Genetic Architecture of Vegetative Phase Change in Maize.

The transition from juvenile to adult vegetative tissue is a fundamental process that must occur prior to the vegetative to reproductive transition.  In maize, the vegetative phase transition from juvenile to adult tissue is under genetic control independent of the vegetative to reproductive transition and the mechanisms controlling this transition are not well characterized. One way to distinguish juvenile vegetative tissue from adult vegetative tissue is the presence of epicuticular wax found on the surface of juvenile leaves.  To begin to understand the genetic mechanism underlying vegetative phase change we scored the last leaf with epicuticular wax on 3875 recombinant inbred lines from the Nested Association Mapping population (NAM) as well as 267 intermated B73 x Mo17 recombinant inbred lines (IBM).  The last leaf with epicuticular wax varied in the NAM RILs ranged from leaf 5 to leaf 14.25, and ranged from leaf 5.4 to 11 in the IBM RILs.  We mapped QTLs within the 25 NAM families as well as the IBMs using composite interval mapping and detected 56 QTL across all NAM populations and 5 QTL in the IBM population.  Three major QTL were located on chromosomes 2, 3, and 9. The combined average additive effects of these three alleles equate to almost a 3 leaf difference in transition, or near 40% of the variation observed in the NAM population.  The chromosome 9 region contains the gene Glossy15, involved in expression of juvenile leaf traits; however, this candidate gene has not been confirmed to be the QTL in this region. The chromosome 2 QTL was detected in 22 of the 25 NAM populations as well as in the IBM population, explaining between 5-55% of the variation in the NAM populations and 16% in the IBM population. These results demonstrate that several major QTL underlie natural variation for this important developmental trait.