Stimulation of defense reactions in Medicago truncatula by antagonistic lipopeptides from Paenibacillus sp. strain B2

Paenibacillus as a Biocontrol Agent for Fungal Phytopathogens: Is P. polymyxa the Only One Worth Attention?

Control of fungal phytopathogens is a significant challenge in modern agriculture. The widespread use of chemical fungicides to control these pathogens often leads to environmental and food contamination. An eco-friendly alternative that can help reduce reliance on these chemicals is plant growth-promoting bacteria (PGPB), particularly those of the genus Paenibacillus, which appear to be highly effective. The review aims to summarize the existing knowledge on the potential of Paenibacillus spp. as fungal biocontrol agents, identify knowledge gaps, and answer whether other species of the genus Paenibacillus, in addition to Paenibacillus polymyxa, can also be effective biocontrol agents. Paenibacillus spp. can combat plant phytopathogens through various mechanisms, including the production of lipopeptides (such as fusaricidin, paenimyxin, and pelgipeptin), the induction of systemic resistance (ISR), hydrolytic enzymes (chitinase, cellulase, and glucanase), and volatile organic compounds. These properties enable Paenibacillus strains to suppress the growth of fungi such as Fusarium oxysporum, F. solani, Rhizoctonia solani, Botrytis cinerea, or Colletotrichum gloeosporioides. Notably, several strains of Paenibacillus, including P. polymyxa, P. illinoisensis KJA-424, P. lentimorbus B-30488, and P. elgii JCK1400, have demonstrated efficacy in controlling fungal diseases in plants. Importantly, many formulations with Paenibacillus strains have already been patented, and some are commercially available, but most of them contain only P. polymyxa. Nevertheless, considering the data presented in this review, we believe that other strains from the Paenibacillus genus (besides P. polymyxa) will also be commercialized and used in plant protection in the future. Importantly, there is still limited information regarding their impact on the native microbiota, particularly from the metataxonomic and metagenomic perspectives. Expanding knowledge in this area could enhance the effectiveness of biocontrol agents containing Paenibacillus spp., ensuring safe and sustainable use of biological fungicides.

Bacillomycin D inhibits growth of Rhizopus stolonifer and induces defense-related mechanism in cherry tomato

The inhibitory effect of Bacillomycin D, a cyclic lipopeptide, on Rhizopus stolonifer colonization of cherry tomato was studied, and its possible mechanism of action was explored. Bacillomycin D showed a direct inhibitory effect on R. stolonifer spore germination and mycelial growth in vitro. It conferred both a direct inhibitory effect on R. stolonifer growth in cherry tomato in vivo and induced host resistance in cherry tomato. Moreover, Bacillomycin D treatment significantly increased the activities of plant defense-related enzymes, including chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), and peroxidase (POD). Real-time PCR (RT-PCR) showed that defense-related genes involved in the salicylic acid defense signaling pathway and genes encoding pathogenesis-related proteins were up-regulated in Bacillomycin D treatment. Furthermore, Bacillomycin D-C16 resulted in direct inhibition and a remarkable induced resistance to R. stolonifer which was higher than as induced by Bacillomycin D-C14. Together, the data indicated that Bacillomycin D can control the growth of R. stolonifer through both the direct inhibition of the fungus and the activation of defense-related genes and enzymes in cherry tomato.

An Extract Purified from the Mycelium of a Tomato Wilt-Controlling Strain of Fusarium sambucinum Can Protect Wheat against Fusarium and Common Root Rots

An approach to manage seed-transmitted Fusarium crown-foot-root rot (FCR, Fusarium spp.) and common root rot (CRR, Bipolaris sorokiniana) on wheat, avoiding environmental risks of chemicals, is seed treatments with microbial metabolites. F. sambucinum strain FS-94 that induces resistance to tomato wilt was shown by this study to be a source of non-fungitoxic wheat-protecting metabolites, which were contained in a mycelium extract purified by gel-chromatography and ultrafiltration. Plant-protecting effect of the purified mycelial extract (PME) was demonstrated in vegetation experiments using a rolled-towel assay and by small-plot field trials. To elucidate mechanisms putatively underlying PME protective activity, tests with cultured Triticum aestivum and T. kiharae cells, particularly the extracellular alkalinization assay, as well as gene expression analysis in germinated wheat seeds were used. Pre-inoculation treatments of seeds with PME significantly decreased the incidence (from 30 to 40%) and severity (from 37 to 50%) of root rots on seedlings without any inhibition of the seed germination and potentiation of deoxynivalenol (DON), DON monoacetylated derivatives and zearalenon production in FCR agents. In vegetation experiments, reductions in the DON production were observed with doses of 0.5 and 1 mg/mL of PME. Pre-sowing PME application on seeds of two spring wheat cultivars naturally infected with FCR and CRR provided the mitigation of both diseases under field conditions during four growing seasons (2013⁻2016). PME-induced ion exchange response in cultured wheat cells, their increased survivability, and up-regulated expression of some defensins' genes in PME-exposed seedlings allow the suggestion of the plant-mediated character of disease-controlling effect observed in field.

Secondary and sucrose metabolism regulated by different light quality combinations involved in melon tolerance to powdery mildew

We evaluated the effect of different light combinations on powdery mildew resistance and growth of melon seedlings. Light-emitting diodes were used as the light source and there were five light combinations: white light (420-680 nm); blue light (460 nm); red light (635 nm); RB31 (ratio of red and blue light, 3: 1); and RB71 (ratio of red and blue light, 7: 1). Compared with other treatments, blue light significantly decreased the incidence of powdery mildew in leaves of melon seedlings. Under blue light, H₂O₂ showed higher accumulation, and the content of phenolics, flavonoid and tannins, as well as expression of the genes involved in synthesis of these substances, significantly increased compared with other treatments before and after infection. Lignin content and expression of the genes related to its synthesis were also induced by blue light before infection. Melon irradiated with RB31 light showed the best growth parameters. Compared with white light, red light and RB71, RB31 showed higher accumulation of lignin and lower incidence of powdery mildew. We conclude that blue light increases melon resistance to powdery mildew, which is dependent on the induction of secondary metabolism that may be related to H₂O₂ accumulation before infection. Induction of tolerance of melon seeds to powdery mildew by RB31 is due to higher levels of sucrose metabolism and accumulation of lignin.

Evaluation of the biocontrol efficacy of a Serratia marcescens strain indigenous to tea rhizosphere for the management of root rot disease in tea

The aim of the present study is to evaluate plant growth promoting and biocontrol efficacy of a Serratia marcescens strain ETR17 isolated from tea rhizosphere for the effective management of root rot disease in tea. Isolated bacterial culture ETR17 showed significant level of in vitro antagonism against nine different foliar and root pathogens of tea. The phenotypic and molecular characterization of ETR17 revealed the identity of the bacterium as Serratia marcescens. The bacterium was found to produce several hydrolytic enzymes like chitinase, protease, lipase, cellulase and plant growth promoting metabolites like IAA and siderophore. Scanning electron microscopic studies on the interaction zone between pathogen and antagonistic bacterial isolate revealed severe deformities in the fungal mycelia. Spectral analyses (LC-ESI-MS, UV-VIS spectrophotometry and HPLC) and TLC indicated the presence of the antibiotics pyrrolnitrin and prodigiosin in the extracellular bacterial culture extracts. Biofilm formation by ETR17 on polystyrene surface was also observed. In vivo application of talc-based formulations prepared with the isolate ETR17 in tea plantlets under green house conditions revealed effective reduction of root-rot disease as well as plant growth promotion to a considerable extent. Viability studies with the ETR17 talc formulation showed the survivability of the isolate up to six months at room temperature. The sustenance of ETR17 (concentration of 8-9x108 cfu g-1) in the soil after the application of talc formulation was recorded by ELISA. Safety studies revealed that ETR17 did not produce hemolysin as observed in pathogenic Serratia strains. The biocontrol strain reported in this study can be used for field application in order to minimize the use of chemical fungicides for disease control in tea gardens.

Systemic Induction of the Defensin and Phytoalexin Pisatin Pathways in Pea (Pisum sativum) against Aphanomyces euteiches by Acetylated and Nonacetylated Oligogalacturonides

Oligogalacturonides (OGs) are known for their powerful ability to stimulate the plant immune system but little is known about their mode of action in pea (Pisum sativum). In the present study, we investigated the elicitor activity of two fractions of OGs, with polymerization degrees (DPs) of 2-25, in pea against Aphanomyces euteiches. One fraction was nonacetylated (OGs - Ac) whereas the second one was 30% acetylated (OGs + Ac). OGs were applied by injecting the upper two rachises of the plants at three- and/or four-weeks-old. Five-week-old roots were inoculated with 10⁵ zoospores of A. euteiches. The root infection level was determined at 7, 10 and 14 days after inoculation using the quantitative real-time polymerase chain reaction (qPCR). Results showed significant root infection reductions namely 58, 45 and 48% in the plants treated with 80 µg OGs + Ac and 59, 56 and 65% with 200 µg of OGs - Ac. Gene expression results showed the upregulation of genes involved in the antifungal defensins, lignans and the phytoalexin pisatin pathways and a priming effect in the basal defense, SA and ROS gene markers as a response to OGs. The reduction of the efficient dose in OGs + Ac is suggesting that acetylation is necessary for some specific responses. Our work provides the first evidence for the potential of OGs in the defense induction in pea against Aphanomyces root rot.

Cyclic LIPopeptides from Bacillus subtilis ABS-S14 elicit defense-related gene expression in citrus fruit

Effects of cyclic lipopeptides (CLPs) obtained from Bacillus subtilis ABS-S14 on eliciting defense-related gene transcription and activity of defense-related enzymes; glucanase (GLU), chitinase (CHI), peroxidase (POX) and lipoxygenase (LOX) in Citrus sinensis cv. Valencia fruit were determined. The maximum level of GLU transcripts induced in fruit treated with fengycin was significantly greatest among treatments at 48 h. Surfactin enhanced the LOX and POX transcripts. In parallel, corresponding enzyme activities were correlated with changes in gene expression observed in fruit inoculated with Penicillium digitatum following treatment with individual CLPs. Synergistic effects of fengycin and iturin A, fengycin and surfactin were shown in gene transcript of GLU and CHI, respectively, and surfactin induced POX and LOX gene expression of citrus flavedo without pathogen infection. These results suggest that fengycin and surfactin act as elicitors of defense-related gene expression in “Valencia” fruit following infection.

Transgenic expression of plant chitinases to enhance disease resistance

Crop plants have evolved an array of mechanisms to counter biotic and abiotic stresses. Many pathogenesis-related proteins are expressed by plants during the attack of pathogens. Advances in recombinant DNA technology and understanding of plant-microbe interactions at the molecular level have paved the way for isolation and characterization of genes encoding such proteins, including chitinases. Chitinases are included in families 18 and 19 of glycosyl hydrolases (according to www.cazy.org ) and they are further categorized into seven major classes based on their aminoacid sequence homology, three-dimensional structures, and hydrolytic mechanisms of catalytic reactions. Although chitin is not a component of plant cell walls, plant chitinases are involved in development and non-specific stress responses. Also, chitinase genes sourced from plants have been successfully over-expressed in crop plants to combat fungal pathogens. Crops such as tomato, potato, maize, groundnut, mustard, finger millet, cotton, lychee, banana, grape, wheat and rice have been successfully engineered for fungal resistance either with chitinase alone or in combination with other PR proteins.