10.9. Search for molecular mechanisms related to Zn accumulation and tolerance in Thlaspi caerulescens

Public examination of a doctoral dissertation in the field of plant biotechnology

Doctoral candidate: M.Sc.Marjo Tuomainen 

Date and venue: 10.9.2011, at 12 noon, L22, Snellmania, Kuopio campus 

ABSTRACT

Zinc is an essential element for all organisms. Plants have evolved various mechanisms to maintain optimal levels of Zn in their cells, and balance between deficiency and excess. If better characterized, these mechanisms could have practical applications in improving the nutritional value of agronomically important crop plants or in developing plants for use in phytoremediation to clean up Zn-contaminated sites. 

The aim of this study was to explore proteins underlying Zn accumulation and tolerance in the hyperccumulator plant Thlaspi caerulescens (currently: Noccaea caerulescens) using accessions originating from different geographic areas and showing pronounced variation in these traits. Protein profiles of three T. caerulescens accessions were clearly different, whereas the effects of Zn exposures were less pronounced. The 48 tentatively identified proteins with differences among the accessions, exposures or both represented diverse metabolic pathways including photosynthesis, energy and carbohydrate metabolism, oxidative stress, regulation and signaling. 

To reduce the variation detected in the proteomes among the accessions, possibly evolved due to environmental factors other than metals, protein patterns of two T. caerulescens accessions and their five cross-derived lines with contrasting Zn accumulation traits were studied. The number of protein spots showing differential levels between the lines was lower than that between the parental accessions, indicating that crossing was able to reduce the variation between the accessions. The levels of four proteins showed co-segregation with Zn accumulation traits both in the parental accessions and cross-derived lines: glutathione S-transferase, S-formylglutathione hydrolase, manganese superoxide dismutase and elongation factor. As there are other possible explanations to the co-segregation, further studies would be needed to prove the relevance of these proteins in Zn accumulation or tolerance. 

Glyoxalase 1 and metallothioneins (MT) were selected for further studies based on previous findings. Glyoxalase 1 was initially spotted in proteomic studies by its higher levels in the shoots of the more Zn-tolerant T. caerulescens accession compared to the less tolerant accessions. A homolog of glyoxalase 1-encoding gene, named as TcGLX1, was isolated and sequenced from three T. caerulescens accessions. Extensive transcript and protein analyses in T. caerulescens accessions and several inter-accession cross lines with differential Zn tolerance and accumulation traits showed no clear correlation between TcGLX1 expression and Zn tolerance or accumulation. When the expression of TcGLX1 homologue ATGLX1 was interrupted by T-DNA insertion in A. thaliana, no change in Zn accumulation or tolerance was found. Therefore, in contrast to the previous literature, this evidence did not support a significant role for glyoxalase 1 either in Zn tolerance or accumulation. 

Previous evidence suggests that MTs play a role in metal homeostasis or in the protection of cells from oxidative damage. Transcript levels of TcMT2s and TcMT3 in T. caerulescens accessions and inter-accession cross lines showed no definitive correlation with either Zn tolerance or accumulation. Ectopic expression of TcMT2a and TcMT3 in A. thaliana did not alter its Zn accumulation or tolerance characteristics. MT2 was localized in the root in epidermal cells and root hairs, especially in the root tip, which would support its role in metal buffering. Overall the data suggest that MTs in T. caerulescens could be involved in establishing and modulating the metallicolous phenotype. 

In conclusion, glyoxalase 1 or MTs appear not to be direct determinants in Zn accumulation or tolerance in T. caerulescens. Proteomic profiling revealed many other proteins that may be involved in Zn-related processes and modulate the hyperaccumulation trait of T. caerulescens

The doctoral dissertation of Marjo Tuomainen entitled Search for molecular mechanisms related to Zn accumulation and tolerance in Thlaspi caerulescens” will be examined at the Faculty of Science and Forestry. The opponent in the public examination is Professor Eevi Rintamäki from the University of Turku and the custos is Professor Sirpa Kärenlampi of the University of Eastern Finland.

Photo available for download at http://www.uef.fi/vaitoskuvat

Contact: Marjo Tuomainen, marjo.tuomainen@uef.fi, tel. +358 40 355 3825

Publishing year: 2011

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