Bacillus sp. LC390B from the Maize Rhizosphere Improves Plant Biomass, Root Elongation, and Branching and Requires the Phytochromes PHYA and PHYB for Phytostimulation

The plant microbiomes include root-associated beneficial species, which rely on carbon supplements to survive and exert their diverse attributes. Here, we isolated and identified through 16S rRNA analysis a novel plant growth promoting rhizobacterium belonging to the Bacillus genus ( Bacillus sp. LC390B) from a collection from root isolates of maize plants grown in an alkaline-calcareous soil. The LC390B strain enhanced shoot and root fresh weights and chlorophyll content in leaves upon direct cocultivation with Arabidopsis roots in medium lacking sucrose or supplemented with 0.6% sucrose, and upon transfer to artificial soil led plants to produce more fruits. These effects coincided with faster cell elongation, enhanced primary root growth and higher branching capacity, and increase in local auxin response within lateral root primordia. Mutant analysis indicated that specific elements in the auxin pathway control either primary root elongation or branching in bacterized plants, while most of the probiotic attributes were missing in a double mutant defective on the phytochromes PHYA and PHYB . The bacteria remained viable in interactions with roots up to seven days, supporting its long lasting effects to plants. Our data show that the newly isolated LC390 strain has potential to stimulate the growth of plants and unveil a critical function of specific elements in the auxin and phytochrome signaling network in plant–bacteria interaction.