DIFFERENTIAL EXPRESSION OF GLUTAMINE-SYNTHETASE GENES DURING THE SENESCENCE OF ARABIDOPSIS-THALIANA ROSETTE LEAVES

Foliar senescence was investigated in Arabidopsis thaliana with regard to conventional parameters as well as the expression of glutamine synthetase (GS) genes. Protein, chlorophyll and nucleic acids reached the highest levels in the rosette when the first flower primordia emerged. There was a marked loss of protein concomitant with the development of the fruits. As much as 90% of the total protein and 70% of the nucleic acids were lost by the completion of yellowing of the rosette. Losses of protein and chlorophyll were hastened upon exposure of the plants to continuous darkness. A. thaliana contains three genes encoding cytosolic isoforms of GS. The mRNA pool of one of these isoforms originally identified to be expressed preferentially in root tissue [1] was found to be selectively increased in the senescent rosette. Its abundance was also found to be increased upon exposure to continuous darkness. The plastidic isoform was strongly expressed in young leaves whereas the mRNA vanished when senescence was initiated. The senescence specific isoform of GS is likely to be responsible for the re-assimilation of ammonia resulting from the catabolism of proteins and nucleic acids.