Endophytes as plant growth-promoting agents

One of the most important abilities of endophytes is plant growth promotion. This can be achieved by such mechanisms as nitrogen fixation, production of antifungal compounds and phytohormones, regulating ethylene production in roots or Induced Systemic Resistance (Gunjal et al. 2018).

One of the most important abilities of endophytes is plant growth promotion. It can be achieved by the production of phytohormones. The most frequent compound that endophytic bacteria synthesizes is indoleacetic acid (IAA), which belongs to auxins, which induce main root elongation and lateral root production (Reinhold-Hurek and Hurek 1998, Strobel 2003, Klama 2004). The same effect was observed by Cacciari et al. (1989) during the growth of pearl millet (Pemisetum americanum) inoculated by Arthrobacter giacomelloi and Azospirillum brasilense, which have the ability to produce other phytohormones, namely, gibberellins and cytokinins (Cacciari et al. 1989). Also, positive modification of black nightshade roots caused by the production of IAA was observed by Long et al. (2008) (Long et al. 2008). Also, endophytic microorganisms can produce biocontrol agents and therefore, indirectly promote plant growth. The growth of such a plant pathogen as Cryphonectriaparasitica (chestnuts blight) can be inhibited by endophytic Bacillus subtilis, isolated from the xylem sap of chestnut trees, thanks to its antifungal propeities (Arthofer and Schafleitner 1997, Gunjal et al. 2018).

For plants, nitrogen deficiency is most adversely affected. It causes poor tillering, yellowing of leaves, weakness or stiffness of tissues (Chodaii et al. 1980). However, this state can be easily resolved by the presence of endophytic microorganisms which have nitrogenase activity, namely, diazotrophs. Thanks to nitrogenase activity, microorganisms may fix the atmospheric nitrogen, due to its reduction to NH3, thus significantly increasing nitrogen uptake by plants. Endophytes that have nitrogenase activity belong to genus Azospirillum, Acetobacter, Herbaspirillum, Azoarcus or Azotobacter (Pisarska and Pietr 2014). Rodrigues et al. (2008) in their research inoculated the Oryza sativa L. (seed rice) plants with diazotrophic endophytic bacteiia Azospirillum amazonense. They noticed that the presence of diazotrophic bacteria increases the uptake of nitrogen by the plant from 3.5 up to 18.5%; also, it increased the biomass of tested plants (Rodrigues et al. 2008). Worth mentioning is the fact that sugar cane grows in some areas of Brazil without any nitrogen fertilization. A great amount, namely 50-80% of this element, is taken from the atmosphere, and this is possible due to endosymbiosis with the diazotrophic bacterium Gluconacetobacter diazotrophicus (Blaszczyk 2010, Chawla et al. 2014). The nitrogen-fixation by endophytic bacteria has been reported by Elbeltagy et al. (2001) for Enterobacter cloacae, Alcaligenes, A. diazotrophicus, Azospirillum, Azoarcus, Herbaspirillum seropedicae and Ideonella dechloratans (Elbeltagy et al. 2001, Gunjal et al. 2018).

Iron is one of the most important elements for plant health due to its participation in cellular respiration, intermediary metabolism, oxygen transport, DNA stability and repair, and photosynthesis. Iron is an element commonly found in soil; however, because of the form in which it occurs, it remains inaccessible to plants (Lopez-Milan et al. 2013, Li and Lan 2017). It is dangerous for plants because deficiency of iron may cause adverse changes in root formation, changes in enzyme activity, and chlorosis. Nevertheless, the bioavailability of iron for plants can be increased by endophytic bacteiia that can produce siderophores. Siderophores are low molecular mass compounds that have a high chelating affinity to Fe3ri In this form, iron’s bioavailability is increased significantly and plants can easily absorb it. Also, siderophores may exhibit static properties against different phytopathogens (Lacava et al. 2008).

Conclusions

There are numerous reports on the possibility of using endophytic microorganisms as degradation agents. However, there is still a great number of plants to explore, looking for microorganisms with special features. Discovery of new endophytic species isolated from a plant growing in raderal environments may give an opportunity to source microorganisms with high metabolic activity towards many contaminants and biosurfactants production. Using endophytic microorganisms as a tool for bioremediation can help us to prevent our planet from ecological calamity.

 
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