Identification of a novel screening strategy of rice resistance breeding through phytoalexin content

Bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae poses a significant threat to rice production worldwide. The identification of resistant varieties is crucial for the development of rice varieties that can withstand this disease. Currently, the identification of resistant varieties primarily relies on resistance genes. However, due to vertical resistance of resistance genes, varieties identified based on this criterion still encounter significant challenges. In this study, we collected eight major cultivated rice varieties in Jiangsu Province, China, along with two experimental varieties, ZH11 and NP. Employing Liquid Chromatograph-Mass Spectrometer (LC-MS), we quantified the accumulation levels of 5,10-diketo-casbene (DKC), a casbene-type diterpene phytoalexin, in these ten rice varieties following treatment with methyl jasmonate (MeJA). These results demonstrated that despite the exogenous application of MeJA, which is known to trigger the biosynthesis of DKC in both shoots and roots, the accumulation levels of this crucial diterpene phytoalexin varied significantly among the ten cultivars. In addition to assessing DKC levels, we also evaluated the resistance of the ten varieties to bacterial leaf blight. Our analysis revealed a positive correlation between DKC content and resistance to this devastating pathogen. To further investigate the molecular basis of this resistance, we selected NG9108 and MN1332 as representative varieties with the higher and weaker resistance to bacterial leaf blight, respectively, and then examined their transcriptional responses to MeJA treatment. By comparing the transcriptional profiles of these two contrasting varieties, we found that genes involved in the synthesis of other diterpene phytoalexins, such as CPS2 and CPS4, were markedly up-regulated in the NG9108 variety as compared to MN1332. The contrasting resistance to bacterial leaf blight between NG9108 and MN1332 can be attributed, at least in part, to their disparate capabilities in diterpene phytoalexin synthesis. In conclusion, our research not only highlighted the importance of phytoalexin synthesis in rice resistance but also offers a practical framework for utilizing phytoalexin content as a criterion in the screening process for rice resistance breeding in the face of escalating disease challenges.