Assessment of affinity and specificity of 'Azospirillum' for plants

Background: 'Azospirillum' spp. are the most studied plant growth-promoting bacteria (PGPB). The genus represents a common model for plant-bacteria interactions. This genus was initially isolated and tested on cereals and was subsequently commercialized. Aims: Despite claims of plant specificity, particularly towards cereals, data over the past 40 years does not appear to substantiate claims of such specificity/affinity of 'Azospirillum' species. Consequently an evaluation of the specificity/affinity of the genus 'Azospirillum' across all plants, in general, and cereals, in particular, was undertaken. Results: Although the majority of studies focused on cereals, 'Azospirillum' spp. increase growth of 113 plant species across 35 botanical families, including 14 species of cereals. Amongst 'Azospirillum' spp., several well studied strains have been effective in several plant species, making these organisms potentially valuable for further study. Conclusions: This review demonstrates that azospirilla are not cereal-specific at the genus and species levels. 'Azospirillum' serves as a general PGPB to every plant species tested so far. Given the paucity of widespread screening, affinity of strains to a plant genotype, cultivar, or plant species cannot be overruled. Definitive conclusions concerning such specificity require molecular and cross-inoculation studies, using various strains of bacteria, and re-isolation after growth of the plants in different plant species.