Surface Barriers of Mandarin ‘Okitsu’ Leaves Make a Major Contribution to Canker Disease Resistance

Abstract

Field evaluations have shown that Satsuma mandarin (Citrus unshiu) ‘Okitsu’ is one of the mandarin cultivars that shows substantial resistance to Xanthomonas citri subsp. citri (X. citri), the causal agent of citrus bacterial canker disease. However, the mechanisms underlying this resistance are not well understood. In this study, we have shown that ‘Okitsu’ leaves are nevertheless susceptible to X. citri infection during a period of their development; however, this period is shorter than that seen in the susceptible mandarin ‘Clemenules’ (C. clementina). Under controlled growth conditions, the resistance of ‘Okitsu’ to X. citri was associated with the age of the leaf and was evident in spray-inoculated plants but not in those inoculated by infiltration. Furthermore, X. citri showed reduced attachment and biofilm formation in ‘Okitsu’ leaves compared with ‘Clemenules’. Taken together, our data suggest that structural features of the ‘Okitsu’ leaf surface, such as the physical properties of the cuticle, are involved in the resistance to X. citri.

Resistance to citrus canker induced by a variant of Xanthomonas citri ssp. citri is associated with a hypersensitive cell death response involving autophagy‐associated vacuolar processes

“…Proper biofilm formation is a requirement for the achievement of maximal X. citri virulence (Malamud et al ., ), and the ability of canker‐resistant Citrus spp. to interfere with this process has been reported in ‘Okitsu’ mandarin (Favaro et al ., ). In this work, we examined whether the impaired ability of X. citri A T to cause disease in C. limon was associated with its inability to develop biofilms.…”

“…a). These are similar structures to those reported previously for X. citri biofilms formed on the susceptible genotypes C. limon and C. clementina (Favaro et al ., ; Rigano et al ., ). Moreover, inoculation of both strains onto C. clementina leaves led to the development of cankerous lesions at 20 dpi (data not shown).…”

“…New shoots of approximately 1 cm in length, with at least five leaves, were selected for pathogenicity assays after pruning the plants. All the leaves on a new shoot were considered to be of the same ontological age (Favaro et al ., ).…”

“…Biofilm formation in vivo was examined using GFP‐tagged X. citri strains and an inverted confocal laser scanning microscope as described previously (Favaro et al ., ). Simulated three‐dimensional images and sections were generated by the software Nikon EZ‐C1 3.9 Free Viewer (Nikon Instruments Inc., Melville, NY, USA).…”

“…citri is a hemibiotrophic pathogen that grows and persists as epiphytes, forming biofilms on the host surface prior to endophytic colonization of the intercellular spaces of the mesophyll tissue through natural openings, such as stomata, or through wounds (Rigano et al ., ). A balance between biofilm formation and bacterial dispersion is essential to enhance the epiphytic persistence of bacteria prior to colonization and to circumvent the plant defence response (Favaro et al ., ; Vojnov and Marano, ).…”