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

Virulence is not directly related to strain success in planta in Clavibacter nebraskensis

Goss's wilt and leaf blight of maize is an economically important disease caused by the Gram-positive bacterium, Clavibacter nebraskensis (Cn). Little is known about the ecology and pathogenesis of this bacterium. Here, we used phenotypic assays and a high-throughput whole-genome sequencing approach to explore among-strain variation in virulence and multistrain reproductive success in planta. Our survey of 41 strains revealed that more recently sampled strains tended to have higher virulence than strains sampled before 2010 and tended to be more genetically divergent from the reference strain, isolated in 1971. More detailed assays with a representative sample of 13 of these strains revealed that host genotype (resistant or susceptible) did not strongly affect strain success and that strain success in planta in multi-strain communities was not closely associated with virulence in single-strain assays. Two weakly virulent strains, CIC354 and CIC370, had the greatest reproductive success, whereas the most highly virulent strains did not significantly change in frequency in any host genotype. A genomic analysis revealed candidate genes, including putative virulence factors (i.e., a secreted cellulase), responsible for among-strain variation in reproductive success.IMPORTANCENon-pathogenic strains of many bacterial pathogens are reported to coexist with pathogenic strains in symptomatic plants. To understand the ecology and pathogenesis of the pathogen population, it is essential to study strain dynamics in the context of the host. We created a community of 13 strains exhibiting diverse virulence phenotypes and used this community to infect the host plant. We compared the strain frequency of these strains before and after the host infection. Contrary to our hypothesis of highly virulent strains being selected by the susceptible host, we found that weakly virulent strains were selected by both resistant and susceptible host lines. We identified several genes associated with strain frequency shifts suggesting their role in strain colonization, virulence, and fitness.

The HD-Zip I transcription factor MdHB-7 negatively regulates resistance to Glomerella leaf spot in apple

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola (Cf), has been one of the main fungal diseases afflicting apple-producing areas across the world for many years, and it has led to substantial reductions in apple output and quality. HD-Zip transcription factors have been identified in several species, and they are involved in the immune response of plants to various types of biotic stress. In this study, inoculation of MdHB-7 overexpressing (MdHB-7-OE) and interference (MdHB-7-RNAi) transgenic plants with Cf revealed that MdHB-7, which encodes an HD-Zip transcription factor, adversely affects GLS resistance. The SA content and the expression of SA pathway-related genes were lower in MdHB-7-OE plants than in 'GL-3' plants; the content of ABA and the expression of ABA biosynthesis genes were higher in MdHB-7-OE plants than in 'GL-3' plants. Further analysis indicated that the content of phenolics and chitinase and β-1, 3 glucanase activities were lower and H2O2 accumulation was higher in MdHB-7-OE plants than in 'GL-3' plants. The opposite patterns were observed in MdHB-7-RNAi apple plants. Overall, our results indicate that MdHB-7 plays a negative role in regulating defense against GLS in apple, which is likely achieved by altering the content of SA, ABA, polyphenols, the activities of defense-related enzymes, and the content of H2O2.

Identification of loci conferring resistance to 4 foliar diseases of maize

Foliar diseases of maize are among the most important diseases of maize worldwide. This study focused on 4 major foliar diseases of maize: Goss's wilt, gray leaf spot, northern corn leaf blight, and southern corn leaf blight. QTL mapping for resistance to Goss's wilt was conducted in 4 disease resistance introgression line populations with Oh7B as the common recurrent parent and Ki3, NC262, NC304, and NC344 as recurrent donor parents. Mapping results for Goss's wilt resistance were combined with previous studies for gray leaf spot, northern corn leaf blight, and southern corn leaf blight resistance in the same 4 populations. We conducted (1) individual linkage mapping analysis to identify QTL specific to each disease and population; (2) Mahalanobis distance analysis to identify putative multiple disease resistance regions for each population; and 3) joint linkage mapping to identify QTL across the 4 populations for each disease. We identified 3 lines that were resistant to all 4 diseases. We mapped 13 Goss's wilt QTLs in the individual populations and an additional 6 using joint linkage mapping. All Goss's wilt QTL had small effects, confirming that resistance to Goss's wilt is highly quantitative. We report several potentially important chromosomal bins associated with multiple disease resistance including 1.02, 1.03, 3.04, 4.06, 4.08, and 9.03. Together, these findings indicate that disease QTL distribution is not random and that there are locations in the genome that confer resistance to multiple diseases. Furthermore, resistance to bacterial and fungal diseases is not entirely distinct, and we identified lines resistant to both fungi and bacteria, as well as loci that confer resistance to both bacterial and fungal diseases.