MYB36 controls differentiation of the endodermis into an ion-selective barrier

Roots are the primary organ that acquires the water and mineral nutrients from the soil essential for plant growth and development. A specialized cell layer in the root, called the endodermis, plays a vital role in these processes by regulating entry of water and mineral nutrients into the vascular system of the plant for transport to the shoot. Of critical important to these functions are Casparian strips, structures that form tight seals between cells, blocking nutrients and water leaking between. The only route for nutrients and water into the vascular system is thus through the endodermis. By blocking leakage around cells Casparian strips allow the endodermis to provide cellular control of nutrient and water uptake by roots. Furthermore, to allow the transport of only the nutrients required for growth and development the endodermis contains proteins that allow specific nutrients to be transported directionally across the endodermis from the soil into the vascular system. Despite its importance, the molecular mechanisms involved in how this cell layer becomes specialized to perform these functions are largely unknown. Here, we propose to identify these molecular mechanisms using as a tool the newly discovered MYB36 transcription factor that is know to be involved in controlling the differentiation of the endodermis through regulating the development of Casparian strips.

The endodermal cell layer in roots acts as a critical checkpoint controlling water and mineral nutrient transport into and out of the root vasculature. To enable this gatekeeping function lignin-based Casparian strips form a transcellular seal between endodermal cells to block uncontrolled extracellular movement of nutrients and water. Construction of Casparian strips requires the deposition of extracellular lignin rings that encircle endodermal cells in the primary cell wall and that are anchored to the plasma membranes of adjacent cells. Many of the molecular details of this process remain to be discovered. Furthermore, to facilitate selective transcellular transport into the vasculature the endodermis contains laterally polarised influx and efflux carriers, though the full extent and function of this set of carriers is unknown. The function of SHORTROOT (SHR) and SCARECROW (SCR) in endodermal specification are well established. However, our understanding of the molecular players that control the programme of differentiation that sets up the endodermis to function as a bidirectional check point for water and mineral nutrient transport remains very limited. This project builds on the PI's recent discovery of the MYB36 transcription factor, a master regulator of Casparian strip formation, published in Proc Natl Acad Sci USA, and highlighted in the same issue with a commissioned commentary. With the discovery of MYB36 the PI now has a new avenue to start investigating the molecular processes involved in endodermal differentiation.

Results from the proposed research should provide an important molecular mechanistic underpinning to efforts to improve mineral nutrient and water use efficiencies and enhanced stress tolerance (e.g. salinity, flooding, drought, nutrient deficiencies, trace element toxicities) in agricultural and horticultural crops. Commercial farmers could potentially benefit from such developments through improved and sustainable yields with less inputs (fertilizers and water). Further, such improvements in agricultural and horticultural crops could also potentially benefit subsistence farmers with limited access to inorganic fertilizers (primarily nitrogen, phosphate and potassium, secondarily sulphur and magnesium), helping to reduce the cost burden such fertilizers impose. Improved water use efficiency and stress tolerance will also improve yields for subsistence farmers cultivating marginal lands. In addition, reduced utilization of fertilizers, achieved through improved mineral nutrient use efficiencies, will limit the environmental and ecological damage their production and excess use causes, potentially benefiting the general public through enhanced quality of life.

An improved understanding of how roots acquire important trace element and minerals should also provide an important molecular mechanistic underpinning to efforts to improve food quality by helping to increase the content of essential mineral nutrients and reduce toxic trace elements in food crops. For most of the world's population plants are the major source of essential minerals such as calcium, potassium, manganese, iron and zinc, and therefore efforts to improve the mineral nutrient content of staple foods such as rice, maize and cassava will have a positive impact on public health both in the UK and internationally. Plants are also the primary entry point for a variety of toxic minerals into the food chain such as arsenic and cadmium, and being able to limit their accumulation in food would also have a positive impact on public health both in the UK and internationally.