Harnessing Multifunctional Bacillus thuringiensis for Plant Growth Promotion and Biocontrol Potential Against...

Bacillus thuringiensis is the most widely used microbial bioinsecticide; however, only a few strains possess additional traits such as plant growth promotion and biocontrol potential. This study characterized 25 B. thuringiensis isolates obtained from soil and infected insect cadavers to identify multifunctional strains with insecticidal and plant growth-promoting (PGP) properties. Microscopic examination revealed diverse parasporal crystal morphologies, while 16S rRNA sequencing confirmed their identity with 99%-100% similarity to reference strains. Enzymatic profiling showed protease activity in 24 strains and lipase activity in 21 strains, whereas all isolates produced lecithinase and chitinase. All strains produced indole-3-acetic acid (11.22-31.87 µg mL^-1) and ammonia (26.74-77.68 µg mL^-1), while phosphate solubilization and siderophore production were observed in 21 and 11 strains, respectively. Bioassays against Spodoptera frugiperda identified five highly virulent strains causing more than 90% larval mortality. Among these, NBAIR Bt25 showed the highest efficacy, causing 92% mortality within 96 h with an LC₅₀ of 39.75 µg mL^-1. PCR analysis revealed that NBAIR Bt25 carries multiple pesticidal genes (cry1A, cry1D, cry1E, cry1I, cry2A, and vip3A). Paper towel and glasshouse assays demonstrated that strain NBAIR Bt25 significantly enhanced shoot growth and secondary root development. Overall, NBAIR Bt25 emerged as a promising multifunctional strain with strong insecticidal activity, plant growth-promoting ability, and biocontrol potential, making it a suitable candidate for sustainable pest management and climate-smart agriculture.