pleiotropic effects of genes involved in cell wall lignification on agronomic characters

摘要

Brown midrib (bm) mutants in maize, which are characterized with altered lignin composition, reduced lignin content and thus with enhanced cell wall digestibility (CWD), are often associated with inferior agronomic traits. To understand how the undesirable associations happen would help us better design strategies to manipulate the cell wall lignification for CWD enhancement without sacrificing biomass yield. First, I reviewed the current knowledge and approaches to study the nature of trait correlations. We proposed that dissection of the trait correlations into DNA polymorphism level is beneficial for plant breeding as intergenic, intragenic, or true pleiotropy will have different impact. Secondly, three new bm mutants were identified by allelism test and were designated as bm5, bm6, and bm7. Then I focused on characterization of bm6, which was revealed to increase CWD but suppress plant height in F2 population. With large mapping populations with about 1000 brown F2 plants, its underlying gene was delimited into a ~180kb interval referring to B73 genome, wherein 10 predicted gene models reside. Besides using natural mutants, we also employed candidate association approach, which suggested that the pleiotropic effects of monolignol biosynthetic genes are most likely due to intragenic linkage of quantitative trait polymorphisms (QTPs). Therefore, optimal haplotypes which combine QTPs beneficial for both CWD and biomass yield might exist. In order to build up suitable materials to confirm this finding, we characterized the COMT gene sampled from the germplasm enhancement of maize (GEM). Much higher genetic diversity of COMT alleles at both DNA and predicted amino acid level and extensive lower linkage disequilibrium (LD) were observed in GEM than in inbred lines. The higher genetic variation suggests GEM is a valuable genetic resource to broaden genetic variation for breeding. And the extensive lower LD indicates the higher resolution of association mapping in GEM derived materials.