Analysis of the Bacterial Diversity Associated with the Roots of Maize (Zea mays L.) through Culture-Dependent and Culture-Independent Methods

The present study investigated bacterial diversity associated with the roots of maize through the use of culture-dependent and culture-independent methods. Bacterial 16S–23S rDNA internal transcribed spacer sequences (ITS) primers were used to amplify sequences obtained directly from the root matrix by Percoll gradient separation. This assay showed that γ-Proteobacteria within Enterobacter, Erwinia, Klebsiella, Pseudomonas, and Stenotrophomonas genera were predominant groups. The culturable component of the bacterial community was also assessed, revealing that the predominant group was Firmicutes, mainly of Bacillus genus, while Achromobacter, Lysinibacillus, and Paenibacillus genera were rarely found in association with the roots. Only two genera within γ-Proteobacteria, Enterobacter and Pseudomonas, were found in the culture collection. Differences in richness and diversity between the rhizospheric and endophytic bacterial communities were also evidenced. The spectrum of bacteria naturally associated with maize roots is wide and the magnitude of such diversity will depend on the methods chosen for analysis. The knowledge of this spectrum will facilitate the search of microorganisms capable of exerting antagonism to diverse pathogens or detecting plant growth enhancers. 1. Introduction Maize (Zea mays L.) is one of the three most important agronomic crops in terms of world production, together with rice and wheat. As world cereal consumption tends to increase due to a constantly growing population, productivity should be significantly improved through different strategies that allow an optimization of yields without implicating an increased sown area [1]. Despite the partial success of synthetic chemical pesticides and fertilizers in achieving this goal, a change to environmentally friendly and conservative alternatives is required to protect biodiversity and sustainability of agroecosystems and natural systems all over the world. Soil microbial communities play an integral and often unique role in ecosystem functions and are among the most complex, diverse, and important assemblages in the biosphere [2]. The study of plant-associated microorganisms is of great importance for biotechnological applications, for example, biological control of plant pathogens, plant growth promotion, or isolation of active compounds [3, 4]. Most studies on rhizospheric and endophytic bacteria and their community structure have been performed by using culture-dependent approaches. Isolation of culturable bacteria is appropriate for functional analysis; however, as a high