Root formation in plants involves the continuous interpretation of positional cues. Physiological studies have linked root formation to auxins. An auxin response element displays a maximum in the Arabidopsis root and we investigate its developmental significance. Auxin response mutants reduce the maximum or its perception, and interfere with distal root patterning. Polar auxin transport mutants affect its localization and distal pattern. Polar auxin transport inhibitors cause dramatic relocalization of the maximum, and associated changes in pattern and polarity. Auxin application and laser ablations correlate root pattern with a maximum adjacent to the vascular bundle. Our data indicate that an auxin maximum at a vascular boundary establishes a distal organizer in the root.

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1 , the ethylene response mutant etr1 , and the SAR regulatory mutant npr1 , we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1 , etr1 , and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance. INTRODUCTIONPlants of which the roots have been colonized by selected strains of nonpathogenic fluorescent Pseudomonas spp develop an enhanced level of protection against pathogen attack (reviewed in van Loon et al., 1998). Strain WCS417r of P. fluorescens is a biological control strain that has been shown to trigger an induced systemic resistance (ISR) response in several plant species, including carnation (van Peer et al., 1991), radish (Leeman et al., 1995), tomato (Duijff et al., 1996), and Arabidopsis (Pieterse et al., 1996). In Arabidopsis, P. fluorescens WCS417r-mediated ISR has been demonstrated against the bacterial leaf pathogen P. syringae pv tomato , the fungal root pathogen Fusarium oxysporum f sp raphani (Pieterse et al., 1996;van Wees et al., 1997), and the fungal leaf pathogen Peronospora parasitica (J. Ton and C.M.J. Pieterse, unpublished data), indicating that this type of biologically induced resistance is effective against different types of pathogens.ISR-inducing rhizobacteria show host specificity in regard to eliciting resistance (Leeman et al., 1995;van Wees et al., 1997), which indicates that specific recognition between protective bacteria and the plant is a prerequisite for the activation of the signaling cascade leading to ISR. The downstream signaling event...

Microarray transcript profiling and RNA interference are two new technologies crucial for large-scale gene function studies in multicellular eukaryotes. Both rely on sequence-specific hybridization between complementary nucleic acid strands, inciting us to create a collection of gene-specific sequence tags (GSTs) representing at least 21,500 Arabidopsis genes and which are compatible with both approaches. The GSTs were carefully selected to ensure that each of them shared no significant similarity with any other region in the Arabidopsis genome. They were synthesized by PCR amplification from genomic DNA. Spotted microarrays fabricated from the GSTs show good dynamic range, specificity, and sensitivity in transcript profiling experiments. The GSTs have also been transferred to bacterial plasmid vectors via recombinational cloning protocols. These cloned GSTs constitute the ideal starting point for a variety of functional approaches, including reverse genetics. We have subcloned GSTs on a large scale into vectors designed for gene silencing in plant cells. We show that in planta expression of GST hairpin RNA results in the 16 Present address:

Meristems are distinctive regions of plants that have capacity for continuous growth. Their developmental activity generates the majority of plant organs. It is currently unknown how cell division and cell differentiation are orchestrated in meristems, although genetic studies have demonstrated the relevance of a proper balance between the two processes. Root meristems contain a distinct central region of mitotically inactive cells, the quiescent centre, the function of which has remained elusive until now. Here we present laser ablation and genetic data that show that in Arabidopsis thaliana the quiescent centre inhibits differentiation of surrounding cells. Differentiation regulation occurs within the range of a single cell, in a manner strikingly similar to examples in animal development, such as during delamination of Drosophila neuroblasts. Our data indicate that pattern formation in the root meristem is controlled by a balance between short-range signals inhibiting differentiation and signals that reinforce cell fate decisions.

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1 , the ethylene response mutant etr1 , and the SAR regulatory mutant npr1 , we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1 , etr1 , and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance. INTRODUCTIONPlants of which the roots have been colonized by selected strains of nonpathogenic fluorescent Pseudomonas spp develop an enhanced level of protection against pathogen attack (reviewed in van Loon et al., 1998). Strain WCS417r of P. fluorescens is a biological control strain that has been shown to trigger an induced systemic resistance (ISR) response in several plant species, including carnation (van Peer et al., 1991), radish (Leeman et al., 1995), tomato (Duijff et al., 1996), and Arabidopsis (Pieterse et al., 1996). In Arabidopsis, P. fluorescens WCS417r-mediated ISR has been demonstrated against the bacterial leaf pathogen P. syringae pv tomato , the fungal root pathogen Fusarium oxysporum f sp raphani (Pieterse et al., 1996;van Wees et al., 1997), and the fungal leaf pathogen Peronospora parasitica (J. Ton and C.M.J. Pieterse, unpublished data), indicating that this type of biologically induced resistance is effective against different types of pathogens.ISR-inducing rhizobacteria show host specificity in regard to eliciting resistance (Leeman et al., 1995;van Wees et al., 1997), which indicates that specific recognition between protective bacteria and the plant is a prerequisite for the activation of the signaling cascade leading to ISR. The downstream signaling event...

environmental effects as a cause of father-son phenotypic similarity by rearing young in groups (Table 1).A slightly smaller mean body size of wild satellites has been interpreted as suggesting that poorly growing chicks disproportionately develop into satellites'. Small body size differences between morphs developed among our captives, despite ad libi-

No abstract

SummaryIn plants, sugars act as signalling molecules that control many aspects of metabolism and development. Arabidopsis plants homozygous for the recessive sucrose uncoupled-6 (sun6) mutation show a reduced sensitivity to sugars for processes such as photosynthesis, gene expression and germination. The sun6 mutant is insensitive to sugars that are substrates for hexokinase, suggesting that SUN6 might play a role in hexokinase-dependent sugar responses. The SUN6 gene was cloned by transposon tagging and analysis showed it to be identical to the previously described ABSCISIC ACID INSENSITIVE-4 (ABI4) gene. Our analysis suggests the involvement of abscisic acid and components of the abscisic acid signal transduction cascade in a hexokinase-dependent sugar response pathway. During the plant life cycle, SUN6/ABI4 may be involved in controlling metabolite availability in an abscisic acid-and sugardependent way.