| Summary: | To characterize cultivar variation in resistance gene (R-gene)-mediated calcium signaling and hormonal regulation in effector-triggered immunity (ETI) and disease susceptibility, <i>Xanthomonas campestris</i> pv. <i>campestris</i> (<i>Xcc</i>) was inoculated in two <i>Brassica napus</i> cultivars (cvs. Capitol and Mosa). At 14 days post inoculation (DPI) with <i>Xcc</i>, there was a necrotic lesion in cv. Mosa along with the significant accumulation of H<sub>2</sub>O<sub>2</sub> and malondialdehyde (MDA), whereas no visual symptom was observed in cv. Capitol. The cultivar variations in the R-gene expressions were found in response to <i>Xcc</i>. <i>ZAR1</i> is a coiled-coil-nucleotide binding site-leucine-rich repeat (CC-NB-LRR)-type R-gene that is significantly induced in cv. Capitol, whereas toll/interleukin-1 receptor-nucleotide binding site-leucine-rich repeat (TIR-NB-LRR)-type R-gene, <i>TAO1</i>, is significantly upregulated in cv. Mosa <i>Xcc</i>-inoculated plants. The defense-related gene’s non-race-specific disease resistance 1 (<i>NDR1</i>) and mitogen-activated protein kinase 6 (<i>MAPK6</i>) were enhanced, whereas calcium-dependent protein kinase (<i>CDPK5</i>) and calcium-sensing protein 60g (<i>CBP60g)</i> were depressed in cv. Capitol <i>Xcc</i> inoculated plants, and opposite results were found in cv. Mosa. The calcium-sensing receptor (<i>CAS</i>), calmodulin (<i>CaM</i>), expression was induced in both the cultivars. However, the <i>CAS</i> induction rate was much higher in cv. Mosa than in cv. Capitol in response to <i>Xcc</i>. The phytohormone salicylic acid (SA) and jasmonic acid (JA) levels were significantly higher in cv. Capitol along with the enhanced SA receptors (<i>NPR3 </i>and <i>NPR4</i>) and JA synthesis and signaling-related gene expression (<i>LOX2, PDF1.2</i>), whereas the JA level was significantly lower in cv. Mosa <i>Xcc</i> inoculated plants. The SA synthesis and signaling-related genes (<i>ICS1, NPR1</i>) and SA were present at higher levels in cv. Mosa; additionally, the SA level present was much higher in the susceptible cultivar (cv. Mosa) than in the resistant cultivar (cv. Capitol) in response to <i>Xcc</i>. These results indicate that <i>ZAR1</i> mediated the coordinated action of SA and JA synthesis and signaling to confirm ETI, whereas <i>TAO1</i> enhanced the synthesis of SA through <i>CAS</i> and <i>CBP60g</i> to antagonize JA synthesis and signaling to cause disease susceptibility in the <i>Brassica napus</i>−<i>Xcc </i>pathosystem.
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