A Plant Stress-Responsive Bioreporter Coupled With Transcriptomic Analysis Allows Rapid Screening for Biocontrols of Necrotrophic Fungal Pathogens.

Katharina Belt,Louise F Thatcher,Cathryn A O’Sullivan,Rhonda C Foley,Margaret M Roper,Karam B Singh

doi:10.3389/fmolb.2021.708530

Abstract

Streptomyces are soil-borne Actinobacteria known to produce a wide range of enzymes, phytohormones, and metabolites including antifungal compounds, making these microbes fitting for use as biocontrol agents in agriculture. In this study, a plant reporter gene construct comprising the biotic stress-responsive glutathione S-transferase promoter GSTF7 linked to a luciferase output (GSTF7:luc) was used to screen a collection of Actinobacteria candidates for manipulation of plant biotic stress responses and their potential as biocontrol agents. We identified a Streptomyces isolate (KB001) as a strong candidate and demonstrated successful protection against two necrotrophic fungal pathogens, Sclerotinia sclerotiorum and Rhizoctonia solani, but not against a bacterial pathogen (Pseudomonas syringe). Treatment of Arabidopsis plants with either KB001 microbial culture or its secreted compounds induced a range of stress and defense response-related genes like pathogenesis-related (PR) and hormone signaling pathways. Global transcriptomic analysis showed that both treatments shared highly induced expression of reactive oxygen species and auxin signaling pathways at 6 and 24 h posttreatment, while some other responses were treatment specific. This study demonstrates that GSTF7 is a suitable marker for the rapid and preliminary screening of beneficial bacteria and selection of candidates with potential for application as biocontrols in agriculture, including the Streptomyces KB001 that was characterized here, and could provide protection against necrotrophic fungal pathogens.

Highlights

Pathogens and pests cause severe damage to crops and are estimated to cost the global economy billions of dollars each year (Bradshaw et al, 2016; Savary et al, 2019)
KB001 showed the highest expression across most genes tested, especially at 48 h post Sclerotinia infection, making it the strongest candidate for further analysis
By cloning a GSTF7:luc promoter:luciferase reporter construct into Arabidopsis plants and selecting for GSTF7 response in real time, we developed a suitable screening platform to identify Actinobacteria strains with potential for biocontrol

Summary

Introduction

Pathogens and pests cause severe damage to crops and are estimated to cost the global economy billions of dollars each year (Bradshaw et al, 2016; Savary et al, 2019). Beneficial plant-interacting microbes offer new possibilities to protect the farming industry from pathogens Many of these microbes create symbiotic relationships with plants and promote plant growth and development, nutrient uptake, and tolerance to abiotic and biotic stresses (Rey and Dumas 2017; Woo and Pepe, 2018; Reis et al, 2020; Romano et al, 2020). These microbe–plant associations play major roles in the ecosystem and can take place at the root level or the shoot level, known as the rhizosphere (in soil) and the phyllosphere (plant aerial parts), respectively. The effectiveness of a biocontrol can differ between laboratory and field conditions due to landscape composition and interactions with other organisms (Perez-Alvarezet al., 2019)

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