Yeasts produce substances that promote active cell division in the fast growing parts of the plants like the roots. A more extensive root system facilitates absorption of more water and nutrients from the soil that speed up plant growth. Greater surface area for photosynthesis is made available by growing numerous or wider leaves. Thus, more starch will be produced by the plant.
These beneficial microorganisms are the principal agents used in natural farming, a highly sustainable farming technique that brings back the lost properties of the soil. More can be learned about natural farming in "Facts About Natural Farming". Indigenous beneficial microorganisms can be produced by following the procedures in "How to Make a Concoction of Indigenous Microorganisms".