Plant-Growth Promotion and Biocontrol Properties of Three Streptomyces spp. Isolates to Control Bacterial Rice Pathogens.

Zulma Rocío Suárez-Moreno,Diana Marcela Vinchira-Villarraga,Giuliano Degrassi,Leonardo Castellanos,Freddy A Ramos,Diana Isabel Vergara-Morales,Vittorio Venturi,Nubia Moreno-Sarmiento,Corrado Guarnaccia

doi:10.3389/fmicb.2019.00290

Abstract

Bacterial Panicle Blight caused by Burkholderia glumae is a major disease of rice, which has dramatically affected rice production around the world in the last years. In this study we describe the assessment of three Streptomyces isolates as biocontrol agents for B. glumae. Additionally, the presence of other plant-growth promoting abilities and their possible beneficial effects upon their inoculation on rice plants was evaluated as an ecological analysis for their future inoculation in rice crops. Two isolates (A20 and 5.1) inhibited growth of virulent B. glumae strains, as well as a wide range of bacterial and fungal species, while a third strain (7.1) showed only antifungal activity. In vitro tests demonstrated the ability of these strains to produce siderophores, Indoleacetic acid (IAA), extracellular enzymes and solubilizing phosphate. Greenhouse experiments with two rice cultivars indicated that Streptomyces A20 is able to colonize rice plants and promote plant growth in both cultivars. Furthermore, an egfp tagged mutant was generated and colonization experiments were performed, indicating that Streptomyces A20 –GFP was strongly associated with root hairs, which may be related to the plant growth promotion observed in the gnotobiotic experiments. In order to characterize the antimicrobial compounds produced by strain A20 bacteria, mass spectrometry analyses were performed. This technique indicated that A20 produced several antimicrobial compounds with sizes below 3 kDa and three of these molecules were identified as Streptotricins D, E and F. These findings indicate the potential of Streptomyces A20 as a biocontrol inoculant to protect rice plants against bacterial diseases.

Highlights

Bacterial Panicle Blight (BPB) caused by Burkholderia glumae is a bacterial disease of rice with an increasing occurrence in South America since its first detection in 2007 (Correa et al, 2007; Castillo et al, 2011; Ham et al, 2011)
Sixty actinobacterial-like isolates were obtained from Colombian soils upon enrichment with CaCO3 only three isolates denoted as A20, 5.1 and 7.1 showed antimicrobial activity against either two plant pathogenic bacterial strains of B. glumae and P. fuscovaginae, or fungal phytopathogens
Biochemical tests indicated that these three strains were catalase positive; oxidase negative, and their carbon source analysis indicated that all strains were able to metabolize glucose, but were unable to use rhamnose, arabinose, xylose or mannitol as carbon source (Table 2)

Summary

Introduction

Bacterial Panicle Blight (BPB) caused by Burkholderia glumae is a bacterial disease of rice with an increasing occurrence in South America since its first detection in 2007 (Correa et al, 2007; Castillo et al, 2011; Ham et al, 2011). B. glumae causes bacterial wilt in a wide variety of plant hosts, but disease in rice is the most studied due to the dramatic economic effects of BPB in rice yields (Jeong et al, 2003). Major symptoms of this BPB include panicle blight, seedling blight, and sheath rot, with a linear lesion extending downward from the leaf blade collar forms on the flag leaf. Affected panicles may have one or all of their florets blighted with grains not filling or aborting, which causes typical upright brown panicles due to the failure of grain filling (Goto and Ohata, 1956; Kurita and Tabei, 1967; Goto et al, 1987; Zeigler and Alvarez, 1987)

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