New Insights into the Transcription Factor Regulatory Networks Driving Peel Coloration under Hormone Induction Analyzed by Transcriptomics and Metabolomics in Tangor 'Murcot'

Yang Chen ^1,2 Lei Yang ^1,3 Shuang Li ¹ Min Wang ^1* Jianjun Yu ¹ Wenqin Bai ^1,2,3* Lin Hong ^1,3*

• ¹ Chongqing Academy of Agricultural Sciences, Chongqing, China
• ² Chongqing Key Laboratory of Adversity Agriculture Research, Chongqing, China
• ³ Key Laboratory of Evaluation and Utilization for Special Crops Germplasm Resource in the Southwest Mountains, Ministry of Agriculture and Rural Affairs, Chongqing, China

Fruit color is a crucial quality factor strongly influencing consumer preference for citrus. The colouration of citrus fruit is primarily determined by carotenoids, which produce a range of hues. Gibberellic acid (GA) and ethylene are critical in fruit colouration during the ripening process. Nevertheless, the underlying mechanisms remain poorly understood. The present study utilized transcriptomic and metabolomic analyses to investigate the molecular regulatory mechanisms affecting peel pigment 2 metabolism in tangors (Citrus reticulata ×Citrus sinensis) following GA and ethephon (ETH) treatments. Collectively, our findings indicate that GA inhibits chlorophyll degradation and the accumulation of numerous carotenoids, including five violaxanthin esters (violaxanthin palmitate, violaxanthin myristate-caprate, violaxanthin myristate-laurate, violaxanthin dilaurate, violaxanthin myristate) and two β-cryptoxanthin derivatives (β-cryptoxanthin laurate, β-cryptoxanthin myristate), while ethephon (ETH) promotes these processes. Furthermore, GA inhibited the downregulation of lutein, the predominant carotenoid in immature fruits. Notably, integrated transcriptomic and metabolomic analyses identified 33 transcription factors associated with pigment metabolism. Of these, two novel transcription factors, the ethylene-responsive transcription factor ABR1 and the HD-Zip transcription factor ATHB7, were uncovered through both transcriptomic analysis and weighted gene co-expression network analysis. These two transcription factors positively regulated the colouration process, as validated by transient overexpression assays in tobacco.Taken together, our findings elucidated the global carotenoid changes and transcriptional alterations in regulating citrus peel color under hormone induction, with significant implications for improving citrus production.