T-DNA insertional mutagenesis for functional genomics in rice

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

T-DNA insertional mutagenesis for activation tagging in rice

We have developed a new T-DNA vector, pGA2715, which can be used for promoter trapping and activation tagging of rice (Oryza sativa) genes. The binary vector contains the promoterless beta-glucuronidase (GUS) reporter gene next to the right border. In addition, the multimerized transcriptional enhancers from the cauliflower mosaic virus 35S promoter are located next to the left border. A total of 13,450 T-DNA insertional lines have been generated using pGA2715. Histochemical GUS assays have revealed that the GUS-staining frequency from those lines is about twice as high as that from lines transformed with the binary vector pGA2707, which lacks the enhancer element. This result suggests that the enhancer sequence present in the T-DNA improves the GUS-tagging efficiency. Reverse transcriptase-PCR analysis of a subset of randomly selected pGA2715 lines shows that expression of the genes immediately adjacent to the inserted enhancer is increased significantly. Therefore, the large population of T-DNA-tagged lines transformed with pGA2715 could be used to screen for promoter activity using the gus reporter, as well as for creating gain-of-function mutants.

FLAGdb/FST: a database of mapped flanking insertion sites (FSTs) of Arabidopsis thaliana T-DNA transformants

A large collection of T-DNA insertion transformants of Arabidopsis thaliana has been generated at the Institute of Agronomic Research, Versailles, France. The molecular characterisation of the insertion sites is currently performed by sequencing genomic regions flanking the inserted T-DNA (FST). The almost complete sequence of the nuclear genome of A.thaliana provides the framework for organising FSTs in a genome oriented database, FLAGdb/FST (http://genoplante-info.infobiogen.fr). The main scope of FLAGdb/FST is to help biologists to find the FSTs that interrupt the genes in which they are interested. FSTs are anchored to the genome sequences of A.thaliana and positions of both predicted genes and FSTs are shown graphically on sequences. Requests to locate the genomic position of a query sequence are made using BLAST programs. The response delivered by FLAGdb/FST is a graphical representation of the putative FSTs and of predicted genes in a 20 kb region.

Rice as a model for comparative genomics of plants

Rapid progress in rice genomics is making it possible to undertake detailed structural and functional comparisons of genes involved in various biological processes among rice and other plant species, such as Arabidopsis. In this review, we summarize the current status of rice genomics. We then select two important areas of research, reproductive development and defense signaling, and compare the functions of rice and orthologous genes in other species involved in these processes. The analysis revealed that apparently orthologous genes can also display divergent functions. Changes in functions and regulation of orthologous genes may represent a basis for diversity among plant species. Such comparative genomics in other plant species will provide important information for future work on the evolution of higher plants.

A new moss genetics: targeted mutagenesis in Physcomitrella patens

The potential of moss as a model system to study plant biology is associated with their relatively simple developmental pattern that nevertheless resembles the basic organization of the body plan of land plants, the direct access to cell-lineage analysis, their similar responses to plant growth factors and environmental stimuli as those observed in other land plants, and the dominance of the gametophyte in the life cycle that facilitates genetic approaches. Transformation studies in the moss Physcomitrella patens have revealed a totally unique feature for plants, i.e., that foreign DNA sequences integrate in the genome preferentially at targeted locations by homologous recombination, enabling for the first time in plants the application of the powerful molecular genetic approaches used routinely in bacteria, yeast, and since 1989, the mouse embryonic stem cells. This article reviews our current knowledge of Physcomitrella patens transformation and its unique suitability for functional genomic studies.

Toward a cytological characterization of the rice genome

Rice (Oryza sativa L.) will be the first major crop, as well as the first monocot plant species, to be completely sequenced. Integration of DNA sequence-based maps with cytological maps will be essential to fully characterize the rice genome. We have isolated a set of 24 chromosomal arm-specific bacterial artificial chromosomes to facilitate rice chromosome identification. A standardized rice karyotype was constructed using meiotic pachytene chromosomes of O. sativa spp. japonica rice var. Nipponbare. This karyotype is anchored by centromere-specific and chromosomal arm-specific cytological landmarks and is fully integrated with the most saturated rice genetic linkage maps in which Nipponbare was used as one of the mapping parents. An ideogram depicting the distribution of heterochromatin in the rice genome was developed based on the patterns of 4',6-diamidino-2-phenylindole staining of the Nipponbare pachytene chromosomes. The majority of the heterochromatin is distributed in the pericentric regions with some rice chromosomes containing a significantly higher proportion of heterochromatin than other chromosomes. We showed that pachytene chromosome-based fluorescence in situ hybridization analysis is the most effective approach to integrate DNA sequences with euchromatic and heterochromatic features.

Gene tagging in rice: a high throughput system for functional genomics

Rice, with its small genome size and well-characterized molecular information, is an ideal model plant for cereal genomics research. Sequence of the rice (Oryza sativa) genome will be determined by the International Rice Genome Sequencing Project (IRGSP) in the near future. Therefore, a large population of mutant plants should be required for adequately assigning function to the abundant sequence information. Here we summarize strategies as well as the progress that has been made in producing gene tags that may be invaluable for understanding the functional genomics of rice.

Gene targeting in Physcomitrella patens

Gene-targeting efficiency in the land plant Physcomitrella patens (Bryophyta) can only be compared with that observed in Saccharomyces cerevisiae. Sequencing programs and microbiological molecular genetic approaches are now being developed to unravel the precise function of plant genes. Physcomitrella patens, as the new 'green yeast', might well become a major tool for functional genomic studies of multicellular eukaryotes.