The Molecular Mechanisms Underlying Iron Deficiency Responses in Rice

Abstract

Iron (Fe) is an essential element required for plant growth and development. Under Fe-deficientconditions, plants have developed two distinct strategies (designated as strategy I and II) to acquire Fe from soil. As a graminaceous species, rice is not a typical strategy II plant, as it not only synthesizes DMA (2'-deoxymugineic acid) in roots to chelate Fe³⁺ but also acquires Fe²⁺ through transporters OsIRT1 and OsIRT2. During the synthesis of DMA in rice, there are three sequential enzymatic reactions catalyzed by enzymes NAS (nicotianamine synthase), NAAT (nicotianamine aminotransferase), and DMAS (deoxymugineic acid synthase). Many transporters required for Fe uptake from the rhizosphere and internal translocation have also been identified in rice. In addition, the signaling networks composed of various transcription factors (such as IDEF1, IDEF2, and members of the bHLH (basic helix-loop-helix) family), phytohormones, and signaling molecules are demonstrated to regulate Fe uptake and translocation. This knowledge greatly contributes to our understanding of the molecular mechanisms underlying iron deficiency responses in rice.

Keywords: Fe acquisition; phytohormones; rice (Oryza sativa), Fe deficiency; strategy II; transcription factors; transporters.

Figure 1

A simplified model of Fe uptake from the rhizosphere in rice. Rice not only synthesizes DMA in roots to chelate Fe³⁺ but also acquires Fe²⁺ through transporters OsIRT1 and OsIRT2. During the synthesis of DMA, there are three sequential enzymatic reactions catalyzed by the enzyme, OsNAS, OsNAAT, and OsDMAS. The TOM (transporter of mugineic acid family phytosiderophores) and OsYSL transporters are required for Fe uptake from the rhizosphere. OsPEZ2s are phenolic efflux transporters responsible for the transport of protocatechuic acid/caffeic acid (PCA/CA). The root cell is shown in the grey rounded rectangle. The vesicle is shown in the light green ellipse. Transporters are shown in the light orange ellipses.

Figure 2

A simplified model of internal Fe translocation in rice. Citrate, NA, and DMA are the main chelators used to bind Fe within rice. OsFRDL1 encodes a citrate effluxer. OsYSL transporters are responsible for the translocation of Fe³⁺-DMA and Fe²⁺-NA from xylem to phloem. Xylem and phloem are shown in rounded rectangles. Transporters are shown in light green ellipses.

Figure 3

The regulatory networks of the genes involved in Fe uptake and translocation in rice roots. Transcription factors are shown in light purple ellipses. Other regulatory proteins are shown in pink rounded rectangles. Hormones and signaling molecules are shown in light blue rounded rectangles. The genes involved in Fe uptake and translocation are shown in light orange rounded rectangles. Positive regulation is indicated by black arrows. Negative regulation is indicated by black blunt arrows. Broken lines indicate regulation with unknown mechanisms. Orange lines indicate that Fe signals are sensed by IDEF1, OsHRZ1, and OsHRZ2.