Calcium redistribution contributes to the hard-to-cook phenotype and increases PHA-L lectin thermal stability in common bean low phytic acid 1 (lpa1) mutant seeds.

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 (Phaseolus vulgaris L.) 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.