First Report of Bacterial Stem and Root Rot of Sweetpotato Caused by a Dickeya sp. (Erwinia chrysanthemi) in China

RNA-Sequencing Analysis Revealed Genes Associated with Sweet Potato (Ipomoea batatas (L.) Lam.) Responses to Stem Rot during Different Infection Stages

The sweet potato, which is an important tuber crop in China, is susceptible to a variety of pathogens and insect pests during cultivation and production. Stem rot is a common sweet potato disease that seriously affects tuber yield and quality. Unfortunately, there have been relatively few studies on the mechanism mediating the stem rot resistance of sweet potatoes. In this study, a transcriptome sequencing analysis was completed using Xushu 48 samples at different stages (T1, T2, and T3) of the stem rot infection. The T1 vs. T2, T1 vs. T3, and T2 vs. T3 comparisons detected 44,839, 81,436, and 61,932 differentially expressed genes (DEGs), respectively. The DEGs encoded proteins primarily involved in alanine, aspartate, and glutamate metabolism (ko00250), carbon fixation in photosynthetic organisms (ko00710), and amino sugar and nucleotide sugar metabolism (ko00520). Furthermore, some candidate genes induced by phytopathogen infections were identified, including gene-encoding receptor-like protein kinases (RLK5 and RLK7), an LRR receptor-like serine/threonine protein kinase (SERK1), and transcription factors (bHLH137, ERF9, MYB73, and NAC053). The results of this study provide genetic insights that are relevant to future explorations of sweet potato stem rot resistance, while also providing the theoretical basis for breeding sweet potato varieties that are resistant to stem rot and other diseases.

Research Progress in the Mechanisms of Resistance to Biotic Stress in Sweet Potato

Sweet potato (Ipomoea batatas (L.) Lam.) is one of the most important food, feed, industrial raw materials, and new energy crops, and is widely cultivated around the world. China is the largest sweet potato producer in the world, and the sweet potato industry plays an important role in China's agriculture. During the growth of sweet potato, it is often affected by biotic stresses, such as fungi, nematodes, insects, viruses, and bacteria. These stressors are widespread worldwide and have severely restricted the production of sweet potato. In recent years, with the rapid development and maturity of biotechnology, an increasing number of stress-related genes have been introduced into sweet potato, which improves its quality and resistance of sweet potato. This paper summarizes the discovery of biological stress-related genes in sweet potato and the related mechanisms of stress resistance from the perspectives of genomics analysis, transcriptomics analysis, genetic engineering, and physiological and biochemical indicators. The mechanisms of stress resistance provide a reference for analyzing the molecular breeding of disease resistance mechanisms and biotic stress resistance in sweet potato.

Synergistic Action of Biosynthesized Silver Nanoparticles and Culture Supernatant of Bacillus amyloliquefacience against the Soft Rot Pathogen Dickeya dadantii

Nanomaterials are increasingly being used for crop growth, especially as a new paradigm for plant disease management. Among the other nanomaterials, silver nanoparticles (AgNPs) draw a great deal of attention because of their unique features and multiple usages. Rapid expansion in nanotechnology and utilization of AgNPs in a large range of areas resulted in the substantial release of these nanoparticles into the soil and water environment, causing concern for the safety of ecosystems and phytosanitary. In an attempt to find an effective control measure for sweet potato soft rot disease, the pathogen Dickeya dadantii was exposed to AgNPs, the cell-free culture supernatant (CFCS) of Bacillus amyloliquefaciens alone, and both in combination. AgNPs were synthesized using CFCS of Bacillus amyloliquefaciens strain A3. The green synthesized AgNPs exhibited a characteristic surface plasmon resonance peak at 410-420 nm. Electron microscopy and X-ray diffraction spectroscopy determined the nanocrystalline nature and 20-100 nm diameters of AgNPs. Release of metal Ag⁺ ion from biosynthesized AgNPs increases with time. AgNPs and CFCS of B. amyloliquefaciens alone exhibited antibacterial activity against the growth, biofilm formation, swimming motility, and virulence of strain A3. The antibacterial activities elevated with the elevation in AgNPs and CFCS concentration. Similar antibacterial activities against D. dadantii were obtained with AgNPs at 50 µg·mL^-1, 50% CFCS alone, and the combination of AgNPs at 12 µg·mL^-1 and 12% CFCS of B. amyloliquefaciens. In planta experiments indicated that all the treatments reduced D. dadantii infection and increased plant growth. These findings suggest that AgNPs along with CFCS of B. amyloliquefaciens can be applied to minimize this bacterial disease by controlling pathogen-contaminated sweet potato tuber with minimum Ag nano-pollutant in the environment.

Biocontrol of Soft Rot Dickeya and Pectobacterium Pathogens by Broad-Spectrum Antagonistic Bacteria within Paenibacillus polymyxa Complex

Polymyxin-producing bacteria within the Paenibacillus polymyxa complex have broad-spectrum activities against fungi and bacteria. Their antibacterial activities against soft rot Dickeya and Pectobacterium phytopathogens containing multiple polymyxin-resistant genes were not clear. Here, we selected nine strains within the P. polymyxa complex having broad-spectrum antagonistic activities against phytopathogenic fungi and a polymyxin-resistant D. dadantii strain causing stem and root rot disease of sweet potato and did antagonistic assays on nutrient agar and sweet potato tuber slices. These strains within the P. polymyxa complex showed clear antagonistic activities against D. dadantii in vitro and in vivo. The most effective antagonistic strain P. polymyxa ShX301 showed broad-spectrum antagonistic activities against all the test Dickeya and Pectobacterium strains, completely eliminated D. dadantii from sweet potato seed tubers, and promoted the growth of sweet potato seedlings. Cell-free culture filtrate of P. polymyxa ShX301 inhibited D. dadantii growth, swimming motility, and biofilm formation and disrupted D. dadantii plasma membranes, releasing nucleic acids and proteins. Multiple lipopeptides produced by P. polymyxa ShX301 may play a major role in the bactericidal and bacteriostatic actions. This study clarifies that the antimicrobial spectrum of polymyxin-producing bacteria within the P. polymyxa complex includes the polymyxin-resistant Dickeya and Pectobacterium phytopathogens and strengthens the fact that bacteria within the P. polymyxa complex have high probability of being effective biocontrol agents and plant growth promoters.

Identification and characterization of opportunistic pathogen Pectobacterium polonicum causing potato blackleg in China

Blackleg and aerial stem rot of potato (Solanum tuberosum L.), caused by soft rot enterobacteria of the genera Pectobacterium and Dickeya, has recently increased years in Hebei Province, China. Field surveys were performed during the 2021 potato growing season in Hebei to identify and characterize bacterial pathogens. Sixteen potato plants showing blackleg or aerial stem rot were collected from three potato-producing areas, and ten representative pectinolytic bacteria were isolated from symptomatic plants. 16S rDNA sequencing and multilocus sequence analysis were performed to determine the taxonomic position of the bacterial isolates. The isolates belonged to the genus Pectobacterium, including Pectobacterium atrosepticum, Pectobacterium carotovorum, Pectobacterium brasiliense, and Pectobacterium parmentieri. The exceptions were isolates BY21311 and BY21312, which belonged to a new species of Pectobacterium polonicum previously found in groundwater. The taxonomy of isolate BY21311 was confirmed using whole genome-based analysis. P. polonicum has only been identified in potato plants on one farm in Baoding region in China. Isolates BY21311 and BY21312 displayed similar physiological and biochemical traits to the type strain DPMP315^T. Artificial inoculation assays revealed that isolate BY21311 fulfilled Koch's postulates for potato blackleg. These findings represent the first time P. polonicum, a water-associated Pectobacterium species may be the cause of blackleg in the field. Interestingly, P. polonicum BY21311 has reduced ability to macerate potato tubers when compared to P. atrosepticum, P. brasiliense, P. versatile, and P. parvum, which is more virulent in tubers than the type strain DPMP315^T. The host range of isolate BY21311 was determined by injection method, which can impregnate five plants. Although the genome of isolate BY21311 harbors gene clusters encoding a type III secretion system, it did not elicit a hypersensitive response (HR) in Nicotiana benthamiana or N. tabacum leaves. T3SS effector AvrE and T4SS effector PilN were obtained by predicting isolate BY21311 genome. P. polonicum appears to show significant variations in gene content between two genomes, and gene content varies between isolates BY21311 and DPMP315^T, with strain specific-genes involved in many aspects, including lipopolysaccharide biosynthesis, substrate translocation, T4SS and T6SS among others, suggesting that isolates BY21311 and DPMP315^T might represent distinct clades within the species.

Three Highly Sensitive and High-Throughput Serological Approaches for Detecting Dickeya dadantii in Sweet Potato

Sweet potato stem and root rot is an important bacterial disease and often causes serious economic losses to sweet potato. Development of rapid and sensitive detection methods is crucial for diagnosis and management of this disease in field. Here, we report the production of four hybridoma cell lines (25C4, 16C10, 9B1, and 9H10) using Dickeya dadantii strain FY1710 as an immunogen. Monoclonal antibodies (MAbs) produced by these four hybridoma cell lines were highly specific and sensitive for D. dadantii detection. Indirect enzyme-linked immunosorbent assay (indirect-ELISA) results showed that the four MAbs 25C4, 16C10, 9B1, and 9H10 could detect D. dadantii in suspensions diluted to 4.89 × 10⁴, 4.89 × 10⁴, 9.78 × 10⁴, and 9.78 × 10⁴ CFU/ml, respectively. Furthermore, all four MAbs can react strongly and specifically with all four D. dadantii strains used in this study, not with the other seven tested bacterial strains. Using these four MAbs, three different serological approaches, triple-antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA), dot-ELISA, and tissue-print-ELISA, were developed for detection of D. dadantii in crude extracts prepared from field-collected sweet potato plants. Among these three methods, TAS-ELISA and dot-ELISA were used to detect D. dadantii in suspensions diluted up to 1.23 × 10⁴ and 1.17 × 10⁶ CFU/ml, respectively, or in sweet potato crude extracts diluted up to 1:3,840 and 1:1,920 (wt/vol, grams per milliliter), respectively. Surprisingly, both TAS-ELISA and dot-ELISA serological approaches were more sensitive than the conventional PCR. Analyses using field-collected sweet potato samples showed that the newly developed TAS-ELISA, dot-ELISA, or tissue-print-ELISA were reliable in detecting D. dadantii in sweet potato tissues. Thus, the three serological approaches were highly valuable for diagnosis of stem and root rot in sweet potato production.

Understanding Root Rot Disease in Agricultural Crops

Root rot diseases remain a major global threat to the productivity of agricultural crops. They are usually caused by more than one type of pathogen and are thus often referred to as a root rot complex. Fungal and oomycete species are the predominant participants in the complex, while bacteria and viruses are also known to cause root rot. Incorporating genetic resistance in cultivated crops is considered the most efficient and sustainable solution to counter root rot, however, resistance is often quantitative in nature. Several genetics studies in various crops have identified the quantitative trait loci associated with resistance. With access to whole genome sequences, the identity of the genes within the reported loci is becoming available. Several of the identified genes have been implicated in pathogen responses. However, it is becoming apparent that at the molecular level, each pathogen engages a unique set of proteins to either infest the host successfully or be defeated or contained in attempting so. In this review, a comprehensive summary of the genes and the potential mechanisms underlying resistance or susceptibility against the most investigated root rots of important agricultural crops is presented.

Rapid diagnosis of Ralstonia solanacearum infection sweet potato in China by loop-mediated isothermal amplification

Bacterial wilt of sweet potato is caused by Ralstonia solanacearum, which is distributed in southern China and causes significant economic losses each year. The pathogen is soil- and rhizome-borne, and thus its rapid detection may prevent the occurrence and spread of the disease. R. solanacearum has been listed as a quarantine disease in China. With the advent of molecular biology, many novel tools have been explored for the rapid identification of plant pathogens. In this study, a strain-specific detection method was developed for this specific pathogen that infects sweet potato using loop-mediated isothermal amplification (LAMP). A set of new LAMP-specific primers was designed from the orf428 gene, which can specifically detect the R. solanacearum bacterium that infect sweet potato. The LAMP reaction consisted of 8.0 mmol·L^-1Mg²⁺, 1.4 mmol·L^-1 dNTPs, and 0.32U μL^-1 Bst 2.0 DNA polymerase and was performed at 65 °C for 1 h. The amplification products were detected by visualizing a mixture of color changes using SYBR Green I dye and assessing ladder-like bands by electrophoresis. Our method has specificity, i.e., it only detected R. solanacearum in sweet potato, and it has high sensitivity, with a detection limit of 100 fg·μL^-1 genomic DNA and 10³ CFU·mL^-1 of bacterial fluid. In addition, R. solanacearum could be directly detected in infected sweet potato tissues without the need for DNA extraction. The LAMP method established in this study is a highly specific, sensitive, and rapid tool for the detection of bacterial wilt in sweet potato caused by R. solanacearum.

Lemon-Fruit-Based Green Synthesis of Zinc Oxide Nanoparticles and Titanium Dioxide Nanoparticles against Soft Rot Bacterial Pathogen Dickeya dadantii

Edible plant fruits are safe raw materials free of toxicants and rich in biomolecules for reducing metal ions and stabilizing nanoparticles. Zinc oxide nanoparticles (ZnONPs) and titanium dioxide nanoparticles (TiO₂NPs) are the most produced consumer nanomaterials and have known antibacterial activities but have rarely been used against phytopathogenic bacteria. Here, we synthesized ZnONPs and TiO₂NPs simply by mixing ZnO or TiO₂ solution with a lemon fruit extract at room temperature and showed their antibacterial activities against Dickeya dadantii, which causes sweet potato stem and root rot disease occurring in major sweet potato planting areas in China. Ultraviolet–visible spectrometry and energy dispersive spectroscopy determined their physiochemical characteristics. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction spectroscopy revealed the nanoscale size and polymorphic crystalline structures of the ZnONPs and TiO₂NPs. Fourier-transform infrared spectroscopy revealed their surface stabilization groups from the lemon fruit extract. In contrast to ZnO and TiO₂, which had no antibacterial activity against D. dadantii, ZnONPs and TiO₂NPs showed inhibitions on D. dadantii growth, swimming motility, biofilm formation, and maceration of sweet potato tuber slices. ZnONPs and TiO₂NPs showed similar extents of antibacterial activities, which increased with the increase of nanoparticle concentrations, and inhibited about 60% of D. dadantii activities at the concentration of 50 µg∙mL⁻¹. The green synthetic ZnONPs and TiO₂NPs can be used to control the sweet potato soft rot disease by control of pathogen contamination of seed tubers.

Green Synthesis of Silver Nanoparticles with Culture Supernatant of a Bacterium Pseudomonas rhodesiae and Their Antibacterial Activity against Soft Rot Pathogen Dickeya dadantii

Bacterial stem and root rot disease of sweet potato caused by Dickeya dadantii recently broke out in major sweet potato planting areas in China and calls for effective approaches to control the pathogen and disease. Here, we developed a simple method for green synthesis of silver nanoparticles (AgNPs) using bacterial culture supernatants. AgNPs synthesized with the cell-free culture supernatant of a bacterium Pseudomonas rhodesiae displayed the characteristic surface plasmon resonance peak at 420–430 nm and as nanocrystallites in diameters of 20–100 nm determined by transmission electron microscopy, scanning electron microscopy, and X-ray diffraction spectroscopy. Functional groups associated with proteins in the culture supernatant may reduce silver ions and stabilize AgNPs. The AgNPs showed antibacterial activities against D. dadantii growth, swimming motility, biofilm formation, and maceration of sweet potato tubers whereas the culture supernatant of P. rhodesiae did not. AgNPs (12 µg∙ml⁻¹) and AgNO₃ (50 µg∙ml⁻¹) showed close antibacterial activities. The antibacterial activities increased with the increase of AgNP concentrations. The green-synthesized AgNPs can be used to control the soft rot disease by control of pathogen contamination of sweet potato seed tubers.