Novel Molecular Markers for Late Phases of the Growth Cycle of Arabidopsis thaliana Cell-Suspension Cultures Are Expressed during Organ Senescence

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Abstract
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, SRG1, 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 SRG1 protein is a new member of the Fe(II)/ascorbate oxidase superfamily, and that SRG2 codes for a protein with significant homology to β-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 SRG1 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.