Sphingomonas panaciterrae PB20 increases growth, photosynthetic pigments, antioxidants, and mineral nutrient contents in spinach (Spinacia oleracea L.).

Plant growth promoting rhizobacteria (PGPR) have been intensively investigated in agricultural crops for decades. Nevertheless, little information is available on the application of Sphingomonas spp. as a PGPR particularly in vegetables, despite of potential plant growth promoting traits of this group. This study investigated the role of Sphingomonas panaciterrae (PB20) on growth and nutritional profile of spinach applied through seed priming (SP), soil drenching (SD), foliar application (FA), and bacterial culture filtrate foliar (BCF) applications. The results showed that, depending on different methods of application, PB20 significantly increased plant height (19.57-65.65 %), fresh weight (7.26-37.41 %), total chlorophyll (71.14-192.54 %), carotenoid (67.10-211.67 %) antioxidant (55.99-207.04), vitamin C (8.1-94.6 %) and protein content (6.7-21.5 %) compared to control in the edible part of spinach. Among the mineral nutrients, root nitrogen (N) showed greater response to bacterial application (18.65%-46.15 % increase over control) than shoot nitrogen (6.70%-21.52 % increased over control). Likewise, in all methods of application, phosphorus (P) content showed significant increase over control both in root (42.79-78.48 %) and in shoot (3.57-27.0 %). Seed priming and foliar application of PB20 increased the shoot calcium (Ca) content compared to control. BCF foliar application yielded maximum magnesium (Mg), iron (Fe) and zinc (Zn) in shoot. However, seed priming resulted in maximum Fe in root. Overall, seed priming outperformed in growth, vitamin C, antioxidants, N and P uptake, while BCF foliar application resulted in better uptake of several nutrients. Multivariate analysis validated the positive association of most of the growth parameters with SP while several nutrients with FA and BCF. Based on the findings it is evident that this rhizobacteria PB20 has the potentiality to be applied as a biofertilizer to produce nutrient-enriched spinach with an improved yield. Farmers can conveniently incorporate PR20 through seed priming before planting of spinach, with additional benefits through foliar spray.

The fungal endophyte Metarhizium anisopliae (MetA1) coordinates salt tolerance mechanisms of rice to enhance growth and yield.

The implementation of salt stress mitigation strategies aided by microorganisms has the potential to improve crop growth and yield. The endophytic fungus Metarhizium anisopliae shows the ability to enhance plant growth and mitigate diverse forms of abiotic stress. We examined the functions of M. anisopliae isolate MetA1 (MA) in promoting salinity resistance by investigating several morphological, physiological, biochemical, and yield features in rice plants. In vitro evaluation demonstrated that rice seeds primed with MA enhanced the growth features of rice plants exposed to 4, 8, and 12 dS/m of salinity for 15 days in an agar medium. A pot experiment was carried out to evaluate the growth and development of MA-primed rice seeds after exposing them to similar levels of salinity. Results indicated MA priming in rice improved shoot and root biomass, photosynthetic pigment contents, leaf succulence, and leaf relative water content. It also significantly decreased Na+/K+ ratios in both shoots and roots and the levels of electrolyte leakage, malondialdehyde, and hydrogen peroxide, while significantly increasing proline content in the leaves. The antioxidant enzymes catalase, glutathione S-transferase, ascorbate peroxidase, and peroxidase, as well as the non-enzymatic antioxidants phenol and flavonoids, were significantly enhanced in MA-colonized plants when compared with MA-unprimed plants under salt stress. The MA-mediated restriction of salt accumulation and improvement in physiological and biochemical mechanisms ultimately contributed to the yield improvement in salt-exposed rice plants. Our findings suggest the potential use of the MA seed priming strategy to improve salt tolerance in rice and perhaps in other crop plants.

Role of salicylic acid in improving the yield of two mung bean genotypes under waterlogging stress through the modulation of antioxidant defense and osmoprotectant levels.

Waterlogging (WL) is a major hindrance to the growth and development of leguminous crops, including mung bean. Here, we explored the effect of salicylic acid (SA) pretreatment on growth and yield output of two elite mung bean genotypes (BU Mung bean-4 and BU Mung bean-6) subjected to WL stress. SA pretreatment significantly improved shoot dry weight, individual leaf area, and photosynthetic pigment contents in both genotypes, while those improvements were higher in BU Mung bean-6 when compared with BU Mung bean-4. We also found that SA pretreatment significantly reduced the reactive oxygen species-induced oxidative burden in both BU Mung bean-6 and BU Mung bean-4 by enhancing peroxidase, glutathione S-transferase, catalase, and ascorbate peroxidase activities, as well as total flavonoid contents. SA pretreatment further improved the accumulation of proline and free amino acids in both genotypes, indicating that SA employed these osmoprotectants to enhance osmotic balance. These results were particularly corroborated with the elevated levels of leaf water status and leaf succulence in BU Mung bean-6. SA-mediated improvement in physiological and biochemical mechanisms led to a greater yield-associated feature in BU Mung bean-6 under WL conditions. Collectively, these findings shed light on the positive roles of SA in alleviating WL stress, contributing to yield improvement in mung bean crop.

Effect of Metarhizium anisopliae (MetA1) on growth enhancement and antioxidative defense mechanism against Rhizoctonia root rot in okra.

Rhizoctonia solani is an important necrotrophic pathogenic fungus that causes okra root disease and results in severe yield reduction. Many biocontrol agents are being studied with the intent of improving plant growth and defense systems and reducing crop loss by preventing fungal infections. Recently, a member of the Hypocrealean family, Metarhizium anisopliae, has been reported for insect pathogenicity, endophytism, plant growth promotion, and antifungal potentialities. This research investigated the role of M. anisopliae (MetA1) in growth promotion and root disease suppression in okra. The antagonism against R. solani and the plant growth promotion traits of MetA1 were tested in vitro. The effects of endophytic MetA1 on promoting plant growth and disease suppression were assessed in planta. Dual culture and cell-free culture filtrate assays showed antagonistic activity against R. solani by MetA1. Some plant growth promotion traits, such as phosphate solubilization and catalase activity were also exhibited by MetA1. Seed primed with MetA1 increased the shoot, root, leaves, chlorophyll content, and biomass content compared to control okra plants. The plants challenged with R. solani showed the highest hydrogen peroxide (H2O2) and lipid peroxidation (MDA) contents in the leaves of okra. Whereas MetA1 applied plants showed a reduction of H2O2 and MDA by 5.21 and 14.96%, respectively, under pathogen-inoculated conditions by increasing antioxidant enzyme activities, including catalase (CAT), peroxidase (POD), glutathione S-transferase (GST), and ascorbate peroxidase (APX), by 30.11, 10.19, 5.62, and 5.06%, respectively. Moreover, MetA1 increased soluble sugars, carbohydrates, proline, and secondary metabolites, viz., phenol and flavonoid contents in okra resulting in a better osmotic adjustment of diseases infecting plants. MetA1 reduced disease incidence by 58.33% at 15 DAI compared to the R. solani inoculated plant. The results revealed that MetA1 improved plant growth, elevated the plant defense system, and suppressed root diseases caused by R. solani. Thus, MetA1 was found to be an effective candidate for the biological control program.

Biocontrol potential of native isolates of Beauveria bassiana against cotton leafworm Spodoptera litura (Fabricius).

The entomopathogenic fungus (EPF), Beauveria bassiana, is reported as the most potent biological control agent against a wide range of insect families. This study aimed to isolate and characterize the native B. bassiana from various soil habitats in Bangladesh and to evaluate the bio-efficacy of these isolates against an important vegetable insect pest, Spodoptera litura. Seven isolates from Bangladeshi soils were characterized as B. bassiana using genomic analysis. Among the isolates, TGS2.3 showed the highest mortality rate (82%) against the 2nd instar larvae of S. litura at 7 days after treatment (DAT). This isolate was further bioassayed against different stages of S. litura and found that TGS2.3 induced 81, 57, 94, 84, 75, 65, and 57% overall mortality at egg, neonatal 1st, 2nd, 3rd, 4th, and 5th instar larvae, respectively, over 7 DAT. Interestingly, treatment with B. bassiana isolate TGS2.3 resulted in pupal and adult deformities as well as decreased adult emergence of S. litura. Taken together, our results suggest that a native isolate of B. bassiana TGS2.3 is a potential biocontrol agent against the destructive insect pest S. litura. However, further studies are needed to evaluate the bio-efficacy of this promising native isolate in planta and field conditions.

Pseudomonas and Burkholderia inhibit growth and asexual development of Phytophthora capsici.

The objective of this study was to isolate and characterize antagonistic rhizobacteria from chili against a notorious phytopathogen Phytophthora capsici. Among the 48 bacteria isolated, BTLbbc-02, BTLbbc-03, and BTLbbc-05 were selected based on their inhibitory activity against P. capsici. They were tentatively identified as Burkholderia metallica BTLbbc-02, Burkholderia cepacia BTLbbc-03, and Pseudomonas aeruginosa BTLbbc-05, respectively, based on their 16S rRNA gene sequencing. All inhibited the growth of P. capsici at varying levels by inducing characteristic morphological alterations of P. capsici hyphae. The cell-free culture supernatant of all three isolates impaired motility (up to 100%) and caused lysis (up to 50%) of the halted zoospores. Bioassays revealed that Pseudomonas sp. had higher antagonism and zoospore motility-inhibitory effects against P. capsici compared with two other isolates, Burkholderia spp. and B. metallica, which caused vacuolation in mycelium. All three bacteria suppressed sporangium formation and zoosporogenesis of P. capsici, and improved the seed germination and growth of cucumber. Our findings suggest that epiphytic bacteria, B. metallica, B. cepacia, and P. aeruginosa, could be used as potential biocontrol agents against P. capsici. A further study is required to ensure conformity with the existing regulations for soil, plant, and human health.