Biopriming of seed with plant growth-promoting bacteria for improved germination and seedling growth

Several seed priming methods can be used to improve seed germination, seedling vigor, and to overcome abiotic stress. In addition to these benefits, only the biopriming method provides the additional benefit of biotic stress management, earning it special attention. Seed biopriming is useful in almost all crops around the world and is an environmentally friendly alternative to chemical fungicides. Biopriming usually refers to use of beneficial microorganisms, in particular plant growth-promoting bacteria (PGPB) able to survive under various harsh environmental conditions. In this study, various bacterial strains were isolated from samples of different origins, i.e., rhizospheric soil, desert sand, and sea mud. Preliminary screening of 156 bacterial isolates was conducted on the basis of their potassium (K), phosphorus (P) solubilization ability, and production of plant growth hormone, i.e., indole acetic acid (IAA). The most efficient bacteria were identified by 16S rRNA gene nucleotide sequences and further examined for their ACC deaminase activity, ammonia production, and biocontrol activity (defined via chitinolytic activity, HCN, and siderophores production). Finally, carrot seed germination assay was conducted with 10 shortlisted most potent isolates. 68.6, 58.3, and 66.7% of tested bacterial isolates were capable of P, K, and Zn solubilization, respectively. Klebsiella aerogenes AF3II1 showed the highest P and K solubilization, while isolate AF4II5, AF7II3, and PC3 showed the highest IAA synthesis ability. Serratia plymuthica EDC15 and Pseudomonas putida AF1I1 showed the strongest chitinolytic and siderophore production activity, respectively. Seven isolates demonstrated strong HCN production ability. Five isolates improved carrot seed germination. Only selected isolates with plant growth-promoting properties can improve carrot germination. The results of this study demonstrate that mainly auxins are involved in seed germination. Furthermore, the data suggest that phosphate solubilization ability may play an additional role in seed germination.

Psychrotolerant Mesorhizobium sp. Isolated from Temperate and Cold Desert Regions Solubilizes Potassium and Produces Multiple Plant Growth Promoting Metabolites

Soil potassium (K) supplement depends intensively on the application of chemical fertilizers, which have substantial harmful environmental effects. However, some bacteria can act as inoculants by converting unavailable and insoluble K forms into plant-accessible forms. Such bacteria are an eco-friendly approach for enhancing plant K absorption and consequently reducing utilization of chemical fertilization. Therefore, the present research was undertaken to isolate, screen, and characterize the K solubilizing bacteria (KSB) from the rhizosphere soils of northern India. Overall, 110 strains were isolated, but only 13 isolates showed significant K solubilizing ability by forming a halo zone on solid media. They were further screened for K solubilizing activity at 0 °C, 1 °C, 3 °C, 5 °C, 7 °C, 15 °C, and 20 °C for 5, 10, and 20 days. All the bacterial isolates showed mineral K solubilization activity at these different temperatures. However, the content of K solubilization increased with the upsurge in temperature and period of incubation. The isolate KSB (Grz) showed the highest K solubilization index of 462.28% after 48 h of incubation at 20 °C. The maximum of 23.38 µg K/mL broth was solubilized by the isolate KSB (Grz) at 20 °C after 20 days of incubation. Based on morphological, biochemical, and molecular characterization (through the 16S rDNA approach), the isolate KSB (Grz) was identified as Mesorhizobium sp. The majority of the strains produced HCN and ammonia. The maximum indole acetic acid (IAA) (31.54 µM/mL) and cellulase (390 µM/mL) were produced by the isolate KSB (Grz). In contrast, the highest protease (525.12 µM/mL) and chitinase (5.20 µM/mL) activities were shown by standard strain Bacillus mucilaginosus and KSB (Gmr) isolate, respectively.

Drought and Plant Community Composition Affect the Metabolic and Genotypic Diversity of Pseudomonas Strains in Grassland Soils

Climate and plant community composition (PCC) modulate the structure and function of microbial communities. In order to characterize how the functional traits of bacteria are affected, important plant growth-promoting rhizobacteria of grassland soil communities, pseudomonads, were isolated from a grassland experiment and phylogenetically and functionally characterized. The Miniplot experiment was implemented to examine the mechanisms underlying grassland ecosystem changes due to climate change, and it investigates the sole or combined impact of drought and PCC (plant species with their main distribution either in SW or NE Europe, and a mixture of these species). We observed that the proportion and phylogenetic composition of nutrient-releasing populations of the Pseudomonas community are affected by prolonged drought periods, and to a minor extent by changes in plant community composition, and that these changes underlie seasonality effects. Our data also partly showed concordance between the metabolic activities and 16S phylogeny. The drought-induced shifts in functional Pseudomonas community traits, phosphate and potassium solubilization and siderophore production did not follow a unique pattern. Whereas decreased soil moisture induced a highly active phosphate-solubilizing community, the siderophore-producing community showed the opposite response. In spite of this, no effect on potassium solubilization was detected. These results suggest that the Pseudomonas community quickly responds to drought in terms of structure and function, the direction of the functional response is trait-specific, and the extent of the response is affected by plant community composition.

Multiple Potential Plant Growth Promotion Activities of Endemic Streptomyces spp. from Moroccan Sugar Beet Fields with Their Inhibitory Activities against Fusarium spp

The characterized 10 Streptomyces isolates were previously selected by their abilities to solubilize phosphates. To investigate whether these isolates represent multifaceted plant growth-promoting rhizobacteria (PGPR), their potassium-solubilizing, auxin-producing and inhibitory activities were determined. The 10 Streptomyces spp. yielded a variable biomass in the presence of insoluble orthoclase as the sole potassium (K) source, indicating that they were able to extract different amounts of K from this source for their own growth. Three strains (AZ, AYD and DE2) released soluble K from insoluble orthoclase in large amounts into the culture broth. The production levels ranged from 125.4 mg/L to 216.6 mg/L after 5 days of culture. Only two strains, Streptomyces enissocaesilis (BYC) and S. tunisiensis (AI), released a larger amount of soluble K from orthoclase and yielded much more biomass. This indicated that the rate of K released from this insoluble orthoclase exceeded its consumption rate for bacterial growth and that some strains solubilized K more efficiently than others. The results also suggest that the K solubilization process of AZ, AYD and DE2 strains, the most efficient K-solubilizing strains, involves a slight acidification of the medium. Furthermore, these 10 Streptomyces spp. were able to secrete indole acetic acid (IAA) in broth medium and ranged from 7.9 ± 0.1 µg/mL to 122.3 ± 0.1 µg/mL. The results of the antibiosis test proved the potential of the 10 tested strains to limit the growth of fungi and bacteria. In dual culture, S. bellus (AYD) had highest inhibitory effect against the three identified fungal causal agents of root rot of sugar beet: Fusarium equiseti and two F. fujikuroi at 55, 43 and 36%, respectively. Streptomyces enissocaesilis (BYC), S. bellus (AYD) and S. saprophyticus (DE2) exhibited higher multifaceted PGPR with their potassium-solubilizing, auxin-producing and inhibitory activities, which could be expected to lead to effectiveness in field trials of sugar beet.

Comparative Genomics and In Vitro Plant Growth Promotion and Biocontrol Traits of Lactic Acid Bacteria from the Wheat Rhizosphere

This study aimed to isolate lactic acid bacteria (LAB) from wheat rhizosphere, to characterize their in vitro plant growth promoting activities and to differentiate plant-associated LAB from those associated with foods or human disease through comparative genomic analysis. Lactococcus lactis subsp. lactis and Enterococcus faecium were isolated using de Man-Rogosa-Sharpe (MRS) and Glucose Yeast Peptone (GYP) as enrichment culture media. Comparative genomic analyses showed that plant-associated LAB strains were enriched in genes coding for bacteriocin production when compared to strains from other ecosystems. Isolates of L. lactis and E. faecium did not produce physiologically relevant concentrations of the phyto-hormone indolacetic acid. All isolates solubilized high amount of phosphate and 12 of 16 strains solubilized potassium. E. faecium LB5, L. lactis LB6, LB7, and LB9 inhibited the plant pathogenic Fusarium graminearum to the same extent as two strains of Bacillus sp. However, the antifungal activity of the abovementioned LAB strains depended on the medium of cultivation and a low pH while antifungal activity of Bacillus spp. was independent of the growth medium and likely relates to antifungal lipopeptides. This study showed the potential of rhizospheric LAB for future application as biofertilizers in agriculture.