EXPRESSION OF NUCLEAR AND CHLOROPLAST PHOTOSYNTHESIS-SPECIFIC GENES DURING LEAF SENESCENCE

During senescence of the primary leaf of bean (Phaseolus vulgaris L. cv. Kinghorn), total RNA declines by almost-equal-to 10-fold, soluble protein declines by almost-equal-to 3-fold and there is a decline in photosynthetic capability. Pulse-labelling studies with S-35-methionine have indicated that there is also a decline in synthesis of both nuclear- and chloroplast-encoded photosynthetic proteins, specifically the D-1 protein of Photosystem Il (PSII) and the ribulose 1,5 bisphosphate carboxylase/oxygenase (RubisCO) large subunit (LSU), which are chloroplast-encoded, and the 26 kD light harvesting chlorophyll binding protein (LHCP) and RubisCO small subunit (SSU), which are nuclear-encoded. Dot blot and Northern blot hybridization studies with gene-specific probes have indicated that transcript levels for the chloroplast genes RRN, psbA and rbcL, which encode chloroplast rRNA, the D-1 protein of PSII and LSU, respectively, remain a constant proportion of total RNA throughout senescence. By contrast, transcript levels for cab and rbcS, which are nuclear genes encoding the 26 kD protein of LHCP and SSU, respectively, comprise a progressively decreasing proportion of total RNA as senescence progresses, whereas the transcript for peroxidase, a non-photosynthetic nuclear-encoded protein, remains a constant proportion of total RNA over the same period. The data suggest that nuclear photosynthetic genes and chloroplast genes are differentially regulated in senescing leaves and follow patterns of expression similar to those observed at other stages of development.