Genome analysis and hyphal movement characterization of the hitchhiker endohyphal Enterobacter sp. from Rhizoctonia solani.

Bacterial-fungal interactions are pervasive in the rhizosphere. While an increasing number of endohyphal bacteria have been identified, little is known about their ecology and impact on the associated fungal hosts and the surrounding environment. In this study, we characterized the genome of an Enterobacter sp. Crenshaw (En-Cren), which was isolated from the generalist fungal pathogen Rhizoctonia solani, and examined the genetic potential of the bacterium with regard to the phenotypic traits associated with the fungus. Overall, the En-Cren genome size was typical for members of the genus and was capable of free-living growth. The genome was 4.6 MB in size, and no plasmids were detected. Several prophage regions and genomic islands were identified that harbor unique genes in comparison with phylogenetically closely related Enterobacter spp. Type VI secretion system and cyanate assimilation genes were identified from the bacterium, while some common heavy metal resistance genes were absent. En-Cren contains the key genes for indole-3-acetic acid (IAA) and phenylacetic acid (PAA) biosynthesis, and produces IAA and PAA in vitro, which may impact the ecology or pathogenicity of the fungal pathogen in vivo. En-Cren was observed to move along hyphae of R. solani and on other basidiomycetes and ascomycetes in culture. The bacterial flagellum is essential for hyphal movement, while other pathways and genes may also be involved.IMPORTANCEThe genome characterization and comparative genomics analysis of Enterobacter sp. Crenshaw provided the foundation and resources for a better understanding of the ecology and evolution of this endohyphal bacteria in the rhizosphere. The ability to produce indole-3-acetic acid and phenylacetic acid may provide new angles to study the impact of phytohormones during the plant-pathogen interactions. The hitchhiking behavior of the bacterium on a diverse group of fungi, while inhibiting the growth of some others, revealed new areas of bacterial-fungal signaling and interaction, which have yet to be explored.

Molecular characterization of a divergent genetic variant of wheat Eqlid mosaic virus from a Texas wheat field.

Investigations conducted during the spring 2020 season to diagnose the associated viral agent of a severe mosaic disease of wheat in a Texas Panhandle field revealed the presence of wheat Eqlid mosaic virus (WEqMV; genus Tritimovirus, family Potyviridae) in the analyzed samples. The complete genome sequences of two WEqMV isolates were determined, and each was found to be 9,634 nucleotides (nt) in length (excluding the polyA tail) and to contain 5' and 3' untranslated regions of 135 nt and 169 nt, respectively, based on rapid amplification of cDNA ends (RACE) assays. Both sequences contained an open reading frame (ORF) of 9,330 nt encoding a polyprotein of 3,109 amino acids (aa). The ORF sequences of the two isolates were 100% identical to each other, but only 74.7% identical to that of the exemplar WEqMV-Iran isolate, with 85.7% aa sequence identity in the encoded polyprotein. The Texas WEqMV isolates also diverged significantly from WEqMV-Iran in the individual proteins at the nt and aa levels. This is the first report of WEqMV in the United States and the first report of this virus outside of Iran, indicating an expansion of its geographical range.

Two New Bacterial Pathogens of Peanut, Causing Early Seedling Decline Disease, Identified in the Texas Panhandle.

Peanut (Arachis hypogaea L.) is cultivated in tropical and subtropical regions of the world as an important source of oil and protein. Until now, bacterial wilt, caused by Ralstonia solanacearum, was the only known bacterial disease of peanut. In 2020, widespread incidence of poor stand establishment was observed in multiple production fields planted to the Spanish-type peanut varieties in the Texas Panhandle. The observed symptoms included seed rot, pre- and postemergence damping-off, poor seedling vigor, poorly developed root systems with little or no nodule formation, and death. Subsequent diagnosis of symptomatic seedlings recovered two bacterial species identified by BLAST using 676- and 661-bp 16S rRNA fragments as a Ralstonia sp. and a Pantoea sp., respectively. To investigate a possible causative role of these bacteria in the observed peanut disease, the pathogenicity of the two isolates was evaluated under greenhouse conditions relying on Koch's postulates. Cell suspensions of the two bacteria, separately and in combination, were used to inoculate seeds of a Valencia-type peanut variety with no history of the disease and found to be pathogenic on the resultant seedling plants. Symptoms that developed on the inoculated plants were similar to the symptoms initially observed in the field, including seed rot, pre- and postemergence damping-off, poor seedling vigor, and root establishment. The two bacteria were also successfully recovered from inoculated and symptomatic plants, thus satisfying Koch's postulates. Given the early onset of symptom development on affected seeds and seedlings, a seedborne origin of the disease, described here as early-decline bacterial disease of peanut, was investigated in the same batches of peanut seeds that were planted, as well as seeds later harvested in some of the affected fields. Identical bacterial species, on the basis of 16S rRNA identity, were recovered from all of the seeds evaluated indicating that the bacteria are both seedborne and seed-transmissible. Multilocus sequence analysis involving six genes (dnaK, fumC, gyrB, murG, trpB, and tuf) showed that these new strains are most closely related to R. pickettii and Pantoea dispersa, but also phylogenetically distinct. The two bacteria were designated Ralstonia sp. strain B265 and Pantoea sp. strain B270. Losses from the disease in affected fields in 2020 averaged 50% (US$1.12 million) from a total of nine production fields. Findings from this study provide evidence for two new bacterial pathogens of peanuts capable of infecting Spanish and Valencia peanut varieties.

The broadly effective recessive resistance gene xa5 of rice is a virulence effector-dependent quantitative trait for bacterial blight.

Mutations in disease susceptibility (S) genes, here referred to as recessive resistance genes, have promise for providing broad durable resistance in crop species. However, few recessive disease resistance genes have been characterized. Here, we show that the broadly effective resistance gene xa5,for resistance to bacterial blight of rice (Oryza sativa), is dependent on the effector genes present in the pathogen. Specifically, the effectiveness of xa5 in preventing disease by strains of Xanthomonas oryzae pv. oryzae is dependent on major transcription activation-like (TAL) effector genes, and correlates with reduced expression of the cognate S genes. xa5 is ineffective in preventing disease by strains containing the TAL effector gene pthXo1, which directs robust expression of the S gene OsSWEET11, a member of sucrose transporter gene family. Incompatibility is associated with major TAL effectors that target the known alternative S genes OsSWEET14 and OsSWEET13. Incompatibility is defeated by transfer of pthXo1 to otherwise xa5-incompatible strains or by engineering a synthetic designer TAL effector to boost SWEET gene expression. In either case, compatible or incompatible, target gene expression and lesion formation are reduced in the presence of xa5. The results indicate that xa5 functions as a quantitative trait locus, dampening effector function, and, regardless of compatibility, target gene expression. Resistance is hypothesized to occur when S gene expression, and, by inference, sucrose leakage, falls below a threshold level.

Effect of Cultivation and Timing of Nitrogen Fertilization on Large Patch Disease of Zoysiagrass.

Large patch of zoysiagrass (Zoysia spp.) is caused by Rhizoctonia solani anastomosis group 2-2 LP. The effects of summer cultivation (core-aerification, verticutting, and sand topdressing) and spring and fall versus summer nitrogen (N) fertilization on large patch in fairway height 'Meyer' zoysiagrass were investigated from 2008 to 2011 in Manhattan, Haysville, and Olathe, KS. Disease was assessed by measuring patch diameters or analyzing digital images of affected plot areas to determine the percentage of non-green turfgrass within patches. Cultivation did not affect thatch temperature, soil temperature, soil water content, or turf recovery from large patch in early summer. Furthermore, cultivation did not result in overall significant reductions in patch diameters or average weekly rate of patch diameter increase among plots at the three experimental locations. In some site-year combinations, spring and fall N fertility was associated with lower percentages of non-green turf within affected plot areas in Manhattan and Haysville. In some cases, applications of N during spring and fall may alleviate large patch symptoms.