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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.

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.

Iron deficiency affects most of the world population both in developed and developing countries, especially people who consume high amount of phytate-containing food. Phytic acid (PA), the main form used by seeds to store phosphorus, is an antinutritional factor for its ability to chelate minerals important for human nutrition. We previously isolated and characterized a lpa1 (low phytic acid) mutant in common bean with 90% reduction of PA. The lpa1 beans have been shown to improve iron status in human trials, but some adverse gastrointestinal effects have been reported and linked to the stability upon cooking of a bean lectin, the PHA-L, contained in these seeds (Petry et al., 2016). Here we will present data on the distribution of globoids and of minerals, by µPIXE analysis, in wt and lpa1 seeds. We have also studied the influence of the lpa1 mutation and of the cooking temperature on the thermal hydrolysis of bean PHAs by analysing beans, both wt and lpa1, carrying different PHA alleles (PHA-E, PHA-L and PHA-E,L), cooked at sub-optimal (95°C) or normal temperature (100°C). Our results show that the lpa1 mutation strongly influences the stability of the PHA-L, but not that of the PHA-E. We show that the mechanism by which the lpa1 mutation affects PHA-L stability involves an excess of free cations (most likely Ca2+) generated by the low levels of PA and we also show a redistribution of Ca2+ ions that in the lpa1 mutant are more concentrated in the cell wall middle lamella area of the parenchyma cells, in accordance with the proposed theory for the development of the HTC defect (Kruger et al., 2015). Petry et al. (2016). Journal of Nutrition 146, 970-975 Kruger et al. (2005). Food Chemistry 174, 365-371