Identification of the Citrus Superoxide Dismutase Family and Their Roles in Response to Phytohormones and Citrus Bacterial Canker

Abstract

Superoxide dismutases (SODs) play critical roles in plants, especially in the maintenance of redox homeostasis. The response of SODs in Citrus (Citrus sinensis (L.) Osbeck) to citrus bacterial canker (CBC) infection were investigated. The CsSODs were identified, and their gene structures, phylogeny, conserved domains and motifs, predicted interactions, and chromosomal distribution were analyzed. CsSOD expression in response to stress-related plant hormones (salicylic acid, SA; methyl jasmonate, MeJA; and abscisic acid, ABA) and Xanthomonas citri subsp. citri (Xcc) infection were also investigated. Thirteen CsSODs were identified in C. sinensis, including four Fe/MnSODs and nine Cu/ZnSODs with typical functional domains. The CsSODs were distributed on chromosomes 3, 5, 7, and 8. Specific hormone-response motifs were identified in the gene promoter regions. Ten genes were induced by MeJA treatment, as shown by qRT-PCR, and were upregulated in the CBC-susceptible Wanjincheng citrus variety, while CsSOD06 and CsSOD08 were upregulated by ABA in both the Wanjincheng and the CBC-resistant Kumquat varieties. Xcc infection significantly altered the levels of most CsSODs. The overexpression of CsSOD06 and CsSOD08 resulted in increased hydrogen peroxide levels and SOD activity. Our findings highlight the significance of SOD enzymes in the plant response to pathogen infection and have a potential application for breeding CBC-tolerant citrus varieties.