Domestication Impacts the Wheat-Associated Microbiota and the Rhizosphere Colonization by Seed- and Soil-Originated Microbiomes, Across Different Fields

The seed-transmitted microorganisms and the microbiome of the soil in which the plant grows are major drivers of the rhizosphere microbiome, a crucial component of the plant holobiont. The seed-borne microbiome can be even coevolved with the host plant as a result of adaptation and vertical transmission over generations. The reduced genome diversity and crossing events during domestication might have influenced plant traits that are important for root colonization by seed-borne microbes and also rhizosphere recruitment of microbes from the bulk soil. However, the impact of the breeding on seed-transmitted microbiome composition and the plant ability of microbiome selection from the soil remain unknown. Here, we analyzed both endorhiza and rhizosphere microbiome of two couples of genetically related wild and cultivated wheat species (Aegilops tauschii/Triticum aestivum and T. dicoccoides/T. durum) grown in three locations, using 16S rRNA gene and ITS2 metabarcoding, to assess the relative contribution of seed-borne and soil-derived microbes to the assemblage of the rhizosphere microbiome. We found that more bacterial and fungal ASVs are transmitted from seed to the endosphere of all species compared with the rhizosphere, and these transmitted ASVs were species-specific regardless of location. Only in one location, more microbial seed transmission occurred also in the rhizosphere of A. tauschii compared with other species. Concerning soil-derived microbiome, the most distinct microbial genera occurred in the rhizosphere of A. tauschii compared with other species in all locations. The rhizosphere of genetically connected wheat species was enriched with similar taxa, differently between locations. Our results demonstrate that host plant criteria for soil bank’s and seed-originated microbiome recruitment depend on both plants’ genotype and availability of microorganisms in a particular environment. This study also provides indications of coevolution between the host plant and its associated microbiome resulting from the vertical transmission of seed-originated taxa.

Polycistronic Artificial microRNA-Mediated Resistance to Cucumber Green Mottle Mosaic Virus in Cucumber

Cucumber green mottle mosaic virus (CGMMV), as a typical seed-borne virus, causes costly and devastating diseases in the vegetable trade worldwide. Genetic sources for resistance to CGMMV in cucurbits are limited, and environmentally safe approaches for curbing the accumulation and spread of seed-transmitted viruses and cultivating completely resistant plants are needed. Here, we describe the design and application of RNA interference-based technologies, containing artificial microRNA (amiRNA) and synthetic trans-acting small interfering RNA (syn-tasiRNA), against conserved regions of different strains of the CGMMV genome. We used a rapid transient sensor system to identify effective anti-CGMMV amiRNAs. A virus seed transmission assay was developed, showing that the externally added polycistronic amiRNA and syn-tasiRNA can successfully block the accumulation of CGMMV in cucumber, but different virulent strains exhibited distinct influences on the expression of amiRNA due to the activity of the RNA-silencing suppressor. We also established stable transgenic cucumber plants expressing polycistronic amiRNA, which conferred disease resistance against CGMMV, and no sequence mutation was observed in CGMMV. This study demonstrates that RNA interference-based technologies can effectively prevent the occurrence and accumulation of CGMMV. The results provide a basis to establish and fine-tune approaches to prevent and treat seed-based transmission viral infections.

Highly efficient intercellular spreading of protein misfolding mediated by viral ligand-receptor interactions

Protein aggregates associated with neurodegenerative diseases have the ability to transmit to unaffected cells, thereby templating their own aberrant conformation onto soluble homotypic proteins. Proteopathic seeds can be released into the extracellular space, secreted in association with extracellular vesicles (EV) or exchanged by direct cell-to-cell contact. The extent to which each of these pathways contribute to the prion-like spreading of protein misfolding is unclear. Exchange of cellular cargo by both direct cell contact or via EV depends on receptor-ligand interactions. We hypothesized that enabling these interactions through viral ligands enhances intercellular proteopathic seed transmission. Using different cellular models propagating prions or pathogenic Tau aggregates, we demonstrate that vesicular stomatitis virus glycoprotein and SARS-CoV-2 spike S increase aggregate induction by cell contact or ligand-decorated EV. Thus, receptor-ligand interactions are important determinants of intercellular aggregate dissemination. Our data raise the possibility that viral infections contribute to proteopathic seed spreading by facilitating intercellular cargo transfer.

Seed-transmission of Cowpea mild mottle virus on several varieties of soybean in Indonesia

Sutrawati M, Hidayat SH, Suastika G, Sukarno BPW, Nurmansyah A. 2021. Seed-transmission of Cowpea mild mottle virus on several varieties of soybean in Indonesia. Biodiversitas 22: 4182-4185. Seeds and infected plants play important role as source of disease in the field for seed-transmitted virus, such as Cowpea mild mottle virus (CPMMV). Research was conducted to determine seed transmission nature of CPMMV on 10 soybean varieties based on growing on test method and dot immunobinding assay to confirm CPMMV infection. Field experiment was conducted to evaluate the efficiency of seed-transmitted CPMMV as the source of initial inoculum in the field. Soybean var. ‘Anjasmoro’ from 3 cultivation areas (Cianjur, Bogor, and Cirebon) was used for field experiment. Seed transmission of CPMMV was confirmed on soybean var. ‘Detam 2’, ‘Detam 3’, ‘Malika’, ‘Anjasmoro’, and ‘Argomulyo’; but was not found on ‘Detam 1’, ‘Detam 4’, ‘Wilis’, ‘Grobogan’, and ‘Dena 1’. The infection of CPMMV did not show symptom, either on the seedcoat and the unifoliolate leaves. Infection rate of CPMMV on seeds were relatively high, ranged between 27 to 86%. Disease incidence on var. ‘Anjasmoro’ from Cianjur, Cirebon, and Bogor varied from 32.9 to 75% and 57.9 to 81.3% in screenhouse and field experiment, respectivelly.

Pea seed-borne mosaic virus (PSbMV) Risk Analysis of Field Pea based on Susceptibility, Yield Loss and Seed Transmission

Pea seed-borne mosaic virus (PSbMV), a non-persistently aphid-transmitted potyvirus, has been reported in field pea (Pisum sativum L.) growing regions worldwide. In 2014, PSbMV was first identified in field peas in North Dakota. Susceptibility and yield losses attributed to PSbMV infection are influenced by viral pathotype and host genotype. Isolate ND14-1, recovered from North Dakota infected seed and identified as pathotype 4 (P4), was mechanically inoculated onto 20 field pea cultivars under greenhouse conditions. PSbMV susceptibility, number of seeds and pods per plant, yield, symptom expression, and PSbMV seed transmission rates were assessed by cultivar. A risk assessment was developed based on cultivar susceptibility, yield reduction, and PSbMV seed transmission. Risk factors were weighted based on perceived importance to commercial field pea producers. Three cultivars were classified as low risk, seven cultivars were classified as intermediate risk and ten cultivars were classified as high risk. Two of the low risk cultivars, Aragorn and Cruiser, were confirmed to be resistant to this isolate of PSbMV. Cultivar Arcadia was susceptible to PSbMV infection with mild expression of symptoms, but classified as low risk based on a low seed transmission rate and diminished yield losses. This risk assessment could prove a useful tool for growers in field pea cultivar selection where PSbMV is prevalent.

Possible Overestimation of Seed Transmission in the Spread of Pospiviroids in Commercial Pepper and Tomato Crops Based on Large-Scale Grow-Out Trials and Systematic Literature Review

Several outbreaks of pospiviroids have been reported in pepper and tomato crops worldwide. Tracing back the origin of the infections has led to different sources. In some cases, the infections were considered to result from seed transmission. Other outbreaks were related to transmission from ornamental crops and weeds. Pospiviroids, in particular potato spindle tuber viroid, are regulated by many countries because they can be harmful to potatoes and tomatoes. Seed transmission has been considered an important pathway of introduction and spread. However, the importance of this pathway can be questioned. This paper presents data on seed transmission from large-scale grow-out trials of infested pepper and tomato seed lots produced under standard seed-industry conditions. In addition, it presents the results of a systematic review of published data on seed transmission and outbreaks in commercial pepper and tomato crops. Based on the results of the grow-out trials and review of the literature, it was concluded that the role of seed transmission in the spread of pospiviroids in practice is possibly overestimated.

Maize Seed Contamination and Seed Transmission of Maize Chlorotic Mottle Virus in Kenya

Maize chlorotic mottle virus (MCMV) causes maize lethal necrosis disease in combination with a cereal infecting potyvirus, leading to high yield losses. There is limited information on seed infection or contamination rate by MCMV and its comparison to transmission rate to maize seedlings. This study was conducted to determine the extent of seed contamination in seed lots from MCMV-infected maize fields in Kenya and the transmission of MCMV from seeds to seedlings. To determine the contamination levels, whole seeds were ground, and the extract tested for the presence of MCMV using double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Seedling grow-outs were tested for seed transmission of MCMV using DAS-ELISA and real-time reverse transcription polymerase chain reaction (real-time RT-PCR) methods. The seed contamination rates of the four seed lots tested ranged from 4.9 to 15.9%. MCMV transmission frequency for 37,617 seedlings, tested in 820 pools of varying seed amounts by DAS-ELISA, was 0.17%, while a transmission frequency of 0.025% was obtained from 8,322 seedlings tested in 242 pools by real-time RT-PCR. Seeds from plants mechanically inoculated with MCMV had an overall seed transmission rate of 0.04% in 7,846 seedlings tested in 197 pools. The study showed that even with substantial contamination of maize seed with MCMV, the transmission of the virus from the seed to seedlings was low. Nevertheless, even low rates of transmission can be significant under field conditions where insect vectors can further spread the disease from infected seedlings, unless diseased plants are detected in time and properly managed.

↵​Urdbean Leaf Crinkle Disease- Assessment of Seed Transmissibility and Its Effect on Yield and Seed Quality in Urdbean [Vigna mungo (L.) Hepper]

Background: Urdbean leaf crinkle disease (ULCD) is one of the most devastating biotic stress factors which is responsible for drastic yield loss in urdbean [Vigna mungo (L.) Hepper]. The etiology of ULCD is still unknown and there are conflicting reports on its transmission through seed, insect vectors, grafting and infected sap. Methods: Per cent seed transmission of ULCD had been studied during kharif, rabi and summer during the year 2019-2020 under natural and insect proof conditions. The effect on yield and seed quality parameters due to ULCD infection was studied. Urdbean cultivar VBN 8 was used in the present study. Result: The per cent seed transmission of ULCD was recorded ranging from 52.88 to 71.64% under natural conditions. In the insect proof glasshouse condition, the seed transmission was 83.62%. All the yield and seed quality parameters were adversely affected upon ULCD infection. The reduction in yield was recorded as 64.36% over the uninfected healthy plants. In ULCD infected seeds, the germination was reduced (84%) as compared to healthy seeds (92%).