Expression of photosynthesis- and senescence-related genes during leaf development and senescence in silver birch (Betula pendula ) seedlings

Abstract

Leaf development and senescence were studied in greenhouse-grown silver birch (Betula pendula Roth) seedlings over a period of 7 weeks. Prior to the experiment, leaves from 100 seedlings were marked for five sampling dates. Timing of the developmental phases was studied with biochemical analyses of total soluble protein, Rubisco protein, chlorophyll concentration and at the level of gene expression related to photosynthesis, energy metabolism, ethylene synthesis and protein degradation. During the sampling period, an initial increase in photosynthetic capacity could be seen, when expression of the Rubisco small subunit gene (RbcS) and Rubisco protein (EC 4.1.1.39) were examined. Down-regulation of photosynthesis, visible as a decrease in Rubisco protein and RbcS mRNA, started soon after full expansion of the leaves and processes related to senescence followed, mRNA accumulation for the ethylene-forming enzyme 1-aminocyclopropane-1-carboxylic acid oxidase increased first during the onset of senescence. Protein degradation was observed as a loss of soluble proteins, with a simultaneous increase in the leucine aminopeptidase (EC 3.4.11.1) mRNA levels. The mRNA levels of ribonuclease-like pathogenesis-related protein 10 also increased clearly during senescence, but the mitochondrial phosphate translocator mRNA showed only a slight increase. Chlorophyll concentration of the leaves decreased after the mRNA levels of these senescence-related genes had become more abundant.