**1. Introduction**

The rhizosphere is characterized as the soil in close contact with plant roots and is where the root microbiome recruitment occurs through exudation of plant molecular signals, especially secondary metabolites [1,2]. Plants are usually benefited by the interactions with soil microbes in their rhizospheres, improving plant nutrient acquisition, pathogen resistance, and stress tolerance [3,4]. The so-called 'Plant Growth-Promoting Rhizobacteria' (PGPR) are non-pathogenic bacteria that live in the rhizosphere soil and are able to promote plant development [3,5–7]. Some PGPR are able to provide essential nutrients to plants using a compound that is synthesized by the bacterium or making some macro and micro-nutrients available that were immobilized in mineral and organic compounds, and were not free

for plant uptake [8]. The biological nitrogen fixation and the phosphate solubilization/mineralization are examples of the direct promotion of plant growth by PGPR [9]. The use of PGPR in biofertilization is an open field for research on sustainable agriculture, as it optimizes crop yields based on beneficial plant-microbe interactions [1,10]. The use of PGPR has been tested with success in different plants such as maize, potato, wheat, and many other economically important ones [11].

A large number of biotic and abiotic factors influence the microbial communities in the rhizosphere. Physicochemical characteristics of the soil directly modulate microbial communities [2] and plant physiology and genetics also control rhizosphere composition [1,12]. The impact of plant genotypes on rhizosphere microbiome composition varies depending on soil context and plant species studied [13]. Therefore, understanding mutual adaptation between microbes and plants in response to different environmental conditions can contribute to crop breeding and management programs.

Sweet potato plants (*Ipomoea batatas* L.) present a complex root system composed by fibrous roots specialized in nutrient absorption - and tuberous roots - specialized in nutrient storage. The transition from fibrous root to tuberous root is related to starch accumulation [14,15]. The tuberous roots represent one of the major nutrient sources for countries in development, and they also contribute to a supply of vitamins B, C, and E and minerals, including iron, calcium, zinc, and selenium, in the human diet (International Potato Center – [16]). Sweet potato is cultivated worldwide, with Asia and Africa together producing 95% of all roots commercialized globally [17]. Sweet potato is used for food, animal feed, and processing (as food, starch, and other products) and its biggest global producer and consumer is China. Among Latin America countries, Brazil is in the spotlight as the main sweet potato producer. Sweet potato is considered to be the country's fourth most consumed vegetable crop, and in the northeast of the country, the culture is important both economically and socially [18].

For sweet potato, as for other crops, high root yields are desirable for good crop productivity. Usually, chemical fertilizers are used, but the high inorganic fertilizer input contributes negatively to production costs and to environment pollution [19]. They are expensive, non-eco-friendly, cause eutrophication, reduce organic matter and microbial activity in soil, and are hazardous to health [20]. Therefore, there is increasing interest in the use of biofertilizers for sustainable agriculture. PGPR can help to increase quality of the soil by providing nutrients required for benefit of the plants [8]. Dawwam, et al. [21] have already demonstrated the beneficial effect of PGPR isolated from the roots of sweet potato. Different isolates having abilities for IAA production and phosphate solubilization were tested as bioinoculants to potato tubers, and the inoculated plants showed significant differences in various parameters such as vegetative growth, photosynthetic pigments, and N, P, and K concentrations.

According to other studies, the growth of sweet potato can be significantly improved by PGPR [19,22–24]. However, information about the PGPR communities during the sweet potato growth and their behavior depending on the genotype of sweet potato studied is still limited. In previous studies, the total bacterial communities from the rhizosphere and endosphere of the tuberous roots of three sweet potato genotypes (IPB-149, IPB-137, and IPB-052) were characterized by Marques, et al. [12,25]. The results showed a strong rhizosphere effect in the soil surrounding the sweet potato tuberous roots, showing an influence of plant age and genotypes in bacterial communities. However, the effects of either plant age or genotypes specifically in the PGPR communities (nitrogen fixing and phosphate mineralizing bacteria) found in the rhizosphere of these sweet potato genotypes have never been described before. Therefore, in the present study, we used cultivation-independent methods (PCR-DGGE and quantitative PCR) based on *alp* gene (alkaline phosphatase coding gene) and on *nifH* gene (nitrogenase coding gene) to analyze the structure and the density (gene copy numbers) of the bacterial communities related to biofertilization (phosphate mineralization and nitrogen fixation) in rhizosphere soil. The analyses were performed to compare the distribution of *alp* and *nifH* genes among the sweet potato genotypes sampled three and six months after planting. The data generated in this study could be important for increasing knowledge on the productivity of these sweet potato genotypes under field conditions.
