Genome-wide screening of AP2/ERF transcription factors involved in Citrus maxima 'Sanhongmiyou' exocarp coloring

Pummelo (Citrus maxima) belong to subfamily Aurantioideae of Rutaceae. In production, 'Sanhongmiyou' exocarp turned out red after bagging but remained yellow green without bagging. Carotenoids were the main color in pummelo exocarp. The AP2/ERF family has been confirmed to be associated with fruit coloring by regulating genes related to carotenoid metabolism in multiple species. To better understand the molecular mechanisms of exocarp coloration and carotenoid accumulation, a total of 126 AP2/ERF transcription factor family members distributed on 9 chromosomes were screened from pummelo genome database and divided into 3 subfamilies including 110 ERF subfamilies, 13 AP2 subfamilies and 3 RAV subfamilies. 26 pairs of genes from the AP2/ERF transcription factor family were colinear. 18 pairs of genes were evolutionarily conserved and functionally different after tandem repetition events. Multi-species collinearity analysis showed close evolutionary relationship between pummelo, Actinidia chinensis and Eriobotrya japonica. Cis-acting elements prediction in the promoter region revealed that members of AP2/ERF family are rich in cis-acting elements which respond to light and plant hormones. Color parameters L*, a*, b* and CCI values were measured. Chlorophyll a,chlorophyll b, carotenoid content in fruit exocarp were determined. Eight ERF genes related to the carotenoid metabolism were screened out and tested by qRT-PCR. The expression pattern of structure genes in carotenoid metabolism were also measured with qRT-PCR. The expression patterns of CitERF23, CitERF27 and CitERF32 appeared correlation with pigment content and color indexes. Meanwhile, CitERF23 showed significant positively correlation with ZDS and CDD4. CitERF27 showed significant positively correlation with CDD1. CitERF32 showed significant positively correlation with PDS. CitERF120 showed significant positively correlation with CDD1. CitERF23, CitERF27 and CitERF32 were proposed as a candidate gene regulating carotenoid metabolism in the AP2/ERF family.