| Three new Arabidopsis thaliana leaf senescence-associated genes (SAGs) were isolated. The expression patterns of these and several other SAGs in attached and/or detached leaves were examined in response to various stress and hormone treatments. Responses varied from gene to gene and with leaf age, but most of the SAGs were induced by most of the treatments, particularly in older leaves. Some of the SAGs were induced quickly by the treatments, suggesting a direct response. Others were induced more slowly, suggesting an indirect response. One of the SAGs, SAG12, was only observed to be induced by treatments that caused visible senescence, and with kinetics that paralleled that senescence, and so appears to be senescence-specific. Antibodies were made to two of the more promiscuously-induced SAGs, ERD1 and BCB, and their expression examined at the protein level. BCB protein and mRNA levels paralleled each other, with both induced by darkness, dehydration, and ethylene, particularly in the older leaves. ERD1 protein and mRNA expression patterns, however, differed greatly from each other, with older leaves having high levels of mRNA but undetectable protein, and younger leaves having high levels of protein but low mRNA. It was concluded that BCB likely plays a broad role in both stress and senescence responses, but that ERD1 may not play a role in senescence, despite what its mRNA expression suggests. Some of the SAGs were then used as molecular markers to examine darkness-induced senescence. Darkness is often referred to as an inducer of senescence, but most of the work that has been done on the phenomenon so far has focused on detached leaves. When instead whole plants were placed in the darkness it was observed that senescence was not visibly induced, and in fact was repressed, and that although certain SAGs were induced the most senescence-specific one, SAG12, was not. When individual leaves were darkened, in contrast, senescence was induced by all criteria measured. It was concluded that darkness can have two separate affects on senescence, depending on the level at which it is perceived: whole plant darkness inhibits senescence, individual leaf darkness promotes it. |
