Associative Nitrogen Fixation

Publisher Summary This chapter provides a short overview of the best-studied plant-associated N 2 -fixing bacteria. Because the tissue is not yet differentiated at the root tip, they may reach the site of the developing stele before the formation of the endodermis, which will later constitute a thick-walled boundary. This explains how these bacteria are able to invade xylem vessels. The chapter describes in detail colonization by Azospirillum. The N 2 fixation phenotype is dependent on the plant species and can even differ according to the cultivar. Another example of some kind of specificity is the limited host range found for Azoarcus species. Presently, eight Azospirillum species have been described: A. brasilense, A. lipoferum, A. halopraeferens, A. irakense, A. largimobile, A. doebereinerae, A. oryzae, and A. amazonense. Azospirillum has been shown to positively influence plant growth, crop yields, and the N content of the plant. Herbaspirillum spp. are the aggressive colonizers of the root interior, establishing themselves not only in the cortex and vascular tissues of roots but also systemically in the whole plant. In endophytic associations where the substantial amounts of fixed N are transferred to the plant (e.g. sugarcane), it would be interesting to find out which factors influence the release of fixed N 2 .