Auxin signaling. Homing In with targeted genetics

Transgenic poplar expressing Arabidopsis YUCCA6 exhibits auxin-overproduction phenotypes and increased tolerance to abiotic stress

YUCCA6, a member of the YUCCA family of flavin monooxygenase-like proteins, is involved in the tryptophan-dependent IAA biosynthesis pathway and responses to environmental cues in Arabidopsis. However, little is known about the role of the YUCCA pathway in auxin biosynthesis in poplar. Here, we generated transgenic poplar (Populus alba × P. glandulosa) expressing the Arabidopsis YUCCA6 gene under the control of the oxidative stress-inducible SWPA2 promoter (referred to as SY plants). Three SY lines (SY7, SY12 and SY20) were selected based on the levels of AtYUCCA6 transcript. SY plants displayed auxin-overproduction morphological phenotypes, such as rapid shoot growth and retarded main root development with increased root hair formation. In addition, SY plants had higher levels of free IAA and early auxin-response gene transcripts. SY plants exhibited tolerance to drought stress, which was associated with reduced levels of reactive oxygen species. Furthermore, SY plants showed delayed hormone- and dark-induced senescence in detached leaves due to higher photosystem II efficiency and less membrane permeability. These results suggest that the conserved IAA biosynthesis pathway mediated by YUCCA family members exists in poplar.

Synthetic plant promoters containing defined regulatory elements provide novel insights into pathogen- and wound-induced signaling

Pathogen-inducible plant promoters contain multiple cis-acting elements, only some of which may contribute to pathogen inducibility. Therefore, we made defined synthetic promoters containing tetramers of only a single type of element and present evidence that a range of cis-acting elements (boxes W1, W2, GCC, JERE, S, Gst1, and D) can mediate local gene expression in planta after pathogen attack. The expression patterns of the promoters were monitored during interactions with a number of pathogens, including compatible, incompatible, and nonhost interactions. Interestingly, there were major differences in the inducibilities of the various promoters with the pathogens tested as well as differences in the speed of induction and in the basal expression levels. We also show that defense signaling is largely conserved across species boundaries at the cis-acting element level. Many of these promoters also direct local wound-induced expression, and this provides evidence for the convergence of resistance gene, nonhost, and wound responses at the level of the promoter elements. We have used these cis-acting elements to construct improved synthetic promoters and show the effects of varying the number, order, and spacing of such elements. These promoters are valuable additions to the study of signaling and transcriptional activation during plant-pathogen interactions.

Molecular genetics of auxin signaling

The plant hormone auxin is a simple molecule similar to tryptophan, yet it elicits a diverse array of responses and is involved in the regulation of growth and development throughout the plant life cycle. The ability of auxin to bring about such diverse responses appears to result partly from the existence of several independent mechanisms for auxin perception. Furthermore, one prominent mechanism for auxin signal transduction involves the targeted degradation of members of a large family of transcriptional regulators that appear to participate in complex and competing dimerization networks to modulate the expression of a wide range of genes. These models for auxin signaling now offer a framework in which to test how each specific response to auxin is brought about.

A mutation in the Arabidopsis HYL1 gene encoding a dsRNA binding protein affects responses to abscisic acid, auxin, and cytokinin

Both physiological and genetic evidence indicate interconnections among plant responses to different hormones. We describe a pleiotropic recessive Arabidopsis transposon insertion mutation, designated hyponastic leaves (hyl1), that alters the plant's responses to several hormones. The mutant is characterized by shorter stature, delayed flowering, leaf hyponasty, reduced fertility, decreased rate of root growth, and an altered root gravitropic response. It also exhibits less sensitivity to auxin and cytokinin and hypersensitivity to abscisic acid (ABA). The auxin transport inhibitor 2,3,5-triiodobenzoic acid normalizes the mutant phenotype somewhat, whereas another auxin transport inhibitor, N-(1-naph-thyl)phthalamic acid, exacerbates the phenotype. The gene, designated HYL1, encodes a 419-amino acid protein that contains two double-stranded RNA (dsRNA) binding motifs, a nuclear localization motif, and a C-terminal repeat structure suggestive of a protein-protein interaction domain. We present evidence that the HYL1 gene is ABA-regulated and encodes a nuclear dsRNA binding protein. We hypothesize that the HYL1 protein is a regulatory protein functioning at the transcriptional or post-transcriptional level.

A mutation in the Arabidopsis HYL1 gene encoding a dsRNA binding protein affects responses to abscisic acid, auxin, and cytokinin

Both physiological and genetic evidence indicate interconnections among plant responses to different hormones. We describe a pleiotropic recessive Arabidopsis transposon insertion mutation, designated hyponastic leaves (hyl1), that alters the plant's responses to several hormones. The mutant is characterized by shorter stature, delayed flowering, leaf hyponasty, reduced fertility, decreased rate of root growth, and an altered root gravitropic response. It also exhibits less sensitivity to auxin and cytokinin and hypersensitivity to abscisic acid (ABA). The auxin transport inhibitor 2,3,5-triiodobenzoic acid normalizes the mutant phenotype somewhat, whereas another auxin transport inhibitor, N-(1-naph-thyl)phthalamic acid, exacerbates the phenotype. The gene, designated HYL1, encodes a 419-amino acid protein that contains two double-stranded RNA (dsRNA) binding motifs, a nuclear localization motif, and a C-terminal repeat structure suggestive of a protein-protein interaction domain. We present evidence that the HYL1 gene is ABA-regulated and encodes a nuclear dsRNA binding protein. We hypothesize that the HYL1 protein is a regulatory protein functioning at the transcriptional or post-transcriptional level.