ENDOPHYTIC ESTABLISHMENT OF DIAZOTROPHIC BACTERIA IN AUXIN-INDUCED TUMORS OF CEREAL CROPS

Gramineous crops such as wheat (triticum aestivum), maize (tea mays), and rice (oryza sativa) develop tumorous structures (para-nodules) alo ng primary and secondary roots when treated with low concentrations of various auxins. Rice forms additional tumors along its hypocotyle. Hi stologically, auxin-induced tumors appear as cancerous grown out root meristems and thus are comparable in origin and structure to stem nodu les of the legume sesbania rostrata. Auxin-affected root meristems do not recover and:develop further to large nodule-like organs. Introduce d diazotrophs (Azospirillum spp., Azorhizobium caulinodans, Rhizobium spp.) potentially inhabit tissues of both stem and root tumors with th e central meristem as a major colonization niche. Evidence is given th at infecting bacteria follow a 'crack entry' invasion at sites where d eveloping tumors have emerged through the root cortex and epidermis. B acteria are shown to establish with high cell numbers inside intercell ular spaces of cortical and meristematic tissues. Plant-cell infection of tumor cells takes place with bacteria found inside the cell-cytopl asm surrounded by membrane-like structures. Once inhabiting induced tu mor tissues introduced diazotrophs colonize endophytically with high c ell numbers. Mutant, ammonium-excreting and thus ecologically disadvan taged A. brasilense is shown to survive inside para-nodulating maize a nd rice plants with a dense population. Micro-aerobic nitrogenase acti vities of tumor inhabiting diazotrophic bacteria (A. brasilense, Azoto bacter vinelandii, A. caulonidans) are in general highly increased whe n compared with untreated control plants. Additionally, bacterial nitr ogenase activity is less sensitive to an increased oxygen tension in t he root environment. The host plants benefit from the enhanced nitroge n fixation in their para-nodulating roots. Highest rates of incorporat ion of fixed nitrogen into host plant material is reported for para-no dule inhabiting ammonium excreting A. brasilense strain C3. The host p lant 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 plant s are potentially capable of developing an endophytical diazotrophic s ymbiosis through para-nodule formation.