Plant-Growth-Promoting Effect by Cell Components of Purple Non-Sulfur Photosynthetic Bacteria

Rhodobacter sphaeroides, a purple non-sulfur photosynthetic bacterium (PNSB), was disrupted by sonication and fractionated by centrifugation into the supernatant and pellet. The effects of the supernatant and pellet on plant growth were examined using Brassica rapa var. perviridis (komatsuna) in the pot experiments. Both fractions showed growth-promoting effects: the supernatant at high concentrations (1 × 10⁷ to 4 × 10⁷ cfu-equivalent mL^-1) and the pellet at a low concentration of 2 × 10³ cfu-equivalent mL^-1). We expected lipopolysaccharide (LPS) to be the active principle of the pellet fraction and examined the effects of LPS on the growth of B. rapa var. perviridis. The growth of the plants was significantly enhanced by the foliar feeding of R. sphaeroides LPS at concentrations ranging from 10 to 100 pg mL^-1. The present study is the first report indicating that LPS acts as one of the active principles of the plant-growth-promoting effect of PNSB.