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Large Expression Differences in Genes for Iron and Zinc Homeostasis, Stress Response, and Lignin Biosynthesis Distinguish Roots of Arabidopsis thaliana and the Related Metal Hyperaccumulator Thlaspi caerulescensJudith E. van de Mortel, Laia Almar Villanueva, Henk Schat, Jeroen Kwekkeboom, Sean Coughlan, Perry D. Moerland, Emiel Ver Loren van Themaat, Maarten Koornneef, and Mark G.M. AartsPlant Physiology (2006) November; 142(1) 1127–1147
Tuesday, April 13, 2010
420 genes not expressed in root
Little variation in expression among conditions
Less expression in PDF genes
Less lignin biosynthesis genes
Less cellular process
Less transport process
Less stress response
420 genes expressed in root
High variation variation in expression among conditions
More expression in PDF genes
More lignin biosynthesis genes
More cellular process
More transport process
More stress response
More transcription2,272 genes were found to be highly express in T. caerulescens than A. thaliana
T. caerulescens is able to maintain nontoxic zinc levels while translocating high amounts of zinc to the leaves
An unexpected event occurs and that is that iron accumulates in the roots of Arabidopsis and T. caerulescens at increasing zinc concentrations
The effect found in both species suggests that the increase in iron uptake is due to prevent possible risks of iron deficiency in leaves.
Some genes known to be involved in zinc homeostasis are ZIP2, 4, 5 and 9, NAS2 and HMA2 genes
Highly expressed in zinc deficiency include ZIP1, 3, and 10, IRT3, MTP2, and NAS4
These transporters are involved in the transport of cations across plasma membrane. Not all of them are involved in the uptake of zinc in the same tissue.
It is likely that these transporters do similar functions in different parts of the roots or are found in intracellular membrane.
NAS2 and NAS4 genes are highly expressed in roots under deficiency rather than sufficiency.
YSL genes are also induced by zinc deficiency.
YSL genes are implicated in the transport of NA metal chelates within the plant and the entry of metals to the phloem and xylem.
YSL2 and YSL3 are slightly affected by different zinc treatments, which in turn lead them to find that genes were slightly induced by lower zinc concentrations.
Arabidopsis was grown under zinc deficient conditions
This was a significant observation because FRD3 has been known to be implicated for the most part with iron homeostasis.
FRO4 and FRO5 approximate the ferric chelate reductase gene FRO2, in the contrary to FRO2 their expression was not induced in the root of Arabidopsis upon iron deficiency.
The expression of these genes is related to a defense against oxidative stress caused by the treatment of zinc.
Large fraction of these genes was found to have an expressed comparison between wild type Arabidopsis and fit1 mutant.
For this species the response to zinc deficiency and zinc excess is quite different from Arabidopsis.
Unlike Arabidopsis, T. caerulescens expresses the ZIP family (ZIP3, 4, and 9) under zinc sufficient conditions
Similar to Arabidopsis, T. caerulescens also expresses a cluster of genes in zinc deficient conditions, but this cluster is quite smaller. The probable cause for this is differences in hybridization efficiency.
The expression of these three NAS genes in T. caerulescens suggest that they are a major function for metal adaptation.
The presence of these genes indicates that there will be flexibility when it comes to NAS gene expression
There are more than 2200 genes which are quite significant and differentially expressed in the three zinc treatments
50% of the genes found in T. caerulescens are of unknown function.
Stress respond genes expressed in T. caerulescens are different from Arabidopsis.
Several expressed genes were 100-fold on T. caerulescens. These genes were defensin genes or PDF genes
These genes included 15 genes which 4 were PDF genes. One of these PDF genes included one that was close to being 1000-fold which was expressed in both deficient and excess zinc. (PDF1.1)
The biological role of defensin is unclear.
IRT1, IRT2, and FRO2 (iron homeostasis genes) were not induced in T. caerulescens upon excess treatment of zinc.
Testing with RT-PCR did not detect the expression of TCIRT1 except in roots at lower levels of zinc.
This suggests that T. caerulescens is able to regulate its iron and zinc homeostasis independently, unlike Arabidopsis, or that continued expression of zinc transporters enables low efficiency but enough iron uptake in T. caerulescens.
High expression of 24 genes suggested a function in lignin biosynthesis, and 13 genes are involved in suberin biosynthesis in T. caerulescens.
These genes included (CER3, CER6,and 11 LTP genes)
CER3 is known to be expressed in the roots of Arabidopsis, but the expression of similar gene CER6 in the roots of T.caerulescens is quite different.
High expression of lignin and suberin biosynthesis concurs well with the U-shaped lignification and suberinization of the endodermis cells and the occasional presence of second endodermal layer found in the roots of T.caerulescens.
This U-shape apparenece is uncommon in plants, this usually occurs at older sections of the root hairs when they are no longer active.
Suggesting that this layer helps to prevent the eflux of metals from the vascular cylinder .