Promotion of growth of bean (Phaseolus vulgaris L.) in a calcareous soil by a phosphate-solubilizing, rhizosphere-competent isolate of Micromonospora endolithica

The aim of this study was to determine the potential of non-streptomycete actinomycetes (NSA) to solubilize insoluble phosphates in soil and to promote plant growth. Thirty-one NSA were isolated using Streptomyces phage and dry heat techniques, from a calcareous soil deficient in available phosphorus (P) in the United Arab Emirates. Nine of these NSA isolates solubilized powdered rock phosphate (PRP) in solid and liquid media. Five isolates were initially selected based on their rhizosphere competence. Among them, an isolate of Micromonospora endolithica caused a significant drop in pH in a liquid medium amended with PRP, solubilized considerable amounts of P, produced acid and alkaline phosphatases, as well as a variety of organic acids. In addition, this isolate of M. endolithica was chosen from among the five isolates because it also showed exceptional rhizosphere competence and a strong ability to colonize bean (Phaseolus vulgaris L.) roots up to a depth of 14 cm. In the greenhouse, the application of M. endolithica to soil amended with either single super-phosphate (SP) or PRP significantly promoted the growth of roots and shoots of bean plants compared with those of plants grown in non-inoculated soil amended with SP or PRP. This was also evident in the significant increases in the concentration of available P in the soil and in the levels of N, P, K, S, Mg, Fe and Zn in the roots and those of N, P, K, S, Mg and Fe in the shoots of inoculated plants. The plant growth promotion by M. endolithica was most pronounced in the presence of SP as soil amendment compared to PRP. In comparison, a non-phosphate-solubilizing, non-rhizosphere-competent NSA isolate of M. olivasterospora failed to increase available soil P, nutrient levels in roots and shoots or to promote plant growth. Plant growth regulators tested do not appear to be involved in the plant growth stimulation observed. Neither M. endolithica nor M. olivasterospora produced detectable levels of indole-acetic acid, indole-pyruvic acid, gibberellic acid, isopentenyl adenine, isopentenyl adenoside or zeatin in vitro. This study is the first published report to demonstrate the potential of phosphate-solubilizing NSA to promote plant growth. In addition, it is also the first published report of rhizosphere-competent actinomycetes capable of solubilizing SP or PRP, in soils.