Cloning and identification of <i>CmCC</i>-<i>NB</i>-<i>ARC</i>,<italic/> a chrysanthemum white rust resistance gene

Abstract

Chrysanthemums are important perennial commercial and ornamental flowers worldwide. Chrysanthemum white rust caused by the fungus <italic>Puccinia horiana</italic> is a destructive disease. Recent studies have aimed to identify sources of resistance to this disease, whereas genes related to the resistance to the fungus were rarely reported. In this study, we used highly resistant and susceptible chrysanthemum cultivars whose disease sensitivity was confirmed over years of observation. Using a conserved nucleotide binding site (NBS) domain sequence that is associated with resistance to rust in other plants as a query in a BLAST search against the chrysanthemum reference database, we identified the gene <italic>CHR00059759</italic>. There were eight consistent nonsynonymous mutations between the highly resistant and susceptible cultivars, five of which occurred in the NB-ARC conserved domain. The expression level of <italic>CmCC</italic>-<italic>NB</italic>-<italic>ARC</italic> increased and then decreased in the susceptible cultivar 'LZ08-61', whereas decreased and then increased in the resistant cultivar 'C029' in the eight time-points after inoculation with the fungus. In order to verify the gene function, we constructed an overexpression vector <italic>35Spro:CmCC</italic>-<italic>NB</italic>-<italic>ARC</italic> and transformed into the susceptible cultivar 'Jinba'. Compared with 'Jinba', the relative expression levels of the gene in the transgenic plants increased significantly at 48h after inoculation. Compared with three overexpression lines, the diseased spots appeared in 'Jinba' at 15d after inoculation. Hence, we speculated the gene <italic>CmCC</italic>-<italic>NB</italic>-<italic>ARC</italic> conferred resistance to chrysanthemum white rust. This study provides insight into the mechanism of resistance to chrysanthemum white rust and might help inform the breeding of resistant cultivars.