Search for Metal-Responsive Genes in Plants: Putative Roles in Metal Tolerance or Accumulation (Kasvien metallien sietoon ja kertymiseen vaikuttavien geenien etsintä)
Year: 2009
Type: dissertation
Abstract: A number of plant species have developed mechanisms to tolerate and/or accumulate quantities of heavy metals that would be toxic to most other plants.The molecular mechanisms are not fully understood, even though these important traits contribute both to the micronutrient content and toxic metal content of our food.T h e a i m o f t h i s t h e s i s w a s t o i s o l a t e z i n c -r e s p o n s i v e g e n e s f r o m t h e c a d m i u m / z i n c hyperaccumulator Thlaspi caerulescens, which might contribute to the metal accumulation or tolerance trait.Using Differential Display, 16 differentially expressed genes were isolated from two T. caerulescens accessions, including metallothionein (MT) type 2 and type 3 genes, a MRP-like transporter gene as well as genes of unknown function.The role of the metal-binding proteins, metallothioneins (MTs), was studied in more detail in three plant species: the hyperaccumulator T. caerulescens, the non-hyperaccumulator metallophyte Silene vulgaris, and hybrid aspen (Populus tremula x tremuloides).The expression levels of MTs associated with the origins of the T. caerulescens and S. vulgaris accessions, with higher expression in accessions originating from metal-enriched soils.The same observation was made with intraspecies crosses, in which the F 3 lines that harbored an allele from the coppertolerant S. vulgaris or from a superior cadmium/zinc-accumulating T. caerulescens accession had higher SvMT2b or TcMT2a and TcMT3 expression levels compared to the corresponding sensitive or less accumulating plants, respectively.The expression of TcMTs did not co-segregate with zinc accumulation capacity in T. caerulescens.Neither did the SvMT2b expression co-segregate with copper tolerance in crosses between sensitive and tolerant plants.However, SvMT2b expression cosegregated with copper tolerance in families derived from crosses between moderately tolerant S. vulgaris plants, suggesting that higher MT2b expression was able to increase copper tolerance, albeit depending on the genetic background, MT2b thus being a candidate for hypostatic enhancer of copper tolerance.In yeast, MTs were able to increase cadmium and copper tolerance.Moreover, in hybrid aspen grown on a metal-contaminated site, PttMT2b expression correlated positively with foliar cadmium and zinc concentrations.However, Arabidopsis plants transformed with TcMT2 or TcMT3 did not show increased metal tolerance or accumulation; instead, they showed reduced root growth a t l o w e x t e r n a l c a d m i u m o r z i n c e x p o s u r e , i m p l y i n g t h a t t h e T c M T 2 a a n d T c M T 3 , w h e n overexpressed, might disturb metal homeostasis at low metal availability.In conclusion, these results indicate that TcMT2a and TcMT3 are not primary determinants of zinc accumulation in Thlaspi.However, the elevated expression levels in the metallicolous accessions suggest that they contribute to the metal-adapted phenotype, possibly through improving copper homeostasis at high zinc and cadmium body burdens.Alternatively, they might function as hypostatic enhancers of copper, zinc or cadmium tolerance.
Author: Viivi H. Hassinen
Language: Finnish
Subfield: Plant Science
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W3095566537