Plant Growth-Promoting Rhizobium: Mechanisms and Biotechnological Prospective

Effects of sublethal stress application on the survival of bacterial inoculants: a systematic review

The use of commercial bacterial inoculants formulated with plant-growth promoting bacteria (PGPB) in agriculture has shown significant prominence in recent years due to growth-promotion benefits provided to plants through different mechanisms. However, the survival and viability of bacterial cells in inoculants are affected during use and may decrease their effectiveness. Physiological adaptation strategies have attracted attention to solve the viability problem. This review aims to provide an overview of research on selecting sublethal stress strategies to increase the effectiveness of bacterial inoculants. The searches were performed in November 2021 using Web of Science, Scopus, PubMed, and Proquest databases. The keywords "nitrogen-fixing bacteria", "plant growth-promoting rhizobacteria", "azospirillum", "pseudomonas", "rhizobium", "stress pre-conditioning", "adaptation", "metabolic physiological adaptation", "cellular adaptation", "increasing survival", "protective agent" and "protective strategy" were used in the searches. A total of 2573 publications were found, and 34 studies were selected for a deeper study of the subject. Based on the studies analysis, gaps and potential applications related to sublethal stress were identified. The most used strategies included osmotic, thermal, oxidative, and nutritional stress, and the primary cell response mechanism to stress was the accumulation of osmolytes, phytohormones, and exopolysaccharides (EPS). Under sublethal stress, the inoculant survival showed positive increments after lyophilization, desiccation, and long-term storage processes. The effectiveness of inoculant-plants interaction also had positive increments after sublethal stress, improving plant development, disease control, and tolerance to environmental stresses compared to unappealed inoculants.

Salinity of irrigation water selects distinct bacterial communities associated with date palm (Phoenix dactylifera L.) root

Saline water irrigation has been used in date palm (Phoenix dactylifera L.) agriculture as an alternative to non-saline water due to water scarcity in hyper-arid environments. However, the knowledge pertaining to saline water irrigation impact on the root-associated bacterial communities of arid agroecosystems is scarce. In this study, we investigated the effect of irrigation sources (non-saline freshwater vs saline groundwater) on date palm root-associated bacterial communities using 16S rDNA metabarcoding. The bacterial richness, Shannon diversity and evenness didn’t differ significantly between the irrigation sources. Soil electrical conductivity (EC) and irrigation water pH were negatively related to Shannon diversity and evenness respectively, while soil organic matter displayed a positive correlation with Shannon diversity. 40.5% of total Operational Taxonomic Units were unique to non-saline freshwater irrigation, while 26% were unique to saline groundwater irrigation. The multivariate analyses displayed strong structuring of bacterial communities according to irrigation sources, and both soil EC and irrigation water pH were the major factors affecting bacterial communities. The genera Bacillus, Micromonospora and Mycobacterium were dominated while saline water irrigation whereas contrasting pattern was observed for Rhizobium, Streptomyces and Acidibacter. Taken together, we suggest that date-palm roots select specific bacterial taxa under saline groundwater irrigation, which possibly help in alleviating salinity stress and promote growth of the host plant.

An Assessment of the Lolium perenne (Perennial Ryegrass) Seedborne Microbiome across Cultivars, Time, and Biogeography: Implications for Microbiome Breeding

Research into the bacterial component of the seed microbiome has been intensifying, with the aim of understanding its structure and potential for exploitation. We previously studied the intergenerational seed microbiome of one cultivar of perennial ryegrass with and without one strain of the commercially deployed fungal endophyte Epichloë festucae var. lolii. The work described here expands on our previous study by exploring the bacterial seed microbiome of different commercial cultivar/Epichloë festucae var. lolii combinations in collections of single seeds from the harvest year 2016. In this dataset, a cultivar effect could be seen between the seed microbiomes from cultivars Alto and Trojan. The bacterial component of the seed microbiome from pooled seeds from a single cultivar/E. festucae var. lolii combination harvested from 13 seed production farms around Canterbury in the year 2018 was also studied. This dataset allows the effect of different production locations on the bacterial seed microbiome to be examined. By comparing the two sets of data, bacteria from the genera Pantoea, Pseudomonas, Duganella, Massilia, and an unknown Enterobacteriaceae were observed to be in common. This core bacterial microbiome was stable over time but could be affected by supplemental taxa derived from the growth environment of the parental plant; differing microbiomes were seen between different seed production farms. By comparison to a collection of bacterial isolates, we demonstrated that many of the members of the core microbiome were culturable. This allows for the possibility of exploiting these microbes in the future.

Rhizosphere community selection reveals bacteria associated with reduced root disease

BACKGROUND

Microbes benefit plants by increasing nutrient availability, producing plant growth hormones, and protecting against pathogens. However, it is largely unknown how plants change root microbial communities.

RESULTS

In this study, we used a multi-cycle selection system and infection by the soilborne fungal pathogen Rhizoctonia solani AG8 (hereafter AG8) to examine how plants impact the rhizosphere bacterial community and recruit beneficial microorganisms to suppress soilborne fungal pathogens and promote plant growth. Successive plantings dramatically enhanced disease suppression on susceptible wheat cultivars to AG8 in the greenhouse. Accordingly, analysis of the rhizosphere soil microbial community using deep sequencing of 16S rRNA genes revealed distinct bacterial community profiles assembled over successive wheat plantings. Moreover, the cluster of bacterial communities formed from the AG8-infected rhizosphere was distinct from those without AG8 infection. Interestingly, the bacterial communities from the rhizosphere with the lowest wheat root disease gradually separated from those with the worst wheat root disease over planting cycles. Successive monocultures and application of AG8 increased the abundance of some bacterial genera which have potential antagonistic activities, such as Chitinophaga, Pseudomonas, Chryseobacterium, and Flavobacterium, and a group of plant growth-promoting (PGP) and nitrogen-fixing microbes, including Pedobacter, Variovorax, and Rhizobium. Furthermore, 47 bacteria isolates belong to 35 species were isolated. Among them, eleven and five exhibited antagonistic activities to AG8 and Rhizoctonia oryzae in vitro, respectively. Notably, Janthinobacterium displayed broad antagonism against the soilborne pathogens Pythium ultimum, AG8, and R. oryzae in vitro, and disease suppressive activity to AG8 in soil.

CONCLUSIONS

Our results demonstrated that successive wheat plantings and pathogen infection can shape the rhizosphere microbial communities and specifically accumulate a group of beneficial microbes. Our findings suggest that soil community selection may offer the potential for addressing agronomic concerns associated with plant diseases and crop productivity. Video Abstract.

Rhizosphere Microbiome Cooperations: Strategies for Sustainable Crop Production

Interactions between microorganisms and host plants determine the growth and development as well as the health of the host plant. Various microbial groups inhabit the rhizosphere, each with its peculiar function. The survival of each microbial group depends to a large extent on its ability to colonize the plant root and outcompete the native organisms. The role of the rhizospheric microbiome in enhancing plant growth has not been fully maximized. An understanding of the complexities of microbial interactions and factors affecting their assembly in the community is necessary to benefit maximally from the cooperations of various microbial communities for sustainable crop production. In this review, we outline the various organisms associated with the plant rhizosphere with emphasis on their interactions and mechanisms used in plant growth promotion.