Iron Nutrition

Plants are limited in the amount of iron they can pick up from the soil because iron, despite being one of the most abundant elements on Earth, is mostly found in its insoluble ferric, Fe (III), form. Most organisms can directly uptake only Fe (II). To facilitate the transport of insoluble Fe (III) into the cells, both plants and microbes produce chemicals called phytosiderophores, which bind to Fe (III) and transform it into a soluble compound that can be transported into the cells. Plant siderophores, or phytosiderophores, are used by many plants to uptake iron. These compounds are much more able to re-enter the plant cells than other microbial siderophores.

The traditional method of increasing iron in plants involves genetically modifying the plants to increase their phytosiderophore production so they can pick up and bring in more iron from the environment. This is done by either amplifying the genes for phytosiderophore production that the plant already has, or by inserting the genes from another plant that is generally has higher iron content and therefore more efficient phytosiderophore genes.

In one study, iron uptake was increased in rice through the upregulation of the gene nicotianamine synthase (NAS), which is one of the genes of phytosiderophore production, and was then fed to anemic rice. The mice that consumed the modified rice recovered more rapidly from anemia than ones fed control rice [3]. In another study, corn was modified to overexpress the binding protein ferritin in order to increase the iron content in the corn. The corn seeds were then ground into a flour paste, which had a significantly higher level of total iron content than flour paste ground from control plants [4].

By attempting to increase the iron content in plants through modification of the rhizosphere, we are trying to shift the paradigm of engineering transgenic plants to engineering the microbial community around these plants. It is known that microbes already support plant health by increasing the bioavailability of nutrients, helping plants become resistant to disease, floods, and drought, facilitating root growth, and aiding in their defense against pathogens [1]. The efficacy of microbial phytosiderophores was proven in a study with iron-starved tomatoes (Radzki 2013). They were able to restore healthy iron levels in tomatoes by the direct application of Chryseobacterium C138 and with media containing just the microbially produced phytosiderophores. Granted, this study used iron-starved conditions, we still believe the use of microbial phytosiderophores is essential to iron uptake in normal plants.

We believe that microbes can be engineered to provide even more benefits to plants, and we hope that our project will make more people aware of the crucial role of the rhizosphere to plant health and nutrition. Ideally, this awareness will encourage further scientific research into the specific roles that each type of microbial organism plays and advocate for the systematic management of microbial soil ecologies.