Two novel species of genus Streptomyces isolated from eucalyptus tissues grown in saline soil and their potential as plant growth promoters

Two endophytic actinobacteria of the genus Streptomyces —EKL1.1 T and EKS8.28 T —were isolated from the surface-sterilized leaf and twig of a red gum tree ( Eucalyptus camaldulensis Dehn.) grown in highly saline soil, as reported in a previous study. These strains were aerobic and feature well-developed substrate mycelia and aerial mycelia with spiral spore chains. Strains EKL1.1 T and EKS8.28 T shared the highest 16S rRNA gene sequence similarity with Streptomyces mexicanus NRRL B-24196 T (99.2%) and Streptomyces glomeratus LMG 19903 T (99.0%), respectively. The comparative genome study of strain EKL1.1 T and its closest type strain, S. mexicanus NRRL B-24196 T , showed the highest dDDH, ANIb, and ANIm values of 31.8, 85.3, and 88.5%, respectively. The comparative genome study of strain EKS8.28 T and its closest type strain, Streptomyces cynarae HUAS 13-4 T , had the highest dDDH, ANIb, and ANIm values of 41.8, 89.6, and 91.4%, respectively. The genotypic and phenotypic properties of strains EKL1.1 T and EKS8.28 T distinguished these two strains from the closely related species with validly published names. The name proposed for the new species of strain EKL1.1 T is Streptomyces kalasinensis (= NRRL B-65753 T = TBRC 19936 T ). The name proposed for the novel species of strain EKS8.28 T is Streptomyces phytorum (= NRRL B-65754 T = TBRC 19937 T ). Strain EKL1.1 T could only inhibit one fungus, Cladosporium sp. LB1, moderately (35%). Strain EKS8.28 T could inhibit Fusarium sp. RE1 (71.1%), Curvularia sp. LB12 (62.2%), and Cladosporium sp. LB1 (50%). Strains EKL1.1 T and EKS8.28 T could produce indole acetic acid (IAA) and solubilize phosphate. The optimum spore inoculum of strains EKL1.1 T and EKS8.28 T to promote seedling growth parameters of eucalyptus was 10 7 and 10 8 spores/mL, respectively. Strains EKL1.1 T and EKS8.28 T facilitated the development of eucalyptus seedlings under saline conditions. They also enhanced the growth of eucalyptus seedlings in planta by augmenting shoot length and fresh weight. Genome mining of these strains reveals their in vitro and in planta characteristics, including biosynthetic gene clusters of bioactive compounds and several genes associated with plant growth enhancement under stressful conditions. They can be formulated as inoculants to improve eucalyptus plantations for sustainable agriculture in the future.