Summary Plant growth responds rapidly to developmental and environmental signals, but the underlying changes in cell division activity are poorly understood. A labile cyclin‐GUS reporter was developed to facilitate the spatio‐temporal analysis of cell division patterns. The chimeric reporter protein is turned over every cell cycle and hence its histochemical activity accurately reports individual mitotic cells. Using Arabidopsis plants transformed with cyclin‐GUS, we visualized patterns of mitotic activity in wounded leaves which suggest a role for cell division in structural reinforcement.

Localized attack by a necrotizing pathogen induces systemic acquired resistance (SAR) to subsequent attack by a broad range of normally virulent pathogens. Salicylic acid accumulation is required for activation of local defenses, such as pathogenesis-related protein accumulation, at the initial site of attack, and for subsequent expression of SAR upon secondary, distant challenge. Although salicylic acid moves through the plant, it is apparently not an essential mobile signal. We screened Agrobacterium tumefaciens transfer DNA (tDNA) tagged lines of Arabidopsis thaliana for mutants specifically compromized in SAR. Here we show that Defective in induced resistance 1-1 (dir1-1) exhibits wild-type local resistance to avirulent and virulent Pseudomonas syringae, but that pathogenesis-related gene expression is abolished in uninoculated distant leaves and dir1-1 fails to develop SAR to virulent Pseudomonas or Peronospora parasitica. Petiole exudate experiments indicate that dir1-1 is defective in the production or transmission from the inoculated leaf of an essential mobile signal. DIR1 encodes a putative apoplastic lipid transfer protein and we propose that DIR1 interacts with a lipid-derived molecule to promote long distance signalling.

SummaryThe ATR and ATM protein kinases are known to be involved in a wide variety of responses to DNA damage. The Arabidopsis thaliana genome includes both ATR and ATM orthologs, and plants with null alleles of these genes are viable. Arabidopsis atr and atm mutants display hypersensitivity to c-irradiation. To further characterize the roles of ATM and ATR in response to ionizing radiation, we performed a short-term global transcription analysis in wild-type and mutant lines. We found that hundreds of genes are upregulated in response to cirradiation, and that the induction of virtually all of these genes is dependent on ATM, but not ATR. The transcript of CYCB1;1 is unique among the cyclin transcripts in being rapidly and powerfully upregulated in response to ionizing radiation, while other G 2 -associated transcripts are suppressed. We found that both ATM and ATR contribute to the induction of a CYCB1;1:GUS fusion by IR, but only ATR is required for the persistence of this response. We propose that this upregulation of CYCB1;1 does not reflect the accumulation of cells in G 2 , but instead reflects a still unknown role for this cyclin in DNA damage response.

During postembryonic plant development, cell division is coupled to cell growth. There is a stringent requirement to couple these processes in shoot and root meristems. As cells pass through meristems, they transit through zones with high rates of cell growth and proliferation during organogenesis. This transition implies a need for coordinate regulation of genes underpinning these two fundamental cell functions. Here, we report a mechanism for coregulation of cell division control genes and cell growth effectors. We identified a GCCCR motif necessary and sufficient for high-level cyclin CYCB1;1 expression at G 2/M. This motif is overrepresented in many ribosomal protein gene promoters and is required for high-level expression of the S27 and L24 ribosomal subunit genes we examined. p33 TCP20 , encoded by the Arabidopsis TCP20 gene, binds to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes in vitro and in vivo. We propose a model in which organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting teosinte-branched, cycloidea, PCNA factor (TCP) genes in the distal meristem boundary zone where cells become mitotically quiescent before expansion and differentiation.