Abstract
Plant associated endophytic actinobacteria may contribute to plant growth and defense by direct or indirect methods. Our aim was to evaluate the plant growth promoting and antifungal activities of endophytic actinobacteria associated with Camellia spp. and related genera, Eurya to find potent plant growth promoting strains that could be applied in future microbe based bioformulations. We isolated 46 endophytic actinobacteria based on morphological characteristics of the isolates. 16S rRNA gene sequence analysis showed that the strains represented nine actinobacterial genera, Nocardia, Amycolatopsis, Streptomyces, Pseudonocardia, Kribbella, Actinomadura, Microbispora, Rothia and Saccharomonospora. In vitro functional characterization of the isolates for plant growth promoting (PGP) traits revealed many potent PGP isolates such as, SA1 and S43 which showed all the tested PGP traits, i.e., phosphate solubilization, indole-3-acetic acid (IAA), ammonia, siderophore and chitinase production. Out of the 46 endophytic actinobacteria isolates, 21 showed inhibition against atleast one test fungal phytopathogen and, isolates SA25 and SA29 exhibited broad spectrum antifungal activity against all the tested phytopathogens. Most of the endophytic actinobacteria isolates having antifungal activity were positive for the presence of chitinase, NRPS (Non-ribosomal peptides synthetase) or PKS-1 (Polyketide Synthase) gene, suggesting the presence of distinctive mechanisms to inhibit the growth of pathogenic plant fungi. ARDRA (Amplified Ribosomal DNA Restriction analysis) and BOX-PCR fingerprinting analysis of the potent isolates with antagonistic activity grouped the isolates into 5 and 4 separate clusters, respectively. In addition, an assessment using bonitur scale revealed the top ranked isolates based on their PGP and biocontrol traits. Further detection of IAA production by the top ranked actinobacterial isolates namely, SA1, T1LA3 and S85 by using thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) was done. Endophytic actinobacteria isolates, namely, SA1, T1LA3, and SA14 were further tested for their efficacy in promoting the growth of commercial tea clones, namely, TV1, TV9, TV18, and TV22 in nursery conditions. All the endophytic isolates tested showed significant differences (P ≤ 0.05) in terms of plant growth promoting parameters in the treated plants compared to untreated control and may, thus be, deemed as potential candidates for application in bioformulations for tea growth.
Highlights
Endophytic bacteria inhabit plant tissues and can boost plant growth under normal and strenuous environments by aiding plants in uptake of metabolites and by regulating phytohormone levels; improvement of plant health by aiming pathogens and pest with antagonistic metabolites, hydrolytic enzymes and nutrient limitation (Afzal et al, 2019)
A total of 46 endophytic actinobacteria were isolated from roots and leaf tissue of five different Camellia spp. and related genera
T1LA3 isolate showed the highest amount of ammonia production of 6.5 μmol ml−1. 15 (32.6%) isolates out of the total isolated endophytic actinobacterial isolates were able to grow on N2 free medium
Summary
Endophytic bacteria inhabit plant tissues and can boost plant growth under normal and strenuous environments by aiding plants in uptake of metabolites and by regulating phytohormone levels; improvement of plant health by aiming pathogens and pest with antagonistic metabolites, hydrolytic enzymes and nutrient limitation (Afzal et al, 2019). Endophytic actinobacteria are recognized as potential agents for their PGP effects and their role in nutrient uptake in host plants by producing biologically active secondary metabolites that enhance the fitness and resilience of the plants against environmental stress (Dinesh et al, 2017; Singh and Dubey, 2018). Endophytic actinobacteria are known to play a significant role in host plant protection from infectious pathogen by producing bioactive antifungal metabolites (Qin et al, 2011), polymer hydrolyzing enzymes like cellulases, siderophores and chitinase which cause degradation of fungal cell wall aiding in lysis of hyphae and limiting spore germination (Liu et al, 2017). Endophytic actinobacteria are reported as beneficial to plant health by reducing stress hormone ethylene, with the activity of an enzyme, 1-aminocyclopropane1-carboxylate deaminase (Yoolon et al, 2019) (ACCD) that converts ACC into ethylene
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