Molecular evolution of two consecutive carotenoid cleavage dioxygenase genes in strigolactone biosynthesis in plants

Strigolactones are newly discovered plant hormones that perform various functions, from signaling in symbiotic interactions with arbuscular mycorrhizal fungi to controlling outgrowth of axillary buds. We examined the phylogenetic relationships of two carotenoid cleavage dioxygenase genes (CCD7 and CCD8) that are involved in consecutive upstream steps of the proposed strigolactone biosynthesis pathway. The CCD7 and CCD8 sequences from 11 model species, divided into two clades, correspond to sequences from monocotyledons and dicotyledons. However, the sequences from the primitive moss, Physcomitrella patens, appeared to be evolutionarily distinct from those of the angiosperms. CCD7 and CCD8 are much conserved, since no significant positive selection was detected among these plants. Ks values indicated that CCD7 and CCD8 diverged about 290 to 430 million years ago. As essential genes in the strigolactone pathway, the divergence timing of the conserved CCD7 and CCD8 genes reflects the approximate time of generation of strigolactone as a regulatory substance. This timing calculation also coincides with initiation of symbiosis between plants and microorganisms, inferred from the fossil record. Molecular evolution analyses of genes in metabolic pathways can provide insight concerning gene evolution.