Observation of microbial colonization on the surface of rice roots along with their development and degradation

Microbial colonization on the surface of rice roots along with their development and degradation was examined throughout the growth period of rice plants with the naked eye, by light microscopy, and scanning electron microscopy. Four stages were recognized in the development and degradation processes of rice roots and microbial colonization pattern. At stage I, both crown and lateral roots were undergoing development, white, and covered with a mucigel layer. Microbial colonization was rarely observed. At stage II, lateral roots developed further, and they stained brownish due to the deposition of iron oxides/hydroxides on the root surface. Microbial colonies commonly developed along the grooves between epidermal cells. At stage III, lateral roots ceased to develop, and the root surfaces were widely covered with deposits of iron oxides/hydroxides. Microbial colonies were found inside epidermal and exodermal cells as well as on epidermal cells. At stage IV, the roots became dark brown or transparent. Most of the epidermal and exodermal cells disappeared from the root surface. Roots went through stage I to stage IV as they aged, and respective degradation stages were observed from the nodal roots from higher nodes to the nodal roots from lower nodes. There were fewer microorganisms on the surface of lateral roots than on crown roots. About 50% of the roots were at stages I and II on June 20 and July 20, which corresponded to the early to middle growth stages, while most of the roots were at stages III or IV from August 10.