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By analogy with N₂-fixing sugar cane, achieving an effective N₂-fixing association between cereals and diazotrophs may require an endophytic mode of colonization, allowing better protection from oxygen and improved access to carbon substrates. Using nifA-lacZ as a genetic marker on a broad host-range plasmid pVK100, inserted into a range of free-living and associative N₂-fixing organisms, it has been possible to define their mode of colonization of the roots of wheat seedlings in addition to measuring the associated rates of nitrogen fixation. Of these bacterial strains, only 'Herbaspirillum seropedicae', 'Azorhizobium caulinodans' and a mutant strain of 'Azospirillum brasilense' (Sp7-S) displayed significant endophytic colonization of 2,4-D-treated wheat seedlings. By contrast, 'Acetobacter diazotrophicus', 'Azotobacter vinelandii', 'Derxia gummosa' and other 'Azospirillum' strains colonized at the rhizoplane. This chapter discusses these modes of colonization and the probable need for a stepwise process of facilitated evolution of selected diazatrophs and plants before an effective association can be achieved. |
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