Background: Nicotiana tabacum is an important economic crop. Topping, a common agricultural practice employed with flue-cured tobacco, is designed to increase leaf nicotine contents by increasing nicotine biosynthesis in roots. Many genes are found to be differentially expressed in response to topping, particularly genes involved in nicotine biosynthesis, but comprehensive analyses of early transcriptional responses induced by topping are not yet available. To develop a detailed understanding of the mechanisms regulating nicotine biosynthesis after topping, we have sequenced the transcriptomes of Nicotiana tabacum roots at seven time points following topping. Results: Differential expression analysis revealed that 4830 genes responded to topping across all time points. Amongst these, nine gene families involved in nicotine biosynthesis and two gene families involved in nicotine transport showed significant changes during the immediate 24 h period following topping. No obvious preference to the parental species was detected in the differentially expressed genes (DEGs). Significant changes in transcript levels of nine genes involved in nicotine biosynthesis and phytohormone signal transduction were validated by qRT-PCR assays. 549 genes encoding transcription factors (TFs), found to exhibit significant changes in gene expression after topping, formed 15 clusters based on similarities of their transcript level time-course profiles. 336 DEGs involved in phytohormone signal transduction, including genes functionally related to the phytohormones jasmonic acid, abscisic acid, auxin, ethylene, and gibberellin, were identified at the earliest time point after topping. Conclusions: Our research provides the first detailed analysis of the early transcriptional responses to topping in N. tabacum, and identifies excellent candidates for further detailed studies concerning the regulation of nicotine biosynthesis in tobacco roots.

Powdery mildew, caused by fungal pathogen Blumeria graminis f. sp. tritici, is an agronomically important and widespread wheat disease causing severe yield losses. Deployment of broad‐spectrum disease resistance genes is the preferred strategy to prevent this pathogen. Chinese wheat landrace Honghuaxiaomai (HHXM) was resistant to all 23 tested B. graminis f. sp. tritici isolates at the seedling stage. The F₁, F₂, and F_2:3 progenies derived from the cross HHXM × Yangmai 158 were used in this study, and genetic analysis revealed that a single dominant gene, designated PmHHXM, conferred resistance to B. graminis f. sp. tritici isolate E09. Bulked segregant analysis and molecular mapping initially located PmHHXM to the distal region of chromosome 4AL. To fine map PmHHXM, we identified two critical recombinants from 592 F₂ plants and delimited PmHHXM to a 0.18-cM Xkasp475200 to Xhnu552 interval covering 1.77 Mb, in which a number of disease resistance–related gene clusters were annotated. Comparative mapping of this interval revealed a perturbed synteny among Triticeae species. This study reports the new powdery mildew resistance gene PmHHXM, which seems different from three known quantitative trait loci/genes identified on chromosome 4AL and has significant values for further genetic improvement. Analysis of the polymorphisms of 13 cosegregating markers between HHXM and 170 modern wheat cultivars indicates that Xhnu227 and Xsts478700 developed here are ideal for marker-assisted introgression of this locus in wheat breeding.

Powdery mildew, caused by the biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt), is a globally important wheat disease causing severe yield losses, and deployment of resistant varieties is the preferred choice for managing this disease. Chinese wheat landrace Datoumai was resistant to 22 of 23 Bgt isolates at the seedling stage. Genetic analysis based on the inoculation of Bgt isolate E09 on the F₁, F₂, and F_2:3populations derived from the cross Datoumai × Huixianhong revealed that the powdery mildew resistance of Datoumai is controlled by a single dominant gene, temporarily designated as PmDTM. Bulked segregant analysis and simple sequence repeat mapping with 200 F₂plants showed that PmDTM was located in the same genetic region as Pm24 on chromosome 1DS. To fine map PmDTM, 12 critical recombinants were identified from 1,192 F₂plants and delimited PmDTM to a 0.5-cM Xhnu58800 to Xhnu59000 interval covering 180.5 Kb (38,728,125 to 38,908,656 bp) on chromosome 1DS, and only one highly confident gene, TraesCS1D02G058900, was annotated within this region. TraesCS1D02G058900 encodes a receptor-like serine/threonine-protein kinase (STK), and a 6-bp deletion in exon 5 may confer the resistance to powdery mildew. Allele frequency analysis indicated that the STK allele with 6-bp deletion was only present in three landraces (Datoumai, Chiyacao [Pm24], and Hulutou) and was absent in all of the 353 Chinese modern cultivars and 147 foreign cultivars. These results demonstrate that PmDTM is mapped to the same locus as Pm24 and can be widely used to enhance powdery mildew resistance in wheat growing regions worldwide.