Evidence for post-translational control in the expression of a gene encoding a plastidic thioredoxin during leaf development in Solanum tuberosum plants

The chloroplastic drought-induced stress protein of 32 kDa (CDSP32) is composed of two thioredoxin modules and participates in plastid defence against oxidative damage. Previous data indicated that the CDSP32 gene is induced at the transcript level by environmental and oxidative stress conditions. We further investigated CDSP32 expression in relation to leaf age and to stress conditions in Solanum tuberosum wild-type (WT) and CDSP32-overexpressing plants. Under control conditions, no noticeable variation in CDSP32 mRNA level was observed depending on leaf age in both plant types. A higher protein amount was noticed in developing leaves of WT and the protein abundance decreased in a leaf age-dependent manner. Similar data with a higher magnitude in the protein level were observed in CDSP32-overexpressing plants. Immunoprecipitation experiments revealed no difference between the CDSP32 synthesis rates in young and in mature leaves in WT as well as in transgenic plants. Under water deficit conditions, a strong increase in CDSP32 mRNA level was observed in leaves from WT whereas no change in transcript abundance was noticed in transgenic plants. Similar CDSP32 levels were observed whatever the leaf developmental stage in water-stressed WT and transgenic plants. These data show that, under normal growth conditions, post-transcriptional events likely related to protein turnover play a fundamental role in regulating the expression of CDSP32 during leaf development. Further, they also indicate that the CDSP32 thioredoxin likely participates in the protection of the photosynthetic structures in developing leaves.