Iron delivery to the growing leaves associated with leaf chlorosis in mugineic acid family phytosiderophores-generating graminaceous crops

Abstract

Graminaceous cereal crops as well as all other green plants require iron (Fe), as Fe is a component of heme and [Fe-S] clusters in the photosystems (PSI, PSII) containing green pigments (chlorophylls). The interveinal yellowing (Fe chlorosis) of growing leaves is caused by insufficient Fe uptake and internal delivery. In the 1970s, Sei-ichi Takagi discovered Fe3+-chelating mugineic acid family phytosiderophores (MAs), which are released from graminaceous roots to directly absorb Fe-MAs complexes. However, the mechanisms underlying the Fe delivery from root cells to terminal interveinal mesophyll cells need intensive investigation. This review first overviews the roles of metal-chelating compounds, i.e., MAs and nicotianamine (NA), and the Fe-chelate transporters involved in primary Fe partitioning; then, the delivery of Fe to the growing leaves via phloem and into the developing chloroplasts/thylakoids via symplastic diffusion and membrane transport is discussed. Fe-MAs are absorbed into the root epidermis cells by YSL transporters and transformed to Fe-NA for symplastic radial movement between root parenchyma cells and into the xylem. If heavy metal ions such as Co2+ and Cu2+ are simultaneously present, competition may occur at their complex formation with NA prior to the radial movement that causes leaf chlorosis because of heavy metal-induced restriction of Fe availability. In xylem saps, large fractions of Fe form Fe-MAs, as in barley plants (Hordeum vulgaris), instead of Fe-citrate, which is predominant in rice plants (Oryza sativa). The former complexes are transferred to the phloem at the stem nodes for the growing leaves, while Fe-citrate is partitioned to the mature leaves by transpiration. Although the major routes of Fe supply to the growing leaves by phloem transport have been confirmed, the mechanisms underlying the synthesis of phloem Fe-compounds, delivery through the phloem system, unloading at the growing sink leaves and Fe trafficking and utilization in the chloroplasts/thylakoids still require intensive investigation. Thus, insufficient root uptake of Fe-MAs and insufficient phloem delivery of Fe to the growing leaves and finally to the chloroplasts may cause the reduced chlorophyll‒Fe-protein assembly (chlorosis). However, the schemes of iron phloem delivery presented in this review must be confirmed in future studies.