SummaryThe Ralstonia solanacearum hrp gene cluster is organized in five transcriptional units. Expression of transcriptional units 2, 3 and 4 is induced in minimal medium and depends on the hrp regulatory gene hrpB, which belongs to unit 1. This regulatory gene also controls the expression of genes, such as popA, located to the left of the hrp cluster. Here, we show that, upon co-culture with Arabidopsis thaliana and tomato cell suspensions, the expression of the hrp transcriptional units 1, 2, 3 and 4 is induced 10-to 20-fold more than in minimal medium. This induction is not triggered by diffusible signals but requires the presence of plant cells. Moreover, we show that this specific plant cell induction of hrp genes is controlled by a gene, called prhA (plant regulator of hrp genes), located next to popA. This gene codes for a putative protein of 770 amino acids, which shows similarities with TonB-dependent outer membrane siderophore receptors. Expression of prhA and hrp genes is not regulated by iron status, and we postulate that iron is not the signal sensed by PrhA. In prhA mutants, the induction of hrpB and other hrp genes is abolished in co-culture with Arabidopsis cells, partially reduced in co-culture with tomato cells and not modified in minimal medium. prhA mutants are hypoaggressive on Arabidopsis (accessions Col-0 and Col-5) but remain fully pathogenic on tomato plants, suggesting that the co-culture assays mimic the in planta conditions. A model suggesting that PrhA is a receptor for plant specific signals at the top of a novel hrp regulatory pathway is discussed.

In an Arabidopsis thaliana T87-C3 cell-suspension culture, entry into the growth-arrest phase is rapidly followed by a loss of cell viability. Three cDNA clones, SRGI, SRG2, and SRG3, corresponding to genes with transcripts that accumulate during these late phases, were isolated by the mRNA differential display method. Amino acid sequence analysis shows that the putative SRCl protein is a new member of the Fe(ll)/ascorbate oxidase superfamily, and that SRG2 codes for a protein with significant homology to P-glucosidases. Significantly, all three SRG genes are expressed in senescing organs of Arabidopsis plants. Two previously characterized genes, SAG2 and SAG4, induced during natural senescence in Arabidopsis, were also found to be expressed in cell-suspension cultures and have expression kinetics similar to those observed for the SRGl gene. Taken together these findings suggest that certain molecular events are common to both plant senescence and growth arrest in Arabidopsis cell suspensions. Both internucleosomal cleavage of nDNA and an apparent compaction of chromatin, two characteristic features of programmed cell death in animal cells, have been observed in Arabidopsis cell cultures at a stage corresponding to loss of cell viability. ~~ ~Cultured cells are now universally recognized as appropriate model systems with which to investigate the molecular responses of plants to stimuli that affect growth, such as nutrient availability (e.g. Roitsch et al., 1995) and plant growth factor requirements (e.g. Teyssendier de la Serve et al., 1985;Dominov et al., 1992;Lee and Chen, 1993). For example, limitations in phosphate concentration (Kodama et al., 1990;Kock et al., 1995; Malboobi and Lefebvre, 1995), nitrogen supply (Kodama et al., 1990), carbon source (Kodama et al., 1990;Sheu et al., 1994;Hsieh et al., 1995;Tseng et al., 1995), or hormone depletion (Crowell and Amasino, 1991) result in modifications in gene expression, leading to changes in the cellular mRNA pattern.After nutrient exhaustion, certain cell cultures undergo rapid growth arrest, followed by a dramatic loss of viabil- ity. We are currently studying these late phases of the cell culture growth cycle, which exhibit a number of morphological symptoms similar to those that occur in senescing plant tissues, which have recently been described in sugarstarved, cultured rice cells (Chen et al., 1994).As a first step toward characterizing the molecular events associated with growth arrest and viability loss, we have attempted to select genes with a relative expression that is enhanced at the end of the growth phase of the cell culture. For this purpose, we chose an Arabidopsis fkaliana cell line (Axelos et al., 1992) with a particularly short growth-arrest period. Transcripts that accumulated at the end of the growth phase of the culture were characterized by the mRNA differential display method (Liang and Pardee, 1992). Three cDNA clones were isolated and the expression patterns of the corresponding genes were analyzed both throughout the cell culture gr...

New tissues and organs are generally initiated throughout the life of the plant from small, highly organized groups of cells called meristems (Medford, 1992). Occasionally, however, adventitious organ development occurs as a consequence of the reactivation of cell division in previously quiescent cells of differentiated tissues (Yang et al., 1994). A better understanding of how developing plants achieve such a tight control of proliferative activity in meristematic and differentiated tissues will come from the elucidation of the molecular events involved in the reentry and progression through the cell cycle in the plant cell (e.g.

The characterisation of an Arabidopsis thaliana cytochrome P450-encoding cDNA clone, B72, preferentially expressed during the hypersensitive response (HR) provoked by the bacterial pathogen Pseudomonas syringae pathovar maculicola, is reported. The B72 cDNA clone corresponded to the CYP76C2 gene, which belongs to a small multigene family comprising four genes. HR-triggering bacteria harbouring different avirulence genes induced the accumulation of transcripts of this P450 gene. CYP76C2 gene expression was moreover associated with various processes leading to cell death such as leaf senescence, ageing of cell cultures, wounding as well as with treatment with the necrotising heavy metal salt, lead nitrate.z 1998 Federation of European Biochemical Societies.