice system necessarily suggest the incorporation of the suggested agronomic adjustments to reduce N losses and enhance NUE while sustaining rice productivity and environmental safety. Future research on N dynamics is encouraged under the aerobic rice system focusing on the interactive evaluation of shifts among activities and diversity in microbial communities, NUE, and plant demands while applying N management measures, which is necessary for its widespread adaptation in face of the projected climate change and scarcity of resources.