Volatile Organic Compounds Produced by the Rhizobacterium Arthrobacter agilis UMCV2 Modulate Sorghum bicolor (Strategy II Plant) Morphogenesis and SbFRO1 Transcription In Vitro

Different rhizobacteria may regulate plant growth using different mechanisms, including production of signal molecules that modulate plant morphogenesis and gene expression. Iron (Fe) is an essential micronutrient for plant growth and is frequently limited in plants. Plants with Strategy I Fe-uptake systems enhance root ferric reductase (FRO) activity to promote Fe absorption. Plants with Strategy II Fe-uptake systems increase Fe absorption by phytosiderophore production. However, recent reports have shown that plants with Strategy II systems also possess FRO genes that are expressed not in roots, but in shoots. Different rhizobacteria trigger plant Strategy I Fe-uptake systems via emission of volatile organic compounds (VOCs). In this work, we show that the plant growth-promoting rhizobacterium Arthrobacter agilis UMCV2 modulates the morphogenesis and FRO transcription of Sorghum bicolor, a plant with a Strategy II Fe-uptake system, via VOC emission. We found that in a system with separate compartments, VOCs emitted by A. agilis promoted plant growth, caused an increase in chlorophyll concentration, and modified the root architecture system. We tested the effect of the pure bacterial volatile compound dimethylhexadecylamine, produced by the UMCV2 strain, on plant growth and found an increase of 1.8-fold on shoot fresh weight, shoot length, chlorophyll concentration, and lateral root number at a concentration of 8 mu M. This effect was dose dependent and was comparable to the effects produced by A. agilis VOCs. Simultaneously, we analyzed SbFRO1 expression using quantitative polymerase chain reaction and found that SbFRO1 expression was strongly modulated by VOCs produced by A. agilis, specifically dimethylhexadecylamine.