A defective ABC transporter of the MRP family, responsible of the bean lpa1 mutation, affects the regulation of phytic acid pathway, reduces seed myo-inositol and alters ABA sensitivity

By screening an EMS mutagenised common bean population, we had previously identified the lpa (low phytic acid) 280-10 line that carries a monogenic recessive mutation conferring a 90% reduction of InsP6 content and, differently from most other lpa mutants, does not display negative pleiotropic effects associated to the mutation. In the present work we mapped the lpa-280-10 mutation on bean chromosome 1, in a region syntenic to soybean (Glycine max) chromosomes 19 and 3, where the lpa mutant CX1834 was previously mapped and correlated to multi-drug resistance protein (MRP) type ATP-binding cassette (ABC) transporter gene orthologous to Arabidopsis AtMRP5/AtABCC5 and maize ZmMRP4 (lpa1). Fine mapping of bean lpa-280-10 and DNA sequencing data showed the occurrence of a single amino acid substitution (E to K) in the orthologous PvMRP-1 gene. We also provide data on a second MRP gene (PvMRP-2) present both in soybean and bean genomes, which, despite the high sequence identity (90%) and a very similar pattern of expression during seed development to PvMRP-1, is not able to complement the lpa mutation, suggesting that it may have a different role in the plant. Transcriptional and metabolic analysis of phytic acid pathway demonstrated the existence of a negative feedback mechanism of InsP6, or its degradation products, acting on genes coding for key enzymes of phytic acid pathway and leading to a 30% reduction of myo-inositol. Our results show that the reduced content of myo-inositol is responsible of the ABA hypersensitive response found in lpa-280-10 seed germination, indicating a key role of this metabolite in the multiple pleiotropic effects often reported for lpa mutants.