Unlocking a Mystery: Characterizing the First Appearance of Clavibacter nebraskensis in Mexican Cornfields

The Goss's wilt and leaf blight is a disease of maize (Zea mays) caused by Clavibacter nebraskensis, which was widespread in the last several years throughout the Midwest in the United States, south in Texas, and north to Canada. The bacterium is included within the high-risk list of quarantine pathogens by many plant protection organizations and countries including Mexico. Severe blight symptoms on maize plants were found in different provinces from Coahuila and Tlaxcala, Mexico, in 2012 and 2021, respectively. Twenty bacterial isolates with morphology similar to C. nebraskensis were obtained from the diseased maize leaves. The isolates were confirmed by phenotypic tests and 16S rRNA and gyrB sequencing. Two strains were tested for pathogenicity tests on seven hybrid sweet corn cultivars available in Mexico, and the most sensitive cultivar was tested for all the strains to fulfill Koch's postulates. The phylogenetic reconstruction based on two single loci reveals a remarkable clustering of Mexican strains to American strains reported approximately 50 years ago. The presence of this pathogen represents a risk and a significant challenge for plant protection strategies in Mexico and maize diversity.

Goss's Wilt Resistance in Corn Is Mediated via Salicylic Acid and Programmed Cell Death but Not Jasmonic Acid Pathways

A highly aggressive strain (CMN14-5-1) of Clavibacter nebraskensis bacteria, which causes Goss's wilt in corn, induced severe symptoms in a susceptible corn line (CO447), resulting in water-soaked lesions followed by necrosis within a few days. A tolerant line (CO450) inoculated with the same strain exhibited only mild symptoms such as chlorosis, freckling, and necrosis that did not progress after the first six days following infection. Both lesion length and disease severity were measured using the area under the disease progression curve (AUDPC), and significant differences were found between treatments. We analyzed the expression of key genes related to plant defense in both corn lines challenged with the CMN14-5-1 strain. Allene oxide synthase (ZmAOS), a gene responsible for the production of jasmonic acid (JA), was induced in the CO447 line in response to CMN14-5-1. Following inoculation with CMN14-5-1, the CO450 line demonstrated a higher expression of salicylic acid (SA)-related genes, ZmPAL and ZmPR-1, compared to the CO447 line. In the CO450 line, four genes related to programmed cell death (PCD) were upregulated: respiratory burst oxidase homolog protein D (ZmrbohD), polyphenol oxidase (ZmPPO1), ras-related protein 7 (ZmRab7), and peptidyl-prolyl cis-trans isomerase (ZmPPI). The differential gene expression in response to CMN14-5-1 between the two corn lines provided an indication that SA and PCD are involved in the regulation of corn defense responses against Goss's wilt disease, whereas JA may be contributing to disease susceptibility.

Suppression of the maize phytoglobin ZmPgb1.1 promotes plant tolerance against Clavibacter nebraskensis

Suppression of the maize phytoglobin ZmPgb1.1 enhances tolerance against Clavibacter nebraskensis by promoting hypersensitive response mechanisms mediated by ethylene and reactive oxygen species. Suppression of the maize phytoglobin, ZmPgb1.1, reduced lesion size and disease severity in leaves following inoculation with Clavibacter nebraskensis, the causal agent of Goss's bacterial wilt disease of corn. These effects were associated with an increase of the transcriptional levels of ethylene biosynthetic and responsive genes, which resulted in the accumulation of reactive oxygen species (ROS) and TUNEL-positive nuclei in the proximity of the inoculation site. An in vitro system, in which maize cells were treated with induced xylem sap, was employed to define the cause-effect relationship of these events. Phytoglobins (Pgbs) are hemoglobins able to scavenge nitric oxide (NO). Suppression of ZmPgb1.1 elevated the level of NO in cells exposed to the induced xylem sap causing a rise in the transcript levels of ethylene biosynthesis and response genes, as well as ethylene. Accumulation of ethylene in the same cells was sufficient to elevate the amount of reactive oxygen species (ROS), through the activation of the respiratory burst oxidase homologs (Rboh) genes, and trigger programmed cell death (PCD). The sequence of these events was demonstrated by manipulating the content of NO and ethylene in culture through pharmacological treatments. Collectively, our results illustrated that suppression of ZmPgb1.1 evokes tolerance against C. nebraskensis culminating in the execution of PCD, a key step of the hypersensitive response.

Re-classification of Clavibacter michiganensis subspecies on the basis of whole-genome and multi-locus sequence analyses

Although the genus Clavibacter was originally proposed to accommodate all phytopathogenic coryneform bacteria containing B2γ diaminobutyrate in the peptidoglycan, reclassification of all but one species into other genera has resulted in the current monospecific status of the genus. The single species in the genus, Clavibacter michiganensis, has multiple subspecies, which are all highly host-specific plant pathogens. Whole genome analysis based on average nucleotide identity and digital DNA-DNA hybridization as well as multi-locus sequence analysis (MLSA) of seven housekeeping genes support raising each of the C. michiganensis subspecies to species status. On the basis of whole genome and MLSA data, we propose the establishment of two new species and three new combinations: Clavibacter capsici sp. nov., comb. nov. and Clavibacter tessellarius sp. nov., comb. nov., and Clavibacter insidiosus comb. nov., Clavibacter nebraskensis comb. nov. and Clavibacter sepedonicus comb. nov.