Isolation of the Endophytic Fungus Aspergillus terreus from a Halophyte (Tetraena qatarensis) and Assessment of Its Potential in Tomato Seedling Protection

Abstract

Living in diverse environmentally harsh conditions, the plant exhibits a unique survival mechanism. As a result, the endophytes residing within the plant produce specific compounds that promote the plant's growth and defend it against pathogens. Plants and algae symbiotically harbor endophytes, i.e., microbes and microorganisms living within them. The objective of this study is to isolate endophytic fungi, specifically strains of Aspergillus terreus, from the leaves of the salt-tolerant plant Tetraena qatarensis and to explore the salt tolerance, antagonistic activity, and growth promotion properties. Strain C A. terreus (ON117337.1) was screened for salt tolerance and antagonistic effects. Regarding salt tolerance, the isolate demonstrated the ability to thrive in a concentration of up to 10% NaCl. A. terreus showed inhibitory activity against four fungal phytopathogens, namely Fusarium oxysporum, Alternaria alternata, Colletotrichum gloeosporioides, and Botrytis cinerea. The GC-MS investigation of the fungal (strain C Aspergillus terreus) extract showed the presence of about 66 compounds (secondary metabolites). Secondary metabolites (SMs) are produced, like Hexadecanoic acid, which aids in controlling phytopathogens. Also produced is lovastatin, which is used to treat hypercholesterolemia. Strain C, which showed salinity tolerance and the highest inhibitory activity, was further analyzed for its effect on tomato seed germination under pathogen stress from Fusarium oxysporum. The greenhouse experiment indicated that the fungi increased the length of tomato seedlings and the plant biomass. Therefore, the selected endophytes derived from Tetraena qatarensis were scrutinized for their potential as biocontrol agents, aiming to thwart fungal pathogens and stimulate plant growth. The in vitro and in vivo assessments of strain C (Aspergillus terreus) against Fusarium oxysporum in this investigation indicate the promising role of endophytes as effective biological control agents. Investigating novel bio-products offers a sustainable approach to agriculture, gradually reducing dependence on chemical fungicides.

Keywords: antagonism; biocontrol; endophyte; growth promotor; halophyte.

Figure 1

Molecular phylogenetic tree analysis for Aspergillus terreus by maximum likelihood method with the value of (1000) bootstrap comparisons. Evolutionary analyses were conducted by [MEGA 11].

Figure 2

Mycelium growth of strain C (A. terreus) on PDA plates mixed with different salinity gradients at 25 °C. (A) 0% NaCl, (B) 2% NaCl, (C) 4% NaCl, (D) 6% NaCl, (E) 8% NaCl, and (F) 10% NaCl.

Figure 3

Antifungal activity of endophytic isolate C Aspergillus terreus assessed by dual-culture assay against phytopathogens. (1) Colletotrichum gloeosporioides, (2) Alternaria alternata, (3) Botrytis cinerea, and (4) Fusarium oxysporum.

Figure 4

Endophytes and their metabolites.

Figure 5

GC-MS scan mood chromatogram of dichloromethane extract of Aspergillus terreus.

Figure 6

A photo of tomato seedlings after 8 weeks of treatment. (T1) Untreated control, (T2) seedlings inoculated with endophyte strain C, (T3) seedlings inoculated with the pathogen Fusarium oxysporum, (T4) seedlings inoculated first (one week earlier) with endophyte strain C and then inoculated with the plant pathogen F. oxysporum.

Figure 7

The impact of endophyte and pathogen treatments on the shoot length and chlorophyll content of tomato seedlings was assessed after an 8-week treatment period (N = 4). Graph (A) represents the shoot length in (cm). Graph (B) represents the chlorophyll content in the SPAD unit. The error bars depict the standard errors of the means. Shared letter(s) among values indicate no significance at p ≤ 0.05, as determined by Tukey’s test (N = 4). (T1) Untreated control, (T2) seedlings inoculated with endophyte, A. terreus strain C, (T3) seedlings inoculated with the pathogen Fusarium oxysporum, (T4) seedlings inoculated first (one week earlier) with endophyte strain C and then inoculated with the plant pathogen F. oxysporum.

Figure 8

Effect of endophyte and pathogen treatments on the biomass of below-ground and aboveground biomass of tomato seedlings after 8 weeks of treatment (N = 4). (A) represents the fresh weight biomass both above and below ground. (B) represents the dry weight biomass both above and below ground. The error bars depict the standard errors of the means. Shared letter(s) among values indicate no significance at p ≤ 0.05, as determined by Tukey’s test (N = 4). (T1) Untreated control, (T2) seedlings inoculated with endophyte, A. terreus strain C, (T3) seedlings inoculated with the pathogen Fusarium oxysporum, (T4) seedlings inoculated first (one week earlier) with endophyte strain C and then inoculated with the plant pathogen F. oxysporum.

Figure 9

FTIR spectrum of dried leaf powder of tomato under different treatments. (T1) Untreated control, (T2) seedlings inoculated with endophyte, A. terreus strain C, (T3) seedlings inoculated with the pathogen Fusarium oxysporum, (T4) seedlings inoculated first (one week earlier) with endophyte strain C and then inoculated with the plant pathogen F. oxysporum.