## 2 Results  
### Phenotypic Investigation of Resistance and Susceptibility to Petiole Spot in Chinese Cabbage  
Two Chinese cabbage varieties, Han Chunwa C8-1 and 4-2-3-4, known for their extreme characteristics of petiole spots, were employed as research materials. The variety of Han Chunwa C8-1(R) is deemed highly resistant due to few spots being observed on the petiole. Meanwhile, the variety of Han Chunwa 4-2-3-4 (S) exhibited a susceptible phenotype, which has many spots of inconsistent size on the petiole (Figure 1A). Six spots with different sizes (I-VI) were selected from the susceptible variety for microscope observation. The findings revealed that the spots are indeed brown and initially gather in the intercellular or extracellular regions. Subsequently, they gradually extend into intracellular areas, and finally, they affect multiple cells as the size of the spots increases (Figure 1B). Additionally, cells without petiole spots (CK) and those with petiole spots (petiole spot) on Chinese cabbage were examined using an electron microscope, and it was found that petiole spot showed significant cellular changes. These alterations involve the distortion of the cell wall, the presence of multiple vesicles, and the accumulation of black particulate matter, as compared to that which is observed in normal cells (Figure 2A). These distinctive features seem to correspond with the typical characteristics related to cell death in plants. Trypan blue staining was used to validate the hypothesis. The result showed that the cells within the petiole spot as well as their neighboring cells were observed to be blue in color, whereas the normal cells were not stained, verifying that cell death occurred within and around the areas with spots (Figure 2B). Transcriptome and Proteome Profiling of Resistant and Susceptible Chinese Cabbage Lines upon Nitrogen Treatment  
Previous research has indicated that petiole spot on Chinese cabbage worsened with high levels of nitrogen treatment. To investigate the mechanisms of the high nitrogen levels involved in petiole spot, we conducted transcriptomic and proteomic analyses on resistant and susceptible Chinese cabbage lines upon nitrogen treatment. Samples treated with ddH\({}_{2}\)O for the resistant variety are named as CKA, and ddH\({}_{2}\)O-treated samples for the susceptible variety are named as CKB. Similarly, samples treated with 10 mM NH\({}_{4}\)NO\({}_{3}\) for the resistant variety are named as TA, and those for the susceptible variety are named as TB. A total of 83.06 Gb of clean data were obtained from 12 transcriptome samples, with each sample's clean data amounting to 6.00 Gb. The percentage of Q30 bases was 94.05% or higher. FPKM values were utilized to standardize the expression in each sample and examine the gene expression pattern further. Pairwise comparisons of each treatment group revealed 177, 150, 1485, and 1234 up-regulated genes and 236, 95, 1023, and 1264 down-regulated genes in the CKA vs. TA, CKB vs. TB, CKA vs. CKB, and TA vs. TB comparisons,  
Figure 1: Characteristic of petiole spots. (**A**) Phenotype of petiole spot in Han ChunwaC8-1 (R) and Han Chunwa 4-2-3-4 (S). Han Chunwa C8-1 is resistant to petiole spots, and Han Chunwa 4-2-3-4 (S) is susceptible to petiole spots. (**B**) Microscope image of petiole spots of various sizes on Chinese cabbage (I–VI). respectively (Figure 3A). Most of the differentially expressed genes were identified in CKA vs. CKB and TA vs. TB, with only a small number observed in CKA vs. TA and CKB vs. TB comparisons (Figure 3B). Additionally, proteomic assays were conducted to illuminate the mechanisms associated with elevated nitrogen levels contributing to the development of petiole spot. A total of 120,752 peptides were identified, with 91,781 being specific peptides. Specifically, 7718 quantifiable proteins were detected.