Minisymposium 1: Mineral nutrition

Abs # 11004: Arabidopsis Yellow Stripe-Like genes encode membrane transporters of nicotianamine-metal complexes.

Presenter:	Walker, Elsbeth L, ewalker@bio.umass.edu
Authors	Walker, Elsbeth L (A)   DiDonato, Raymond J (A)   Roberts, Louis A (A)   Sanderson, Tamara  (A)
Affiliations:	(A): Biology Department, UMass, Amherst, Amherst, MA 01003

The Yellow Stripe-Like (YSL) family of proteins was identified on the basis of strong sequence similarity with the primary iron transporter of maize, Yellow Stripe1 (ZmYS1). ZmYS1 transports iron, but only when that iron is bound by specific plant-derived chelators called phytosiderophores (PS). PS are produced by certain plants as a response to iron deficiency, and constitute an important part of one strategy by which these plants make sparingly soluble iron in the soil bioavailable for uptake by root cells. Only grasses make and use PS; other plant species use alternative strategies for primary iron uptake. With this in mind, it is surprising that the Arabidopsis genome encodes eight YSL proteins bearing strong, near-full-length sequence similarity to ZmYS1. The YSL proteins of Arabidopsis cannot have a function identical to that of ZmYS1, yet the sequence similarity among family members suggests a closely related function for these proteins. However, the physiological role of the YSL proteins in Arabidopsis (AtYSLs) is distinct from that of ZmYS1. Instead of a function in primary iron uptake from the soil, the YSL proteins appear to accomplish metal distribution within the plant body. Metal distribution in plants is a complex and poorly understood process, but by understanding the function of each member of the Arabidopsis YSL family, we hope to elucidate some of the mechanisms plants use to correctly partition metals into particular cells and tissues. We will present evidence that AtYSL proteins transport iron and copper, but only when these metals are chelated by nicotianamine (NA). Nicotianamine is a strong complexor of various transition metals, particularly Fe(II), and NA is found in all higher plants examined. YSL genes are expressed in many cell types in both roots and shoots, suggesting that diverse cell types obtain metals as metal-nicotianamine complexes. We will further focus on the YSL2 gene, whose transcription is regulated by the levels of iron and copper in the growth medium. YSL2 has an unusual pattern of distribution within the plasma membranes of vascular parenchyma cells that is consistent with a function in moving metals laterally within veins. Based on its expression pattern, the major function of YSL2 seems to be in lateral movement of metals within the vasculature. This movement appears to be most critical when iron is abundantly available, since YSL2 expression under iron-deficient growth conditions is reduced.