Leafy vegetables, pulses, grains and potatoes are rich in iron, but this is not always bioavailable for human nutrition.
Plants do not produce much haem, therefore iron in plants is classified as ‘non-haem’ by nutritionists. For a plant biochemist working on iron cofactors this seems a rather crude classification.
The forms of iron found in plants include FeS cluster proteins in leafy vegetables and lots of ferritin in pulses. These iron species fall apart in the digestive tract and the iron is taken up in the gut.
Unfortunately, plants contain high levels of anti-nutrients, such as polyphenols in tea and phytic acid in grains, which severely inhibit iron uptake.
As part of a collaborative project funded by a grant from the BBSRC DRINC2 programme, the Balk lab have been able to show that iron bioavailability from bread is improved by sourdough fermentation (Rodriguez-Ramiro et al., 2017); that the iron from plant ferritin can be taken up by different pathways (Perfecto et al., J. Nutr, 2018); that frozen garden peas are a better source of bioavailable iron than mature peas (Moore et al., 2018); and that cabbages and garden peas, but not spinach, are good sources of bioavailable iron (manuscripts in preparation).
Iron biofortification of wheat
Wholegrain wheat has plenty of iron (~30 µg/g), but it is concentrated in the bran which is also rich in phytic acid, a phosphorus storage form for plants, but a strong iron chelator and anti-nutrient in the human intestine. White flour is milled mainly from the starchy endosperm of the grain, which has less phytic acid but is low in iron (~6 µg/g). In the UK and many other countries, wheat flour is fortified with iron salts or iron powder to help combat iron deficiency. Low iron levels leading to iron deficiency anaemia is a global health problem affecting up to 2 billion people, mainly women and children (WHO reports).
Using their knowledge of genes and iron in plants, the Balk lab changed the expression of a wheat gene encoding a vacuolar iron transporter, such that it became highly active in the endosperm during grain ripening (Connorton et al., 2017b). This strategy was highly effective: the total iron content in white flour went up to 20 µg/g, well above the legal requirement of 16.5 µg/g in the UK. Preliminary studies have shown that the iron tends to be more bioavailable, which is correlated with low phytate levels in the endosperm. A new grant (BB/P019072/1) will enable them to study the underlying molecular mechanisms as well as develop other biofortification strategies.
Iron sensing and regulation
Jorge Rodriguez-Celma joined the group with a Marie Sklodowska Curie fellowship (2015 – 2017) to investigate the biochemistry of hemerythrin E3 ligases in plants, which act as negative regulators of the iron deficiency response.
The research continues in 2018 with funding from the BBSRC (BB/N001079/1) in collaboration with Nick Le Brun at the University of East Anglia.
Iron in symbiotic root nodules
Legumes and a few other plant species are able to form a symbiosis with nitrogen-fixing bacteria. Small root outgrowths (nodules) house the bacteria in a low-oxygen environment created by large amounts of leghaemoglobin protein. The nodules have a high demand for iron, both for the synthesis of haem by the host plant and for bacterial iron-sulfur enzymes.
Jenny Walton has been funded by a Sainsbury Studentship from the Gatsby Charitable Foundation to investigate how iron is delivered from the host plant, Medicago truncatula, to the Sinorhizobium bacteria.