Diversity of Plant Growth-Promoting Endophytic Bacteria, Genome Analysis of Strain Sx8-8 and Its Rice Germination Promoting Activity

Twenty-eight endophytic bacteria isolated from stems and leaves of healthy plants in Kanchanaburi Province, Thailand, were characterized and evaluated for their plant growth-promoting activity. Based on their phenotypic characteristics and 16S rRNA gene sequence similarity, the isolates were identified as Pantoea (6 isolates), Priestia (4 isolates), Pseudomonas (3 isolates), Enterobacter (2 isolates), Acinetobacter (2 isolates), Novosphingobium (2 isolates), Curtobacterium (2 isolates), as well as Bacillus , Peribacillus , Sphingobium , Staphylococcus , Brevibacillus , Aneurinibacillus , and Pseudarthrobacter (1 isolate each). Seven isolates produced indole-3-acetic acid (IAA) in nitrogen-free broth supplemented with 0.01% L-tryptophan, eleven isolates fixed nitrogen, and twenty-three isolates solubilized phosphate and zinc. Sphingobium sp. Sx8-8 produced the highest IAA at 67.29 µg mL –1 , followed by Novosphingobium sp. SI8-3 and Pantoea sp. S5-1 at 33.06 and 29.4 µg mL –1 , respectively. The maximum IAA produced by the Sphingobium sp. Sx8-8 increased to 232.1 µg mL –1 at optimized conditions. In in vitro rice germination, the Sphingobium sp. Sx8-8 and Pantoea sp. S5-1 increased root length, number of lateral roots, and shoot length more than uninoculated control and synthetic IAA as a standard. The genome size of Sphingobium sp. Sx8-8 was 4.67 Mb (83 contigs), N50 size and average G + C content were 150 389 bp and 64.2 mol %, respectively. This study indicated that endophytic bacteria could potentially promote plant growth and be utilized as bioinoculant or biofertilizers in agriculture.