Citron C-05 inhibits both the penetration and colonization of Xanthomonas citri subsp. citri to achieve resistance to citrus canker disease

Abstract

Citrus canker, caused by <i>Xanthomonas citri</i> subsp. <i>citri</i> (<i>Xcc</i>), is a serious bacterial disease that affects citrus production worldwide. Citron C-05 (<i>Citrus medica</i>) is the only germplasm in the <i>Citrus</i> genus that has been identified to exhibit strong resistance to <i>Xcc</i>. However, it has not been determined when, where, and how <i>Xcc</i> is restricted in the tissues of Citron C-05 during the infection process. In the present study, we investigated the spatiotemporal growth dynamics of an <i>eGFP</i>-labeled virulent <i>Xcc</i> (<i>eGFP-Xcc</i>) strain in Citron C-05 along with five susceptible biotypes (i.e., lemon, pummelo, sour orange, sweet orange, and ponkan mandarin) upon inoculation via the spraying or leaf infiltration of a bacterial suspension. The results from extensive confocal laser scanning microscopy analyses showed that while <i>Xcc</i> grew rapidly in plants of all five susceptible genotypes, <i>Xcc</i> was severely restricted in the epidermal and mesophyll cell layers of the leaves of Citron C-05 in the early stage of infection. Not surprisingly, resistance against <i>Xcc</i> in Citron C-05 was found to be associated with the production of reactive oxygen species and hypersensitive response-like cell death, as well as greater upregulation of several defense-related genes, including a pathogenesis-related gene (<i>PR1</i>) and a glutathione S-transferase gene (<i>GST1</i>), compared with sweet orange as a susceptible control. Taken together, our results not only provide further valuable details of the spatiotemporal dynamics of the host entry, propagation, and spread of <i>Xcc</i> in both resistant and susceptible citrus plants but also suggest that resistance to <i>Xcc</i> in Citron C-05 may be attributed to the activation of multiple defense mechanisms.