Plants have evolved a tremendous ability to respond to environmental changes by adapting their growth and development. The interaction between hormonal and developmental signals is a critical mechanism in the generation of this enormous plasticity. A good example is the response to the hormone ethylene that depends on tissue type, developmental stage, and environmental conditions. By characterizing the Arabidopsis wei8 mutant, we have found that a small family of genes mediates tissue-specific responses to ethylene. Biochemical studies revealed that WEI8 encodes a long-anticipated tryptophan aminotransferase, TAA1, in the essential, yet genetically uncharacterized, indole-3-pyruvic acid (IPA) branch of the auxin biosynthetic pathway. Analysis of TAA1 and its paralogues revealed a link between local auxin production, tissue-specific ethylene effects, and organ development. Thus, the IPA route of auxin production is key to generating robust auxin gradients in response to environmental and developmental cues.

SUMMARYOne of the most information-rich aspects of gene functional studies is characterization of gene expression profiles at cellular resolution, and subcellular localization of the corresponding proteins. These studies require visualization of the endogenous gene products using specific antibodies, or, more commonly, generation of whole-gene translational fusions with a reporter gene such as a fluorescent protein. To facilitate the generation of such translational fusions and to ensure that all cis-regulatory sequences are included, we have used a bacterial homologous recombination system (recombineering) to insert fluorescent protein tags into genes of interest harbored by transformation-competent bacterial artificial chromosomes (TACs). This approach has several advantages compared to other classical strategies. First, the researcher does not have to guess what the regulatory sequences of a gene are, as tens of thousands of base pairs flanking the gene of interest can be included in the construct. Second, because the genes of interest are not amplified by PCR, there are practically no limits to the size of a gene that can be tagged. Third, there are no restrictions on the location in which the fluorescent protein can be inserted, as the position is determined by sequence homology with the recombination primers. Finally, all of the required strains and TAC clones are publically available, and the experimental procedures described here are simple and robust. Thus, we suggest that recombineering-based gene tagging should be the gold standard for gene expression studies in Arabidopsis.

BackgroundRp1 is a complex locus of maize, which carries a set of genes controlling race-specific resistance to the common rust fungus, Puccinia sorghi. The resistance response includes the “Hypersensitive response” (HR), a rapid response triggered by a pathogen recognition event that includes localized cell death at the point of pathogen penetration and the induction of pathogenesis associated genes. The Rp1-D21gene is an autoactive allelic variant at the Rp1 locus, causing spontaneous activation of the HR response, in the absence of pathogenesis. Previously we have shown that the severity of the phenotype conferred by Rp1-D21 is highly dependent on genetic background.ResultsIn this study we show that the phenotype conferred by Rp1-D21 is highly dependent on temperature, with lower temperatures favoring the expression of the HR lesion phenotype. This temperature effect was observed in all the 14 genetic backgrounds tested. Significant interactions between the temperature effects and genetic background were observed. When plants were grown at temperatures above 30°C, the spontaneous HR phenotype conferred by Rp1-D21 was entirely suppressed. Furthermore, this phenotype could be restored or suppressed by alternately reducing and increasing the temperature appropriately. Light was also required for the expression of this phenotype. By examining the expression of genes associated with the defense response we showed that, at temperatures above 30°C, the Rp1-D21 phenotype was suppressed at both the phenotypic and molecular level.ConclusionsWe have shown that the lesion phenotype conferred by maize autoactive resistance gene Rp1-D21 is temperature sensitive in a reversible manner, that the temperature-sensitivity phenotype interacts with genetic background and that the phenotype is light sensitive. This is the first detailed demonstration of this phenomenon in monocots and also the first demonstration of the interaction of this effect with genetic background. The use of temperature shifts to induce a massive and synchronous HR in plants carrying the Rp1-D21 genes will be valuable in identifying components of the defense response pathway.

Ethylene is a gaseous plant hormone involved in several important physiological processes throughout a plant's life cycle. Decades of scientific research devoted to deciphering how plants are able to sense and respond to this key molecule have culminated in the establishment of one of the best characterized signal transduction pathways in plants. The ethylene signaling pathway starts with the perception of this gaseous hormone by a family of membrane-anchored receptors followed by a Raf-like kinase CTR1 that is physically associated with the receptors and actively inhibits downstream components of the pathway. A major gap is represented by the mysterious plant protein EIN2 that genetically works downstream of CTR1 and upstream of the key transcription factor EIN3. Transcriptional regulation by EIN3 and EIN3-family members has emerged as a key aspect of ethylene responses. The major components of this transcriptional cascade have been characterized and the involvement of post-transcriptional control by ubiquitination has been determined. Nevertheless, many aspects of this pathway still remain unknown. Recent genomic studies aiming to provide a more comprehensive view of modulation of gene expression have further emphasized the ample role of ethylene in a myriad of cellular processes and particularly in its crosstalk with other important plant hormones. This review aims to serve as a guide to the main scientific discoveries that have shaped the field of ethylene biology in the recent years.

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