FIGURE 1.

Proposed model for OsNAC5 role in senescence and metal remobilization. The model is based on indirect evidence provided by several studies, especially on Sperotto et al. (2009) and Jeong et al. (2013), and shows only Fe remobilization, although a similar pathway is likely to be involved in Zn and other metals remobilization. (A) A signal is sensed by the cell, triggering the senescence-associated cellular components degradation, including chloroplasts, the main site of Fe concentration in the cell. (B) OsNAC5 transcription is up-regulated by the senescence downstream signaling pathway. (C) OsNAC5 protein is produced and triggers the up-regulation of OsNAS2 and OsYSL2 transcription (based on microarray data presented in the work performed by Jeong et al., 2013). The increased transcription observed is either directly or indirectly regulated by OsNAC5. (D) OsNAS2 protein increases intracellular NA concentration, which in turn chelates free Fe coming from chloroplast and other cellular components degradation. (E) The Fe–NA complex is transported across the plasma membrane of the senescent cell, and then transported into phloem by OsYSL2, which allows Fe–NA complex long distance translocation. It is important to point the possibility that Fe and NA are exported independently from the cell, interacting in the apoplast and then transported into the phloem by OsYSL2.