Isolation and characterization of plant growth promoting bacteria (PGPB) from anaerobic digestate and their effect on common wheat seedling growth

Anaerobic digestion of organic wastes produces biogas and a nutrient-rich digestate. Digestate contains partially-degraded organic matter, inorganic plant nutrients and microbial biomass, therefore it can be used as soil conditioner or fertilizer on agricultural field (Alburquerque et al., 2012). The use of digestate as a fertilizer is considered eco-friendly since it recycles plant nutrients in the organic waste and thus reduces large scale use of chemical fertilizers. Furthermore, plant nutrients are present in inorganic plant-available forms in digestate at a markedly higher level compared to undigested organic wastes, because of the mineralization of organic nutrients during anaerobic digestion process (Umetsu et al., 2002). Previous researches have documented the beneficial effects of digestate as organic fertilizer on plant growth and nutrients uptake, and soil structure and microbial activity (Muscolo et al., 2017; Risberg et al., 2017; Solé-Bundó et al., 2017; Tampio et al., 2016).

Plant growth promoting bacteria (PGPB) represent a wide variety of bacteria, which occupy the rhizosphere of many plant species and promote host plant growth directly by solubilizing minerals such as phosphorus, producing siderophores that chelate iron and producing phytohormones (Grobelak et al., 2015). Phosphorus (P) is one of the major macronutrients required for growth and development of plant. Generally, soils have large reserves of total P, but the amount available to plants is low as majority of soil P is found in insoluble forms (Ahemad and Kibret, 2014; Vessey, 2003). PGPB could make phosphorus available to plants by solubilizing and mineralizing inorganic and organic phosphorus in soils (Ahemad and Kibret, 2014). Iron is also an essential nutrient plant growth. However, iron exists mainly as Fe3+ in aerobic environment and is likely to form insoluble hydroxides and oxyhydroxides which are not unavailable to plants (Rajkumar et al., 2010). The siderophores, which are low-molecular mass iron chelators, secreted by some PGPB could solubilize iron from minerals or organic compounds under conditions of iron limitation to make iron accessible to plants (Indiragandhi et al., 2008). Indole-3- acetic acid (IAA) is the primary phytohormone produced by RGPB and has various effects on plant growth promotion such as cell division and elongation, stimulation of seed germination, and increase root development (Ahemad and Kibret, 2014).

PGPB can also stimulate plant growth indirectly by suppressing phytopathogens in forms of producing antibiotics, siderophores, and fungal cell wall-lysing enzymes (Ji et al., 2014). The largest groups of PGPB are Pseudomonas, Bacillus, Enterobacter, and Erwinia (Grobelak et al., 2015). Majority of researched PGPB are isolated from rhizosphere and they are generally known as plant growth promoting rhizobacteria (PGPR) (Khalid et al., 2004). However, anaerobic digestates are host to numerous PGPB and little attention has been focused on the isolation and characterization of PGPB from anaerobic digestate.

In the present study, two groups of PGPB: Bacillus and Pseudomonas isolated from two types of anaerobic digestate were screened on plant growth promoting traits including phosphate solubilization, siderophore production and phytohormone production, as well as antifungal activity. Selected bacterial isolates were further evaluated for their growth promoting activity on common wheat (Triticum aestivum).