EARLY IRON-DEFICIENCY STRESS-RESPONSE IN LEAVES OF SUGAR-BEET

Iron nutrient deficiency was investigated in leaves of hydroponically grown sugar beets (Beta vulgaris) to determine how ribulose-1,5-bispho sphate carboxylase/oxygenase (Rubisco) gene expression is affected whe n thylakoid components of photosynthesis are diminished. Rubisco polyp eptide content was reduced by 60% in severely iron-stressed leaves, an d the reduction was directly correlated to chlorophyll content. The co ncentration of Rubisco protein in iron-stressed leaves was found to be regulated by availability of mRNAs, and CO2 fixation by Rubisco was r educed from 45 mu mol CO2 m(-2) s(-1) in extracts from iron-sufficient leaves to 20 mu mol CO2 m(-2) s(-1) in extracts from severely stresse d leaves. The rate of CO2 fixation was directly correlated to leaf chl orophyll content. Rubisco in iron-sufficient control leaves was 59% ac tivated, whereas in severely stressed leaves grown under the same ligh t, Rubisco was 43% activated. RNA synthesis was reduced by about 50% i n iron-deficient leaves, but 16S and 25S rRNA and ctDNA were essential ly unaffected by iron stress.