Nicotianamine synthase 2 localizes to the vesicles of iron-deficient rice roots, and its mutation in the YXXφ or LL motif causes the disruption of vesicle formation or movement in rice.

Graminaceous plants release mugineic acid family phytosiderophores (MAs) to acquire iron from the soil. Here, we show that deoxymugineic acid (DMA) secretion from rice roots fluctuates throughout the day, and that vesicles accumulate in roots before MAs secretion. We developed transgenic rice plants that express rice nicotianamine (NA) synthase (NAS) 2 (OsNAS2) fused to synthetic green fluorescent protein (sGFP) under the control of its own promoter. In root cells, OsNAS2-sGFP fluorescence was observed in a dot-like pattern, moving dynamically within the cell. This suggests that these vesicles are involved in NA and DMA biosynthesis. A tyrosine motif and a di-leucine motif, which have been reported to be involved in cellular transport, are conserved in all identified NAS proteins in plants. OsNAS2 mutated in the tyrosine motif showed NAS activity and was localized to the vesicles; however, these vesicles stuck together and did not move. On the other hand, OsNAS2 mutated in the di-leucine motif lost NAS activity and did not localize to these vesicles. The amounts of NA and DMA produced and the amount of DMA secreted by OsNAS2-sGFP plants were significantly higher than in non-transformants and domain-mutated lines, suggesting that OsNAS2-sGFP, but not the mutated forms, was functional in vivo. Overall, the localization of NAS to vesicles and the transport of these vesicles are crucial steps in NA synthesis, leading to DMA synthesis and secretion in rice.
(© 2013 The Authors The Plant Journal © 2013 John Wiley & Sons Ltd.)