Promoter Variation Results in Differential Phytoalexin Accumulation in Two Maize Inbred Lines

Crop varieties exhibited phenotype variations during domestication and breeding. Such phenotype variations include secondary metabolism differentiation; however, the underlying mechanisms remain to be clarified. Here, two elite maize inbred lines Mo17 and HZ4 were analyzed to determine constitutive phytoalexin accumulation including zealexins and kauralexins. Both inbred lines produced phytoalexins in above- and belowground tissues in sterile culture. HZ4 accumulated much more kauralexin A3 than Mo17, which was consistent with higher gene expression of kauralexin biosynthesis in HZ4. Promoter cloning and sequence analysis disclosed a number of sequence variations including fragment insertion/deletion and nucleotide substitution in promoter regions of both inbred lines. Further analysis showed that one key biosynthetic gene (KSL5) of maize phytoalexins exhibited higher promoter activity in HZ4 than in Mo17. The underlying mechanism was explored and promoter mutation in both inbred lines accounted for such promoter activity difference. Specifically, one W-box element with a positive effect in KSL5 promoter from HZ4 was identified; meanwhile, a 413-bp fragment in KSL5 promoter from Mo17 played a negative role in gene expression. Both inbred lines accumulated these sequence mutations in promoters during breeding, which resulted in different gene expression and phytoalexin production, potentially contributing to basic resistance.