The DL gene system in common bean: a possible mechanism for control of root-shoot partitioning

Crosses between certain genotypes of common bean result in dwarfing of F1 p lants and lethal dwarfing in a proportion of the F2 population. This is und er the control of the semi-dominant alleles, DL1 and DL2 at two complementa ry loci which are expressed in the root and shoot respectively. The various DL genotypes can be simulated by grafting. The graft combination DL(1)DL(1 )dl(2)dl(2)/dl(1)dl(1)DL(2)DL(2) was found to have a significantly higher r oot dry matter fraction than either parent. Lethally dwarfed plants (DL1DL1 DL2DL2) and the analogous lethal, graft combination (dl(1)dl(1)DL(2)DL(2)/D L(1)DL(1)dl(2)dl(2)) exhibit failure of root growth and have very low root fractions. Hybrids or graft combinations with failed roots ceased growth an d accumulated large amounts of starch throughout their hypocotyls. In steri le culture, both lethal dwarfs and lethal graft combinations were able to g row roots if sucrose was added to the growth medium. This indicates that a failure of sucrose translocation to the roots is probably responsible for f ailed root growth. Data from screening the DL genotypes of 49 cultivars cou ld be fully explained using the DL system hypothesis, and grafting proved t o be efficient for identifying DL genotype. The DL system might be of funda mental importance in root-shoot partitioning. Current evidence favours the hypothesis that failure of root growth is the outcome of excessively high s ink strength of shoots compared to roots, which might arise from signalling incompatibilities between the genotypes.