Some bacteria that live in the rhizosphere define how the volume of soil influenced by the roots has the ability to promote plant growth [1] [2]. These microorganisms referred as PGPR drew attention to the need to reduce chemicals particularly in the context of sustainable agriculture and environmental protection [3]. PGPRs influence plant health and productivity through a variety of mechanisms that involve mineral solubilization, nitrogen fixation, phytohormone synthesis, hydrolytic enzyme synthesis and balance modulation plant hormone by deamination of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) [4] [5]. These rhizobacteria can improve the elongation and branching of the root system which promotes the absorption of water and minerals from the soil to the host plant including wheat, barley, corn and rice that are necessary for plant survival [6]. The benefits of PGPR inoculation on plant growth and productivity are well documented and have been correlated with phytohormone production and higher nutrient intake [7] [8] [9]. Various species of PGPR have been considered PGPR to stimulate plant growth and some of them marketed in recent years belong to the genera Pseudomonas, Azospirillum, Azotobacter, Bacillus, Enterobacter, Rhizobium, Mycobacterium, Enterobacter, Caulobacter, Serratia, Flavobacterium, Actinobacteria sp. [10] [11].

Maize is a staple crop in Benin but soils are poor and inappropriate use of mineral fertilizers prevents producers from achieving the potential yield of this crop. Several research studies have reported that the excessive use of chemical fertilizers in agronomic systems aiming at improving yield, is less efficient in terms of stabilizing chemical elements that are quickly leached just after their application at ground level [12] [13]. This represents a deficit of absorption and nutritional assimilation in plants.

In Benin, the phenomenon of soil degradation is increasing especially in central regions dominated by ferruginous soils. This contributes to the depletion of organic matter in the soil as well as the nutrients essential for plant development [14]. The role of PGPRs in resolving several constraints such as soil degradation, water stress, and declining soil fertility that limit agricultural production has been widely proven [15]. The use of plant growth-promoting bacteria (PGPR) can be a promising alternative that can reduce the application of chemicals and improve crop yields and plant health [16] [17] [18]. For example, [19] has shown that PGPRs can increase nutrient bioavailability in the rhizosphere. They fix nutrients and prevent them from washing. In the work undertaken [20], the improvement in wheat yield was attributed to the co-inoculation of Bacillus thuringiensis and Serratia sp. Similarly, [21] [22] noted that inoculation of corn seeds with strains of Bacillus polymyxa, Pseudomonas alcaligenes and Actinomycetes sp. (O19-AHB12) significantly improved the dry weight of maize plants by 19% to 52%, total maize biomass by 38% and the weight of 1000 corn seeds by 74.72% compared to controls. To date, little work has been done in Benin on the use of PGPRs to improve maize growth and yield. The objective of this study is to investigate the effects of nine (09) isolated rhizobacteria (PGPR) identified in central and northern Benin on the growth of greenhouse maize on ferruginous soil in central Benin.

Various research has focused on the assessment of rhizobacteria for their ability to induce improvements in growth and yield parameters on controlled cereals (Qaisrani et al., 2014) [36] (Karnwal, 2017) [37]. Based on the analysis of the data presented at the level of Figure 2, it is noted that the plants that benefited from the treatments B. thuringiensis, B. thurenthonicus, B. panthothenicus, S. marcescenss, P. cichorii, P. putida, P. syringae and P. putida, B. thuringiensis S. marcesens all in group 4 showed the best growth in height with an increase of 20.15% compared to control plants in the group 1. The promoter effect of these treatments exceeds by 12.11% that induced by B. anthracis, B. circulens, P. putida, P. cichorii, S. marcescens, P. putida, S. marcescens and P. putida, B. thuringiensis du gropue 2. It is the same for B. polymyxa, P. syringae, B. panthothenicus and B. polymyxa “B. anthracis” B. circulatens, B. thuringiensis B. panthethonicus classified in group 3. The same trend was observed on the diameter at the collar of the plants. Indeed, the highest neck diameter values were obtained by group 4 rhizobacteria. These treatments resulted in a 21% improvement over the diameter at the collar of non-inoculated group 1 plants. Our experimental results are consistent with those of Noumavo et al. (2013) [38], which reported increases of 26.77% and 30.25% respectively on the diameter at the collar as well as on the height of the corn plants inoculated with the combination of P. fluorescens, P. putida versus the witness. In addition, the work of Emami et al. (2019)

[39] have recently demonstrated that co-inoculation of eight bacterial strains of different taxa (Pseudomonas, Bacillus, Stenotrophomonas, Serratia, Nocardia and Microbacterium) with several PGP traits allows for increased growth of plants than mono-inoculation. From an agronomic point of view, the diameter at the collar appears as a very important parameter for the plants. The more robust they are, the more they will be able to withstand the environmental conditions that have become increasingly difficult in recent years as a result of climate change (Duponnois et al., 2005) [40] (Biaou et Gnimadi, 2012) [41].

Among the parameters of agronomic interest is also the leaf surface used by researchers to measure the photosynthetic capacity of plants. In this study, inoculation of plants by rhizobacteria of different taxa had a highly beneficial effect on the leaf surface of the plants 30 days after sowing (Figure 4). The best leaf surfaces were recorded at the plant level both by mono-inoculation (B. thuringiensis) and by co-inoculation of group 4 rhizobacteria strains with an improvement of more than 32.77% over the control. Group 2 and 3 plants had leaves with an improvement of 21.29% and 17.92% respectively compared to non-inoculated plants. This means that the inoculation of group 4 rhizobacteria resulted in a leaf area gain of 11.48% and 14.85% respectively compared to the group 3 and group 2 plants. Such results were reported by Noumavo et al. (2013) [38] with inoculation of corn seeds on ferralitic soil in southern Benin. Other scientists have also observed increased plant height and leaf surface through inoculation with bacterial combinations (Govindappa et al., 2011) [28]. Increased growth was also recorded on other crops such as wheat (Salantur et al., 2006) [42], rice (Nandakumar et al., 2001) [43] and on tomatoes (Oluwambeet Kofoworola, 2016) [44] due to co-inoculation of rhizobacteria. Akhtar et al. (2018) [45] justify the broadest leaves carried by corn plants under the influence of rhizobacteria combinations by better development of the root system of the plants; this is conducive to better absorption not only of nutrients but also of water and therefore of better productivity.

In terms of biomass yield, the dry weight of the aerial and underground biomass of the plants measured during our harvest study was significantly improved with inoculation compared to control plants (Figure 5). The results of the variance analysis applied to the average weights at the level of each group indicated a significant effect (Table 5) and a highly significant effect (Table 6) respectively for dry air biomass and dry underground biomass. Indeed, plants treated with B. thuringiensis and other combinations of group 4 rhizobacteria induced the best weight of aerial biomass (5.53 - 1.02 g) with an increase of 30.73% compared to the control. In contrast, the highest subterranean biomass dry weight (3.42 - 0.42 g) was obtained with B. polymyxa, P. syringae, B. panthetonicus, and B. anthracis, B. circulans, B. thuringiensis, B. panthetonicus, B. polymyxa) in group 3 followed by group 3 treatments 4. They induced an improvement rate of 26.19% and 12.91% respectively over the group 1 control plants. Similarly, Akhtar et al. (2018) [45] reported that the combination of 4 rhizobacteria (Bacillus megaterium, Pseudomonas aeruginosa, Serratia spp., and

Pseudomonas fluorescens) significantly stimulated the dry weight of the aerial part of the 33.85% increase over controls, while the lowest dry aerial biomass was recorded at the control corn feet. Significant increases in the biomass of wheat plants and improved absorption of nutrients have also been reported by Emami et al. (2018) [46] with greenhouse inoculation of Pseudomonas strains characterized by multiple PGP activities. The results of the dry weight of plant biomass obtained in our study show that dry matter production has been improved by inoculation. El Fakhri et al. (2010) [47] reported that dry matter is used for the production of new roots, their proliferation (root volume), their lengthening (length growth) and their maintenance.

The classification obtained on the basis of the dendrogram (Figure 1(b)) shows that most individually tested strains, for example P. putida, P. cichorii, P. syringae, B. circulans, B. anthracisand S. marcescens, did not express a high capacity of stimulation for all the parameters assessed compared to the performance of the combinations of the so-called strains. On the other hand, the inoculation of B. thuringiensis induced very interesting effects comparable to those produced by B. thuringiensis, B. panthethonicus, S. marcescens, P. cichorii, P. putida, P. syringae and P. putida, B. thuringiensis, S. marcesens. Several studies have reported the ability to promote the growth of plants in greenhouses or in the field conditions of Bacillus strains, alone or in combination with other strains (Lim and Kim, 2009) [48] (Win et al., 2018) [49] Akinrinlola et al., 2018 [50]. Our results are consistent with those of Ishizawa et al. (2017) [51] who stated that the effect of combining different strains of PGPR was similar to the effect of individual strains on Lemna minor growth stimulation. On the other hand, the results obtained in our study are at odds with those reported by Yamakawa et al. (2018) [52]. The authors of this work noted that the effect of combining two strains of PGPR promoting lentucle growth was less than their mono-inoculation. Previous studies have previously reported the stimulation capacity of B. thuringiensis on various crop species (Goes, 2012) [53] (Raddadi etal., 2008) [54] because of its ability to produce metabolites of agricultural interest (Raddadi et al., 2008) [54]. This corroborates the results of work done in Canada by Emad et al. (2019) [55] which showed that R10 (B. thuringiensis) produced auxine (AIA), significantly improves the growth of Lemna Minor. Many authors have suggested that the effects of co-inoculation on plants depend on the particularity of the combinations used (Barea et al., 1998) [56] Molina-Romero et al., (2017) [57]. The work done by Agbodjato et al. (2018) [25] has demonstrated the ability of the majority of strains tested in our study to solubilize inorganic phosphate and produce metabolites of agricultural interest including acetic indole acid (AIA). The best results recorded on the parameters measured during our work would be related to the ability of PGPR strains to produce AIA, the siderophore, to solubilize phosphorus as it has been the case in many scientific works (Backer et al., 2018) [58] (Amogou et al., 2018) [59] (Won et al., 2019) [60].

PGPRs are used in agriculture as soil biofertilizers. These microorganisms have been shown to mediate interactions between plants and allow them to exploit soil resources. The results of this work confirm the importance of PGPR strains on height growth, neck diameter and dry weights of the aerial and underground portion of corn plants on depleted ferruginous soil in central Benin. Depending on whether combined or not, the best performance on the above parameters was recorded with B. thuringiensis; B. thuringiensis B. panthethonicus S. marcescens and Pseudomonas cichorii, Pseudomonas putida, and Pseudomonas syringae inoculation. Thus, the combined application of several strains of rhizobacteria to promote plant growth may have an effect comparable to that induced by mono-inoculation. These rhizobacteria presented desirable traits which might suggest promise for future field application to promote the growth of maize, thus contributing to sustainable agricultural practices in Benin.

Authors extend their thanks to the West Africa Agricultural Productivity Program (WAAPP/PPAAO), the “Centre National de Spécialisationsur le Maïs (CNS-Maïs)” and the “Fond National pour la Recherche Scientifique et de l’Innovation Technologique (FNRSIT)” of the Benin government for funding this project.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Amogou, O., Agbodjato, N.A., Dagbénonbakin, G., Noumavo, P.A., Sina, H., Sylvestre, A.A., Adoko, M.Y., Nounagnon, M., Kakaï, R.G., Adjanohoun, A. and Baba-Moussa, L. (2019) Improved Maize Growth in Condition Controlled by PGPR Inoculation on Ferruginous Soil in Central Benin. Food and Nutrition Sciences, 10, 1433-1451. https://doi.org/10.4236/fns.2019.1012102