**5.2 Beneficial soil bacteria and crop production**

Apart from fungi, there are several groups of soil bacteria that are important to plant growth. Some bacteria have the ability to fix atmospheric nitrogen and form symbiotic relationship with plants [101]. In tropical soils, phosphate-solubilizing microorganisms indirectly provide phosphorus for plants by solubilizing phosphorus precipitated with iron, aluminum and calcium thus making it important for plant growth and development [102].

## *5.2.1 Root colonization by bacteria*

Root colonization is the microbial attachment to and proliferation on roots. It is an essential factor in the beneficial interactions used for biofertilization, microbiological control, phytoremediation and phytostimulation as well as in plant pathogenesis of soil borne microbes [103]. PGPR may colonize the rhizosphere, root surface, or even superficial intercellular spaces [104].

Howie *et al*. [105] hypothesized two phase processes in which bacterium can attach itself to the plant and soil. In the phase I, bacteria on the seed are attached to the emerging root tip where they are passively transported into the soil. During root growth, some bacteria cells remain associated with the tip while others are left behind on the older parts of the root and the rhizosphere. In phase II, bacteria deposited along the root zone multiply and form microcolonies in nutrient-rich microsites, where they compete with indigenous microflora in order to avoid displacement. Both phases occur simultaneous on different root parts [106]. Root colonization can be influenced by both biotic (such as genetic traits of the host plant and the colonizing organism) and abiotic (such as soil humidity, growth substrate, soil and rhizosphere pH and temperature) factors. Changes in plant physical and chemical composition in the rhizosphere can strongly influence root colonization and competence [107]. Root exudates and mucilage-derived nutrients attract beneficial and neutral bacteria as well as harmful bacteria allowing them to colonize and reproduce in the rhizosphere [108].
