Bacteria—plant associations

The origin and evolutionary development of higher plants has occurred in environments that were already colonised by bacteria, resulting in the co-evolution of a range of bacteria—plant associations. The associated microbes may be broadly considered in two main categories: epiphytic bacteria (present on the outside of the plant) and internal bacteria (infecting the plant tissue).

Epiphytic bacteria

These are associated with the plant surface, which is generally divided into root (rhizosphere) and aerial (phyllosphere) regions. A wide range of bacteria are adapted to various microenvironments at the soil and air interface, and are important in such aspects as nutrient uptake, frost damage, and biological control of plant pathogens. Many of these epiphytic bacteria are saprophytes, obtaining complex nutrients from the plant. Some epiphytic bacteria are also parasites, spending part of their life cycle on the plant surface, and part within the plant tissue.

Infective bacteria: parasites and symbionts

Parasitic bacteria are able to invade plant tissue, where they grow and multiply, and cause localised or general deterioration in the health of the plant. The great majority of these parasites are extracellular, multiplying within intercellular spaces but not penetrating plant cell walls or entering protoplasts. The relatively few parasitic bacteria that are able to penetrate the higher plant cell include members of the genus Agrobacterium (with the ability to transfer part of the genome into the plant cell) and Rhizobium (where the whole organism enters the plant cell).