Ms Jian Wu: "Paving the way for genetic improvement for zinc accumulation in Brassica rapa"

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29 Oct 2007 13:30
Unit: Wageningen University
Location: Aula, building 362, Gen. Foulkesweg 1, Wageningen
Organisation: Wageningen university
Promotor: prof.dr.ir. M. Koornneef (Genetics)
Co Promotor: Dr. M.G.M. Aarts, Prof.dr. Xiaowu Wang (Institute for vegetables and Flowers, Beijing) (co-promotor)

 Brassica rapa comprises several vegetable crops, some of which are among the most important vegetables in China. The main objective of the work is to explore the genetic potential for the improvement of Zn accumulation in B. rapa vegetables.

Screening of a germplasm collection with 188 accessions in total revealed that there was marked variation in accumulation of Zn (23.2-159.9 µg g-1), Fe (60.3-350.1 µg g-1) and Mn (20.9-53.3 µg g-1) in B. rapa leaves. A two-fold variation was found for dry-biomass based tolerance to Zn deficiency or excessive Zn.  Quantitative Trait Locus（QTL） analysis was carried out for the accumulation of 11 minerals in leaves and for tolerance to deficient or toxic Zn supplies by using a doubled haploid (DH) population of B. rapa. Seven QTLs were detected for Na, Mg, P, Mn, Zn and Sr leaf concentration and one QTL was detected for shoot dry biomass under Zn excess stress. The fact that no major QTL was detected in our study indicates that the genetic improvement of these traits by classical breeding will be very complicated, as it will require a combination of favourable alleles at many of the genes contributing to the traits.

Metal transporters play important roles in maintaining metal homeostasis in plants. We cloned four metal transporter full-length cDNAs by screening cDNA libraries of Thlaspi caerulescens, a Zn/Cd/Ni hyperaccumulator. The expression difference of these four genes (TcZNT5 TcZNT6, TcNRAMP3 and TcNRAMP3) between T. caerulescens and non-accumulator Arabidopsis thaliana and between two T. caerulescens accessions with contrasting Cd accumulation indicated that these genes were involved in metal hyperaccumulation. Furthermore, we showed inactivation of AtZIP5 increased Arabidopsis tolerance to Cd, but overexpression of the T. caerulescens orthologue TcZNT5 in Arabidopsis did not lead to any change in phenotype. It is in contrast for AtZIP6/TcZNT6. The Arabidopsis double mutant nramp3nramp4 showed hypersensitivity to Cd and toxic Zn which can be rescued by expression of TcNRAMP3. However, expression of TcNRAMP3 in nramp3nramp4 plants or TcNRAMP4 in wild-type plants did not lead to enhanced Zn or Cd tolerance. The data presented in this thesis indicate that enhanced Zn accumulation, Zn tolerance or Cd tolerance probably requires elevated and controlled expression of a set of genes, rather than only single metal transporter genes.

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