Phenotypic, Antioxidant and Physiological response of Citrus japonica (kumquat) leaves exposed to Xanthomonas citri subsp. citri

OLIVELLA, L.; BUSTOS, D.; ALISIO, M.; REYES, M.; CECCOLI, G.; DERITA, M.; DAURELIO, L.

Congreso; 4th Edition of International Conference Plant Productivity and Food Safety; 2024

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is a bacterial disease that affects all citrus species. However, susceptibility to canker varies among different citrus types and their relatives. Almost of citrus commercial cultivars are susceptible hosts or exhibit moderate resistance, while kumquats (C. japonica Thunb) demonstrate significant resistance. This study aimed to investigate the phenotypic, antioxidant, and physiological responses of kumquat leaves exposed to citrus canker during the early stages of infection. In phenotipic assays, C. japonica leaves 1 months old were inoculated by pricking and immediately rubbing the wounds with a suspension of Xcc (1x10 8 CFU/mL), a mutant strain of X. citri subsp. citri in secretion system type III (Hrp-, basal response), X. campestris pv. vesicatoria (Xcv, non-host response), and sterile demineralized water (control) using a cotton swab. All treatments were applied to different areas of the same leaf, which were collected at 8, 24, 48, 72, and 144 hours post-infection (hpi). The inoculated regions were examined using a LEICA model EZ4E stereoscopic microscope. The leaves from four different plants analyzed for each treatment at each tpi (n=4). To assess the antioxidant response, we evaluated lipid peroxidation (MDA) levels, ferric reducing antioxidant power (FRAP), and pigment content. The complete leaves of four plants were pressure infiltrated on the abaxial side using a syringe without a needle for each strain as the before mentioned, and collected at 8, 24, and 48 hpi (n=4). The results showed that kumquat leaves did not exhibit canker development at any tested time point. Yellowish-brown necrotic zones around the puncture sites may indicate apoptosis induced by a hypersensitive reaction. The accumulation of MDA in kumquats inoculated with different bacterial strains remained constant at 8 and 24 hpi, with no significant differences observed. However, at 48 hpi, a significant increase (mean 0.05, p=0.009) in control leaves was noted, contrasting with a decrease in MDA levels in bacterial strain-inoculated leaves. For FRAP measurements no significant differences were observed in any of the treatments. Additionally, the content of photosynthetic pigments remained stable at 8 and 24 hpi but dropped significantly at 48 and 72 hpi. Specifically, chlorophyll a and carotenoid levels decreased markedly in the Xcv reatment compared to the control. These findings suggested that C. japonica displays distinct response to Xcc at phenotypic and physiological level, between basal and non-host responses, at early (8 hours) and relatively later (24-48 hours) time post treatment.