Specific mRNA and rRNA Levels in Greening Pea Leaves during Recovery from Iron Stress

Abstract

Hydroponically grown pea seedlings (Pisum sativum L., cv Alaska) were subjected to Fe stress for 10 to 16 days to produce mature chlorotic leaves. Greening was initiated by adding Fe to the nutrient solution. The levels of chlorophylls, chloroplast, and cytoplasmic rRNAs, and specific chloroplast- and nucleus-encoded mRNAs were all significantly lower in leaves developing during iron stress than in nonstressed leaves. In plants greening after addition of Fe, nuclear transcripts encoding chlorophyll a/b-binding protein and the small subunit of ribulose bisphosphate carboxylase/oxygenase increased about 5-fold in abundance following an 18 to 24 hour lag, as did the chloroplast-encoded transcript for the large subunit of the carboxylase/oxygenase. Chloroplast rRNA showed an increase over that in continually stressed control leaves only after a 40 hour lag. The chloroplast-encoded transcript encoding the Q(B)-binding 32 kilodalton polypeptide of Photosystem II showed little change during greening. Chlorophyll itself increased gradually after a lag period of 24 hours, with an increase in chlorophyll a slightly preceding that of chlorophyll b. Kinetic considerations suggest that the changes observed represent a coordinate series of events initiated by readdition of Fe and occurring in parallel. Though accumulation of mRNA for light-harvesting, chlorophyll-a/b-binding protein might limit chlorophyll accumulation at the onset, subsequent changes in the mRNA do not parallel chlorophyll changes. All three of the mRNAs showing recovery on addition of Fe to Fe-stressed plants undergo sharp diurnal fluctuations in abundance. Such fluctuations are comparable to those in nonstressed controls (mRNA for light-harvesting protein) or considerably more pronounced (mRNAs for carboxylase large and small subunits). The carboxylase small subunit mRNA and that for light-harvesting chlorophyll-binding protein were measured under constant conditions of light and temperature. Though a rhythm in greening leaves was hard to detect, it was prominent in the Fe-sufficient controls, persisting undamped through three full cycles for both mRNAs, and hence is probably circadian.