Ontogenic changes of thioredoxins f and m, and of their targets fructose-1,6-bisphosphatase and NAD(P)-malate dehydrogenase, of pea plants grown under light stress conditions

The transcript (mRNA) level and the protein content (as determined by enzym e-linked immunosorbent assay) of thioredoxins (Trx) f and m, and of their t argets, chloroplast fructose-1,6-bisphosphatase (FBPase) and NAD(P)-malate dehydrogenase (NADP-MDH), increase over the ontogeny of pea plants grown un der normal conditions, showing their highest values before flowering (40 d growth). The dearest results appear in apical, but also in middle leaves. E nzyme activites of FBPase and NADP-MDH were lowest just before flowering. I n the case of FBPase this was probably a mechanism to facilitate triose-pho sphate export to the cytosol for sucrose synthesis. The likely function of NADP-MDH is to supply the cytosol, via the malate translocator, with the NA D(P)H necessary for UTP regeneration in the sucrose biosynthetic pathway. B oth the F-v/F(m)ratio and the net photosynthetic rate (IRGA) decreased at s aturating irradiance (16 h photoperiod) and under sub-saturating continuous light. However, the F-v/F-m quotient recovered to normal values after seve ral days adaptation to high light. A similar recovery was also observed in net photosynthesis, although normal levels were never obtained. Under light -stress conditions the concentration of Trxs f and m, and of the targets FB Pase and NADP-MDH, were somewhat lower than those of unstressed plants. Eve n though the levels of the corresponding transcripts (mRNAs) are similar in upper leaves from control and light-stressed plants, those of the middle a nd basal leaves from plants grown under high light were substantially highe r than those of the control plants. In addition to the well-documented tran scriptional regulation of nuclear-coded chloroplast proteins, these results seem to indicate the existence of an additional post-transcriptional contr ol.