Translocation Pathways of Micronutrients to Seeds.

Seeds are of immense importance as human and animal food sources. Understanding the basic physiological and molecular processes that result in loading of minerals into seeds will assist in the development of crop varieties with improved nutritional quality.  Two members of the Yellow Stripe-Like family of metal micronutrient-nicotianamine transporters, YSL1 and YSL3, are involved in translocation of Fe, Zn, and Cu through aboveground Arabidopsis tissues.  These genes are expressed near vascular tissues in roots, leaves, silique hulls, and floral tissues.  Null ysl1ysl3 mutants are prone to chlorosis resembling Fe deficiency, and decreased fertility under high light.  These YSL genes have increased expression in senescing leaves, suggesting a role for these genes in remobilization of micronutrients from vegetative tissues.  Quantification of mineral contents over the plant life cycle showed that remobilization from vegetative tissues contributed to seed Cu, Fe and Zn content.  The ysl1ysl3 mutants accumulated excess Fe, Cu and Zn in leaves and silique hulls, but had lower quantities in stems, suggesting that xylem transport was unaffected, while phloem transport of these micronutrients was inhibited.  These results show that these minerals are trafficked through leaves and silique hulls en route to seeds.  The NAM (no apical meristem) genes in wheat are involved in regulation of leaf senescence.  In NAM RNAi lines leaf senescence is delayed, and net remobilization of Fe and Zn from vegetative tissues is inhibited, and less Fe and Zn was found in the grain.  Remobilization accounted for a substantial percent of grain Fe and Zn, however, the decreased NAM expression effect was not limited to remobilization, as newly taken up Zn was translocated to grain less efficiently in the RNAi line than in the control line. This is an indication that even during terminal senescence, root uptake is an important source of micronutrients for developing seeds.