Colonization of lettuce rhizosphere and roots by tagged Streptomyces.

Maria Bonaldi,Xiaoyulong Chen,Marco Saracchi,Cristina Pizzatti,Paolo Cortesi,Andrea Kunova

doi:10.3389/fmicb.2015.00025

Abstract

Beneficial microorganisms are increasingly used in agriculture, but their efficacy often fails due to limited knowledge of their interactions with plants and other microorganisms present in rhizosphere. We studied spatio-temporal colonization dynamics of lettuce roots and rhizosphere by genetically modified Streptomyces spp. Five Streptomyces strains, strongly inhibiting in vitro the major soil-borne pathogen of horticultural crops, Sclerotinia sclerotiorum, were transformed with pIJ8641 plasmid harboring an enhanced green fluorescent protein marker and resistance to apramycin. The fitness of transformants was compared to the wild-type strains and all of them grew and sporulated at similar rates and retained the production of enzymes and selected secondary metabolites as well as in vitro inhibition of S. sclerotiorum. The tagged ZEA17I strain was selected to study the dynamics of lettuce roots and rhizosphere colonization in non-sterile growth substrate. The transformed strain was able to colonize soil, developing roots, and rhizosphere. When the strain was inoculated directly on the growth substrate, significantly more t-ZEA17I was re-isolated both from the rhizosphere and the roots when compared to the amount obtained after seed coating. The re-isolation from the rhizosphere and the inner tissues of surface-sterilized lettuce roots demonstrated that t-ZEA17I is both rhizospheric and endophytic.

Highlights

Roots anchor plants in soil, provide uptake of water and nutrients, and mediate numerous interactions with soil organisms
The Enhanced GFP (EGFP) gene was detected in transformed strains, and its expression was confirmed by fluorescence microscopy observing a strong green fluorescence in all transformants following exposition to fluorescent light (Figure 1), while the corresponding wild-type strains did not fluoresce
Plant beneficial bacteria have a great potential in agriculture as plant growth promoting bacteria (PGPB) and biological control agents (BCAs) and reports about successful control of plant diseases are increasing

Summary

Introduction

Roots anchor plants in soil, provide uptake of water and nutrients, and mediate numerous interactions with soil organisms. Plant growth promoting bacteria are gaining more and more attention in modern agriculture, where sustainable and environmentally friendly strategies of crop cultivation increasingly rely on their use as biofertilizers, phytostimulants, or biopesticides. They employ several mechanisms to improve the plant growth, such as synthesis of phytohormones, nitrogen fixation and increasing availability of nutrients by production of siderophores and solubilization of phosphates (Lugtenberg et al, 2002; Compant et al, 2010). Antagonism is widespread in Bacillus, Pseudomonas, and Streptomyces spp., from which a wide range of biologically active secondary metabolites were isolated (Raaijmakers et al, 2002; Compant et al, 2005)

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