Changes in Patterns of Nucleic Acids and Proteins during Senescence

While senescence is a degradative process, this degradation is not merely the result of increased rates of protein turnover and decreases in the synthesis of proteins and RNA. Although general decreases in total protein and RNA levels are observed in senescing floral and foliar tissues, specific proteins and mRNAs have been found to increase (Brady, 1988; Borochov and Woodson, 1989). Experiments with inhibitors of protein and RNA synthesis have demonstrated that senescence is a genetically programmed process that requires the selective activation of specific RNAs and proteins, and does not merely result from the inhibition of cellular metabolism by declining rates of protein and RNA synthesis. These inhibitor studies have also suggested that transcription and protein synthesis in organelles is not central to the regulation of senescence. In support of the nuclear regulation of senescence, nuclear genes have been found to encode almost all of the mRNAs found to increase during senescence (Nooden, 1988). While the later stages of ripening resemble senescence, the entire process represents more of an interaction between degradative and synthetic processes. In contrast to senescing flowers and leaves, protein levels in fruits remain constant or increase slightly during ripening (Brady, 1988). Specific mRNAs and proteins that increase during ripening have also been identified (Brady, 1988; Gray et al., 1992). The focus of this chapter will be on those mRNAs and proteins that increase during senescence. These will be referred to as senescence-related (SR) genes.

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