Plant growth-promoting Pseudomonas sp. Strains reduce natural occurrence of anthracnose in soybean (Glycine max L.) in central Himalayan region

Root microbiota alters response to root rot in Rhododendron delavayi Franch

Root microbiota have a significant effect on plant health. However, the role of root microbiota in the resistance of Rhododendron against root rot is not known. In this study, we employed amplicon 16S and ITS sequencing to investigate the bacterial and fungal communities associated with four distinct niches (bulk soil, rhizosphere, rhizoplane, and endosphere) of both healthy and diseased Rhododendron plants in the Baili Rhododendron nature reserve in China. The amplicon data analysis identified 182 bacterial genera and 141 fungal genera that were impacted by root rot across all niches. Specifically, the rhizoplane appeared to exert a selective gating effect, resulting in a reduction in the complexity of bacterial communities, but not fungal communities, in wild Rhododendron delavayi Franch roots. Nevertheless, the stress induced by root rot led to alterations in the root microbiota and compromised the gating function of the rhizoplane, thereby significantly increasing the complexity of the bacterial community within the plant root. In the root tissue following root rot outbreak, the relative abundance of the pathogenic species Pezicula brunnea and Diaporthe helianthi was enriched by as much as 6.13% and 1.71%, respectively. These findings provide novel insights into the contribution of enrichment of root-associated microbiota to wild plant hosts under the disease stress of root rot. The root rot-causing pathogenic fungi may interact with beneficial bacteria and induce plants to send out "cry for help" signals, which may encourage the specific assembly of microbiota. In the Rhododendron delavayi Franch root microbiota, we found 23 potentially beneficial microbes. Notably, certain beneficial bacteria, such as Sporolactobacillus and Stenotrophomonas, were found to accumulate in the rhizoplane and endosphere under root rot disease stress. Overall, our results lend support to our hypothesis that Rhododendron recruits protective microbes as a strategy to suppress root rot outbreaks. Future endeavors in isolating beneficial microbes capable of mitigating root rot have the potential to enhance plant resilience against root diseases.

Screening of Siderophore-Producing Bacteria and Their Effects on Promoting the Growth of Plants

Iron deficiency is a major global agricultural problem. Siderophores can help organisms to uptake iron in form of siderophore-Fe³⁺ complexes and then in the cell cytosol, iron is reducted and released in ferrous form. This research aimed to obtain some efficient siderophore-producing bacterial strains and evaluate their plant growth-promoting effects in the iron-deficit environment. Two strains, Brucella sp. E7 and Pseudomonas brassicae W7, were isolated from rhizosphere soil. Both strains could produce maximum siderophores under the optimal conditions. Plant promoting experiment showed that many indicators of Vigna radiata seedling were all increased significantly by strain E7/W7 or the consortium of E7 + W7. Under no-iron and high iron stress, the inoculation treatment also showed growth promotion effects on both Vigna radiata and Lolium multiflorum. These results indicated that the potential ability of strain E7 and W7 in increasing agricultural production as a growth-promoting agent in iron-deficit soil.

Molecular characterization of plant growth promoting rhizobacteria that enhance peroxidase and phenylalanine ammonia-lyase activities in chile (Capsicum annuum L.) and tomato (Lycopersicon esculentum Mill.)

Pythium and Phytophthora species are associated with damping-off diseases in vegetable nurseries and reduce seedling stand and yield. In this study, bacterial isolates were selected on the basis of in vitro antagonism potential to inhibit mycelial growth of damping-off pathogens along with plant growth properties for field assessment in wet and winter seasons. We demonstrate efficacy of bacterial isolates to protect chile and tomato plants under natural vegetable nursery and artificially created pathogen-infested (Pythium and Phytophthora spp.) nursery conditions. After 21 days of sowing, chile and tomato plants were harvested and analysed for peroxidase and phenylalanine ammonia-lyase activities. Pseudomonas sp. strains FQP PB-3, FQA PB-3 and GRP(3 )were most effective in increasing shoot length (P > 0.05%) in both artificial and natural field sites. For example, Pseudomonas sp. FQA PB-3 treatment increased shoot length by 40% in the artificial Pythium 4746 infested nursery site in chile plants in the wet season. The bacterial treatments significantly increased the activity of peroxidase and phenylalanine ammonia-lyase in chile and tomato plant tissues, which are well known as indicators of an active lignification process. Thus, we conclude that treatment with potential bacterial plant growth promoting agents help plants against pathogen invasion by modulating plant peroxidase and phenylalanine ammonia-lyase activities.

Rhamnolipids from the rhizosphere bacterium Pseudomonas sp. GRP(3) that reduces damping-off disease in Chilli and tomato nurseries

A detailed screening of bacterial isolates from the Central Himalayan region for plant growth promotion and antimycelial activity against Pythium and Phytophthora strains afforded seven isolates, of which three were particularly effective against the incidence of damping-off in field trials on chilli and tomato. In this investigation an initial spectroscopic survey of the methanolic extracts of the seven bacterial isolates showed complex mixtures except for Pseudomonas sp. GRP3, one of the most promising isolates on the basis of field studies. Strain GRP3 was selected for structural characterization of its secondary metabolites, particularly glycolipids. The extracellular secondary metabolites were enriched by Amberlite XAD-16 adsorber resin followed by separation and structural analysis using TLC, LC-MS, MS-MS, and NMR spectroscopy. Acquired data show the presence of a number of mono- and dirhamnolipids and include rhamnose (Rha)-C8-C10, Rha-C10-C8, Rha-C10-C10, Rha-C10-C12:1, Rha-C10-C12, Rha-Rha-C8-C10, Rha-Rha-C10-C10, Rha-Rha-C10-C10:1, Rha-Rha-C10-C12, Rha-Rha-C10-C12:1, Rha-Rha-C12-C12:1, and Rha-Rha-C12-C12 in strain GRP3. Since rhamnolipids are involved in the lysis of the plasma membrane of zoospores of fungi, application of such rhamnolipid-producing rhizobacteria could facilitate control of damping-off plant pathogens.

Rhizosphere Pseudomonas sp. strains reduce occurrence of pre- and post-emergence damping-off in chile and tomato in Central Himalayan region

Based on in vitro screening for PGP and anti-mycelial activity against three zoosporic pathogenic oomycetes, Pythium aphanidermatum 123, P. aphanidermatum 4746, and Phytophthora nicotianae 4747, seven bacterial isolates were selected for field trials on tomato and chile to test for plant growth promotion under natural and artificial disease-infested field sites in both winter and wet seasons. The effectiveness of isolates in the field trials correlated with the in vitro antagonism screening data. Pseudomonas sp. FQP PB-3, FQA PB-3 and GRP(3) showed substantial beneficial effects on plant growth promotion and lowered considerably the incidence of pre- and post-emergence damping-off in both the crops under various disease scenarios. For example, seed bacterization with these bacterial strains reduced pre-emergence-damping off by ca. 60-70% in the two natural sites, with and without histories of fungicide use in the winter season, and to a lesser extent, ca. 20-40%, in the warmer wet (high humidity; 85-92%) season. The suppression efficacy for post-emergence damping-off was less compared to pre-emergence damping-off although still significant (P > 0.05). Our data unambiguously show that screening of a large number of bacterial pool identifies promising isolates that show beneficial effects on all stages of plant growth in natural oomycete-infested regimes.