Structural Characterization of ABCB1, the Gene Underlying the d2 Dwarf Phenotype in Pearl Millet, Cenchrus Americanus (L.) Morrone

Pearl millet is an important food crop in arid and semi-arid regions of South Asia and sub-Saharan Africa and is grown in Australia and the United States as a summer fodder crop. The d2 dwarf germplasm has been widely used in the last half-century to develop high-performing pearl millet hybrids. We previously mapped the d2 phenotype to a 1.6 cM region in linkage group (LG) 4 and identified the ABCB1 gene as a candidate underlying the trait. Here, we report the sequence, structure and expression of ABCB1 in tall (D2D2) and d2 dwarf (d2d2) germplasm. The ABCB1 allele in d2 dwarfs differs from that in tall inbreds by the presence of two different high copy transposable elements, one in the coding region and the second located 664 bp upstream of the ATG start codon. These transposons were present in all d2 dwarfs tested that were reported to be of independent origin and absent in the analyzed wild-type tall germplasm. We also compared the expression profile of this gene in different organs of multiple tall and d2 dwarf inbreds, including the near-isogenic inbreds at the d2 locus, Tift 23B (D2D2) and Tift 23DB (d2d2). Heterologous transformation of the tall (Ca_ABCB1) and the d2 dwarf (Ca_abcb1) pearl millet alleles in the Arabidopsis double mutant abcb1abcb19 showed that the pearl millet D2 but not the d2 allele complements the Arabidopsis abcb1 mutation. Our studies also show the importance of the COOH-terminal 22 amino acids of the ABCB1 protein in either protein function or stability.