Genetic and phenotypic characterization of a novel brachytic2 allele of maize

The maize brachytic2 (br2) gene encodes a protein homologous to the Arabidopsis ABCB1 protein, which actively transports chemically diverse substrates across cellular membranes. This includes indole-3-acetic acid (IAA), the most abundant natural form of auxin, a key regulator of plant growth and tropisms. Defects in auxin transport are responsible for the short stature of br2 mutants. We describe a novel br2 mutant allele present in NC238, an inbred maize of medium stature and demonstrate that the mutant allele br2-NC238 is caused by the insertion of a novel transposon in intron IV of the br2 gene. A tall revertant of NC238 (NC238-rev) appeared spontaneously as a result of excision of the transposon from the br2 gene. This event yielded two completely isogenic versions of the NC238 inbred, one brachytic (NC238-ref) and one tall (NC238-rev). The presence of the transposon in br2-NC238 alters normal splicing of the transcript, producing several splicing forms. One splicing form, ABCB1-T02, was abundantly expressed in NC238-ref, while its expression was very weak in tall NC238-rev plants. Differences between NC238-rev and NC238-ref were also detected in the expression pattern of auxin transporter genes. Morphological changes associated with the br2-NC238 mutant allele included reduced length and increased diameter of the lower internodes. The shortening of plant stature was confirmed in the B73 genetic background by introgression of the br2-NC238. Our results demonstrate that br2-NC238 reduces plant height without altering other morphological traits, and might thus be useful in maize breeding programs when reduction of stature is desired.