Box–behnken optimization of indole-3-acetic acid production by endophytic bacteria and its potential role in wheat seed growth promotion

Abstract Endophytic bacteria enhance plant growth and yield. Sugarcane is a key economic crop in Egypt, yet few studies have examined endophytic bacteria impact on its production. This study aimed to isolate endophytic bacteria from sugarcane roots and stems, screen for indole-3-acetic acid (IAA) production, identify top IAA producer isolates, and optimize yield using Box-Behnken design (BBD). This study is considered as the first paper to use this tool for optimizing IAA production by sugarcane’s endophytes. The salt tolerance of the best performing strain was also investigated. Additionally, the effects of IAA-producing bacteria and their IAA on wheat seed germination were separately evaluated. The results showed that thirty-one endophytic bacterial isolates were isolated from sugarcane ( Saccharum sp., cultivar C 9), from which twelve isolates were IAA producers. Among the positive isolates, strain no. IA47 isolated from sugarcane root demonstrated the highest IAA yield by producing 231.52 ± 0.09 mg/L representing the highest IAA production reported for a sugarcane endophyte. Application of the One-Factor-At-a-Time approach and BBD proved tryptone (5 g/L), tryptophan (0.2 g/L), and incubation temperature (35 °C) as the most critical factors improving IAA production. Bacterial strain IA47 was subjected to molecular analysis and was found to be a member of the family Enterobacteriaceae, closely related to Klebsiella. The strain was deposited in the NCBI database under accession number PX101404. This strain was reported as halotolerant strain that could grow and produce notable high concentration of IAA (13.04 mg/L) when growing in 150 mM NaCl concentration. The plant growth stimulation effect of IA47 on wheat was tested in vitro and showed significant growth promotion, especially for the number of lateral roots, shoot length, and chlorophyll a/b ratio, by bacterial coating treatment. The results of the current study provide a valuable bacterial strain promising to be applied as a biofertilizer for promoting the growth and yield of non-host crops.