Silicon application and related changes in soil bacterial community dynamics reduced ginseng black spot incidence in Panax ginseng in a short-term study
This study analyzed the effect of silicon (Si) application on the occurrence of ginseng black spot caused by Alternaria panax. We explored the differences in soil physical and chemical factors and microbial community structure following Si application as well as the key factors that affected the occurrence of ginseng black spot in soil. Potted Panax ginseng plants were used to assess the effect of Si treatment on ginseng black spot. Plants were grown under four kinds of treatment: (1) control
... ment: (1) control (no inoculations), (2) inoculation with A. panax, (3) inoculation wit Si, and (4) inoculation with A. panax + Si, with 18 plants (3 pots) per treatment. Soil physical and chemical properties were comprehensively analyzed. Bacterial communities were analyzed using Illumina HiSeq sequencing targeting the 16S rRNA gene. Using micro-ecological regulation measures, we developed a theoretical basis for the prevention and control of ginseng black spot. Results After inoculation with A. panax, the morbidity (and morbidity index) of ginseng with and without Si was 52% (46) and 83% (77), respectively. Soil physical and chemical analysis showed that under the ginseng black spot inoculation, bacterial communities were mainly affected by pH and available potassium, followed by ammonium nitrogen and available Si. NMDS and PLS-DA analyses and the heat maps of relative abundance revealed that Si application elevated the resistance of ginseng black spot as regulated by the abundance and diversity of bacterial flora in rhizosphere soils. Heatmap analysis at the genus level revealed that A. panax + Si inoculations significantly increased the soil community abundance of Sandaracinus, Polycyclovorans, Hirschia, Haliangium, Nitrospira, Saccharothrix, Aeromicrobium, Luteimonas, and Rubellimicrobium and led to a bacterial community structure with relative abundances that were significantly similar to that of untreated soil. Conclusions Si alleviated the incidence of ginseng black spot by directly and indirectly affecting the structure and diversity of the soil microbial community. Short-term Si application also significantly regulated the structural impact on soil microorganisms caused by ginseng black spot. Our findings indicated that Si applications may possibly be used in the prevention and treatment of ginseng black spot. Background Ginseng black spot, caused by Alternaria panax Whetz, is a common soil-borne disease and one of the most serious diseases affecting the above-ground parts, especially the leaves, of Panax ginseng. This pathogen is distributed widely in the Changbai Mountains of China and other ginseng production regions, and accounts for more than 20-30% of the annual incidence, which is very common in cultivated and wild ginseng. Alternaria panax infestation may lead to 10-20% yield loss of the total crop. Infection first appears as elongated reddish to dark brown crevices in the infected areas. In seedlings, the stems are gradually girdled and thus collapse, resulted in damping-off . In older plants, foliar infections appear later in summer, characterized by rapidly enlarging dark brown necrotic spots (circular, ellipsoid, or wedge-shaped) surrounded by chlorotic margins. Silicon (Si) has been demonstrated to play an important role in enhancing plant resistance to disease. Si deposition has been suggested to create a physical barrier along cell walls and prevent fungal penetration into the plant . Additional studies have indicated that Si is related to plant-pathogen interactions for the control in diseases in different plant species , and aids in the enhancement of plant resistance against disease caused by viruses, fungi, bacteria, and nematodes. Recently, it was suggested that the deposition of Si in the apoplast may prevent fungal effectors from entering the target cells, thus altering the development of the pathogens . Another recently study showed that Si treatment conferred an effective protection of soybean plants against Phytophthora sojae in a hydroponic experiment . Agricultural soil productivity largely depends on microbial diversity and community composition, which significantly affects plant growth and crop quality . The homeostasis of the soil microbial community can suppress pathogens and promote plant growth . Plant-microbe interactions remodel the complex biological and ecological processes in soil, where roots are influenced by the rhizosphere . Many studies have assessed the effect of Si on plantmicrobe interactions and have demonstrated that Si enhances plant resistance to pathogens by activating defense reactions [9, 10]. Recently, a pot experiment demonstrated that Si addition decreased the concentrations of water-soluble and exchangeable arsenic in soil and, therefore, decreased the bioavailability of red soil arsenic in Panax notoginseng . The present study, therefore, aimed to investigate if Si treatment would enhance the resistance of ginseng against A. panax. The study objectives were to evaluate the effect of Si on the prevention the internal transcribed spacer (ITS) region generating 553~554 bp fragments and the GPD gene for 565~566 bp fragments, respectively. The A. panax strain was deposited in the Culture Collection Center of Yangtze University in Jingzhou, China. Spores were flushed from colonies and then resuspended in sterile distilled water at 1 × 10 5 spores/mL. The sterilized surfaces of detached spring ginseng leaves were inoculated with 20 µL conidial suspension and incubated under the same greenhouse conditions for 9 days, when black spot symptoms became visible on the leaves. Plants were grown under four kinds of treatment: ginseng control plants (CK), plants only inoculated with A. panax (A), plants inoculated with A. panax + Si (AS), and plants only inoculated with Si (S), with 18 plants (3 pots) per treatment. To test the prophylactic role of Si, the Si concentration was set at 1.7 mM, i.e., the highest possible concentration of silica acid in solution . Six seedlings of ginseng were randomly selected from each treatment group, and the soils were ML carried out the experimental plan, collected samples, extracted DNA and conducted data analysis.