The brachytic 2 and 3 maize double mutant shows alterations in plant growth and embryo development

In maize there are three types of brachytic mutants (br1, br2 and br3) showing short stature and a gibberellins-insensitive phenotype. So far only the brachytic 2 gene has been cloned and it encodes for a putative protein of the Multidrug Resistant (MDR) class of P-glycoproteins (PGPs) that could be involved in polar movement of auxins: in fact the br2 mutant is insensitive to treatment with auxins and gibberellins. We have isolated a new recessive mutation of br2 gene (named br2-23) and with the aim of study its interactions with the other brachytic mutations we produced a br2 br3 double mutant that showed an additive effect on the stature with respect to the single mutants br2 and br3 and abnormal growth. In the progeny of the selfed double mutant we observed various defective seedlings, mirroring an altered embryo development and growth, which also suggested a role for the br3 gene in auxin transport. Expression analysis of the auxin efflux transporters codified by ZmPIN1 genes supports this finding, showing the up-regulation of the ZmPIN1a gene in the br3 mutant. To our knowledge this is the first report showing the involvement of Br2 and Br3 genes in embryo development. These single and double mutants appear to be useful tools to study the genetics of plant height and to investigate auxin transport in plants.