The Rht-B1b Allele Negatively Impacts Photosynthesis and Modifies Carbon and Nitrogen Partitioning

The genes which formed the basis for the green revolution in wheat were the Reduced Height (Rht) genes. There are currently two widely used mutant alleles, Rht-B1b and Rht-D1b, which reduce plant height by 20%. These alleles are associated with increased plant productivity, though the mechanism of increased productivity is not yet understood. In our experiments, we are testing the degree to which Rht-B1b impacts plant photosynthetic capacity and overall carbon metabolism using tall and semi-dwarf spring wheat NILs varying only for the presence or absence of Rht-B1b. The results demonstrate that Rht-B1b reduces flag leaf photosynthetic rate and chlorophyll content. RuBisCo content is also decreased in the semi-dwarf lines throughout the photoperiod in flag leaves. Global expression analysis during leaf and stem development indicated carbon metabolism is not upregulated in Rht-B1b genotypes. However, measurements of carbon and nitrogen metabolites present in flag leaves at anthesis indicates that Rht-B1b confers an increased ratio of carbohydrates to amino acids, while the opposite is true in developing seeds. These results suggest that the increased productivity of the mutant dwarfing alleles is not due to an increased photosynthetic capacity, but rather more efficient carbon transport.