Editorial: Harnessing Useful Rhizosphere Microorganisms for Pathogen and Pest Biocontrol

摘要

Growing demographic trends require sustainable technologies to improve quality and yield of future food productions. However, there is uncertainty about plant protection strategies in many agro-ecosystems. Pests, diseases, and weeds are overwhelmingly controlled by chemicals which pose health risks and cause other undesirable effects. Therefore, an increasing concern on control measures emerged in recent years. Many chemicals became questioned with regard to their sustainability and are (or will be) banned. Alternative management tools are studied, relying on biological, and low impact solutions.This Research Topic concerns microbial biocontrol agents, root-associated microbiomes, and rhizosphere networks. Understanding how they interact or respond to (a) biotic environmental cues is instrumental for an effective and sustainable impact. The rhizosphere is in this regard a fundamental object of study, because of its role in plant productivity. Microbial volatiles and other compounds Roots are surrounded by a flow of molecules released by microorganisms. Some volatiles may affect growth through different mechanisms, such as biochemical signals eliciting local defense reactions or systemic resistance (Kai et al., 2007 ; Chung et al., 2016 ). Yi et al. studied the effects of 2,3-butanediol produced by a Bacillus subtilis isolate overexpressing the bud operon synthesis encoding genes. They showed that the isolate persisted on pepper roots more than the wild type, thereby interfering with colonization by fungi. Exudates from isolate pre-treated roots inhibited colonization by Trichoderma sp., by another B. subtilis strain, and by the soil-borne pathogen Ralstonia solanacearum . Application of 2,3-butanediol to roots followed by R. solanacearum exposure enhanced the expression of pathogenesis-related (PR) genes. The volatile triggered the secretion of root exudates modulating fitness of soil fungi and bacteria, thus acting as a plant defense inducer.Bacteria metabolism affects nutrient assimilation. Aziz et al. reported a new mechanism in the plant growth-promoting (PGP) Bacillus amyloliquefaciens GB03, which activates genes involved in sulfur assimilation and uptake by Arabidopsis . Other transcripts encoding for proteins involved in the biosynthesis of sulfur-rich aliphatic and indolic glucosinolates were also expressed. The enhanced sulfur assimilation increased plant glucosinolate biosynthesis, and conferred protection against the beet armyworm Spodoptera exigua .Bacterial metabolites may also protect plants by inhibiting herbivores (Mith?fer and Boland, 2012 ). Ganassi et al. reported that also fungal metabolites may have this effect, as shown by a Trichoderma citrinoviride isolate, interfering with feeding of the cereal aphid Rhopalosiphum padi . Different long-chain primary alcohols (LCOHs) showed a phagodeterrent effect restraining aphids from settling on treated leaves. The LCOHs, perceived through taste receptor neurons and effective at low concentrations, hold potential for insect control in synergy with other compounds. Lysobacter spp. may affect soil-borne pathogens through extracellular enzymes and other metabolites (Folman et al., 2003 ). Members of this genus appeared abundant in soil suppressive to the root pathogen Rhizoctonia solani . Some strains showed in vitro activity against R. solani and other phytopathogens such as Pythium ultimum, Aspergillus niger, Fusarium oxysporum , and Xanthomonas campestris . In soil, however, suppression of R. solani damping-off on sugar beet and cauliflower was low, and no PGP effect was found on sugar beet, cauliflower, onion and Arabidopsis thaliana , likely due to poor rhizosphere colonization. Antagonistic Lysobacter spp. are an important source of new enzymes and antimicrobial compounds, although their role in disease suppressiveness needs to be confirmed ( Gómez Expósito et al. ). Soil amendments The characterization of organic matter amendments enhancing biocontrol and influencing resident soil communities is a growing research field (Bailey and Lazarovits, 2003 ; Bonilla et al., 2012 ). Debode et al. studied the effect of chitin on lettuce growth and survival of pathogenic Escherichia coli O157:H7 and Salmonella enterica colonizing leaves. Chitin addition increased yields and reduced phyllosphere survival of both bacteria, increasing fungal and bacterial biomass in the rhizosphere. An increase was observed for bacterial genera Cellvibrio, Pedobacter, Dyadobacter , and Streptomyces and the fungi Lecanicillium and Mortierella . These taxa include species involved in biocontrol, PGP, nitrogen cycle, and chitin degradation, showing a potential for chitin-based amendments.A positive effect of amendments on soil microbial communities was reported by Vida et al. Composted almond shells elicited suppression of Rosellinia necatrix , causal agent of white root rot on avocado, increasing Proteobacteria and Ascomycota, and reducing Acidobacteria and Mortierellales.