Genome-wide analysis of AP2/ERF gene family in Camellia oleifera and its potential roles in ethylene-induced fruit abscission

Camellia oleifera is a commercially important woody edible oil tree species in China. Yet, heavy premature fruit drop causes substantial yield losses and significantly limits the economic potential of the oil tea industry. Ethylene is well recognized as a promoter of organ abscission in plant, and the APETALA2/Ethylene Responsive Factor (AP2/ERF) family plays critical roles in response to ethylene. However, the AP2/ERF gene family in Camellia oleifera remains largely unexplored. Enhanced fruit abscission in Camellia oleifera was associated with increased accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, in fruit pedicels. Compared with the high-yield Camellia oleifera cultivar ‘1712’, the low-yield cultivar ‘143’ exhibited significantly higher fruit abscission rates (e.g., 9.4% vs. 17.2% in August) and was more sensitive to ethylene. Ethephon treatment increased the cumulative fruit drop rates of cultivar ‘143’ to 14.1% at 3 days, while that of cultivar ‘1712’ was only 3.4%. Microscopic observations of the pedicel abscission zone revealed a reduction in lignified cells following abscission-promoting treatments in Camellia oleifera. Genome-wide analysis identified 143 CoAP2/ERF genes distributed across 15 chromosomes, which were classified into 16 subfamilies with 57 duplicated gene pairs. Transcriptomic data revealed tissue-specific expression patterns of CoAP2/ERF family. Among these genes, 79 CoAP2/ERFs were significantly differentially expressed in responses to ethephon. Notably, nine CoAP2/ERFs were highly expressed in pedicel, among which five down-regulated in response to abscission signal. Collectively, these five CoAP2/ERFs serve as the candidate regulators of ethylene-induced fruit abscission in Camellia oleifera, and they likely govern this process by modulating the development of abscission zone tissues. Camellia oleifera shows high sensitivity to ethylene signaling, which is closely correlated with the premature abscission of young fruits. Our study systematically characterized the AP2/ERF gene family and identified five CoAP2/ERF genes as potential key regulators of fruit abscission. These findings deepen the understanding of abscission mechanisms in Camellia oleifera and lay a solid scientific foundation for further functional characterization of CoAP2/ERF gene family.