Plant Growth-Promoting Activity of Bacteria Isolated from Asian Rice (Oryza sativa L.) Depends on Rice Genotype

A review on mechanisms and prospects of endophytic bacteria in biocontrol of plant pathogenic fungi and their plant growth-promoting activities

Endophytic bacteria, living inside plants, are competent plant colonizers, capable of enhancing immune responses in plants and establishing a symbiotic relationship with them. Endophytic bacteria are able to control phytopathogenic fungi while exhibiting plant growth-promoting activity. Here, we discussed the mechanisms of phytopathogenic fungi control and plant growth-promoting actions discovered in some major groups of beneficial endophytic bacteria such as Bacillus, Paenibacillus, and Pseudomonas. Most of the studied strains in these genera were isolated from the rhizosphere and soils, and a more extensive study of these endophytic bacteria is needed. It is essential to understand the underlying biocontrol and plant growth-promoting mechanisms and to develop an effective screening approach for selecting potential endophytic bacteria for various applications. We have suggested a screening strategy to identify potentially useful endophytic bacteria based on mechanistic phenomena. The discovery of endophytic bacteria with useful biocontrol and plant growth-promoting characteristics is essential for developing sustainable agriculture.

Ensifer adhaerens strain OV14 seed application enhances Triticum aestivum L. and Brassica napus L. development

Given the challenges imposed by climate change and societal challenges, the European Union established ambitious goals as part of its Farm to Fork (F2F) strategy. Focussed on accelerating the transition to systems of sustainable food production, processing and consumption, a key element of F2F is to reduce the use of fertilisers by at least 20% and plant protection products by up to 50% by 2030. In recent years, a substantial body of research has highlighted the potential impact of microbial-based applications to support crop production practices through both biotic/abiotic stresses via maintaining or even improving yields and reducing reliance on intensive chemical inputs. Here, we have characterised the ability of a new soil-borne free-living bacterium strain Ensifer adhaerens OV14 (EaOV14) to significantly enhance crop vigour index by up to 50% for monocot (wheat, Triticum aestivum L., p < 0.0001) and by up to 40% for dicot (oilseed rape, Brassica napus L., p < 0.0001) species under in-vitro conditions (n = 360 seedlings/treatment). The beneficial effect was further studied under controlled glasshouse growing conditions (n = 60 plants/treatment) where EaOV14 induced significantly increased seed yield of spring oilseed rape compared to the controls (p < 0.0001). Moreover, using bespoke rhizoboxes, enhanced root architecture (density, roots orientation, roots thickness etc.) was observed for spring oilseed rape and winter wheat, with the median number of roots 55% and 33% higher for oilseed rape and wheat respectively, following EaOV14 seed treatment compared to the control. In addition, EaOV14 treatment increased root tip formation and root volume, suggesting the formation of a more robust root system architecture post-seed treatment. However, like other microbial formulations, the trade-offs associated with field translation, such as loss or limited functionality due to inoculum formulation or environmental distress, need further investigation. Moreover, the delivery method requires further optimisation to identify the optimal inoculum formulation that will maximise the expected beneficial impact on yield under field growing conditions.

The alleviation of salt stress on rice through increasing photosynthetic capacity, maintaining redox homeostasis and regulating soil enzyme activities by Enterobacter sp. JIV1 assisted with putrescine

The detrimental impact of soil salinization on crop productivity and agricultural economy has garnered significant attention. A rhizosphere bacterium with favorable salt tolerance and plant growth-promoting (PGP) functions was isolated in this work. The bacterium was identified as Enterobacter through 16 S rDNA sequencing analysis and designated as Enterobacter sp. JIV1. Interestingly, the presence of putrescine (Put), which had been shown to contribute in reducing abiotic stress damage to plants, significantly promoted strain JIV1 to generate 1-aminocyclopropane-1-carboxylic (ACC) deaminase, dissolve phosphorus and secrete indole-3-acetic acid (IAA). However, the synergy of plant growth promoting rhizobacteria (PGPR) and Put in improving plant salt resistance has not been extensively studied. In this study, strain JIV1 and exogenous Put effectively mitigated the inhibitory impact of salt stress simulated by 200 mM NaCl on rice (Oryza sativa L.) growth. The chlorophyll accumulation, photosynthetic efficiency and antioxidant capacity of rice were also significantly strengthened. Notably, the combined application of strain JIV1 and Put outperformed individual treatments. Moreover, the co-addition of strain JIV1 and Put increased soil protease and urease activities by 451.97% and 51.70% compared to that of salt treatment group. In general, Put-assisted PGPR JIV1 provides a new perspective on alleviating the salt-induced negative impacts on plants.

Pseudomonas mosselii improves cold tolerance of Asian rice (Oryza sativa L.) in a genotype-dependent manner by increasing proline in japonica and reduced glutathione in indica varieties

Cold stress is an important factor limiting rice production and distribution. Identifying factors that contribute to cold tolerance in rice is of primary importance. While some plant specific genetic factors involved in cold tolerance have been identified, the role of the rice microbiome remains unexplored. In this study, we evaluated the influence of plant growth promoting bacteria (PGPB) with the ability of phosphate solubilization on rice cold tolerance and survival. To reach this goal, inoculated and uninoculated 2-week-old seedlings were cold stressed and evaluated for survival and other phenotypes such as electrolyte leakage (EL) and necessary elements for cold tolerance. The results of this study showed that of the five bacteria, Pseudomonas mosselii, improved both indica and japonica varietal plants' survival and decreased EL, indicating increased membrane integrity. We observed different possible cold tolerance mechanisms in japonica and indica plants such as increases in proline and reduced glutathione levels, respectively. This bacterium also improved the shoot growth of cold exposed indica plants during the recovery period. This study confirmed the host genotype dependent activity of P. mosselii and indicated that there is an interaction between specific plant genes and bacterial genes that causes different plant responses to cold stress.

Endophytic bacterial communities in wild rice (Oryza officinalis) and their plant growth-promoting effects on perennial rice

Endophytic bacterial microbiomes of plants contribute to the physiological health of the host and its adaptive evolution and stress tolerance. Wild rice possesses enriched endophytic bacteria diversity, which is a potential resource for sustainable agriculture. Oryza officinalis is a unique perennial wild rice species in China with rich genetic resources. However, endophytic bacterial communities of this species and their plant growth-promoting (PGP) traits remain largely unknown. In this study, endophytic bacteria in the root, stem, and leaf tissues of O. officinalis were characterized using 16S rRNA gene Illumina sequencing. Culturable bacterial endophytes were also isolated from O. officinalis tissues and characterized for their PGP traits. The microbiome analysis showed a more complex structure and powerful function of the endophytic bacterial community in roots compared with those in other tissue compartments. Each compartment had its specific endophytic bacterial biomarkers, including Desulfomonile and Ruminiclostridium for roots; Lactobacillus, Acinetobacter, Cutibacterium and Dechloromonas for stems; and Stenotrophomonas, Chryseobacterium, Achromobacter and Methylobacterium for leaves. A total of 96 endophytic bacterial strains with PGP traits of phosphate solubilization, potassium release, nitrogen fixation, 1-aminocyclopropane-1-carboxylate (ACC) deaminase secretion, and siderophore or indole-3-acetic acid (IAA) production were isolated from O. officinalis. Among them, 11 strains identified as Enterobacter mori, E. ludwigii, E. cloacae, Bacillus amyloliquefaciens, B. siamensis, Pseudomonas rhodesiae and Kosakonia oryzae were selected for inoculation of perennial rice based on their IAA production traits. These strains showed promising PGP effects on perennial rice seedlings. They promoted plants to form a strong root system, stimulate biomass accumulation, and increase chlorophyll content and nitrogen uptake, which could fulfil the ecologically sustainable cultivation model of perennial rice. These results provide insights into the bacterial endosphere of O. officinalis and its application potential in perennial rice. There is the prospect of mining beneficial endophytic bacteria from wild rice species, which could rewild the microbiome of cultivated rice varieties and promote their growth.