A PARTHENOGENESIS allele from apomictic dandelion can induce egg cell division without fertilization in lettuce.

Apomixis, the clonal formation of seeds, is a rare yet widely distributed trait in flowering plants. We have isolated the PARTHENOGENESIS (PAR) gene from apomictic dandelion that triggers embryo development in unfertilized egg cells. PAR encodes a K2-2 zinc finger, EAR-domain protein. Unlike the recessive sexual alleles, the dominant PAR allele is expressed in egg cells and has a miniature inverted-repeat transposable element (MITE) transposon insertion in the promoter. The MITE-containing promoter can invoke a homologous gene from sexual lettuce to complement dandelion LOSS OF PARTHENOGENESIS mutants. A similar MITE is also present in the promoter of the PAR gene in apomictic forms of hawkweed, suggesting a case of parallel evolution. Heterologous expression of dandelion PAR in lettuce egg cells induced haploid embryo-like structures in the absence of fertilization. Sexual PAR alleles are expressed in pollen, suggesting that the gene product releases a block on embryogenesis after fertilization in sexual species while in apomictic species PAR expression triggers embryogenesis in the absence of fertilization.

Plant Breeding: Surprisingly, Less Sex Is Better.

Introduction of apomixis, asexual reproduction through seeds, into crop species has the potential to dramatically transform plant breeding. A new study demonstrates that traits can be stably transferred between generations in newly produced apomictic lines, and heralds a breeding revolution needed to increase food production for the growing planet.

Comparison of two ecotypes of the metal hyperaccumulator Thlaspi caerulescens (J. & C. PRESL) at the transcriptional level.

This paper investigates differences in gene expression among the two Thlaspi caerulescens ecotypes La Calamine (LC) and Lellingen (LE) that have been shown to differ in metal tolerance and metal uptake. LC originates from a metalliferous soil and tolerates higher metal concentrations than LE which originates from a non-metalliferous soil. The two ecotypes were treated with different levels of zinc in solution culture, and differences in gene expression were assessed through application of a cDNA microarray consisting of 1,700 root and 2,700 shoot cDNAs. Hybridisation of root and shoot cDNA from the two ecotypes revealed a total of 257 differentially expressed genes. The regulation of selected genes was verified by quantitative reverse transcriptase polymerase chain reaction. Comparison of the expression profiles of the two ecotypes suggests that LC has a higher capacity to cope with reactive oxygen species and to avoid the formation of peroxynitrite. Furthermore, increased transcripts for the genes encoding for water channel proteins could explain the higher Zn tolerance of LC compared to LE. The higher Zn tolerance of LC was reflected by a lower expression of the genes involved in disease and defence mechanisms. The results of this study provide a valuable set of data that may help to improve our understanding of the mechanisms employed by plants to tolerate toxic concentrations of metal in the soil.

High-throughput detection of induced mutations and natural variation using KeyPoint technology.

Reverse genetics approaches rely on the detection of sequence alterations in target genes to identify allelic variants among mutant or natural populations. Current (pre-) screening methods such as TILLING and EcoTILLING are based on the detection of single base mismatches in heteroduplexes using endonucleases such as CEL 1. However, there are drawbacks in the use of endonucleases due to their relatively poor cleavage efficiency and exonuclease activity. Moreover, pre-screening methods do not reveal information about the nature of sequence changes and their possible impact on gene function. We present KeyPoint technology, a high-throughput mutation/polymorphism discovery technique based on massive parallel sequencing of target genes amplified from mutant or natural populations. KeyPoint combines multi-dimensional pooling of large numbers of individual DNA samples and the use of sample identification tags ("sample barcoding") with next-generation sequencing technology. We show the power of KeyPoint by identifying two mutants in the tomato eIF4E gene based on screening more than 3000 M2 families in a single GS FLX sequencing run, and discovery of six haplotypes of tomato eIF4E gene by re-sequencing three amplicons in a subset of 92 tomato lines from the EU-SOL core collection. We propose KeyPoint technology as a broadly applicable amplicon sequencing approach to screen mutant populations or germplasm collections for identification of (novel) allelic variation in a high-throughput fashion.

The heavy metal hyperaccumulator Thlaspi caerulescens expresses many species-specific genes, as identified by comparative expressed sequence tag analysis.

Thlaspi caerulescens is a natural zinc (Zn), cadmium (Cd) and nickel (Ni) hyperaccumulator and an emerging plant model species to study heavy metal hyperaccumulation and tolerance. This paper describes the analysis of the first expressed sequence tag (EST) collection from T. caerulescens. This collection is a new resource to unravel the molecular basis of plant metal homeostasis, tolerance and hyperaccumulation. In total, 4289 ESTs were generated originating from Zn-exposed root and shoot tissues, leading to 3709 T. caerulescens assembled partial cDNA sequences (unigenes). In comparison to Arabidopsis or other publicly available plant sequences, a fraction of c. 8% of the T. caerulescens unigenes (TcUGs) had no significant similarity with any known DNA sequence and, so far, these sequences are T. caerulescens specific. Three per cent of the TcUGs correspond to Arabidopsis thaliana orthologues that, as yet, have not been found to be expressed. The T. caerulescens transcriptome generally relates very well to the A. thaliana transcriptome, although, compared with other closely related species, a relatively large number of T. caerulescens-specific transcripts were found. T. caerulescens also expresses a relatively large number of genes which are expressed at a very low level in A. thaliana.

Transcription profiling of the metal-hyperaccumulator Thlaspi caerulescens (J. & C. PRESL).

Thlaspi caerulescens is a well-studied metal-hyperaccumulator of zinc, cadmium and nickel, belonging to the Brassicaceae family. Moreover it is one of the few hyperaccumulators that occur on different metalliferous soil types, as well as on nonmetalliferous soils. We are interested in the development of systems to improve phytoremediation of metal contaminated soils through improved metal-accumulation. About 1900 cDNAs isolated from T. caerulescens roots were hybridized with reverse transcribed RNA from zinc-treated T. caerulescens plants of two accessions originating from two different soil types. This comparative transcript profiling of T. caerulescens plants resulted in the identification of genes that are affected by heavy metals. The developed microarray proved to be an appropriate tool for a large scale analysis of gene expression in this metal-accumulator species.