Rhizobial strain-dependent restriction of nitrogen fixation in a legume-Rhizobium symbiosis

Diversity and functions of quorum sensing bacteria in the root environment of the Suaeda glauca and Phragmites australis coastal wetlands

The quorum sensing (QS) system plays a significant role in the bacteria-bacteria or plant-bacteria relationships through signal molecules. However, little is known about the distribution and functional diversity of QS bacteria in the root environment of Suaeda glauca and Phragmites australis in coastal wetlands. We explored the bacterial community by amplicon sequencing and isolated 1050 strains from the rhizosphere soil and root tissues of S. glauca and P. australis in northern China to investigate the bacterial community and AHL producers. AHL activity was found in 76 isolates, and 22 distinct strains were confirmed by 16S rRNA gene sequencing. A substantial number of AHL producers clustered in rhizobiales and sphingomonadale, which derived from the root tissues. AHL producers in the rhizosphere soil mostly belonged to rhodobacterales. The different taxa of AHL producers in the rhizosphere soil and root tissues resulted in a variation of AHL profiles that C6-HSL dominated the AHL profiles in root bacteria compared to the C8-HSL in rhizobacteria, implying different ecological roles for AHL producers in the rhizosphere soil and root tissues. Many AHL producers may form biofilms, and some can degrade DMSP and oil, demonstrating that QS bacteria in the root environment have a wide ecological roles. In our study, for one of the first times here, we explore the distribution and functional variety of AHL producers in the root environment of S. glauca-P. australis. This study expands current knowledge of the relationship between QS bacteria and coastal plants (S. glauca and P. australis), and vital roles of QS bacterial in maintaining the health of coastal wetlands.

Effectiveness of nitrogen fixation in rhizobia

Biological nitrogen fixation in rhizobia occurs primarily in root or stem nodules and is induced by the bacteria present in legume plants. This symbiotic process has fascinated researchers for over a century, and the positive effects of legumes on soils and their food and feed value have been recognized for thousands of years. Symbiotic nitrogen fixation uses solar energy to reduce the inert N2 gas to ammonia at normal temperature and pressure, and is thus today, especially, important for sustainable food production. Increased productivity through improved effectiveness of the process is seen as a major research and development goal. The interaction between rhizobia and their legume hosts has thus been dissected at agronomic, plant physiological, microbiological and molecular levels to produce ample information about processes involved, but identification of major bottlenecks regarding efficiency of nitrogen fixation has proven to be complex. We review processes and results that contributed to the current understanding of this fascinating system, with focus on effectiveness of nitrogen fixation in rhizobia.

Influence of fungicides on activities of enzymes of phenolic metabolism in the early stages of formation and functioning of soybean symbiotic apparatus

We studied the effect of complex seed treatment with fungicides and rhizobium culture on the activity of phenolic metabolism enzymes – polyphenol oxidase and guaiacol peroxidase in the early stages of the formation and functioning of various symbiotic systems Glycine max – Bradyrhizobium japonicum. In the research we used microbiological, physiological, biochemical methods, gas chromatography and spectrophotometry. The objects of the study were selected symbiotic systems formed with the participation of soybean (Glycine max (L.) Merr.), Diamond variety, strains Bradyrhizobium japonicum 634b (active, virulent) and 604k (inactive, highly virulent) and fungicides Maxim XL 035 PS (fludioxonil, 25 g/L, metalaxyl, 10 g/L), and Standak Top (fipronil, 250 g/L, thiophanate methyl, 225 g/L, piraclostrobin, 25 g/L). Before sowing, the seeds of soybean were treated with solutions of fungicides, calculated on the basis of one rate of expenditure of the active substance of each preparation indicated by the producer per ton of seed. One part of the seeds treated with fungicides was inoculated with rhizobium culture for 1 h (the titre of bacteria was 108 cells in 1 ml). The other part of the fungicide-treated seeds was not inoculated by rhizobium culture. As a result of the research, it was revealed that an effective symbiotic system formed with the participation of soybean plants and the active strain rhizobia 634b is characterized by a high level of polyphenol oxidase activity and low guaiacol peroxidase in roots and root nodules in the stages of second and third true leaves. Such changes in the activity of enzymes occurred along with the formation of nodules which actively fixed the molecular nitrogen of the atmosphere. An ineffective symbiotic system (strain 604k) is characterized by an elevated level of polyphenol oxidase activity in the roots and guaiacol peroxidase in the root nodules, which is accompanied by activation of the process of nodulation. Treatment of soybean seeds with fungicides in an effective symbiotic system leads to a change in the activity of the enzymes of the phenolic metabolism, which induced adaptive changes in plant metabolism and growth of nitrogenase activity of the root nodules. The recorded changes in the activity of both enzymes for the action of fungicides in the ineffective symbiotic system can be considered as a kind of response of the plant to the treatment and were observed along with the reduction of the processes of nodulation into the stage of the third true leaf.

Plant health: feedback effect of root exudates-rhizobiome interactions

The well-being of the microbial community that densely populates the rhizosphere is aided by a plant's root exudates. Maintaining a plant's health is a key factor in its continued existence. As minute as rhizospheric microbes are, their importance in plant growth cannot be overemphasized. They depend on plants for nutrients and other necessary requirements. The relationship between the rhizosphere-microbiome (rhizobiome) and plant hosts can be beneficial, non-effectual, or pathogenic depending on the microbes and the plant involved. This relationship, to a large extent, determines the fate of the host plant's survival. Modern molecular techniques have been used to unravel rhizobiome species' composition, but the interplay between the rhizobiome root exudates and other factors in the maintenance of a healthy plant have not as yet been thoroughly investigated. Many functional proteins are activated in plants upon contact with external factors. These proteins may elicit growth promoting or growth suppressing responses from the plants. To optimize the growth and productivity of host plants, rhizobiome microbial diversity and modulatory techniques need to be clearly understood for improved plant health.

Apple rootstocks of different nitrogen tolerance affect the rhizosphere bacterial community composition

AIMS

To select apple rootstocks that are tolerant to low nitrogen and reveal the relationship between the rhizosphere bacterial communities and the low nitrogen tolerance of the apple rootstock.

METHODS AND RESULTS

In total, 235 lines of hybrids of Malus robusta Rehd. × M.9 with low nitrogen stress were cultivated in pots in a greenhouse equipped with a drip irrigation system, and growth characteristics, photosynthesis traits and mineral elements were monitored. The bacterial community structure of the rhizosphere from different rootstocks was determined via Illumina MiSeq sequencing. This study selected three low nitrogen-tolerant (NT) lines that had higher nitrogen concentration, and higher photosynthesis rate than the three low nitrogen-sensitive (NS) lines. The bacterial community structure significantly differed (P ≤ 0·001) among the rootstocks. The bacterial phyla Proteobacteria and Actinobacteria were the dominant groups in the rhizosphere and presented higher abundance in the NT rhizosphere. The N concentration in the apple rootstocks exhibited highly positive Pearson correlations with the bacterial genera Sphingomonas, Pseudoxanthomonas, Bacillus and Acinetobacter, and negative correlations with the bacterial genera Pseudarthrobacter and Bradyrhizobium.

CONCLUSIONS

This study showed that investigated rootstocks achieved increased nitrogen concentration by enhancing their photosynthetic production capacity and shaping their rhizobacteria community structure.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings provide a basis for studying the mechanisms of resistance to low nitrogen stress in apple rootstocks. Based on these beneficial bacteria, microbial inoculants can be developed for use in sustainable agricultural and horticultural production.