No abstract

SummaryPlant nutrition critically depends on the activity of membrane transporters that translocate minerals from the soil into the plant and are responsible for their intra-and intercellular distribution. Most plant membrane transporters are encoded by multigene families whose members often exhibit overlapping expression patterns and a high degree of sequence homology. Furthermore, many inorganic nutrients are transported by more than one transporter family. These considerations, coupled with a large number of so-far non-annotated putative transporter genes, hamper our progress in understanding how the activity of speci®c transporters is integrated into a response to¯uctuating conditions. We designed an oligonucleotide microarray representing 1096 Arabidopsis transporter genes and analysed the root transporter transcriptome over a 96-h period with respect to 80 mM NaCl, K starvation and Ca 2 starvation. Our data show that cation stress led to changes in transcript level of many genes across most transporter gene families. Analysis of transcriptionally modulated genes across all functional groups of transporters revealed families such as V-type ATPases and aquaporins that responded to all treatments, and families ± which included putative non-selective cation channels for the NaCl treatment and metal transporters for Ca 2 starvation conditions ± that responded to speci®c ionic environments. Several gene families including primary pumps, antiporters and aquaporins were analysed in detail with respect to the mRNA levels of different isoforms during ion stress. Cluster analysis allowed identi®cation of distinct expression pro®les, and several novel putative regulatory motifs were discovered within sets of co-expressed genes.

The development of designed site-specific endonucleases boosted the establishment of gene targeting (GT) techniques in a row of different species. However, the methods described in plants require a highly efficient transformation and regeneration procedure and, therefore, can be applied to very few species. Here, we describe a highly efficient GT system that is suitable for all transformable plants regardless of transformation efficiency. Efficient in planta GT was achieved in Arabidopsis thaliana by expression of a sitespecific endonuclease that not only cuts within the target but also the chromosomal transgenic donor, leading to an excised targeting vector. Progeny clonal for the targeted allele could be obtained directly by harvesting seeds. Targeted events could be identified up to approximately once per 100 seeds depending on the target donor combination. Molecular analysis demonstrated that, in almost all events, homologous recombination occurred at both ends of the break. No ectopic integration of the GT vector was found.plant biotechnology | plant breeding | gene technology | double-strand-break repair S ince the first report on gene targeting (GT) in plants was published (1), various approaches were tested to improve the efficiency of the method (2-5), which has been summarized in recent reviews (6, 7). We were able to demonstrate that the integration of a transfer DNA (T-DNA) by homologous recombination (HR) into a specific locus could be enhanced by two orders of magnitude via double-strand-break (DSB) induction using a site-specific endonuclease (8). More recently, by the use of zinc finger nucleases (9), which, in principle, can be used to induce a DSB at any genomic site, endogenous loci have been targeted in Arabidopsis (10), tobacco (11), and maize (12) at high frequencies (13). Some time ago, a method for in vivo targeting was developed in Drosophila. A stably integrated donor precursor molecule is first circularized by the FLP recombinase and subsequently linearized via cutting a single I-SceI recognition site, generating the actual GT vector (14). However, such a technique has not been successfully transferred to plants. We have previously shown that DNA can be efficiently excised from the genome in planta by the use of a site-specific endonuclease (15). To test whether the combination of this approach with DSB-induced recombination might lead to an efficient GT system, that is independent of transformation, we performed a proof-of-concept (POC) experiment in Arabidopsis using I-SceI (16) as a sitespecific nuclease. ResultsGenerating Homozygous Single-Copy GT Lines. Our in planta GT system is based on three different constructs (two shown in Fig. 1A) that were transformed independently by floral dipping. The target locus contains a truncated β-glucuronidase (GUS) gene (uidA) that can be restored via GT. DSB induction at the two ISceI recognition sites flanking a kanamycin-resistance gene would result in excision of the kanamycin-resistance gene and in activation of the target locus for HR. Th...

We provide evidence that the tripeptide thiol glutathione (GSH) participates in the regulation of cell division in the apical meristem of Arabidopsis roots. Exogenous application of micromolar concentrations of GSH raised the number of meristematic cells undergoing mitosis, while depletion of GSH had the opposite effect. A role for endogenous GSH in the control of cell proliferation is also provided by mapping of GSH levels in the root meristem using the GSH-specific dye monochlorobimane and confocal laser scanning microscopy. High levels of GSH were associated with the epidermal and cortical initials and markedly lower levels in the quiescent center. The mechanisms controlling cell division could also be triggered by other reducing agents: ascorbic acid and dithiothreitol. Our data also reveal significant plasticity in the relationship between the trichoblast cell length and the hair it subtends in response to alterations in intracellular redox homeostasis. While mechanisms that control trichoblast elongation are inf luenced by nonspecific redox couples, root hair tip growth has a more specific requirement for sulfhydryl groups. The responses we describe here may represent the extremes of redox control of root plasticity and would allow the root to maintain exploration of the soil under adverse conditions with minimal cell divisions and root hair production or capitalize on a favorable environment by production of numerous long hairs. Redox sensing of the environment and subsequent redox-dependent modulation of growth and development may be crucial components in the strategies plants have evolved for survival in a f luctuating environment.