Abstract
Certain microbes stimulate plant growth. The mechanisms used by such microbes to achieve this are: biofertilization (i.e. increasing the supply of mineral nutrients to the plant); biocontrol (i.e. elimination of the plant's enemies including microbial pathogens, insects and weeds); and direct plant growth promotion (e.g. by delivering plant growth hormones to the plant). In all of these mechanisms, plant root colonization is crucial as the delivery system.