title: Ascending migration of endophytic rhizobia, from roots to leaves, inside rice plants and assessment of benefits to rice growth physiology.
abstract: Rhizobia, the root-nodule endosymbionts of leguminous plants, also form natural endophytic associations with roots of important cereal plants. Despite its widespread occurrence, much remains unknown about colonization of cereals by rhizobia. We examined the infection, dissemination, and colonization of healthy rice plant tissues by four species of gfp-tagged rhizobia and their influence on the growth physiology of rice. The results indicated a dynamic infection process beginning with surface colonization of the rhizoplane (especially at lateral root emergence), followed by endophytic colonization within roots, and then ascending endophytic migration into the stem base, leaf sheath, and leaves where they developed high populations. In situ CMEIAS image analysis indicated local endophytic population densities reaching as high as 9 x 10(10) rhizobia per cm3 of infected host tissues, whereas plating experiments indicated rapid, transient or persistent growth depending on the rhizobial strain and rice tissue examined. Rice plants inoculated with certain test strains of gfp-tagged rhizobia produced significantly higher root and shoot biomass; increased their photosynthetic rate, stomatal conductance, transpiration velocity, water utilization efficiency, and flag leaf area (considered to possess the highest photosynthetic activity); and accumulated higher levels of indoleacetic acid and gibberellin growth-regulating phytohormones. Considered collectively, the results indicate that this endophytic plant-bacterium association is far more inclusive, invasive, and dynamic than previously thought, including dissemination in both below-ground and above-ground tissues and enhancement of growth physiology by several rhizobial species, therefore heightening its interest and potential value as a biofertilizer strategy for sustainable agriculture to produce the world's most important cereal crops.
authors.affiliation: Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 10093, People's Republic of China.
authors.forename: Feng
authors.initials: F
authors.lastname: Chi
authors.forename: Shi-Hua
authors.initials: SH
authors.lastname: Shen
authors.forename: Hai-Ping
authors.initials: HP
authors.lastname: Cheng
authors.forename: Yu-Xiang
authors.initials: YX
authors.lastname: Jing
authors.forename: Youssef G
authors.initials: YG
authors.lastname: Yanni
authors.forename: Frank B
authors.initials: FB
authors.lastname: Dazzo
mesh_terms: D015169:Colony Count, Microbial; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D018613:Microscopy, Confocal; D012275:Oryza / Q000254:growth & development* / Q000382:microbiology*; D018515:Plant Leaves / Q000382:microbiology*; D018517:Plant Roots / Q000382:microbiology*; D012231:Rhizobium / Q000235:genetics / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D049452:Green Fluorescent Proteins