Study of the pleiotropic effects of the common bean low phytic acid1 mutation and mapping of new lpa mutants

Phytic acid (PA), the main form of phosphorus storage present in seeds, is an antinutritional factor for its ability to chelate cations important for human nutrition. Plant breeders have spent many efforts to isolate and develop low phytic acid (lpa) mutants in different important crops. We isolated different common bean lpa mutants with reduction of PA content at different extent. The consumption of common bean seeds harboring the lpa1 mutation, affecting the PvMRP1 transporter and causing a reduction of 90% in PA content, improved iron status of volunteers in human trials, but caused adverse gastrointestinal effects, presumably due to the increased stability of lectin phytohemagglutinin L (PHA-L) in these seeds, compared to the wild type (wt) ones. A hard-to-cook (HTC) defect observed in the lpa1 seeds intensified the problem. We confirmed and quantified the HTC phenotype of the lpa1 common bean seeds in three different genetic backgrounds, giving a genetic demonstration of the so-called "phytase-phytate-pectin" theory and found differences depending on the background. In one of them, we correlated the HTC defect to the redistribution of calcium, whose concentration in all parts of the seed and, particularly in the cell walls, was larger in the lpa1 compared to the wt. Furthermore, the lpa1 mutation, combined with the presence of different PHA alleles, affected the stability of the PHA-L lectin, due to an excess of free cations. Moreover, we showed a decreased seed density in the lpa1 mutant compared to the wt, but only in some of the analyzed genetic backgrounds. All these data suggest that the pleiotropic effect due to the lpa1 mutation are strictly dependent on the genetic background. Other common bean lpa mutants, have been recently isolated with milder effect on PA reduction than the previously described lpa1 one. A candidate gene approach for three of these mutants did not reveal any mutation in known PA biosynthetic genes or in genes coding for PA transport. A mapping approach is underway in order to identify the affected genes. These mutants can be assayed for their cooking and nutritional properties in order to develop useful biofortified beans devoid of negative traits.