Gerontosomes, a Multifunctional Type of Peroxisome in Senescent Leaves

Abstract

Summary In rape cotyledons the short period of expansion and greening is almost immediately followed by senescence, as characterized by progressive losses of chlorophyll, protein and RNA. Transition from mature green to senescent is poorly defined if seedlings develop under a photoperiod, but excision and incubation of mature cotyledons in permanent darkness causes the abrupt induction of a hastened senescence process. Changes in activities of selected peroxisomal enzymes suggest that the senescent cotyledons are equipped with a multifunctional type of peroxisome. Thus, a marked increase of uricase activity points to a function of senescent peroxisomes in purine catabolism in connection with the mobilization of RNA. Increased activities of key enzymes of the glyoxylic acid cycle suggest a role in gluconeogenesis from fatty acids. Changes in uricase and isocitrate lyase are particularly pronounced if the senescence process is hastened upon the incubation of detached cotyledons in darkness. Under a photoperiod, rates of photosynthetic CO 2 -fixation remain high if calculated on the basis of residual chlorophyll. This is compatible with a retention of the photorespiratory key enzyme, glycolate oxidase, to a similar extent as the retention of chlorophyll. Taken together with published data on peroxisomal enzymes and peroxisomes, it appears that senescent leaves possess a type of microbody that is involved in all three major peroxisomal functions: glyoxysomal, peroxisomal and purine catabolic. «Gerontosome» may be an adequate term for this multifunctional type of peroxisome in senescent leaves.