Isolation and identification of pathogens
Aspergillus niger (collar rot) affected seeds with blackish testa and rotted internal tissue, and also mature plants with wilting and rotting symptoms were collected at just below the ground level. The affected portion turned dark, shrunken, and shredded, and later covered by black spores of the pathogen. The pathogen was identified as Aspergillus niger based on microscopic examination (Figure 1). The fungus displayed upright and tiny conidiophores, finishing with globose swelling, holding phialides radiating from the entire surface. The conidia were single-celled, light to dark brown, globose shaped, and produced basipetal (Figure 1a and b). Mycelium was hyaline branched and septate.
The wilt (Fusarium udum) pathogen isolated from the infected plant roots of pigeon pea had slender hyaline hyphae with abundant branching, typically with small aerial growth (Figure 2). Chlamydospores were globose and smooth-walled. Macroconidia were straight to falcate, thin-walled, septate but predominantly 3-septate. Microconidia fusiform to reniform, septate (Figure 2a and b), micro and macroconidia formation were observed under the compound microscope. The isolates' nucleotide sequence has 100 % identity with Aspergillus niger isolate PP2 and Fusarium udum isolate PP1. The sequences were submitted to NCBI-GenBank with the accession numbers MW826097 (A. niger) and MW826105 (F. udum).
Biofabrication of silver nanoparticles and characterization
The majority of metal nanoparticles have optical properties that are proportional to the size and form. The bio-fabrication of silver nanoparticles (AgNPs) has been verified using a notable transition in media color from yellow to brown after 24 hours of incubation (Figure 3), signaling the biotransformation of Ag+ to Ag0 by reductive enzymes (Ahmad et al., 2003; Saifuddin et al., 2009). The variation in the organism physiology and the size and shape of metal particles could explain the current sample's over-color development. The variation in relative behavior in reducing silver nitrate ions to metal nanoparticles due to the proteins' existence may also explain the color shift. Surface plasmon vibration gives silver nanoparticles a peculiar brownish yellow color. Light evokes the free electrons in silver nanoparticles as observed under UV-Vis spectroscopy and transmits to a higher energy level, but the electron is unstable in an excited state and returns to base energy; a photon is emitted simultaneously (Thangaraju et al., 2012). Correspondingly, the spectrum of silver nanoparticles demonstrated the highest surface plasmon resonance at 420 nm. Sharp Surface Resonance Plasmon (SRP) in the 350-600 nm range are commonly used to confirm the presence of silver nanoparticles in an aqueous solution (Sastry et al., 1998; Henglein, 1993). Figure 4 reveals that the SPR is in the 400-500 nm band, suggesting that bio-fabricated silver nanoparticles are smaller and identical in form. As per Mie's theory, only a single SPR band is expected in the absorption spectra of spherical nanoparticles whereas, the number of peaks increases as anisotropy increases (Raut Rajesh et al., 2009). We also recorded a single peak in the present study that indicates the silver nanoparticles are spherical. This finding was the primary confirmation of size, form, and distribution, further verified by TEM analysis.
TEM micrographs demonstrated the size and form of individual bio-fabricated silver nanoparticles (Figure 5). With an average size of 30 nm with slight agglomeration, they are quasi-spherical to spherical in form and anisotropic in nature. These observations follow the findings of the small-sized nanoparticles of distinct shapes found by Abdel-Raouf et al. (2018). When the zeta potential is greater than 30 mV, the particles are stable (Sapsfor et al., 2011), consistent with the high zeta potential observed (- 42.7mV).
The particle size distribution of bio-fabricated silver nanoparticles has been evaluated with Dynamic Light Scattering (DLS). According to the findings, the mean size of bio-fabricated silver nanoparticles was estimated to be approximately 66.0 nm (Figure 6), with the graph peak tend to range from 32 to 115 nm, indicating that variable sizes of AgNPs were found in the solution. The TEM images of 50, 100, and 150 ppm concentrations of silver nanoparticles revealed the sizes as 49.7, 41.3, and 78.5 nm, respectively. This evidence is in good agreement with the DLS measurements. As predicted, due to the variations in measurements by the devices, the DLS calculated value is marginally larger than the TEM calculated value i.e., TEM calculates the number based on the size distribution of the physical dimension without capping agent. At the same time, the DLS measures the hydrodynamic diameter, that is, the particle diameter as well as the ions or molecules that are connected and travel along with them in silver nanoparticles in solution (Huang et al., 2007).
Additional insight into the stability of the bio-fabricated silver nanoparticles was checked using zeta potential. The zeta potential's magnitude determines the colloid's potential stability; particles with more positive than + 30 mV or more negative than -30 mV were presumed stable (Melendrez et al., 2010; and Vimala et al., 2017). We have observed the zeta potential peak at -70 mV, demonstrating the robust stability of silver nanoparticles at pH=7 ± 0.2 (Figure 7). High negative zeta potential might have occurred due to the interaction of silver nanoparticles with protein, peptides, or lipids of bacteria, which increases the negative zeta potential of silver nanoparticles (Shannahan et al., 2013). Our zeta potential results suggest that silver nanoparticles were highly stable and aggregated; similar results were reported by Loza et al. (2014). It thus confirms the presence of highly stable and aggregating AgNPs.
Antifungal potentials of silver nanoparticles
Due to expanded tolerance to fungicides and antibiotics, silver nanoparticles have recently sparked interest in the field of phytopathogenic fungi regulation. The bio-fabricated silver in the current research has demonstrated excellent in vitro inhibition of phytopathogenic fungi. However, because the current research is focused on in vitro petri dish testing, the extrapolation of the results to more general cases is minimal. Nonetheless, these findings include valid preliminary efficacy evidence on silver compounds used in plant-pathogen control. In the present investigation, silver nanoparticles inhibited the growth of Aspergillus niger and Fusarium udum significantly. The inhibition of mycelia formation by AgNPs and AgNO3 at various concentrations was assessed regularly for all of the fungi tested and the negative control. Bacterial filtrate of Pseudomonas fluorescens has no discernible impact on the mycelial growth of two phytopathogens (p>0.05). The bacterial filtrate treatment inhibited A. niger up to 7.7 per cent and F. udum up to 4.2 per cent.
The quantified results revealed that the antifungal activities of the AgNPs varied depending on the fungi studied and the concentrations used. In comparison to the control, AgNPs substantially inhibited the mycelial development of Aspergillus niger and Fusarium udum in a concentration-dependent manner (p<0.05). Fusarium udum had the best mycelial inhibition of 100 per cent among the studied fungi. The least successful concentration of bio-fabricated silver nanoparticles against Fusarium udum was 10 ppm, which inhibited mycelial development by 55.5 per cent. According to the statistical analysis, antifungal effect of AgNO3 was significantly lower than AgNPs on Fusarium udum (p>0.05). Rabab et al. (2018), reported that silver nanoparticles inhibited Fusarium sp. by 68.2 per cent at 150 ppm concentration. We observed 100 per cent antifungal activity against Fusarium udum at the same concentration (150 ppm). In several studies, AgNPs have demonstrated a broad spectrum of action against Fusarium sp., Cadndia tropicalis (Rahisuddin et al., 2015), Trichophyton rubrum (Pereira et al., 2014), Rhizoctonia solani, Colletotrichum sp. (Balakumaran et al., 2015). However, the methods used for biosynthesis and particle size differed depending on the experiment. We should note that the antifungal activity of silver nanoparticles varies depending on the type of fungus and the size of silver nanoparticles (Shafaghat, 2015).
The effect of silver nanoparticles on the mycelial growth of A. niger was slightly lower than the AgNO3. We observed that geometric mean and mode were higher for AgNO3 as compared to AgNPs. The per cent inhibition of Aspergillus niger and Fusarium udum was 87.19 and 74.8 per cent, respectively, at 150 ppm of AgNO3 concentration (Figures 8 and 9). Our results are in agreement with the findings of El-Kadi et al. (2018), who reported that AgNO3 inhibited Aspergillus sp. efficiently than other fungi. Furthermore, none of the isolates exhibited tolerance to silver nanoparticles. However, correlation with other studies is required to confirm the mode of action of AgNPs. It was believed that the antimicrobial activity of silver nanoparticles was due to the binding of silver nanoparticles on the mycelium leads to the formation of pores (Young et al., 2009) and electrolytes leakage from cells. The destabilization of cell membrane integrity (Kim et al., 2009a) and related proteins and lipids leads to respiratory inhibition and, eventually, cell lysis (Durán et al., 2005). Interaction of AgNPs with phosphorous and sulfhydryl groups in the cells leads to DNA damage, transcriptional interference (Feng et al., 2000; and Yamanaka et al., 2005), and protein inactivation (Durán et al., 2005).