Bacterial Endophytes Contribute to Rice Seedling Establishment Under Submergence

Seed Endophytic Bacteria from Manilkara zapota L. and Their Influence as Rice Seed Priming Agents

In recent years, seed endophytes have gained significant attention due to their impact on the ecology, health, and productivity of host plants. Extensive research is being conducted to explore novel endophytic bacteria for sustainable crop improvement. Manilkara zapota L. P. Royen (Sapotaceae) is a highly stress-tolerant tree widely cultivated in tropical countries, yet its associated endophytes remain unexplored. In this study, nine bacterial isolates were obtained from M. zapota seeds, of which three (LA2, LA4, and NS1) were selected based on their IAA production capability which ranged from 2.3, 6.34, and 16.1 µg mL-1, respectively. Identification through 16S rRNA sequencing confirmed LA2 as Pseudomonas rhodesiae, LA4 as Bacillus cereus, and NS1 as Enterobacter cloacae. All isolates exhibited nitrogen-fixing ability, while NS1 uniquely solubilized potassium and phosphorus with KSI and PSI value as 2.9 and 2.3, respectively. Further, the efficacy of the bacterial isolates in promoting rice seedling growth was evaluated, and novel bioformulation was prepared from the consortia of LA2 + LA4, LA2 + NS1, LA4 + NS1, and LA2 + LA4 + NS1. All the isolates and bioformulations were tested as biopriming agents. Rice seedling growth experiments revealed a significant increase in germination percentage, root length, and shoot length following biopriming with individual isolates and consortia. Among treatments, the LA2 + LA4 combination exhibited the highest growth promotion, with a root length increase of 2.1-fold and shoot length increase of 2.3-fold as compared to control. Thus, our results highlighted that bioprospecting microbes from M. zapota seeds can help in nutrient management and seedling establishment.

Isolation and optimization of Bacillus thuringiensis BRB-3 isolated from germinated black upland rice seeds for GABA production

Gamma-aminobutyric acid (GABA) is a non-coded amino acid that recently gained increased attention within the functional food industry. This study aimed to isolate endophytic bacteria from sterilized surfaces of germinated black upland rice seeds for GABA production and optimization. A total of 38 isolates were screened for glutamic acid decarboxylase (GAD) activity using a colorimetric method. Among these, thirteen isolates exhibited positive GAD activity. The BRB-3 strain emerged as the most promising, producing the highest GABA levels (4.54 g/L) at an optimal pH of 6.5. However, the BRB-3 strain was sensitive to pH lower than 5.5; so the pH was adjusted to 6.5 through all experiments. Further identification of the BRB-3 strain was performed using 16S rRNA gene sequence, which placed it in Bacillus cereus group. Subsequent rpoB gene sequence analysis confirmed that the BRB-3 strain was Bacillus thuringiensis BRB-3. GABA production was optimized in MRS broth by investigating carbon, nitrogen, monosodium glutamate (MSG), pH and temperature. Interestingly, the concentration of MSG did not impact GABA production of B. thuringiensis BRB-3. Under optimal conditions of 3 % soy protein as the nitrogen source, 2 % sucrose, no added MSG at 30 °C, pH 6.5 for 12 days, B. thuringiensis BRB-3 achieved a high GABA yield of 25.06 g/L (242.9 mM). Therefore, B. thuringiensis BRB-3 demonstrated significant potential for GABA production and could be utilized in various functional food products, including germinated rice seeds.

Advances in seed hypoxia research

Seeds represent essential stages of the plant life cycle: embryogenesis, the intermittent quiescence phase, and germination. Each stage has its own physiological requirements, genetic program, and environmental challenges. Consequently, the effects of developmental and environmental hypoxia can vary from detrimental to beneficial. Past and recent evidence shows how low-oxygen signaling and metabolic adaptations to hypoxia affect seed development and germination. Here, we review the recent literature on seed biology in relation to hypoxia research and present our perspective on key challenges and opportunities for future investigations.

Amplicon Sequencing Analysis of Submerged Plant Microbiome Diversity and Screening for ACC Deaminase Production by Microbes

Submerged plants can thrive entirely underwater, playing a crucial role in maintaining water quality, supporting aquatic organisms, and enhancing sediment stability. However, they face multiple challenges, including reduced light availability, fluctuating water conditions, and limited nutrient access. Despite these stresses, submerged plants demonstrate remarkable resilience through physiological and biochemical adaptations. Additionally, their interactions with microbial communities are increasingly recognized as pivotal in mitigating these environmental stresses. Understanding the diversity of these microbial communities is crucial for comprehending the complex interactions between submerged plants and their environments. This research aims to identify and screen microbes from submerged plant samples capable of producing 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and to explore microbial diversity through metagenomic analysis. Microbes were isolated and screened for ACC deaminase production, and metagenomic techniques, including co-occurrence network analysis, were used to examine microbial diversity and interactions within the communities. ACC deaminase-producing microbes can significantly enhance plant metabolism under stress conditions. The identification of the culturable bacteria revealed that most of these microbes belong to the genera Pseudomonas, Bacillus, and Acinetobacter. A total of 177 microbial strains were cultured, with molecular identification revealing 79 reductant, 86 non-reductant, and 12 uncultured strains. Among 162 samples screened for ACC deaminase activity, 50 tested positive. To further understand microbial dynamics, samples were collected from both natural sources and artificial pond reservoirs to assess the impact of the location on flood-associated microbiomes in submerged plants. Metagenomic analysis was conducted on both the epiphytic and endophytic samples. By exploring the overall composition and dynamics of microbial communities associated with submerged plants, this research seeks to deepen our understanding of plant-microbe interactions in aquatic environments. The microbial screening helped to identify the diverse microbes associated with ACC deaminase activity in submerged plants and amplicon sequencing analysis paved the way towards identifying the impact of the location in shaping the microbiome and the diversity associated with endophytic and epiphytic microbes. Co-occurrence network analysis further highlighted the intricate interactions within these microbial communities. Notably, ACC deaminase activity was observed in plant-associated microbes across different locations, with distinct variations between epiphytic and endophytic populations as identified through co-occurrence network analysis.

Comparative genomics-based insights into Xanthomonas indica, a non-pathogenic species of healthy rice microbiome with bioprotection function

Xanthomonas species are major pathogens of plants and have been studied extensively. There is increasing recognition of the importance of non-pathogenic species within the same genus. With this came the need to understand the genomic and functional diversity of non-pathogenic Xanthomonas (NPX) at the species and strain level. This study reports isolation and investigation into the genomic diversity and variation in NPX isolates, chiefly Xanthomonas indica, a newly discovered NPX species from rice. The study establishes the relationship of X. indica strains within clade I of Xanthomonads with another NPX species, X. sontii, also associated with rice seeds. Identification of highly diverse strains, open-pan genome, and systematic hyper-variation at the lipopolysaccharide biosynthetic locus when compared to pathogenic Xanthomonas indicates the acquisition of new functions for adaptation. Furthermore, comparative genomics studies established the absence of major virulence genes such as type III secretion system and effectors, which are present in the pathogens, and the presence of a known bacterial-killing type IV secretion system (X-T4SS). The diverse non-pathogenic strains of X. indica and X. sontii were found to protect rice from bacterial leaf blight pathogen, X. oryzae pv. oryzae (Xoo). The absence of phenotype of an X-T4SS mutant suggests redundancy in the genetic basis of the mechanisms involved in the bioprotection function, which may include multiple genetic loci, such as putative bacteriocin-encoding gene clusters and involvement of other factors such as nutrient and niche competition apart from induction of innate immunity through shared microbial-associated molecular patterns. The rice-NPX community and its pathogenic counterpart can be a promising model for understanding plant-microbe-microbiome interaction studies.IMPORTANCEThe Xanthomonas group of bacteria is known for its characteristic lifestyle as a phytopathogen. However, the discovery of non-pathogenic Xanthomonas (NPX) species is a major shift in understanding this group of bacteria. Multi-strain, in-depth genomic, evolutionary and functional studies on each of these NPX species are still lacking. This study on diverse non-pathogenic strains provides novel insights into genome diversity, dynamics, and evolutionary trends of NPX species from rice microbiome apart from its relationship with other relatives that form a sub-clade. Interestingly, we also uncovered that NPX species protect rice from pathogenic Xanthomonas species. The plant protection property shows their importance as a part of a healthy plant microbiome. Furthermore, finding an open pan-genome and large-scale variation at lipopolysaccharide biosynthetic locus indicates a significant role of the NPX community in host adaptation. The findings and high-quality genomic resources of NPX species and the strains will allow further systematic molecular and host-associated microbial community studies for plant health.

Xanthomonas sontii, and Not X. sacchari, Is the Predominant Vertically Transmitted Core Rice Seed Endophyte

Seed endophytes, particularly the abundant, core, and vertically transmitted species, are major areas of focus in host microbiome studies. Apart from being the first members to colonize, they accompany the plant throughout its development stages and to the next generation. Recently published studies have reported the keystone species to be Xanthomonas sacchari, a core endophyte that is vertically transmitted in rice with probiotic properties. Furthermore, the Xanthomonas species was reported to be involved in the assembly of beneficial bacteria after early inoculation in rice seeds. However, the strains discussed in these studies were misclassified as X. sacchari, a well-known pathogen of sugarcane. By including nonpathogenic Xanthomonas species with plant-protective functions reported from rice seeds, we have correctly established the phylogenetic and taxonomic identity of the keystone species as X. sontii. This will enable researchers to use the correct reference or lab strain of X. sontii for further systematic and in-depth studies as a model endophyte in plant-microbe interactions apart from its exploitation in seed health.

Xanthomonas protegens sp. nov., a novel rice seed-associated bacterium, provides in vivo protection against X. oryzae pv. oryzae, the bacterial leaf blight pathogen

Historically, Xanthomonas species are primarily known for their pathogenicity against plants, but recently, there have been more findings of non-pathogenic xanthomonads. In the present study, we report isolates from healthy rice seeds that belong to a new species, Xanthomonas protegens, a protector of the rice plants against a serious pathogenic counterpart, i.e. X. oryzae pv. oryzae upon leaf clip co-inoculation. The new member species is non-pathogenic to rice and lacks a type III secretion system. The pangenome investigation revealed a large number of unique genes, including a novel lipopolysaccharide biosynthetic gene cluster, that might be important in its adaptation. The phylo-taxonogenomic analysis revealed that X. protegens is a taxonomic outlier species of X. sontii, a core, vertically transmitted rice seed endophyte with numerous probiotic properties. Interestingly, X. sontii is also reported as a keystone species of healthy rice seed microbiome. The findings and resources will help in the development of unique gene markers and evolutionary studies of X. sontii as a successful symbiont and X. oryzae as a serious pathogen. Here, we propose X. protegens sp. nov. as a novel species of the genus Xanthomonas with PPL118 = MTCC 13396 = CFBP 9164 = ICMP 25181 as the type strain. PPL117, PPL124, PPL125, and PPL126 are other strains of the species.

Seed bacterial microbiota in post-submergence tolerant and sensitive barley genotypes

Flooding is a predominant abiotic stress for cultivated plants, including barley. This cereal crop shows a large adaptability to different environmental conditions, suggesting the presence of key traits to tolerate adverse conditions. During germination, genetic variations account for dissimilarities in flooding tolerance. However, differences in the seed microbiota may also contribute to tolerance/sensitivity during seedling establishment. This work investigated differences in microbiome among the grains of barley accessions. Two barley phenotypes were compared, each either tolerant or sensitive to a short submergence period followed by a recovery. The study used a metataxonomic analysis based on 16S ribosomal RNA gene sequencing and subsequent functional prediction. Our results support the hypothesis that bacterial microbiota inhabiting the barley seeds are different between sensitive and tolerant barley accessions, which harbour specific bacterial phyla and families. Finally, bacteria detected in tolerant barley accessions show a peculiar functional enrichment that suggests a possible connection with successful germination and seedling establishment.

Interaction studies of Serendipita indica and Zhihengliuella sp. ISTPL4 and their synergistic role in growth promotion in rice

Rapid urbanization and globalization demand increasing agricultural productivity. Soil nutrient supply capacity is continuously decreasing due to soil erosion, degradation, salt deposition, undesired element, metal deposition, water scarcity, and an uneven nutrient delivery system. Rice cultivation requires a large amount of water which is becoming detrimental due to these activities. There is a need to increase its productivity. Microbial inoculants are becoming increasingly important in achieving sustainable agricultural production systems. The current study was conducted to investigate the interaction between the root endophytic fungus Serendipita indica (S. indica) and the actinobacterium Zhihengliuella sp. ISTPL4 (Z. sp. ISTPL4) and their synergistic effects on the growth of rice (Oryza sativa L). Both S. indica and Z. sp. ISTPL4 showed positive interactions. Growth of S. indica was observed at different days after Z. sp. ISTPL4 inoculation, and stimulated growth of S. indica was observed when Z. sp. ISTPL4 was inoculated at 5 dafi (days after fungal inoculation). Z. sp. ISTPL4 promoted the growth of S. indica as it increased spore germination. Furthermore, confocal and scanning electron microscopy (SEM) analyses showed a 27% increase in the spore size of S. indica in the presence of Z. sp. ISTPL4. In a liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis increased production of alanine and glutamic acid was observed in their sequential co-culture as compared with individual cultures. Sequential inoculation of S. indica and Z. sp. ISTPL4 significantly increased the biochemical and physical characteristics of rice as compared with their individual inoculum. Biochemical parameters such as chlorophyll content, total soluble sugar, and flavonoid content in the rice increased by up to 57%, 47%, and 39%, respectively, in the presence of the combined inoculum of S. indica and Z. sp. ISTPL4. This will be the first study, to the best of our knowledge, which shows the fungus and actinobacterium interaction and their synergistic roles in the growth promotion of rice. Furthermore, this novel combination can also be used to boost the growth of other crops to increase the agricultural yield.

Engineered Phomopsis liquidambaris with Fhb1 and Fhb7 Enhances Resistance to Fusarium graminearum in Wheat

Fusarium head blight is one of the most serious diseases caused by Fusarium graminearum in wheat. Here, we developed a new way to prevent and control Fusarium head blight by introducing the resistance genes Fhb1 and Fhb7 into the endophytic fungus Phomopsis liquidambaris, named PL-Fhb1 and PL-Fhb7, respectively, which could colonize wheat. The wheat seedlings were preinoculated with PL-Fhb1 and PL-Fhb7 to enhance the resistance against deoxynivalenol (DON) and PL-Fhb1 and PL-Fhb7 inhibited the growth of F. graminearum by 73% and 49%, respectively. The incidence rate of diseased spikes decreased to 35.2% and 45.4%, and the corresponding DON levels for wheat grains decreased from 13.2 to 1.79 μg/g and from 13.2 μg/g to 0.39 μg/g when the leaves were preinoculated with PL-Fhb1 and PL-Fhb7 after overwintering, respectively. The incidence rates of diseased spikes decreased to 25.7% and 34.7%, and the DON levels for wheat grains decreased from 17.48 μg/g to 1.23 μg/g and from 17.48 μg/g to 0 μg/g when the wheat flowers were inoculated with PL-Fhb1 and PL-Fhb7, and the wheat flowers were subsequently infected with F. graminearum, respectively. It was confirmed that DON was transformed into DON-glutathione (GSH) by PL-Fhb7 using high-performance liquid chromatography-mass spectrometry (HPLC-MS). However, PL-Fhb1 may have increased plant immunity and enhanced the resistance to F. graminearum. This study indicates that engineered endophytes can improve the resistance to Fusarium head blight and presents a new method for the biological control of Fusarium head blight.