Endophytic Streptomyces & Plant Growth
by Roberto
Because of the deleterious effects on soil due to the prolonged use and overuse of synthetic fertilizers, the quest for bacteria able to promote plant growth is an ongoing pursuit. Yet, these searches are usually focused on members of a few genera, prime amongst these Bacillus and Pseudomonas. What about the Streptomyces?
Given their abundance in soil and proclivity to secrete bioactive small molecules, I have often wondered why members of the genus Streptomyces are not investigated more widely for their plant growth promoting capabilities. A paper by Sarah Worsley and colleagues from the groups of Barrie Wilkinson and Matthew Hutchings at the John Innes Center, Norwich, UK makes me feel that there's a bright future in Streptomyces as plant growth promoters. In their studies, the authors started by isolating strains not from bulk soil or even the rhizosphere. They went specifically for endophytic Streptomyces, bacteria living inside or in between the root cells of the model plant Arabidopsis thaliana. Their study is focused on the characteristics of five new Streptomyces strains thus isolated.
Fig. 1. Violin plots showing the total dry weight of A. thaliana plants grown in compost from seeds inoculated with spores of Actinovate, L2, M2, M3, N1, and N2 or a mixture of L2, M2, and M3 Streptomyces spores. Dry weight in grams. Sterile seeds were grown as a control. N = 8 replicate plants per treatment. Box plots show the locations of the medians and quartiles, with whiskers reaching to 1.5 times the interquartile range. ◆ mean values. The width of the outer shaded area illustrates the proportion of data located there (the kernel probability density). Groups labeled with different lowercase letters differ significantly in plant biomass (P < 0.05 in Tukey's HSD tests). Source. Frontispiece: A. thaliana root (red) colonized with an endophytic Streptomyces strain expressing a green fluorescent protein. Source.
The completed genomes offered a glimpse at the synthetic potential of the five strains. Overall, these endophytes had the expected multitude of biosynthetic genes clusters – 21 to 34 per genome – predicted to encode polyketide synthases, nonribosomal peptide synthases, ribosomally encoded posttranslationally modified peptides, and terpenes. Therefore, these bacteria likely have the potential to produce numerous antibacterial and antifungals, to protect the host plant from pathogens. And indeed, these bacteria do inhibit the growth of several bacteria and fungi.
Perhaps more relevant to the strain's potential for growth promotion, the bacteria produce numerous siderophores and plant hormones, e.g., indole-3-acetic acid. But, do they promote growth? When tested on agar plates, where individual seeds were either uninoculated or inoculated with each strain separately, three of the strains showed moderate increases in growth (as determined by the dry weight of 16-day-old seedlings). Two of their strains inhibited growth slightly. And a commercially available strain of Streptomyces lydicus, sold as a biological bactericide/fungicide under the brand name Actinovate, showed no effect on growth. Then they moved on to test growth promoting in soil (compost, actually). Here is where I think their results get really interesting...
None of the strains had an effect on plant growth when applied individually. But the application of a spore mixture of three strains (L2, M2, and M3) increased plant dry weight from an average of 12.69+/-1.94 mg for control plants to 39.29+/-4.39 mg (see Fig. 1). That's a three-fold increase in plant dry weight in 16 days! Being used to plant growth promotion measured in a few percent increase, I was wowed when I saw the 300% increase. That's an impressive amount. Will this much growth promotion scale up to mature crop plants? Of course not. But still, with growth promotion of this magnitude, the future is bright for understanding its molecular basis. It also opens the door to something many out there are after: the generation of stable bacterial consortia that show synergistic effects on plant growth. Endophytic Streptomyces may just be where it's at!
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