Microscopic morphology of nitrogen fixing paranodules on wheat roots

Abstract

Symbiotic nitrogen fixation is an environmentally benign and inexpensive means of providing plants with nitrogen, but is currently not possible in non-legume grain crops. This study examines nitrogen fixing paranodules that developed on wheat (Triticum aestivum) roots after treatment with 2,4-dichlorophenoxyacetic acid (2,4-D) or Bacillus polymyxa 43, a mixed culture of diazotroph bacteria Xanthomonas sp. and Arthrobacter sp. (XA) or their combination. The formation of paranodules 1 mm in diameter was observed two weeks after planting. The size of paranodules did not increase during subsequent plant growth. Light microscopic examination and scanning electron microscope showed a large number of bacteria within the mucigel. In ultra thin sections of the nodule, bacterial cells were found in the intercellular space of the paranodules. In the latter case, most microorganisms were oriented along the plant cell wall. The active functioning of introduced cultures within the paranodules was evidenced by the presence of dividing bacterial cells and bacteria surrounded by zones of lysis. The maximum nitrogen fixing activity was observed in wheat co-treated with 2,4-D and mixed culture of XA. This study supports the ability of a current wheat variety to form root paranodulation and nitrogen fixation by a naturally occurring soil diazotroph, and to benefit from the relation by increased biomass and protein.

Key words: Arthrobacter sp., biological nitrogen fixation, paranodule, wheat, Xanthomonas sp.