Low phytate beans: is seed biofortification getting closer?

Iron deficiency anaemia (IDA) 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 to increase dietary iron for human beings (http://www.harvestplus.org/content/iron-beans). Bean seeds have an iron content between 35 and 90 ?g/g and breeding to increase iron concentration by 60-80% in cultivated varieties has been achieved at CIAT. However, increase in iron content not necessarily translates into increased iron bioavailability, that may be strongly reduced by the presence in the seeds of iron absorption inhibitors, such as phytic acid (PA) and polyphenols (PP). Thus, any strategic approach for bean biofortification should consider nutritional trials to be performed to verify the success and extent of the biofortification strategy. Experiments to assess iron bioavailability in high iron bean (HIB) lines have shown that the benefit of increased iron content may be vanished by the presence of high levels of PA and PP in the seeds. In common 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. Although such type of mutants have been identified in several grain crops, often PA reduction was associated with negative agronomic traits, such as lower seed viability and emergence, reduced plant growth rate and grain yield. These findings 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 (2,3). Compared to wild type genotypes, bean lpa seeds have a 90% PA reduction, seven fold higher free iron extractability in mild acid conditions and 25% less raffinosaccharides. Agronomic analyses of the original lpa mutant and of derived lpa lines have shown that, despite the strong PA reduction in the seed, seedling emergence, seed yield and plant growth were not affected and not statistically different from those of wt and parental genotypes. Work aimed to evaluate iron bioavailability in lpa lines, using an in vitro/digestion Caco-2 cell culture model is in progress. References: Campion, B. et al. (2009). Theor. Appl. Genet. 118, 1211-1221. / Panzeri, D. et al.. (2011). New Phytol. 191, 70-83. Acknowledgements: Research partially supported by Regione Lombardia/CNR agreement, project 2