Polar auxin transport controls multiple developmental processes in plants, including the formation of vascular tissue. Mutations affecting the PIN-FORMED (PIN1) gene diminish polar auxin transport in Arabidopsis thaliana inflorescence axes. The AtPIN1 gene was found to encode a 67-kilodalton protein with similarity to bacterial and eukaryotic carrier proteins, and the AtPIN1 protein was detected at the basal end of auxin transport–competent cells in vascular tissue. AtPIN1 may act as a transmembrane component of the auxin efflux carrier.

The molecular mechanisms underlying gravity perception and signal transduction which control asymmetric plant growth responses are as yet unknown, but are likely to depend on the directional flux of the plant hormone auxin. We have isolated an Arabidopsis mutant of the AtPIN2 gene using transposon mutagenesis. Roots of the Atpin2::En701 null-mutant were agravitropic and showed altered auxin sensitivity, a phenotype characteristic of the agravitropic wav6-52 mutant. The AtPIN2 gene was mapped to chromosome 5 (115.3 cM) corresponding to the WAV6 locus and subsequent genetic analysis indicated that wav6-52 and Atpin2::En701 were allelic. The AtPIN2 gene consists of nine exons defining an open reading frame of 1944 bp which encodes a 69 kDa protein with 10 putative transmembrane domains interrupted by a central hydrophilic loop. The topology of AtPIN2p was found to be similar to members of the major facilitator superfamily of transport proteins. We have shown that the AtPIN2 gene was expressed in root tips. The AtPIN2 protein was localized in membranes of root cortical and epidermal cells in the meristematic and elongation zones revealing a polar localization. These results suggest that AtPIN2 plays an important role in control of gravitropism regulating the redistribution of auxin from the stele towards the elongation zone of roots.

The plant hormone auxin is transported in a polar manner along the shoot-root axis, which requires efflux carriers such as PIN1. Asymmetric localization of PIN1 develops from a random distribution in Arabidopsis early embryogenesis. Coordinated polar localization of PIN1 is defective in gnom embryos. GNOM is a membrane-associated guanine-nucleotide exchange factor on ADP-ribosylation factor G protein (ARF GEF). Thus, GNOM-dependent vesicle trafficking may establish cell polarity, resulting in polar auxin transport.

A widely applicable promoter system is described that allows a gene of interest to be activated in specific plant tissues after a cross between defined transgenic lines. The promoter, pOp, consists of lac operators cloned upstream of a minimal promoter. No expression was detected from this promoter when placed upstream of a ␤-glucuronidase (GUS) reporter gene in transgenic plants. Transcription from the promoter was activated by crossing reporter plants with activator lines that expressed a chimeric transcription factor, LhG4. This factor comprised transcription-activation domain-II from Gal4 of Saccharomyces cerevisiae fused to a mutant lac-repressor that binds its operator with increased affinity. When LhG4 was expressed from the CaMV 35S promoter, the spatial and quantitative expression characteristics of the 35S promoter were exhibited by the GUS reporter. The LhG4͞pOp system may be used to study toxic or deleterious gene products, to coordinate the expression of multiple gene products, to restrict transgene phenotypes to the F1 generation, and to generate hybrid seed. The LhG4 system offers spatially regulated gene expression in the tissues of whole plants growing under normal conditions without the need for external intervention. It complements inducible expression systems that offer temporal control of gene expression in tissues that can be treated with inducing chemicals.

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SummaryDerivatives of the Saccharomyces cerevisiae transcription factor Gal4 which act as effective transcription activators in yeast, Drosophila, mammalian cells and plant protoplasts are shown to direct expression from a GUS reporter construct when expressed in transgenic tobacco. However, in comparison to 35S-GUS controls, Gal4-mediated expression of the reporter gene was relatively weak and extremely variable. GUS expression was lost as plants matured and it was almost undetectable in most of their progeny. Gal4-mediated gene expression could be restored by treating tissues with 5-aza-cytidine, implicating cytosine methylation in the loss of Gal4-mediated expression. Restoration of reporter expression was not accompanied by an increase in steady-state levels of the activator transcript. We propose that the DNA-binding activity of Gal4 is sensitive to methylation of its binding site in plant chromatin. The Gal4-DNA co-crystal predicts that 5-methylcytosine at either of the outer two positions of the binding site will effectively prevent Gal4 binding. We show that these positions become extensively methylated in transgenic plants and that methylation of Gal4-binding sites interferes with Gal4 binding in vitro. These observations suggest that the Gal4 DNA-binding domain is intrinsically sensitive to cytosine methylation and that, despite the success of Gal4-based expression systems in yeast and Drosophila, Gal4 is not ideal for use in plant gene expression technology.