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

Abstract

The transcript (mRNA) level and the protein content (as determined by enzyme-linked immunosorbent assay) of thioredoxins (Trx) f and m, and of their targets, chloroplast fructose-1,6-bisphosphatase (FBPase) and NAD(P)- malate dehydrogenase (NADP-MDH), increase over the ontogeny of pea plants grown under normal conditions, showing their highest values before flowering (40 d growth). The clearest results appear in apical, but also in middle leaves. Enzyme activites of FBPase and NADP-MDH were lowest just before flowering. In the case of FBPase this was probably a mechanism to facilitate triose-phosphate export to the cytosol for sucrose synthesis. The likely function of NADP-MDH is to supply the cytosol, via the malate translocator, with the NAD(P)H necessary for UTP regeneration in the sucrose biosynthetic pathway. Both the F_v/F_m ratio and the net photosynthetic rate (IRGA) decreased at saturating irradiance (16 h photoperiod) and under sub-saturating continuous light. However, the F_v/F_m quotient recovered to normal values after several 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 FBPase and NADP-MDH, were somewhat lower than those of unstressed plants. Even though the levels of the corresponding transcripts (mRNAs) are similar in upper leaves from control and light-stressed plants, those of the middle and basal leaves from plants grown under high light were substantially higher than those of the control plants. In addition to the well-documented transcriptional regulation of nuclear-coded chloroplast proteins, these results seem to indicate the existence of an additional post-transcriptional control.