ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating shoot branching.

CONCLUSIONS

ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase that may mediate strigolactone biosynthesis highly responsive to phosphorus deficiency and undergoes negative selection over domestication from Zea ssp. parviglumis to Zea mays. Carotenoid cleavage dioxygenase 7 (CCD7) functions to suppress shoot branching by controlling strigolactone biosynthesis. However, little is known about CCD7 and its functions in maize and its ancestor (Zea ssp. parviglumis) with numerous shoot branches. We found that ZmCCD7 and ZpCCD7 had the same coding sequence, indicating negative selection of the CCD7 gene over domestication from Zea ssp. parviglumis to Zea mays. CCD7 expression was highly responsive to phosphorus deficiency in both species, especially in the meristematic zone and the pericycle of the elongation zone of maize roots. Notably, the crown root had the strongest ZmCCD7 expression in the meristematic zone under phosphorus limitation. Transient expression of GFP tagged ZmCCD7/ZpCCD7 in maize protoplasts indicated their localization in the plastid. Further, ZmCCD7/ZpCCD7 efficiently catalyzed metabolism of six different linear and cyclic carotenoids in E. coli, and generated β-ionone by cleaving β-carotene at the 9,10 (9',10') position. Together with suppression of shoot branching in the max3 mutant by transformation of ZmCCD7/ZpCCD7, our work suggested that ZmCCD7/ZpCCD7 encodes a carotenoid cleavage dioxygenase mediating strigolactone biosynthesis in maize and its ancestor.