Isolation and Characterization of the d2 Dwarfing Gene in Pearl Millet

Pearl millet, Cenchrus americanus, is an important food crop in Sub-Saharan Africa and India, and is grown for forage in the US and Australia.  Landraces cultivated in Africa are tall, while the majority of improved varieties grown in the rest of the world are semi-dwarfs.  Semi-dwarfism is caused by a recessive mutation in the d2 gene.  We aimed to clone and characterize the d2 gene, which was no small feat because pearl millet has a relatively large genome (C=2,500 Mb) and limited genetic resources.  We used a combination of fine-mapping, comparative analysis and haplotype analysis to identify ABCB1 as a candidate for d2.  ABCB1 encodes a glycoprotein involved in auxin transport, and mutating the ABCB1 gene leads to the dw3 dwarf phenotype in sorghum.  Sequencing of the pearl millet d2 and D2 alleles showed the presence of an LTR retrotransposon in the first exon of the dwarf allele which, presumably, inactivates the gene.  Confirmation of gene identity, however, has been challenging.  We are currently in the process of transforming both the dwarf and tall pearl millet alleles in an Arabidopsis mutant defective for both ABCB1 and ABCB19, a close homolog of ABCB1.  The results of this work and of expression and biochemical analyses in tall and dwarf isogenic pearl millet lines will be reported.