Iron biofortification in bean: is the lpa1 mutant a good candidate?

Iron deficiency is the most prevalent micronutrient deficiency affecting more than 2 billion people throughout of the world. Due to its worldwide consumption and nutritional qualities, common bean has been identified as a strategic target crop for iron biofortification. To this purpose we adopted a strategy aimed to reduce seed phytic acid (PA) and polyphenols (PP) content, being these strong inhibitors of iron absorption. In bean, low PP genotypes can be easily identified, conversely, natural variability in PA content is not very high and the best way to gain significant PA reductions is by obtaining low phytic acid (lpa) mutants. PA reduction is often associated with negative agronomic traits, such as lower seed viability and emergence, reduced plant growth rate and grain yield. These traits may limit the use of lpa mutants for iron biofortification, since acceptable agronomic performance should be guaranteed to small farmers and poor populations that would benefit from biofortified crops. Recently, we have identified a bean lpa mutant and showed it carries a defective MRP type ABC transporter (Pvmrp1) necessary for phytic acid transport to the vacuole. lpa seeds have a 90% PA reduction, seven fold higher free iron and 25% less raffinosaccharides. Assessment of iron bioavailability in lpa lines, using an in vitro Caco-2 cell model, showed a twelve times increase in low PP lpa seeds, while non significant increases were detected in high PP lpa seeds. Agronomic analyses of the original lpa mutant and of derived lpa lines have shown that seedling emergence, seed yield and plant growth were not affected. Contrary to other species, bean possesses a MRP paralog (PvMRP2) that is poorly expressed in seeds but not in other plant parts. This gene may complement the effects of the defective Pvmrp1 copy in vegetative tissues and thus explain the absence of negative phenotypes.