Heat stress results in incomplete C-to-U editing of maize chloroplast mRNAs and correlates with changes in chloroplast transcription rate

The editing status rps14 and rpl20 mRNAs decreased rapidly from nearly 100% edited to about 30% edited when maize plants were shifted from 20 degrees C to 37 degrees C. A decrease in the extent of editing was easily detected within 2 h and decreased to a steady-state level of about 40% C-to-U conversion at 37 degrees C. In contrast, the editing status of these chloroplast mRNAs increased relatively slowly after plants were shifted from 37 degrees C to 20 degrees C. Chloroplasts isolated from maize plants which were grown at 20 degrees C and then shifted to 37 degrees C for 24 h were 5-10 times more transcriptionally active than chloroplasts isolated from maize plants grown continuously at 20 degrees C. Thus, the high transcription rate at 37 degrees C may establish a kinetic condition where the rate of transcription exceeds the capacity of the editing apparatus and results in incomplete editing.