Dartmouth prof. researches iron efficiency in rice plants

Dartmouth professor Mary Lou Guerinot of the Department of Biological Sciences recently discovered that disruption of the OsYSL15 gene leads to iron inefficiency in rice plants. The finding was published last month in Plant Physiology.

“Iron deficiency is the leading human nutritional disorder in the world today, affecting an estimated four to five billion people. In addition, iron is one of the three nutrients that commonly limit plant growth.  Because most people get their iron from eating plants, ensuring that plants have adequate stores of iron will help not only crop productivity but also human health,” said Guerinot.

Using a promoter-GUS fusion molecule that detects OsYSL15 expression, Guerinot and her research team found that when iron was deficient, the OsYSL15 promoter became active in all root tissues and in all leaf tissues except epidermal leaf cells.

High levels of activity were also found in flowers and seeds. Further, when iron was sufficient, activity of OsYSL15 was minimal. These results imply that OsYSL15 may be involved in iron distribution in rice plants.

OsYSL15 seemed to functionally complement yeast strains defective in iron uptake. Over-expression of OsYSL15 increased the iron concentration in leaves and seeds of rice plants. However, when both alleles of the OsYSL15 gene were mutated, the plants had reduced iron concentrations in their shoots, roots, and seeds.

The iron-deficient mutant plants also exhibited chlorotic phenotypes under iron deficiency. Chlorosis is the yellowing of normally green plant tissue due to decreased chlorophyll resulting from nutrient deficiency, which in this case is iron deficiency.

The researchers discovered that treating the chlorotic plants with nitric oxide (NO) could reverse the process under iron limiting conditions. NO treatment did not change the entire plant iron concentration or enhance translocation of iron, but rather it improved the internal availability of iron.

The researchers also discovered that OsYSL15 was localized to the plasma membrane by fusing the gene to green fluorescent protein.

When asked if she is satisfied, Guerinot said, “No, we still have a long way to go in understanding how iron gets from the soil to the seed. At the moment we are focusing on identifying the ‘iron sensor.’  That is, we want to understand how a plant knows whether it is iron deficient.  We know that iron deficient plants turn on a number of key genes necessary for iron uptake and distribution but we don’t yet know how the whole cascade gets started.”