Growth promotion and protection against damping-off of wheat by two rock phosphate solubilizing actinomycetes in a P-deficient soil under greenhouse conditions

Micromonospora aurantiaca- and Streptomyces griseus-related strains isolated from Moroccan phosphate mines (MAMPM and SGMPM) were previously selected for their rock phosphate (RP) solubilizing abilities and their multiple plant growth promoting properties demonstrated in laboratory conditions. In order to assess whether these interesting properties could have a direct effect on plant growth and fitness, seeds of the wheat plant (Triticum durum L. cv. Vitron) coated or not with mycelium of these strains and of the reference strain S. griseus M1323, were grown in a sterile soil deficient in soluble phosphate supplemented or not with soluble phosphate or with the insoluble RP, under greenhouse conditions. These studies revealed that the presence of the actinomycete strains in the soil supplemented with RP significantly promoted the growth of the wheat plants. MAMPM and SGMPM had the greatest stimulatory effect on plant growth with 50–47% and 80–78% weight increase of shoots and roots, respectively, in comparison with the sterile control. This increase correlated with a significant increase in the N and P content of plant tissues. The MAMPM- and SGMPM-dependent growth promotion in the RP supplemented soil was on average 10–13% lower than that achieved by the soluble phosphate supplement. Furthermore, in a soil infested with Pythium ultimum, the mediator of damping-off disease, the coating of wheat seeds with the mycelium of MAMPM strain resulted in a clear protection of the plant. The level of protection achieved by MAMPM was 14% lower than that conferred by the commercial bio-fungicide agent (Mycostop®). This study demonstrated that MAMPM in association with pulverized RP could constitute a novel and non-polluting bio-fertilizer/biocontrol product useful for the development of sustainable agriculture.