Furthermore, Faltin et al. found no correlation between IAA production in vitro and the plant growth-promoting effect on lettuce seedlings of several antagonistic bacteria, including the S. plymuthica strain 3Re4-18. In view of these data, the growth and yield promotion observed might be explained by the ‘additive hypothesis’ , postulating that growth promotion is the result of multiple coordinated mechanisms such as associative nitrogen fixation, modulation of phytohormonal balances, phytohormone biosynthesis and solubilization of phosphate.