Arabidopsis thaliana

We describe the identification and structural characterization of a novel family of Arabidopsis genes related to ATL2 which encode a variant of the RING zinc finger domain, known as RING-H2. Analysis of genes selected by us and of sequences from Arabidopsis stored in databases permitted the prediction of several RING-H2 proteins that contain highly homologous RING domains. The ATL gene family is represented by fifteen sequences that contain, in addition to the RING, a transmembrane domain which is located in most of them towards the N-terminal end. Transgenic Arabidopsis seedlings carrying the ATL2 promoter fused to the GUS reporter gene revealed that the expression of ATL2 is rapidly induced after exposure to chitin or inactivated crude cellulase preparations. Rapid induction of transcript accumulation of another member of the ATL family was also observed under the same conditions. These results suggest that some ATLs may be involved in the early stages of the defense response triggered in plants in response to pathogen attack.

New insights into the phenomenon of systemic acquired resistance have been gained in recent years, by the use of techniques in molecular genetics and biology that have replaced the largely descriptive approach of earlier work. The isolation of mutants in the signal transduction pathway from induction to expression of resistance as well as the use of transgenic plants over-expressing or suppressing the expression of putative candidate genes involved in systemic acquired resistance and its signalling have identified several steps in the establishment of plant resistance. In this review the latest developments implicating salicylic acid as a signal molecule in systemic resistance are discussed and contrasted with new signalling pathways which, seemingly, are based on alternative mechanisms.

Plant pathogen resistance is mediated by a large repertoire of resistance (R) genes, which are often clustered in the genome and show a high degree of genetic variation. Here, we show that anArabidopsis thaliana R-gene cluster is also subject to epigenetic variation. We describe a heritable but metastable epigenetic variant bal that overexpresses the R-like geneAt4g16890 from a gene cluster on Chromosome 4. The balvariant and Arabidopsis transgenics overexpressing theAt4g16890 gene are dwarfed and constitutively activate the salicylic acid (SA)-dependent defense response pathway. Overexpression of a related R-like gene also occurs in the ssi1(suppressor of SA insensitivity1) background, suggesting that ssi1 is mechanistically related to bal.

There is now mounting evidence that heritable variation in ecologically relevant traits can be generated through a suite of epigenetic mechanisms, even in the absence of genetic
variation. Moreover, recent studies indicate that epigenetic variation in natural populations can be independent from genetic variation, and that in some cases environmentally induced epigenetic changes may be inherited by future generations. These novel findings are potentially highly relevant to ecologists because they could significantly improve our understanding of the mechanisms underlying natural phenotypic variation and the responses of organisms to environmental change. To understand the full significance of epigenetic processes, however, it is imperative to study them in an ecological context. Ecologists should therefore start using a combination of experimental approaches borrowed from ecological genetics, novel techniques to analyse and manipulate epigenetic variation, and genomic tools, to investigate the extent and structure of epigenetic variation within and among natural populations, as well as the interrelations between epigenetic variation, phenotypic variation and ecological interactions.

Extensive communication occurs between plants and microorganisms during different stages of plant development in which signaling molecules from the two partners play an important role. Volatile organic compounds (VOCs) emission by certain plant-growth promoting rhizobacteria (PGPR) has been found to be involved in plant growth. However, little is known about the role of bacterial VOCs in plant developmental processes. In this work, we investigated the effects of inoculation with twelve bacterial strains isolated from the rhizosphere of lemon plants (Citrus aurantifolia) on growth and development of Arabidopsis thaliana seedlings. Several bacterial strains showed a plant growth promoting effect stimulating biomass production, which was related to differential modulation of root-system architecture. The isolates L263, L266, and L272a stimulated primary root growth and lateral root development, while L254, L265a and L265b did not significantly alter primary root growth but strongly promoted lateral root formation. VOC emission analysis by SPME-GC-MS identified aldehydes, ketones and alcohols as the most abundant compounds common to most rhizobacteria. Other VOCs, including 1-octen-3-ol and butyrolactone were strain specific. Characterization of L254, L266 and L272a bacterial isolates by 16S rDNA analysis revealed the identity of these strains as Bacillus cereus, Bacillus simplex and Bacillus sp, respectively. Taken together, our data suggest that rhizospheric bacterial strains can modulate both plant growth promotion and root-system architecture by differential VOC emission.

To explore the potential evolutionary relevance of heritable epigenetic variation, the National Evolutionary Synthesis Center recently hosted a catalysis meeting that brought together molecular epigeneticists, experimental evolutionary ecologists, and theoretical population and quantitative geneticists working across a wide variety of systems. The group discussed the methods available to investigate epigenetic variation and epigenetic inheritance, and how to evaluate their importance for phenotypic evolution. We found that understanding the relevance of epigenetic effects in phenotypic evolution will require clearly delineating epigenetics within existing terminology and expanding research efforts into ecologically relevant circumstances across model and nonmodel organisms. In addition, a critical component of understanding epigenetics will be the development of new and current statistical approaches and expansion of quantitative and population genetic theory. Although the importance of heritable epigenetic
effects on evolution is still under discussion, investigating them in the context of a multidisciplinary approach could transform the field.

By selecting for germinating seeds in the progeny of mutagen-treated non-germinating gibberellin responsive dwarf mutants of the ga-1 locus in Arabidopsis thaliana, germinating lines (revertants) could be isolated. About half of the revertants were homozygous recessive for a gene (aba), which probably regulates the presence of abscisic acid (ABA). Arguments for the function of this gene were obtained from lines homozygous recessive for this locus only, obtained by selection from the F2 progeny of revertant X wild-type crosses. These lines are characterized by a reduced seed dormancy, symptoms of withering, increased transpiration and a lowered ABA content in developing and ripe seeds and leaves.

Large-scale and high throughput approaches increasingly play an essential role in the study of biological systems, which are per se highly complex. Therefore, they need to be examined by these extensive methods to receive information about the large genomic and proteomic networks. In plant biology, this purpose has a strong support through the accessability of the complete genome sequence of the model plant Arabidopsis thaliana. This brief review intends to focus on the basics and the state-of-the-art of these high-throughput technologies and their application to plant proteomics. It describes protein microarrays, the use of antibodies, 2-DE and MS methods and the yeast two hybrid system, which are emerging as the major technologies for plant proteomics.