Since bacteria are so common in the air and soil around plants, you might expect that important relationships between plants and bacteria have evolved through time. Certain bacteria can cause plant diseases, but the most wide spread and important relationships between these very different living things may well be the helpful ones. Some plants and bacteria help one another in only casual ways, and each can get along without the other very well. In other relationships, the partners have evolved very complicated ways of operating together that give them great advantages over their competitors or which affect their very survival.
The simplest and least studied bacteria-plant relationships concern bacteria living on leaf surfaces. Nutrients such as sugars and amino acids are given off by plant leaves, providing food for the bacteria. The microorganisms, in breaking down these chemicals to meet their own needs, produce plant hormones which make the plants grow faster. One group of scientists found 46 different strains of bacteria growing on pea leaves and stems. Twenty-six of these could produce an important plant growth hormone called IAA. Over half the bacteria isolated from corn plants were able to manufacture another important plant hormone, auxin, and this auxin appeared to be transferred from the bacteria into the corn plants themselves.