Enhanced nodulation of peanut when co-inoculated with fungal endophyte Phomopsis liquidambari and bradyrhizobium

In peanut continuous cropping soil, the application of fungal endophyte Phomopsis liquidambari B3 showed peanut pod yield promotion and root nodule number increase. P. liquidambari improved soil environment by degrading allelochemicals and thus promoted peanut pod yield. Furthermore, peanut yield promotion is in part due to the root nodule increase since nodular nitrogen fixation provides the largest source of nitrogen for peanut. However, it is unknown whether this nodule number increase is induced by fungal endophyte. We therefore conducted several pot experiments using vermiculite to investigate the effects of P. liquidambari on peanut-bradyrhizobium nodulation. Our results showed that P. liquidambari co-inoculated with bradyrhizobium increased root nodule number and shoot accumulated nitrogen by 28.25% and 29.71%, respectively. Nodulation dynamics analysis showed that P. liquidambari accelerated nodule initiation and subsequent nodule development. Meanwhile, P. liquidambari was able to colonize the peanut root as an endophyte. The dynamics of P. liquidambari and bradyrhizobial root colonization analysis showed that P. liquidambari inoculation significantly increased the rate of bradyrhizobial colonization. Furthermore, P. liquidambari inoculation significantly increased flavonoids synthesis-related enzymes activities, two common types of flavonoid (luteolin and quercetin-peanut rhizobial nod gene inducer) secretion and lateral root (peanut rhizobial infection site) formation, indicating that P. liquidambari altered the peanut nodulation-related physiological and metabolic activities. These obtained results confirmed the direct contribution of P. liquidambari in enhancing peanut-bradyrhizobium interaction, nodulation and yield.

Enhancement of growth and salt tolerance of red pepper seedlings (Capsicum annuum L.) by regulating stress ethylene synthesis with halotolerant bacteria containing 1-aminocyclopropane-1-carboxylic acid deaminase activity

Three 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing halotolerant bacteria were isolated from West Coast soil of Yellow Sea, Incheon, South Korea and evaluated for their efficiency in improving red pepper plant growth under salt stress. The strains RS16, RS656 and RS111 were identified by 16S rRNA gene sequencing as Brevibacterium iodinum, Bacillus licheniformis and Zhihengliuela alba, respectively. Two hour exposure of 100, 150 and 200 mM NaCl stress on 8 day old red pepper seedlings caused 44, 64 and 74% increase ethylene production, while at 150 mM NaCl stress, inoculation of B. licheniformis RS656, Z. alba RS111, and Br. iodinum RS16 reduces ethylene production by 44, 53 and 57%, respectively. Similarly, 3 week old red pepper plants were subjected to salt stress for two weeks and approximately ∼50% reduction in growth recorded at 150 mM NaCl stress compared to negative control whereas bacteria inoculation significantly increase the growth compared to positive control. Salt stress also caused 1.3-fold reduction in the root/shoot dry weight ratio compared to the absence of salt while bacteria inoculation retained the biomass allocation similar to control plants. The salt tolerance index (ratio of biomass of salt stressed to non-stressed plant) was also significantly increased in inoculated plants compared to non-inoculated. Increase nutrient uptakes under salt stress by red pepper further evident that bacteria inoculation ameliorates salt stress effect. In summary, this study indicates that the use of ACC deaminase-producing halotolerant bacteria mitigates the salt stress by reducing salt stress-induced ethylene production on growth of red pepper plants.

Isolation, characterization, and use for plant growth promotion under salt stress, of ACC deaminase-producing halotolerant bacteria derived from coastal soil

In total, 140 halotolerant bacterial strains were isolated from both the soil of barren fields and the rhizosphere of six naturally growing halophytic plants in the vicinity of the Yellow Sea, near the city of Incheon in the Republic of Korea. All of these strains were characterized for multiple plant growth promoting traits, such as the production of indole acetic acid (IAA), nitrogen fixation, phosphorus (P) and zinc (Zn) solubilization, thiosulfate (S2O3) oxidation, the production of ammonia (NH3), and the production of extracellular hydrolytic enzymes such as protease, chitinase, pectinase, cellulase, and lipase under in vitro conditions. From the original 140 strains tested, on the basis of the latter tests for plant growth promotional activity, 36 were selected for further examination. These 36 halotolerant bacterial strains were then tested for 1- aminocyclopropane-1-carboxylic acid (ACC) deaminase activity. Twenty-five of these were found to be positive, and to be exhibiting significantly varying levels of activity. 16S rRNA gene sequencing analyses of the 36 halotolerant strains showed that they belong to 10 different bacterial genera: Bacillus, Brevibacterium, Planococcus, Zhihengliuella, Halomonas, Exiguobacterium, Oceanimonas, Corynebacterium, Arthrobacter, and Micrococcus. Inoculation of the 14 halotolerant bacterial strains to ameliorate salt stress (150 mM NaCl) in canola plants produced an increase in root length of between 5.2% and 47.8%, and dry weight of between 16.2% and 43%, in comparison with the uninoculated positive controls. In particular, three of the bacteria, Brevibacterium epidermidis RS15, Micrococcus yunnanensis RS222, and Bacillus aryabhattai RS341, all showed more than 40% increase in root elongation and dry weight when compared with uninoculated saltstressed canola seedlings. These results indicate that certain halotolerant bacteria, isolated from coastal soils, have a real potential to enhance plant growth under saline stress, through the reduction of ethylene production via ACC deaminase activity.