Growth promotion of bean (Phaseolus vulgaris L.) by a polyamine-producing isolate of Streptomyces griseoluteus

Of seventy-five actinomycetes isolated from a bean rhizosphere in the United Arab Emirates, an isolate of Streptomyces griseoluteus (WT) was found to be capable of producing relatively high levels of putrescine on decarboxylase agar medium and to produce putrescine, spermidine and spermine in liquid decarboxylase medium. In the glasshouse, the application of the WT strain to soil amended with arginine (as a precursor for putrescine) significantly (P < 0.05) promoted the growth of bean plants and increased the fresh and dry weights and lengths of roots and shoots, compared with control plants. Infestation of soil with the WT strain resulted in a significant (P < 0.05) increase in the levels of putrescine, spermidine and spermine, certain endogenous plant growth regulators (PGRs) (indole-acetic acid, and gibberellic acid), chlorophylls (a, b) and carotenoids with a concomitant reduction in the level of abscisic acid in bean plants, compared with control plants. A polyamine non-producing mutant strain (PNPM) obtained from the wild-type isolate (WT), however, failed to promote plant growth. There were no significant (P > 0.05) differences between the levels of polyamines, endogenous PGRs, chlorophylls (a, b), and carotenoids between plants that were not exposed to either of the strain (control) and those grown in soil with the PNPM strain. Both WT and PNPM strains were incapable of producing in vitro detectable levels of PGRs, indole-acetic acid, indole-pyruvic acid, gibberellic acid, isopentenyl adenine and zeatin in the culture filtrates. This study is the first to demonstrate the potential of a polyamine-producing actinomycete to promote plant growth. In addition, it is also the first published report of the production of polyamines by streptomycete actinomycetes.