Chemistry Reference
In-Depth Information
OsYSL2
Aleurone
Companion cell
Seed
OsYSL2
OsIRT1
Companion cell
YSL ?
Mesophyll
Citrate-Fe
3+
Na-Fe
2+
Shoot
YSL ?
MA-Zn
2+
MA-Fe
3+
Fe
2+
ZmYS1
OsYSL15
OsIRT1
OsFRDL1
Citrate
Epidermis
Root
Cortex
Endodermis
Pericycle
Xylem
Phloem
FIGURE 8.9
Fe and Zn are taken up as phytosiderophore chelates by YSL transporters in the epidermis. Fe can also be taken up by OsIRT1.
Metals move through the symplastic space to the vasculature, bypassing the waxy Casparian strip on the endodermis. The citrate effluxer
FRDL1 is important for loading of citrate into the xylem and subsequent Fe transport to the shoot through the transpiration stream. YSL
transporters also may play a role in unloading the xylem into the shoot and the phloem. Fe is unloaded from the phloem by OsYSL2 and OsIRT1
into shoot and seed tissue. The dark brown boxes represent the Casparian strip. MA, mugineic acid; NA, nicotianamine.
(Adapted from Palmer
& Guerinot, 2009.)
which is transported into the chloroplast by HMA1, PAA1, and possibly PIC1. PAA2 is thought to transport Cu
across the thylakoid membrane. Transport of Fe into the chloroplast is known to require reduction by FRO7 and
may involve transport by PIC1. Fe and Cu must be transported into the mitochondria, but very little is known,
although ATM3 is well established as an Fe
S exporter. In seeds, the vacuole plays an important part in storage of
essential metals. Fe is transported into the vacuole by VIT1, Zn is transported into the vacuole by MTP1 (or
MTP3) and HMA3, and Fe is remobilised from the vacuole by NRAMP3 or NRAMP4.
Plants contain phytoferritins, which accumulate in non-green plastids
1
in conditions of iron loading. They are
targeted to the plastids by a putative transit peptide at their N-terminal extremity, and possess the specific residues
for ferroxidase activity and iron nucleation, found in mammalian H-type or L-type ferritin subunits. We already
mentioned the presence of metallothioneins in photosynthetic cyanobacteria, and it comes as no surprise that
metallothioneins as well as phytochelatins are found in plants where they probably function by protecting from
toxic metals.
e
2.
Iron, copper, and zinc homeostasis in plants (
Grotz & Guerinot, 2006
)
In Arabidopsis, the basic helix-loop-helix (BHLH) transcription factor, FIT, orthologous to tomato FER protein
regulates iron-deficiency responses. The expression of FIT in the fer mutant of tomato rescues the ability of the
1. Plastids are members of a family of organelles found in the cytoplasm of eukaryotic cells, all of which contain DNA and are bounded by
a double membrane.
