Does the Clp protease play a role during senescence-associated protein degradation in barley leaves?

Abstract

Leaf senescence has been studied both under natural conditions and in a model system. In the first case flag leaves of field grown barley plants were characterized by different parameters indicating the onset and course of senescence. Decreases in photosystem II efficiency and chlorophyll content have been shown to occur in parallel with decreases in the levels of several transcripts related to photosynthesis (e.g. rbcL, coding for the large subunit of Rubisco; see also K. Humbeck, S. Quast and K. Krupinska, Plant Cell and Environment, 19 (1996) 337–344). Inversely, a senescence specific mRNA represented by the cDNA clone pHvS40 (W. Becker and K. Apel, Planta, 189, (1993) 74–79) has been shown to accumulate during senescence of the flag leaves. In the second case senescence has been studied with barley primary foliage leaves which were induced to senescence by transfer of young plants to darkness. Dark induced decreases of photosystem II efficiency and protein content on the one hand and accumulation of the senescence specific transcript ( pHvS40) on the other hand clearly indicate that transfer of the plants to darkness induces typical senescence processes. By measurement of the same parameters it has been demonstrated that these dark induced senescence processes are reversible by retransfer of the plants into light under the conditions used. Both cases of leaf senescence have been used to investigate whether the plastid homologue of E. coli Clp protease is playing a role in the senescence related protein degradation of plastids. As a first approach, transcript levels of the plastid encoded catalytic subunit P of Clp protease have been analyzed. Surprisingly, Northern blot analyses clearly snow that transcript levels of clpP are high in photosynthetically active leaves and decrease during senescence. These data suggest that Clp protease is not playing a major role for protein degradation during senescence but rather has a function in non-senescing chloroplasts.