Microorganismos endofíticos: Una interacción sutil bajo nuestros pies.

Plant-microbe interactions in the rhizosphere are determinants of plant health, productivity, and soil fertility. Plant growth-promoting bacteria (PGPB) are bacteria that can improve plant growth and protect plants from diseases and abiotic stresses through a wide variety of mechanisms; those that establish close associations with plants, such as endophytes, may be more successful in promoting plant growth. Several important bacterial traits, such as biological nitrogen fixation, phosphate solubilization, ACC deaminase activity, and production of siderophores and phytohormones, can be evaluated as plant growth promoting (PGP) traits. Bacterial inoculants can contribute to increasing agronomic efficiency by reducing production costs and environmental pollution, since the use of chemical fertilizers can be reduced or eliminated if the inoculants are efficient. For bacterial inoculants to be successful in improving plant growth and productivity, several processes involved can influence the efficiency of inoculation, such as plant root exudation, bacterial colonization in the roots, and soil health. This review presents an overview of the importance of soil-plant-microbe interactions for the development of efficient inoculants, since PGPB are widely studied microorganisms, which represent a very diverse group of easily accessible beneficial bacteria. Interactions between plants and bacteria occur through symbiotic, endophytic or associative processes with varying degrees of proximity to the roots and surrounding soil. Endophytic PGPBs are good candidates for inoculants because they colonize roots and create a favorable environment for their development and function. Non-symbiotic endophytic relationships occur within the intercellular spaces of plant tissues, which contain high levels of carbohydrates, amino acids and inorganic nutrients (Bacon and Hinton, 2006). The success and efficiency of PGPBs as inoculants for agricultural crops is influenced by several factors, including the ability of these bacteria to colonize plant roots, plant root exudation, and soil health. PGPB root colonization efficiency is closely associated with microbial competition and survival in soil, as well as modulation of the expression of several genes and cell-cell communication through quorum sensing (Danhorn and Fuqua, 2007; Meneses et al., 2011; Alquéres et al., 2013; Plant roots react to different environmental conditions by secreting a wide range of compounds that interfere with the plant-bacteria interaction, being considered an important factor in the efficiency of inoculants (Bais et al., 2006; Cai et al., 2009, 2012; Carvalhais et al., 2013). Soil health is another important factor that affects inoculation efficiency, due to several characteristics such as type. of soil, nutrient pool and toxic metal concentrations, soil moisture, microbial diversity and soil alterations caused by management practices. In research developed by AGROSAVIA, different microorganisms were evaluated, with contrasting results: Bacillus amyloliquefaciens demonstrated a completely different pattern to Pseudomonas fluorescens. Its colonization was external to the root, generating bacterial clusters, clusters or agglomerates through a matrix or biofilm. Something important to highlight in Bacillus amyloliquefaciens is that in the first post-inoculation times (T0 and T1) the bacterial concentration and signal was higher, contrasting with the final post-inoculation times (T2, T3 and T4) of this study, where very reduced concentrations and signals were evident. The absolute control without microbial inoculum did not show a signal at any of the times evaluated. In our study it was confirmed that the colonization pattern of Pseudomonas fluorescens is endophytic, that is, it can colonize the interior of the roots (Pseudomonas fluorescens) showed few signs and colonization in the roots in the early times T0 and T1 in contrast to from time T2 (96 h) the bacteria began endophytic colonization of the root. It is important to highlight that the longer the time, the greater the concentration and signal of the bacteria within the root, after 15 days post inoculation. Without microbial inoculum, no signal was evident at any of the times evaluated. The contrasting results in the colonization patterns of the two bacteria studied here raise many questions. Bacillus amyloliquefaciens colonizes externally and begins with very copious growth but decreases in the final evaluation times. Otherwise, Pseudomonas starts very timidly, but its interaction is more subtle and certain, because it manages to enter the plant and increases the number of bacteria in the intermediate and final sampling times. Some microbial species may disappear or be drastically reduced while others become more abundant. We suggest that somehow the interaction and exchange of nutrients in the case of Bacillus amyloliquefaciens (external to the root) could be exhausted and that they have a limited temporality. But in the case of Pseudomonas fluorescens, this bacterium manages to enter the plant and probably uses the entire energy, enzymatic and nutrient battery of its host, given that it is already inside the plant. The bacterial cell surface plays a key role in bacterial aggregation, which in turn promotes bacterial dispersal, survival, and the ability to adhere to plant surfaces. It is well documented that bacterial auto aggregation, biofilm development, endophytic colonization and the relationship of these processes with plant colonization depend on both surface bacterial factors and extracellular factors. However, the two rhizobacteria with such contrasting colonization patterns in banana possibly support individual advantages, each in its own establishment style. These rhizobacteria significantly increased growth parameters such as height, pseudostem thickness, leaf area, fresh weight and dry weight in banana seedlings cv. Williams, compared to the absolute control (without microorganisms). Additionally, the bacteria were able to colonize banana roots. [ABSTRACT FROM AUTHOR]