Endophytic Establishment of Diazotrophic Bacteria in Auxin-Induced Tumors of Cereal Crops

Gramineous crops such as wheat (triticum oestivum), maize (zea mays), and rice (oryza sativa) develop tumorous structures (para-nodules) along primary and secondary roots when treated with low concentrations of various auxins. Rice forms additional tumors along its hypocotyle. Histologically, auxin-induced tumors appear as cancerous grown out root meristems and thus are comparable in origin and structure to stem nodules of the legume sesbania rostrata. Auxin-affected root meristems do not recover and develop further to large nodule-like organs. Introduced diazotrophs (Azospirillum spp., Azorhizobium caulinodans, Rhizobium spp.) potentially inhabit tissues of both stem and root tumors with the central meristem as a major colonization niche. Evidence is given that infecting bacteria follow a ‘crack entry’ invasion at sites where developing tumors have emerged through the root cortex and epidermis. Bacteria are shown to establish with high cell numbers inside intercellular spaces of cortical and meristematic tissues. Plant-cell infection of tumor cells takes place with bacteria found inside the cell-cytoplasm surrounded by membrane-like structures. Once inhabiting induced tumor tissues introduced diazotrophs colonize endophytically with high cell numbers. Mutant, ammonium-excreting and thus ecologically disadvantaged A. brasilense is shown to survive inside para-nodulating maize and rice plants with a dense population. Micro-aerobic nitrogenase activities of tumor inhabiting diazotrophic bacteria (A. brasilense, Azotobacter vinelandii, A. caulonidans) are in general highly increased when compared with untreated control plants. Additionally, bacterial nitrogenase activity is less sensitive to an increased oxygen tension in the root environment. The host plants benefit from the enhanced nitrogen fixation in their para-nodulating roots. Highest rates of incorporation of fixed nitrogen into host plant material is reported for para-nodule inhabiting ammonium excreting A. brasilense strain C3. The host plant potentially stimulates the nitrogenase activity of endophytically colonizing diazotrophs by providing energy in the form of a suitable carbon source. In conclusion, it is demonstrated that gramineous plants are potentially capable of developing an endophytical diazotrophic symbiosis through para-nodule formation.