Comparative Genomic Analysis of Hormone-Associated 2OGD Genes in Citrus and Expression Profiling of CreACO Paralogs Under Phytohormone Treatments

The 2-oxoglutarate-dependent dioxygenase (2OGD) superfamily includes nine clades associated with hormone biosynthesis and metabolism and contributes to plant growth and responses to abiotic stress. In this study, we used bioinformatic approaches to identify 2OGD genes in nine citrus and related species (Atalantia buxifolia, Clausena lansium, Citrus medica, Citrus trifoliata, Citrus hindsii, Citrus ichangensis, Citrus maxima, Citrus reticulata, and Citrus sinensis). Segmental duplication accounted for most analyzed DOXC duplication events in C. reticulata (wild mandarin). Among these species, 36 DOXC genes were assigned to nine conserved clades associated with hormone biosynthesis and metabolism. To investigate the ACO members involved in ethylene biosynthesis, we analyzed their expression in different tissues and different fruit developments, as well as their responses to exogenous abscisic acid (ABA), methyl jasmonate (MeJA), and ethephon treatments. CreACO1 and CreACO5 showed preferential expression in roots, whereas CreACO4 expression was highest in stems and lowest in roots. CreACO4a showed no significant differences in expression among roots, stems, and leaves. In fruit, CreACO1 and CreACO4a expression was higher at 180 than at 120 days, whereas CreACO4 and CreACO5 showed the opposite pattern. The four genes showed distinct responses to hormone treatments, with CreACO5 transcript levels approximately 60 times the control level after 12 h of ethephon treatment. Together, these results reveal the transcriptional responsiveness of CreACO genes to exogenous hormone treatments and provide a basis for subsequent functional characterization, whereas the functional involvement of CreACO genes in hormone homeostasis and hormone-mediated physiological responses remains to be validated by further experiments.