Molecular characterisation and plant-growth promotion potential of phosphate solubilising bacteria from roots of selected crops around morogoro municipality, Tanzania

Abstract

Soil infertility is reported to be among the most limiting factors for crop production and yield. Despite being abundant in most soils, only a small proportion of phosphorus is readily available to plants due to its high reactivity with soil constituents and slow release from phosphate compounds. Phosphate solubilising bacteria (PSB) play an important role in phosphorus nutrition. These microbes can solubilise various insoluble phosphate compounds through different mechanisms including production of organic and inorganic acids, production of chelating substances and ammonium assimilation, thus enriching soluble phosphorus into soil solution for plant uptake. Use of phosphate solubilising bacteria in agriculture has been reported to increase crop yield in different crops including maize (Zea mays L.). Other than phosphate solubilisation, PSB can also solubilise micronutrients including zinc (Zn) and iron (Fe). PSB are also known to produce various plant growth promoting substances such as indole acetic acid (IAA) and siderophore, which are important for crop growth and development. This study therefore aimed at evaluating the plant growth promotion potential of phosphate solubilising microorganisms by, in addition to phosphate solubilisation, looking at their potential for zinc solubilisation, siderophore and IAA production and plant growth promotion in general. Purified colonies of bacteria from 19 native PSB isolated from selected field and garden crops grown around Morogoro municipality, Tanzania, were found to be strong phosphate solubilisers, hence were selected for further studies. Morphologically, these bacteria were whitish, yellowish to creamy in colour, rod shaped and gram negative. Based on 16s rRNA gene sequence most of the isolates were found to belong to the bacterial genus Burkholderia while a few others belonged to the genus Ralstonia. All isolates were positive for IAA and siderophore production and zinc solubilisation although at varying levels. On phosphate solubilization, Burkholderia cepacia strain GPY1 isolated from rice was the most promising strain releasing the highest phosphorus concentration (84.8 mg of soluble P L-1) compared to the lowest amount (10.85 mg of soluble P L-1) that was released by Burkholderia territorii strain KBB5 isolated from rice. Similarly, Burkholderia cepacia strain ATCC 25416 isolated from rice was the most promising IAA producer, producing up to 28 mg of L-1, followed by Burkholderia cepacia strain GYP1 isolate from sweet potato which released 21 mg L-1 of IAA. On the other hand, the lowest IAA amount (i.e. 1.072 mg L-1) was from Burkholderia territorii strain S2 isolated from rice. Furthermore, siderophore production as measured in percentage siderophore unit (PSU) was highest (95 %) by Burkholderia sp. QN m1 isolated from sweet pepper, followed by Burkholderia territorii strain KBB5 (94.82%) and Burkholderia territorii strain S2 (93.98%) both isolated from rice, while the lowest percentage siderophore unit was 28.77 % produced by Burkholderia cepacia strain GYP1 isolated from sweet potato. The highest quantinty of zinc soulubilised was 347.5 mg of soluble Zn L-1 by Burkholderia territorii strain KBB5 isolated from sweet potato followed by Burkholderia cepacia strain ATCC (242.1 mg L-1) isolated from sweet potato. Direct application of bacterial cultures to maize seedlings was observed to significantly (P = 0.05) increase both plant height and shoot elongation as compared to a water treated control. Bacteria strains indicated varying abilities in promoting root and shoot elongation. However, strains belonging to B. cepacia were the most promising plant growth promoters as compared to other strains. Overall, the finings of this study imply that bacterial isolates can be used as inoculants for enhancing plant growth and consequently yield. However field trials need to be carried out to evaluate the performance of the strains under field conditions.