Genomic and transcriptomic insights into cytochrome P450 monooxygenase genes involved in nicosulfuron tolerance in maize (Zea mays L.)

Postemergence application of nicosulfuron for weed control in maize fields can cause great damage to certain maize inbred lines and hybrids. Two maize genotypes, tolerant inbred (HBR) and sensitive inbred (HBS), were found to significantly differ in their phenotypic responses to nicosulfuron, with the EC50 (50% effective concentration) values differed statistically (763.6 and 5.9 g a.i. ha(-1), respectively). Pre-treatment with malathion, a known cytochrome P450 inhibitor, increased nicosulfuron injury in both HBR and HBS. Our results support the hypothesis that nicosulfuron selectivity in maize is associated with cytochrome P450 metabolism. Further analysis of the maize genome resulted in the identification of 314 full length cytochrome P450 monooxygenase (CYP) genes. These genes were classified into 2 types, 10 clans and 44 families. The CYP71 clan was represented by all A-type genes (168) belonging to 17 families. Nine clans possessed 27 families containing 146 non-A-type genes. The consensus sequences of the heme-binding regions of A-type and non-A-type CYP proteins are 'PFGXGRRXCPG' and 'FXXGPRXCXG', respectively. Illumina transcriptome sequence results showed that there were 53 differentially expressed CYP genes on the basis of high variation in expression between HBS and HBR, nicosulfuron-treated and untreated samples. These genes may contribute to nicosulfuron tolerance in maize. A hierarchical clustering analysis obtained four main clusters named C1 to C4 in which 4, 15, 21, and 13 CYP genes were found in each respective cluster. The expression patterns of some CYP genes were confirmed by RT-qPCR analysis. The research will improve our understanding of the function of maize cytochrome P450 in herbicide metabolism.