Short introduction to research topic

The role of reactive oxygen species during Gluconacetobacter diazotrophicus – plant interaction

Endophytic nitrogen‐fixing bacteria have been isolated from grasses such as maize, rice and cane sugar (Boddey and Döbereiner, 1995). These microorganisms, mainly through biological nitrogen fixation (BNF), are capable of promoting plant growth. The diazotrophic Alphaproteobacteria Pal5 Gluconacetobacter diazotrophicus (Cavalcante and Döbereiner, 1988) is able to contribute to plant growth and can increase their dry weight by up to 35% (James et al. 1994). This increase in dry weight is mainly due to increased nitrogen availability to the plant from BNF (Sevilla et al., 2001). The BNF process is catalyzed by nitrogenase and requires large amounts of ATP. This energy demand is supplied by high rates of aerobic respiration (Flores‐Encarnacion et al., 1999; Pan and Vessey, 2001). However, nitrogenase activity is extremely sensitive to inhibition by oxygen (Ureta and Nordlund, 2002). In previous studies Alquéres et al. (2010) showed that reactive oxygen species (ROS), a natural byproduct of oxidative metabolism, could also inhibit the activity of nitrogenase strongly. However G. diazotrophicus alters its redox metabolism during BNF, increasing the levels of transcription of antioxidant enzymes, resulting in lower levels of intracellular ROS during BNF. The authors suggested that careful regulation of ROS levels during this critical phase is an adaptive mechanism that allows the fixation of nitrogen.
In the present work, we intend to study the influence of ROS on G. diazotrophicus - plant interaction. We will construct mutants of G. diazotrophicus, unable to produce ROS‐detoxifying enzymes and will analyze the infection and colonization process using sugarcane and rice plantlets as Gramineae models. The processes of colonization and infection will be analyzed using the Fluorescent in situ hybridization (FISH) technique, specifically to evaluate the spatial and temporal behavior of bacteria inoculated in a controlled system. Until now no study to examine this aspect of the interaction of diazotrophic plants from the grass family (or Poaceae) has been made. However, this botanical family is the most important of all plant families for man’s economy, as it includes the main groups of food grown around the world (wheat, rye, barley, oats, sorghum, rice, maize) and some species, such as sugarcane, which are extremely important for sustainable energy production. Elucidating the mechanisms that control ROS signaling during the G. diazotrophicus ‐ Gramineae interaction can help to define a powerful strategy to increase the efficiency of this interaction and therefore to increase productivity and reduce or even eliminate dependence on nitrogen fertilization for the cultivation of grasses.